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Background: The "burden" of atrial fibrillation (AF) detected by screening likely influences stroke risk, but the distribution of burden is not well described., Objectives: This study aims to determine the frequency of AF and the distribution of AF burden found when screening individuals ≥70 years of age with a 14-day electrocardiograph monitor., Methods: This is a cohort study of the screening arm of a randomized AF screening trial among those ≥70 years of age without a prior AF diagnosis (between 2019 and 2021). Screening was performed with a 14-day continuous electrocardiogram patch monitor., Results: Analyzable patches were returned by 5,684 (95%) of screening arm participants; the median age was 75 years (Q1-Q3: 72-78 years), 57% were female, and the median CHA 2 DS 2 -VASc score was 3 (Q1-Q3: 2-4). AF was detected in 252 participants (4.4%); 29 (0.5%) patients had continuous AF and 223 (3.9%) had paroxysmal AF. Among those with paroxysmal AF, the average indices of AF burden were of low magnitude with right-skewed distributions. The median percent time in AF was 0.46% (Q1-Q3: 0.02%-2.48%), or 75 (Q1-Q3: 3-454) minutes, and the median longest episode was 38 (Q1-Q3: 2-245) minutes. The upper quartile threshold of 2.48% time in AF corresponded to 7.6 hours. Age >80 years was associated with screen-detected AF in our multivariable model (OR: 1.46; 95% CI: 1.06-2.02)., Conclusions: Most AF detected in these older patients was very low burden. However, one-quarter of those with AF had multiple hours of AF, raising concern about stroke risk. These findings have implications for targeting populations for AF screening trials and for responding to heart rhythm alerts from mobile devices (GUARD-AF [A Study to Determine if Identification of Undiagnosed Atrial Fibrillation in People at least 70 Years of Age Reduces the Risk of Stroke]; NCT04126486)., Competing Interests: Funding Support and Author Disclosures The GUARD-AF trial was funded by Bristol Myers Squibb (Princeton, New Jersey, USA) and Pfizer (New York, New York, USA). The authors are solely responsible for the drafting and editing of the paper and its final contents. Dr Singer has received research support from Bristol Myers Squibb; has received consulting fees from Bristol Myers Squibb, Fitbit, Medtronic, and Pfizer; and has been supported, in part, by the Eliot B. and Edith C. Shoolman Fund of Massachusetts General Hospital. Dr Atlas has received sponsored research support from Bristol Myers Squibb/Pfizer and the American Heart Association (18SFRN34250007); and he has received consulting fees from Bristol Myers Squibb, Pfizer, and Fitbit. Dr Go has received research support through his institution from Bristol Myers Squibb/Pfizer, Johnson & Johnson, and iRhythm Technologies. Dr Lubitz has received support from National Institutes of Health (NIH) grants R01HL139731 and R01HL157635, and the American Heart Association (18SFRN34250007); has received sponsored research support from Bristol Myers Squibb, Pfizer, Boehringer Ingelheim, Fitbit, Medtronic, Premier, and IBM; has received consulting fees from Bristol Myers Squibb, Pfizer, Blackstone Life Sciences, and Invitae; and is an employee of Novartis as of July 18, 2022. Dr McManus has received sponsored research support from Bristol Myers Squibb, Pfizer, Flexcon, and Boehringer Ingelheim; has received consulting fees from Bristol Myers Squibb, Pfizer, Fitbit, Heart Rhythm Society, and Flexcon; and has received payment to serve on Data Safety Boards for NAMSA and Avania. Dr Rothberg has received consulting fees from Blue Cross/Blue Shield Association. Dr Crosson has formerly been employed at iRhythm Technologies. Dr Aronson, Ms Mills, and Mr Patlakh are employees of Bristol Myers Squibb. Dr Lopes has received grants or contracts from Amgen, Bristol-Myers Squibb, GlaxoSmithKline, Medtronic, Pfizer, and Sanofi-Aventis; has received funding for educational activities or lectures from Pfizer, Bristol-Myers Squibb, Novo Nordisk, and AstraZeneca; and has received consulting fees from Bayer, Boehringer Ingelheim, Bristol-Myers Squibb, Novo Nordisk, and AstraZeneca. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose., (Copyright © 2025 The Authors. Published by Elsevier Inc. All rights reserved.)

Background: Atrial fibrillation (AF) often remains undiagnosed, and it independently raises the risk of ischemic stroke, which is largely reversible by oral anticoagulation. Although randomized trials using longer term screening approaches increase identification of AF, no studies have established that AF screening lowers stroke rates., Objectives: To address this knowledge gap, the GUARD-AF (Reducing Stroke by Screening for Undiagnosed Atrial Fibrillation in Elderly Individuals) trial screened participants in primary care practices using a 14-day continuous electrocardiographic monitor to determine whether screening for AF coupled with physician/patient decision-making to use oral anticoagulation reduces stroke and provides a net clinical benefit compared with usual care., Methods: GUARD-AF was a prospective, parallel-group, randomized controlled trial designed to test whether screening for AF in people aged ≥70 years using a 14-day single-lead continuous electrocardiographic patch monitor could identify patients with undiagnosed AF and reduce stroke. Participants were randomized 1:1 to screening or usual care. The primary efficacy and safety outcomes were hospitalization due to all-cause stroke and bleeding, respectively. Analyses used the intention-to-treat population., Results: Enrollment began on December 17, 2019, and involved 149 primary care sites across the United States. The COVID-19 pandemic led to premature termination of enrollment, with 11,905 participants in the intention-to-treat population. Median follow-up was 15.3 months (Q1-Q3: 13.8-17.6 months). Median age was 75 years (Q1-Q3: 72-79 years), and 56.6% were female. The risk of stroke in the screening group was 0.7% vs 0.6% in the usual care group (HR: 1.10; 95% CI: 0.69-1.75). The risk of bleeding was 1.0% in the screening group vs 1.1% in the usual care group (HR: 0.87; 95% CI: 0.60-1.26). Diagnosis of AF was 5% in the screening group and 3.3% in the usual care group, and initiation of oral anticoagulation after randomization was 4.2% and 2.8%, respectively., Conclusions: In this trial, there was no evidence that screening for AF using a 14-day continuous electrocardiographic monitor in people ≥70 years of age seen in primary care practice reduces stroke hospitalizations. Event rates were low, however, and the trial did not enroll the planned sample size.(Reducing Stroke by Screening for Undiagnosed Atrial Fibrillation in Elderly Individuals [GUARD-AF]; NCT04126486)., Competing Interests: Funding Support and Author Disclosures The Bristol Myers Squibb/Pfizer Alliance funded the GUARD-AF trial but was not involved in data collection, analysis, or interpretation; manuscript preparation; or decision to publish. Dr Lopes has received research grants or contracts from Amgen, Bristol Myers Squibb, GlaxoSmithKline, Medtronic, Pfizer, and Sanofi; has received funding for educational activities or lectures from Pfizer, Daiichi-Sankyo, and Novo Nordisk; and has received funding for consulting or other services from Bayer, Boehringer Ingelheim, Bristol Myers Squibb, and Novo Nordisk. Dr Atlas has received sponsored research support from Bristol Myers Squibb/Pfizer and the American Heart Association (18SFRN34250007); and has consulted for Boehringer Ingelheim, Bristol Myers Squibb, Fitbit, Pfizer, and Premier. Dr Go has received a research grant from Bristol Myers Squibb/Pfizer Alliance. Dr Lubitz is an employee of Novartis. Dr McManus has received grants or research support from Apple, Fitbit, Boehringer Ingelheim, Pfizer, Bristol Myers Squibb, and Flexcon; and has received consulting fees from Fitbit, NAMSA, Avania, and Heart Rhythm Society. Dr Rothberg is a consultant for Blue Cross Blue Shield Association. Prof Crosson is a former employee of iRhythm Technologies,. Dr Aronson and Mr Patlakh are employees of Bristol Myers Squibb. Ms. Mills is a former employee of Bristol Myers Squibb, Inc. Dr O’Brien has received institutional research funding from Pfizer. Dr Singer has received research support from Bristol Myers Squibb; and has consulted for the Bristol Myers Squibb, Pfizer, Medtronic, and Fitbit (Google). All other authors have reported that they have no relationships relevant to the contents of this paper to disclose., (Copyright © 2024 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.)

Background: In the AUGUSTUS trial (An Open-Label, 2 x 2 Factorial, Randomized Controlled, Clinical Trial to Evaluate the Safety of Apixaban vs Vitamin K Antagonist and Aspirin vs Aspirin Placebo in Patients With Atrial Fibrillation and Acute Coronary Syndrome or Percutaneous Coronary Intervention), the combination of dual antiplatelet therapy plus oral anticoagulation increased the risk of bleeding without reducing ischemic events compared with a P2Y12 inhibitor plus oral anticoagulation among patients with atrial fibrillation and acute coronary syndrome or elective percutaneous coronary intervention. However, AUGUSTUS enrolled patients up to 14 days after acute coronary syndrome or percutaneous coronary intervention, and there may be a benefit to dual antiplatelet therapy plus oral anticoagulation early after an ischemic event., Methods: In this secondary analysis of AUGUSTUS, we divided patients into groups based on whether they were enrolled <6 days (early) or ≥6 days (later) after their index acute coronary syndrome or percutaneous coronary intervention, and tested the interaction between time from the index event to enrollment and randomized treatment (apixaban versus vitamin K antagonist and aspirin versus placebo) on 30-day and 6-month clinical outcomes using Cox proportional hazards models., Results: Among 4605 patients enrolled in AUGUSTUS with data available on time from the index event to enrollment, the median time from the index event to enrollment was 6 (range, 0-14) days. There were no significant interactions between time from the index event and aspirin versus placebo on clinical outcomes at 30 days or 6 months, though patients with time from the index event <6 days had a nominally significant reduction in death or ischemic events at 30 days with aspirin (hazard ratio, 0.55 [95% CI, 0.30-0.99]), whereas patients with time from the index event ≥6 days did not (hazard ratio, 0.88 [95% CI, 0.54-1.43]; interaction P =0.23). There were no significant interactions between time from the index event and apixaban versus vitamin K antagonist on clinical outcomes., Conclusions: Among patients with atrial fibrillation with acute coronary syndrome or undergoing percutaneous coronary intervention, there was no difference in the relative benefit of apixaban versus vitamin K antagonist or aspirin versus placebo when patients were enrolled early versus later after their index event., Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT02415400., Competing Interests: Dr Fanaroff: research grants from the American Heart Association, National Institutes of Health, and Cardiovascular Systems Inc; and consulting fees from Intercept Pharmaceuticals and Abbott Laboratories. Dr Granger: research grants from Bayer, Boehringer Ingelheim, Bristol Myers Squibb, Daiichi Sankyo, Janssen, Pfizer, Armetheon, AstraZeneca, US Food and Drug Administration (FDA), GlaxoSmithKline, The Medicines Company, Medtronic Foundation, Medtronic, Inc, and Novartis; consulting fees from Bayer, Boehringer Ingelheim, Boston Scientific, Bristol Myers Squibb, Daiichi Sankyo, Janssen, Pfizer, Abbvie, Armetheon, AstraZeneca, Eli Lilly, Gilead, GlaxoSmithKline, Hoffmann-La Roche, The Medicines Company, National Institutes of Health (NIH), Novartis, Sirtex, Verseon, Apple, Medscape, LLC, Merck, Novo Nordisk, Roche Diagnostics, and Rho Pharmaceuticals. Dr Goodman: research grant support and speaker/consulting honoraria from Amgen, Anthos Therapeutics, AstraZeneca, Bayer, Boehringer Ingelheim, Bristol Myers Squibb, CSL Behring, CYTE, Ltd, Daiichi-Sankyo/American Regent, Eli Lilly, Esperion, Ferring Pharmaceuticals, HLS Therapeutics, JAMP Pharma, Merck, Novartis, Novo Nordisk A/C, Pendopharm/Pharmascience, Pfizer, Regeneron, Sanofi, Servier, Tolmar Pharmaceuticals, and Valeo Pharma; salary support/honoraria from the Heart and Stroke Foundation of Ontario/University of Toronto (Polo) Chair, Canadian Heart Failure Society, Canadian Heart Research Centre and MD Primer, Canadian Virtual Coordinating Centre for Global Collaborative Cardiovascular Research, Cleveland Clinic Coordinating Centre for Clinical Research, Duke Clinical Research Institute, New York University Clinical Coordinating Centre, Baim Institute for Clinical Research, and Thrombolysis in Myocardial Infarction Study Group (Brigham Health). Dr Aronson: employee of Bristol Myers Squibb. Dr Windecker: research, travel, or educational grants to the institution without personal remuneration from Abbott, Abiomed, Amgen, AstraZeneca, Bayer, Braun, Biotronik, Boehringer Ingelheim, Boston Scientific, Bristol Myers Squibb, Cardinal Health, CardioValve, Cordis Medical, Corflow Therapeutics, CSL Behring, Daiichi Sankyo, Edwards Lifesciences, Farapulse, Inc, Fumedica, Guerbet, Idorsia, Inari Medical, InfraRedx, Janssen-Cilag, Johnson & Johnson, MedAlliance, Medicure, Medtronic, Merck Sharp & Dohm, Miracor Medical, Novartis, Novo Nordisk, Organon, OrPha Suisse, Pharming Tech, Pfizer, Polares, Regeneron, Sanofi-Aventis, Servier, Sinomed, Terumo, Vifor, and V-Wave; advisory board member and steering/executive group member of trials funded by Abbott, Abiomed, Amgen, AstraZeneca, Bayer, Boston Scientific, Biotronik, Bristol Myers Squibb, Edwards Lifesciences, MedAlliance, Medtronic, Novartis, Polares, Recardio, Sinomed, Terumo, and V-Wave, with payments to the institution but no personal payments; steering/executive committee group member of several investigator-initiated trials funded by industry without impact on his personal remuneration. Dr Mehran: institutional research grants from AstraZeneca, Bayer, Beth Israel Deaconess, Bristol Myers Squibb/Sanofi, CSL Behring, Eli Lilly/Daiichi Sankyo, Medtronic, Novartis, and OrbusNeich; consulting fees from Boston Scientific, Abbott Vascular, Medscape, Siemens Medical Solutions, Roivant Sciences, Inc, and Sanofi; consulting (no fees) to Regeneron Pharmaceuticals, Inc; institutional consulting fees from Abbott Vascular, Spectranetics/Phillips/Volcano Corporation, Bristol Myers Squibb, Novartis, and Watermark Research; executive committee member for Janssen Pharmaceuticals and Bristol Myers Squibb; <1% equity in Claret Medical and Elixir Medical. Dr Alexander: research grants from Bristol Myers Squibb, Boehringer Ingelheim, AstraZeneca, CryoLife, CSL Behring, US FDA, NIH, Sanofi, and VoluMetrix; consulting fees from Pfizer, Bristol Myers Squibb, AbbVie Pharmaceuticals, CSL Behring, Novo Nordisk, Portola Pharmaceuticals, Quantum Genomics, Teikoku Pharmaceuticals, VA Cooperative Studies, and Zafgen. Dr Lopes: research grants from Bristol Myers Squibb, Pfizer, Amgen, Inc, GlaxoSmithKline, Medtronic PLC, and Sanofi Aventis; consulting fees from Bristol Myers Squibb, Pfizer, Boehringer Ingelheim, and Bayer AG. The other authors report no conflicts.

Background: Screening for atrial fibrillation (AF) is attractive because AF independently raises the risk of ischemic stroke, this risk is largely reversible by long-term oral anticoagulant therapy (OAC), and many patients with AF remain undiagnosed and untreated. Recent trials of one-time brief screening for AF have not produced a significant increase in the proportion of patients diagnosed with AF. Trials of longer-term screening have demonstrated an increase in AF diagnoses, primarily paroxysmal AF. To date, however, no trials have demonstrated that screening for AF results in lower rates of stroke. Clinical practice guidelines conflict in their level of support for screening for AF., Methods: The GUARD-AF individually randomized trial is designed to test whether screening for AF in individuals age 70 years or greater using a 2-week single-lead electrocardiographic patch monitor can identify patients with undiagnosed AF and lead to treatment with OAC, resulting in a reduced rate of stroke in the screened population. The trial's efficacy end point is hospitalization for stroke (either ischemic or hemorrhagic) and the trial's safety end point is hospitalization for a bleeding event. End points will be ascertained via Medicare claims or electronic health records at 2.5 years after study start. Enrollment is based in primary care practices and the OAC decision for screen-detected cases is left to the patient and their physician. The initial planned target sample size was 52,000, with 26,000 allocated to either screening or to usual care., Results: Trial enrollment was severely hampered by the novel coronavirus disease 2019 (COVID-19) pandemic and stopped at a total enrollment of 11,931 participants. Of 5,965 randomized to the screening arm, 5,713 patients (96%) returned monitors with analyzable results. Incidence of screen-detected and clinically detected AF and associated stroke and bleeding outcomes will be ascertained., Conclusions: GUARD-AF is the largest AF screening randomized trial using a longer-term patch-based continuous electrocardiographic monitor. The results will contribute important information on the yield of patch-based AF screening, the "burden" of AF detected (percent time in AF, longest episode), and physicians' OAC decisions as a function of AF burden. GUARD-AF's stroke and bleed results will contribute to pooled trial analyses of AF screening, thereby informing future studies and guidelines., (Copyright © 2022 Elsevier Inc. All rights reserved.)

The present study explored the possibility that an alteration in the transmembrane calcium current (ICa), through its ability to modulate Ca2+ release from the sarcoplasmic reticulum, could contribute to the depressed peak [Ca2+]i we previously observed in hypertrophied failing myocardium. Whole-cell patch clamp was used to measure ICa in single guinea pig ventricular myocytes isolated from hearts of normal guinea pigs and from guinea pig hearts in which hypertrophy and failure were induced by gradually developing left ventricular pressure overload subsequent to ascending aortic banding of young animals. Membrane capacitance (Cm) was significantly greater. and ICa, normalized for Cm, was significantly lower in myocytes from hypertrophied failing hearts. Myocytes from hypertrophied failing hearts did not differ significantly from normal myocytes in terms of the voltage-dependence of the activation variable (d) of ICa (except at -30 mV), the time course of removal of inactivation of ICa, and the time constant of decay of ICa. Measurement of the voltage dependence of the inactivation variable (f) of ICa showed that significantly more steady-state inactivation was present at 0, -10, and -20 mV in myocytes from hypertrophied failing hearts. Multiple regression analysis of all data indicated that ICa density decreased with increasing myocyte membrane area (as reflected by Cm) irrespective of any specific effects of hypertrophy and heart failure. We conclude that ICa, normalized for Cm, is significantly reduced in myocytes isolated from hypertrophied failing hearts, probably by a process associated with increased cell size, per se.

Introduction: Early afterdepolarizations (EADs) can give rise to triggered activity and thereby produce cardiac arrhythmias. We used the whole-cell patch clamp technique to examine the relationship between L-type Ca2+ channel window current and the generation of EADs in single ventricular myocytes isolated from guinea pig hearts., Methods and Results: With a high concentration of EGTA in the internal solution and Na(+)-containing physiologic external solution, EADs were induced in unclamped cells by injecting intracellular depolarizing current pulses. During voltage clamp protocols designed to simulate action potentials interrupted by EADs, we recorded an inward shift in total current up to 0.7 pA/pF over 400 msec at test steps in the range of the take-off potential for EADs. Cd2+ (0.2 mM) blocked most of the inward shift of current during the test steps and abolished EADs. When the same voltage clamp protocol was used following perfusion with an Na(+)-free, K(+)-free external solution, the Cd(2+)-sensitive inward currents recorded during the test steps were similar to those obtained in physiologic external solution. The overlapping range of potentials for partial activation of the d and f variables of L-type Ca2+ current ("window" region) measured in Na(+)-free, K(+)-free external solution was virtually the same as the voltage range of the Cd(2+)-sensitive inward currents., Conclusion: Our experiments suggest that: (1) EADs can arise under conditions of high EGTA buffering of intracellular [Ca2+]; and (2) under these conditions, L-type Ca2+ channel window current plays a major role in the initiation of EADs.

Calcium ions appear to play a central role in the development of ventricular arrhythmias associated with digitalis intoxication. Therefore, the effects of the calcium channel antagonist, verapamil, on ouabain-induced ventricular tachycardia were investigated. Ventricular tachycardia (three or more consecutive wide QRS complexes) was induced in guinea pigs by intravenous infusion of ouabain (1 microgram/min) and bursts of rapid ventricular stimulation. Of seven guinea pigs given the infusion of ouabain, all developed ventricular tachycardia at a dose of 82 +/- 17 micrograms/kg (mean +/- SD) and none developed heart block or asystole. Eight guinea pigs were treated with verapamil (2 micrograms/min for 30 minutes) prior to exposure to ouabain. Of those eight animals, only two developed ventricular tachycardia but six developed heart block or asystole at a significantly higher dose of ouabain (154 +/- 47 micrograms/kg). None of three control guinea pigs given an infusion of normal saline for 90 minutes developed ventricular tachycardia. These results show that pretreatment with verapamil inhibits ouabain-induced ventricular tachycardia in guinea pigs. Combined treatment with verapamil and ouabain is associated with a high incidence of heart block and asystole, which may limit the usefulness of verapamil.

We investigated the basis for impaired left ventricular function of hearts in which hypertrophy was produced by gradual pressure overload. We measured myoplasmic free calcium concentration ([Ca2+]i) with fura-2 and sarcomere shortening in single myocytes isolated from control hearts and hypertrophied failing hearts. Diastolic [Ca2+]i was normal, but [Ca2+]i at the peak of contraction was depressed in myocytes from failing hypertrophied hearts. Increasing drive rate from 0.20 Hz to 5.00 Hz increased both diastolic and peak [Ca2+]i. Norepinephrine (3 x 10(-6) M) increased diastolic [Ca2+]i in all cells and tended to normalize peak [Ca2+]i in myocytes from hypertrophied failing hearts during 5.00 Hz drive. Depressed peak [Ca2+]i in the hypertrophied cells was paralleled by significant decreases in both the velocity and percent of sarcomere shortening, which were measured in cells not loaded with fura-2. Sarcomere length was correlated with estimates of [Ca2+]i in intact cells and with controlled levels of [Ca2+] in chemically "skinned" myocytes. A plot of sarcomere length against [Ca2+] gave a single continuous relationship that spanned resting and peak values at all drive rates in both the control and hypertrophied myocytes. Thus heart failure in this model is reflected in impaired myocyte contraction, which is closely related to reduced levels of [Ca2+]i during systole rather than to depressed myofilament sensitivity to Ca2+.

It is suggested that torsades de pointes may be only one of a group of arrhythmias that are characterized by being pause induced or bradycardia induced. A distinction is made between the cause of the "twisting of the points" and the cause of the action potentials that initiate and sustain the tachycardia. It is pointed out that torsades de pointes and other pause-induced arrhythmias share many features with rhythmic activity arising from early afterdepolarizations. Both are seen after pauses or at low rates, both are seen in quinidine intoxication, and both are seen in hypokalemia. The short-long-short sequence that is seen in torsades de pointes and certain other pause- or bradycardia-induced arrhythmias can be fully explained by the behavior of rhythmic activity initiated and sustained by early afterdepolarizations, as can the abrupt onset and termination of pause-induced arrhythmias and their tendency to show initial warming up and terminal slowing down.

Triggered activity induced by delayed afterdepolarizations has been suggested as a cellular mechanism for some arrhythmias. The development of triggered activity should be favored by conditions that increase myoplasmic Ca2+ and increase membrane resistance. A solution which could create these conditions was devised. After perfusion with normal Tyrode's solution, 24 trabeculae were exposed to the experimental solution. All preparations developed triggered activity after exposure to the experimental solution, 83% developed delayed afterdepolarizations before the onset of triggered activity, and triggered activity stopped in all trabeculae after reperfusion with normal Tyrode's solution. The interaction of ouabain, lidocaine, and three Ca2+ channel blockers with triggered activity induced by the experimental solution is described. Verapamil inhibited triggered activity but not underlying voltage oscillations, whereas nisoldipine and Mn2+ inhibited both triggered activity and voltage oscillations. Lidocaine did not inhibit afterdepolarizations or triggered activity. Exposure to ouabain for 10 min caused delayed afterdepolarizations but not triggered activity. Our results show that the experimental solution induced triggered activity, which was highly reliable and readily reversible. The high reproducibility of this activity enables the study of interactions of pharmacological agents with this triggered activity. This may contribute to the further understanding of the mechanism underlying some arrhythmias.

This study investigates the role of reducing potassium conductance (gK) in generating delayed afterdepolarizations and triggered activity in small preparations of ventricular muscle from guinea-pig hearts. We used agents believed to reduce gK (low or absent K0, tetraethylammonium (TEA), CsCl) and we used ouabain (10(-6) M) to induce delayed afterdepolarizations. Treatment with ouabain only caused subthreshold delayed afterdepolarizations or occasionally non-sustained triggered activity. Exposure to Tyrode's solution with K reduced from 4 to 2 mM or K-free Tyrode's solution, with or without ouabain, caused subthreshold delayed afterdepolarizations and sometimes non-sustained triggered activity. Exposure to Tyrode's solution containing TEA and ouabain caused sustained triggered activity, supporting the hypothesis that accumulation of extracellular K inhibits the development of triggered activity. Presumably, the reduction in gK caused by TEA is not reversed by accumulation of extracellular K so that the delayed afterdepolarizations in the presence of persistently reduced gK are large enough to induce sustained triggered activity. Under extreme conditions, when Cs replaced K and half the NaCl was replaced by TEA, delayed afterdepolarizations occurred in the presence of markedly reduced gK, the result being the rapid development of sustained triggered activity, even at the basic drive rate of 1 Hz. Our results suggest that reduced gK plays an important role in the development of triggered activity.

Previous studies have shown that myocardium of the diabetic rat has impaired myoplasmic calcium metabolism. Delayed afterdepolarizations and triggered activity are potentiated by conditions believed to increase intracellular calcium concentration therefore, we performed this study to investigate the possibility that myocardium of the diabetic rat is more susceptible than normal tissue to develop afterdepolarizations and triggered activity. We used standard microelectrode techniques to record the electrical activity of papillary muscles from hearts of control rats and rats made diabetic with streptozotocin. We compared the response of control and diabetic preparations to conditions presumed to create progressively more severe degrees of myoplasmic calcium loading, viz. perfusion with solutions containing ouabain (5 X 10(-5) M) and increasing concentrations of calcium (2.4, 4.8, 7.2, and 9.6 mM). Our results showed the following. Ventricular muscle from diabetic rats was more prone than normal myocardium to develop delayed afterdepolarizations and triggered activity under conditions believed to cause myoplasmic calcium overload. The external calcium concentration correlated with the incidence but not the magnitude or coupling interval of the delayed afterdepolarizations in fibers of diabetic rats. The action potentials in fibers of diabetic rats decreased markedly in duration after exposure to ouabain, whereas normal action potentials were not affected significantly; as external calcium was increased with ouabain still present, the action potential duration in diabetic fibers decreased slightly more, whereas the action potential duration in normal fibers did not change significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Cells of the coronary sinus of the canine heart can exhibit triggered activity which each action potential arises from a depolarizing after-potential that follows the previous action potential; an early after-hyperpolarization commonly precedes the delayed after-depolarization and both are increased in amplitude by the addition of noradrenaline. The delayed after-depolarization is thought to be caused by an inward current activated by a rise in intracellular Ca2+ that is, in turn, caused by Ca2+-induced release of Ca2+ from the sarcoplasmic reticulum (s.r.). The effects of caffeine and of ryanodine on the electrical activity of the coronary sinus were investigated because each of those agents is thought to affect the handling of intracellular Ca2+ by the s.r. The steady-state effect of exposure to 5 mM-caffeine is to cause the delayed after-depolarization to move much earlier in the cycle, and become too small to give rise to an action potential so that preparations cannot show triggered activity; moreover, if a burst of activity is in progress it is terminated by exposure to 5 mM-caffeine. Exposure to 0.5 mM-caffeine causes the delayed after-depolarization to move earlier in the cycle but to become larger so that triggered activity is more easily induced and longer lasting than in the absence of caffeine. Shortly after the addition (or wash-out) of 5 mM-caffeine the after-depolarization transiently resembles that seen in the presence of 0.5 mM-caffeine so that bursts of triggered activity may occur just after the addition or removal of 5 mM-caffeine. Exposure to 5 mM-caffeine abolishes early rapid repolarization (phase 1), shifts the plateau to a more positive level and retards the completion of repolarization. The effect on phase 1 is mimicked by exposure to solutions low in Cl-; the effect on the plateau is mimicked by exposure to 20 mM-tetraethylammonium (TEA); fibres exposed to solutions containing 20 mM-TEA and 21 mM-Cl- show action potentials very like those of fibres exposed to 5 mM-caffeine. If a fibre already exposed to a low Cl-, TEA-containing solution is then exposed to 5 mM-caffeine, no further change occurs in the action potential but the characteristic effects of caffeine on the after-depolarization appear. Exposure to ryanodine prevents the appearance of the delayed after-depolarization but leads to the appearance of an exceptionally long depolarizing after-potential that begins very early in diastole and, though waning, persists almost throughout diastole.(ABSTRACT TRUNCATED AT 400 WORDS)

We have found that afterpotentials can be induced selectively in hypertrophied myocardium from rats with renal hypertension. Three kinds of afterpotentials were recorded by standard microelectrode techniques: early afterdepolarizations, delayed afterdepolarizations, and early afterdepolarizations. The first two kinds of afterpotentials could give rise to triggered spontaneous activity, whereas the last kind did not. Delayed afterdepolarizations were induced in hypertrophied fibers exposed to Tyrode's solution containing high extracellular calcium ([Ca2+]o = 7.2-12 mM) and early afterdepolarizations occurred in hypertrophied fibers exposed to Tyrode's solution containing tetraethylammonium (TEA = 10-30 mM). Neither of these treatments produced afterpotentials in normal myocardium. We found that delayed afterdepolarizations became larger when the stimulation frequency, number of preceding driven beats, or [Ca2+]o was increased. The coupling interval from the upstroke of the last driven action potential to the peak of the delayed afterdepolarization decreased when the stimulation frequency or number of preceding driven beats increased. Hypertrophied fibers treated with high [Ca2+]o that gave rise to triggered activity showed a characteristic relationship between delayed afterpotential magnitude and drive cycle length. In hypertrophied muscles treated with TEA, triggered activity developed from an early afterdepolarizations but terminated with a delayed afterdepolarization. The occurrence of afterpotentials in hypertrophied, but not in normal myocardium, appears to reflect the development of an important electrophysiological alteration that may predispose hypertrophied fibers to develop arrhythmias.

We studied the distribution and nature of the electrical changes associated with myocardial hypertrophy induced by renal hypertension in rats. Standard microelectrode techniques were used to study transmembrane action potentials recorded from endocardial, papillary muscle, and epicardial stimulation from hypertrophied (HBP) and normal (SHAM) hearts. We also determined the effects of stimulation frequency on the action potentials recorded from these preparations. To assess whether altered intercellular electrical connections contribute to the electrophysiological changes associated with hypertrophy, we analyzed the spatial steady state voltage decrement produced by passing intracellular constant current pulses and determined the effective input resistance (Rin) of endocardial HBP and SHAM preparations. Our results show that the action potential prolongation that accompanies hypertrophy is not uniform. Thus, the entire course of repolarization is prolonged in epicardial and papillary muscle fibers, but only the latter half of repolarization is prolonged in epicardial fibers. Endocardial action potentials is general, and HBP action potentials in particular, have a distinctive steep relation between duration and stimulation frequency which may be due to a difference in the rate dependence of a membrane conductance(s), although relatively greater accumulation of extracellular potassium or altered activity of the Na+-K+ pump cannot be excluded as contributing factors. In addition, the similarity in the profile of spatial voltage decrement and the values for Rin in HBP and SHAM preparations indicates that alterations in electrotonic coupling between cells are unlikely to account for the prolonged action potentials of hypertrophied myocardium.

The early afterdepolarization, which is an interruption of repolarization, can evoke a second upstroke or a salvo of action potentials. It is suggested that the electrophysiological characteristics of the early afterdepolarization can produce a lengthening of the QT interval and that the second upstroke and salvo of activity that may follow, it can explain many features of torsade de pointes and of certain other ventricular tachycardias. The early afterdepolarization, torsade de pointes, and repetitive monomorphic idiopathic ventricular tachycardia are all induced by bradycardia or by a preceding long RR interal. The R-on-T phenomenon is also discussed.

To determine whether the electrical changes associated with cardiac hypertrophy are due to alterations in the membrane properties of individual hypertrophied cells, we recorded action potentials in single myocytes isolated from normal and hypertrophied hearts. Cardiac hypertrophy was produced by a gradual pressure overload created by placing a band around the ascending aorta in young guinea-pigs (200-250 g). Almost half the animals that developed left ventricular (LHV) hypertrophy also developed evidence of cardiac dysfunction. Action potentials were recorded with standard microelectrodes in single ventricular myocytes isolated by enzymatic dispersion of the heart. The action potential duration at 1 Hz was significantly longer in hypertrophied cells than in control cells. The degree of action potential prolongation in isolated cells did not correlate with the degree of hypertrophy but did correlate with the degree of myocardial disease, the duration being longer in hypertrophied myocytes from dyspneic than in those from non-dyspneic animals. The resting potential was significantly lower in hypertrophied myocytes from dyspneic animals than in hypertrophied cells from non-dyspneic animals or control cells stimulated at 5 Hz. The relationship between the frequency of stimulation (0.33, 1, and 5 Hz) and action potential duration was steeper in hypertrophied than normal myocytes. The mean membrane capacitance (cm) of hypertrophied myocytes increased by 31% over the control value. Thus, isolated hypertrophied myocytes retain the prolonged duration of the action potential and the exaggerated dependence of duration on rate observed in intact hypertrophied muscle. The increased duration of the action potential in hypertrophied cells cannot be readily attributed to the observed increase in cm. Our results indicate that the membrane changes responsible for the altered electrical properties of hypertrophied myocardium are due to an effect of hypertrophy on individual myocytes and that the prolonged duration of the action potential is probably due to changes in active currents flowing during repolarization.

The effects of amrinone on conduction in the intact canine heart were studied. Intracardiac His-electrode catheter recordings were used to measure the functional refractory period (FRP) of the AV node and conduction time through the AV node (A2H2 interval) and in the His-Purkinje system (H2V2 interval). Amrinone (2.5 to 10 mg/kg) shortened the FRP and A2H2 in a dose-dependent manner but had no significant effect on H2V2. In hearts where AV conduction was depressed by treatment with verapamil, propranolol, or ouabain, amrinone partially reversed this depression. Amrinone also shortened the recovery time of spontaneous sinoatrial (SA) node activity following a train of rapid atrial stimulation. This effect was also observed after depression of SA nodal recovery with verapamil, propranolol, or ouabain. These results indicate that amrinone enhances AV conduction and SA nodal activity in the normal heart and may favorably influence depressed AV conduction and SA nodal activity induced by a variety of cardioactive agents.

The effect of membrane potential on the manifestations of ouabain-induced alterations in the electrical activity of shortened sheep cardiac Purkinje fibres was studied by a microelectrode technique. The level of membrane potential was modulated by passing intracellular polarizing current. It was found that the toxic electrical effects induced by exposure to ouabain vary according to the level of membrane potential.

We determined the electrical constants of epicardial and endocardial preparations from both normal and hypertrophied rat hearts. This was done by comparative analysis of the spatial decay of steady-state electronic voltage deflection produced by injection of a hyperpolarizing constant-current pulse. We used a two-dimensional finite disk model to obtain the apparent membrane resistance, (Rm)app, and internal longitudinal resistivity (Ri), (Rm)app was significantly larger in epicardial (565 +/- 222 omega . cm2) than endocardial (375 +/- 137) preparations from normal hearts. This regional difference disappeared in hypertrophied hearts (epicardium 421 +/- 138, endocardium 383 +/- 121 omega . cm2). Ri was similar for normal endocardial (272 +/- 169 omega . cm) and epicardial (326 +/- 152) preparations, as well as for hypertrophied endocardial (251 +/- 108) and epicardial (312 +/- 59) preparations. We determined the effective membrane capacity (Ceff) by measuring the ratio of applied charge to the displacement of membrane potential. Ceff was larger for normal hearts (epicardium 9.7 +/- 2.5 micro F/cm2, endocardium 7.5 +/- 3.0) than for hypertrophied hearts (epicardium 4.1 +/- 1.4, endocardium 4.7 +/- 1.2). From the values for Ceff we calculated the effective membrane resistance, (Rm)eff. (Rm)eff was larger for normal (epicardium 5,392 +/- 2,613 omega . cm2, endocardium 3,013 +/- 2,096) than for hypertrophied (epicardium 1,552 +/- 633, endocardium 1,838 +/- 826) preparations. Our results show that the amount of electrically effective membrane area is decreased in hypertrophied myocardium, despite the increased total area per hypertrophied cell. One functional implication of this finding is that activation of contraction by spread of surface electrical depolarization into the T-tubules may be impaired in hypertrophied cardiac muscle.

To investigate the possible role of an alteration in excitation-contraction coupling in cardiac hypertrophy, we compared simultaneously recorded action potentials along with isometric or isotonic contractions of normal and hypertrophied papillary muscles. Hypertrophy was produced by renal hypertension in rats. Hypertrophied papillary muscles were taken from rats that had been hypertensive for 10 [HBP (10)] or 20 [HBP (20)] weeks. Regression of changes induced by hypertrophy was studied in rats that had been hypertensive for 10 weeks and then made normotensive for 10 weeks by removal of the ischemic kidney. Papillary muscles from age-matched, sham-operated rats [SHAM (10), SHAM (20)] were used as controls. We found that HBP (10) rats had significantly longer action potentials than SHAM (10) rats and that difference in the action potential duration recorded during isotonic and isometric contractions was significantly different from SHAM (10) and HBP (10) rats. Peak developed tension was the same in HBP (10) and SHAM (10) muscles, but the duration of isometric contraction and time-to-peak shortening were longer in HBP (10) muscles. Similarly, whereas the peak tension was the same in HBP (20) and SHAM (20) muscles, the duration of the action potential and isometric contraction, as well as the time-to-peak tension, was longer in HBP (20) muscles. The longer values for action potential duration, isometric contraction, and time-to-peak tension in HBP (20) muscles returned to SHAM values in HBP (R) muscles. The functional relationship between contraction and the action potential time course was assessed by plotting action potential duration against four parameters of contraction: peak developed tension, time-to-peak tension, time-to-half relaxation, and time-to-peak shortening. Statistical analysis of these data showed a significant correlation between action potential duration and all four parameters of contraction in SHAM (10) and SHAM (20) muscles. In contrast, HBP (10) muscles showed a significant correlation between action potential duration and only two contractile parameters, whereas action potential duration did not correlate significantly with any of the contractile parameters in HBP (20) muscles. Remarkably, in HBP (R) preparations a significant correlation was restored between action potential duration and three of the four contractile parameters. The results of this study suggest that reversible cardiac hypertrophy is associated with reversible alterations in excitation-contraction coupling. The reversibility of the mechanical and electrical alterations that accompany hypertrophy suggests, in turn, that cardiac hypertrophy is an adaptive process.

Myocardial function, electrophysiologic characteristics, and structure were studied in rats with both renovascular hypertension and streptozotocin-induced diabetes (HD). Ventricular papillary muscles from untreated rats with HD showed a marked slowing of isometric and isotonic contractions. Peak developed tension and peak shortening were preserved, except in one animal with findings of congestive heart failure. Transmembrane action potentials increased fivefold in duration. Myocardial interstitial fibrosis was frequently observed. Physiologic parameters of rats with HD treated by left nephrectomy, captopril, and insulin were very similar to those of age-matched controls. The mortality rate of rats with HD was 43% over 5 to 6 months in the first study. In a second study, spontaneously dying rats with HD were compared with those deliberately killed. A 55% mortality was observed over 7 months. Myocardial structural damage and histologic evidence of congestive heart failure were more frequent in spontaneously dying rats with HD. Combined renovascular hypertension and diabetes in rats appears to be a new preparation of congestive cardiomyopathy.

The results of previous studies of the relation between the surface electrocardiogram and cellular transmembrane potentials have suggested that the T wave configuration of the ECG is the result of a difference in the duration of endocardial and epicardial action potentials. Ventricular hypertrophy induced by renal hypertension in rats is associated with lengthening of action potential duration and a reproducible decrease in T wave magnitude. Therefore, this model was used to study the relation between the surface T wave configuration and regional differences in action potential duration. ECGs were recorded from hypertensive (HBP) and sham-operated (SHAM) rats. The hearts from these animals were removed and transmembrane action potentials were recorded by standard microelectrode techniques from endocardial and epicardial preparations. We found that the normally peaked T waves seen in the ECG of SHAM rats was reduced by 35% in the ECG of HBP rats. This reduction of T wave magnitude was associated with similar duration of epicardial and endocardial action potentials in HBP rats. However, peaked T wave in SHAM ECG was not accompanied by a significant disparity in the duration of the epicardial and endocardial action potentials. Thus, there is no simple, consistent correlation between surface T wave configuration and regional differences in intracellular action potential duration in rats.

Diabetes induced by streptozotocin in rats is associated with changes in the mechanical function of isolated ventricular papillary muscle. Relaxation is slowed and shortening velocity is depressed. The effects of ouabain (10(-7) to 2 x 10(-4)M) and changes in extracellular calcium ([Ca2+]0 = 0.6 to 12 mM) on the mechanical and electrical properties of normal and diabetic papillary muscles were studied. High doses of ouabain caused a rise in resting tension and a fall in developed tension in both diabetic and control muscles. However, these changes were strikingly greater in diabetic muscles which developed partial contractures at 10(-4)M. The altered response to ouabain was observed in chronically (5 to 7 weeks or 3 months) but not acutely (less than 1 week) diabetic animals. Similarly, the response to ouabain was restored to normal after chronic (5 weeks) therapy with insulin but not after acute (4 days) therapy. In both normal and diabetic muscles, the mechanical effects of increasing [Ca2+]0 from 2.4 to 12.0 mM were qualitatively similar to those seen with ouabain at 10(-5) to 10(-4)M. Electrophysiologic studies showed that under control conditions action potentials of diabetic muscles were significantly longer than those of normal muscles. Treatment with progressively higher concentrations of ouabain (10(-7) to 10(-4)M) and [Ca2+]0 (2.4 to 12.0 mM) caused shortening of both normal and diabetic action potentials, but the effects of these interventions were much greater in the diabetics. These results suggest that the response of diabetic muscles to ouabain is markedly different from normal and that this altered response may be due to impaired regulation of intracellular Ca2+ levels in diabetic myocardium.

A circiut was designed for measuring maximum negative or positive potentials occurring during an action potential. The basic functional unit is a peak-holding circuit that is reset following each action potential. The output of the circuit is displayed on a digital meter and may also be used to drive a point plotter for a graphic presentation of data.

Chronic hypophosphataemia is associated with reversible depression in myocardial performance of the intact heart. To define the basis for this, were studied the mechanical and electrical properties of isolated left ventricular papillary muscles from rats with 6 weeks of phosphorus depletion (P6), 6 weeks of phosphorus repletion and age-matched controls (C). Muscles were perfused with Tyrode's solution (Ca2+ 2.4 mmol.liter-1, 7 = 4.0 mmol . litre-1, dextrose = 5.5 mmol . litre-1) and driven at 0.1 Hz. P6 muscles had slowed isometric relaxation (T1/2R), depressed velocity of shortening and relaxation and prolongation of the transmembrane action potential to 75% of complete repolarisation. These results suggest that the slow relaxation phase may be the result of the prolonged APD75 and that the depressed myocardial performance in the intact heart may be based on impaired relaxation and reduced the velocity of shortening associated with hypophosphataemia. This view is supported by our finding that these alterations are reversible with phosphate repletion.

Dopamine shifted the voltage-dependence of the slow outward current that underlies spontaneous diastolic depolarization to a more positive voltage range and increased the rate constants of inactivation of this current at voltages negative to -80 mV. Dopamine increased the magnitude of the slow inward current and the delayed outward current. Dopamine had an inconsistent effect on the positive dynamic current and the time-independent potassium current. These effects were consistent with effects on the slope of diastolic depolarization, plateau voltage, and action potential duration and are similar to those of epinephrine. The effects of dopamine on the slow inward current and on the delayed outward current were abolished by verapamil, a slow channel blocker. This finding substantiates that outward potassium currents are modulated by intracellular calcium.

We studied the effect of ouabain and other inhibitors of the Na-K pump on the pacemaker current (IK2) in sheep cardiac Purkinje fibers. We found that exposure to ouabain (2 X 10(7) M) caused an apparent decrease in the amplitude of the IK2 activation curve. We also found that a similar effect was caused by exposure to dinitrophenol (DNP) or lithium-substituted Tyrode's solution. Our studies showed that after prolonged (more than 60 minutes) exposure to ouabain a characteristic current noise appeared and IK2 was replaced by an oscillatory inward current (IOS). The apparent threshold for IOS was -50 to -60 mV and the peak amplitude of this current increased with increasing levels of depolarization positive to threshold. These results suggest that (1) the apparent decrease in IK2 may be a consequence of ionic redistribution due to Na-K pump inhibition; (2) cardiac glycosides cause abnormal impulse generation by an ionic mechanism separate from IK2 and involving IOS.

1. The two-microelectrode method of voltage clamp was used in single myocytes isolated from guinea-pig ventricles to investigate the mechanism underlying the arrhythmogenic interaction between low external K+ and digitalis. 2. We investigated the effects of ouabain (10(-6) M) with 4 mM-K+ or 2 mM-K+ on the peak magnitude of the inward component of oscillatory current (Iti) recorded upon repolarization to the resting potential after depolarizing clamps to -5 mV, and on the characteristics of the steady-state current-voltage relationship. 3. Whereas ouabain with 4 mM-K+ did not alter the current-voltage relationship from its control shape, ouabain with 2 mM-K+ caused marked changes in the curve: the zero-current intercept was shifted in a negative direction, the region of low slope conductance was extended to more negative potentials, and the curve was shifted downward relative to the control current-voltage relationship. The changes in the current-voltage relationship induced by ouabain with 2 mM-K+ were very similar to those induced by 2 mM-K+ alone. 4. The functional consequence of the changes in the current-voltage relationship induced by ouabain with 2 mM-K+ was a highly significant reduction in the amount of outward current (Ith) needed to reach the threshold for excitation; Ith was reduced from 1.6 +/- 0.4 nA (n = 6) in ouabain with 4 mM-K+ to 0.8 +/- 0.3 nA (n = 5) in ouabain with 2 mM-K+. 5. The mean value for Iti was larger in ouabain with 2 mM-K+ (0.58 +/- 0.41 nA, mean +/- S.D., n = 4) than in ouabain with 4 mM-K+ (0.42 +/- 0.38 nA, n = 5). Although the increase in Iti was not statistically significant because of the large variability of the measurement, it is possible that the increase might be physiologically significant. 6. Our results suggest that the arrhythmogenic interaction between digitalis and low K+ is due to the combined effects of low K+ on the current-voltage relationship and on the size of the peak inward current induced by ouabain. Whereas the effect of low K+ and ouabain on the inward current was highly variable, the effects on the current-voltage curve were far more consistent, pointing to an important role for alterations in the current-voltage relationship in the arrhythmogenic interaction between low K+ and ouabain.

Propagated action potentials can be obtained in canine cardiac Purkinje fibers exposed to Na-free solutions containing no inorganic cation other than Ca and K. Essentially similar action potentials are obtained if Na is replaced by tetraethylammonium (TEA), tetramethylammonium (TMA), or choline. In a solution containing 128 mM TEA and 16.2 mM Ca the characteristics of these electrical responses were: maximum diastolic potential, -59 +/- 3.3 mV; overshoot, 20 +/- 6.8 mV; maximum upstroke velocity, 3.7 +/- 2.3 V/s; conduction velocity, 0.1 m/s; and action potential duration, 360 +/- 45 ms. The magnitude of the overshoot varied with log Ca(o) with a slope of about 30 mV/10-fold concentration change. The upstroke velocity was an approximately linear function of Ca(o). The active response was greatly diminished or abolished by Mn and D-600 but was unaffected by tetrodotoxin. These Ca-dependent responses appeared in a region of transmembrane potential (about -50 mV) at which the rapid Na-dependent upstroke is abolished even when Na is present.

The effect of endotoxin fromSalmonella typhimurium orEscherichia coli was studied on bilayer lipid membranes (1% lecithin-1% cholesterol in n-decane) formed in buffered 0.1M NaCl solution (pH 6.8). Endotoxin was added to the buffered solution either prior to membrane formation or after stable membranes were formed. In both cases, concentrations of 110 to 720 μg/ml endotoxin initiated a decrease in the electrical resistance of the membranes followed by their rupture. A 50 μg/ml concentration of the agent was unable to elicit any response. Also, the addition of an equal volume of buffer solution, serving as a control, caused no decrease in membrane resistance or survival time. Treatment of the endotoxin with alkaline hydroxylamine to remove esterand amide-bound fatty acids likewise abolished the membrane effect. This is the first report of an endotoxin effect on lipid bilayer membranes. The potential correlation of this interaction of bilayer and endotoxin with the diverse biologic effects of endotoxin is discussed.