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Effects of the hyperbaric environment on the autonomic nervous system (ANS) in recreational divers are not firmly settled. Aim of this exploratory study was to (1) assess ANS changes during scuba diving via recordings of electrocardiograms (ECG) and to (2) study whether nitrox40 better improves sympathovagal balance over air. 13 experienced divers (~40yrs) performed two open-water dives each breathing either air or nitrox40 (25m/39min). 3-channel ECGs were recorded using a custom-made underwater Holter-monitor. The underwater Holter system proved to be safe. Air consumption exceeded nitrox40 consumption by 12% (n = 13; p < 0.05). Both air and nitrox40 dives reduced HR (10 vs 13%; p < 0.05). The overall HRV (pNN50: 82 vs 126%; p < 0.05) and its vagal proportion (RMSSD: 33 vs 50%; p < 0.05) increased during the dive. Moreover, low (LF: 61 vs 47%) and high (HF: 71 vs 140%) frequency power were increased (all p < 0.05), decreasing the ratio of LF to HF (22 vs 34%). : Conventional open-water dives distinctly affect the ANS in experienced recreational divers, with sympathetic activation less pronounced than vagal activation thereby improving the sympathovagal balance. Nitrox40 delivered two positive results: nitrox40 consumption was lower than air consumption, and nitrox40 better improved the sympathovagal balance over air.

Introduction: Scuba diving likely has an impact on the autonomic nervous system (ANS). In the course of conducting trials of underwater ECG recording for measurement of heart rate variability, there was an unexpected stressful event; one participant's regulator iced and began to free-flow., Methods: A custom-made, water- and pressure-tight aluminum housing was used to protect a portable Holter monitor. ECGs were recorded in three experienced divers who witnessed an unplanned moderately stressful incident during diving. The ECG signals were analysed for measures of heart rate variability (HRV)., Results: Analysis for different short-term HRV measures provided consistent results if periods of interest were appropriately time-aligned. There was improvement in sympatho-vagal balance. One diver unexpectedly exhibited an increase in both sympathetic and vagal activity shortly after the incident., Conclusions: A conventional open-water dive affected the ANS of experienced recreational divers as measured by HRV which provides a global evaluation of the ANS and alterations in its two branches. The heart rate variability data gathered from several participating divers around the time of this event illustrate the potential utility of this variable in quantifying stress during diving. HRV data may be useful in addressing relevant diving related questions such as effects of cold, exercise or different breathing gases on ANS function., (Copyright: This article is the copyright of the authors who grant Diving and Hyperbaric Medicine a non-exclusive licence to publish the article in electronic and other forms.)

Background: Implantable cardioverter/defibrillators (ICDs) can detect ventricular fibrillation (VF) and terminate it. For determining the optimal defibrillation threshold, ventricular fibrillation is repetitively induced and terminated with DC shocks. Depending on the protocol, several fibrillation/defibrillation sequences are mandatory before the final implantation of an ICD. This procedure provides an elegant human model of circulatory arrest and resuscitation., Patients and Methods: In anesthetized 73 patients (15 females) of on the average 60+/-11 years, the end-expiratory pressure was set to zero. Left ventricular pressure (LVP) was monitored with a microtip-catheter, central venous pressure (CVP) through a cannula which was advanced into the superior V. cava. ECG was recorded. After testing, a monoexponential function was found to best fit the time courses of LVP, CVP and heart rate. Data are mean+/-S.D., Results: After termination of circulatory arrest, peak LVP increased with a time constant tau of 9.2+/-4.2 beats, CVP decreased with tau=2.8+/-1.5 beats, and RR-intervals decreased with tau=4.3+/-3.5 beats. Correlations between prefibrillatory values and steady-state values after termination of fibrillation were high: peak LVP: r=0.78; CVP: r=0.95; RRI: r=0.82., Summary: After DC termination of VF, the heart 'finds' relatively quickly a steady-state rhythm at the prefibrillatory level (22 beats), thereby normalizing CVP almost in parallel (14 beats). Peak LVP plateaus only after about 40 beats, although reasonable arterial pressures are reached within the first beats. Our data are limited to periods of ventricular fibrillation of no longer than 60s, which limits the generalisability to the setting of clinical cardiac arrest.

Aim: We compared protective effects of a ss-adrenoceptor blocker (metoprolol; Met) and a If current (Ivabradine; Iva) in a rabbit model of myocardial infarction., Methods: Experiments were performed on 44 adult New-Zealand-White (NZW) rabbits. The effects of either metoprolol or ivabradine were assessed 15 min after experimental occlusion of a coronary artery (CAO), 28 days after CAO (drug gavage), and in vitro hearts (Langendorff apparatus). The results were compared with sham and placebo hearts., Results: Metoprolol (0.25 mg/kg) slightly reduced heart rate and left ventricular systolic function. Ivabradine (0.25 mg/kg) reduced heart rate significantly (P<0.05) (18% vs control). Both drugs provided advantages over placebo: mortality was significantly (P<0.01)smaller (6/13 Pla animals died, 2/10 Met animals, and 3/11 Iva animals), left ventricular function was better preserved after 28 days (external power; Pla; Met; Iva=56%; 76%; 74%), and dilatation (BNP) was reduced (P<0.05). In the Pla group, the ST segment was significantly (P<0.05) elevated by 0.35 mV after CAO and exhibited in 50% of the animals Q waves after 28 days, while after ivabradine or metoprolol, ST displacement and Q waves had disappeared. The uneconomic myosin isoenzyme V3 predominated in Met hearts and Iva hearts (V3/V1: 63/37% and 62/38%), while it was further increased in Pla hearts (78/21%). External efficiency was lowest in Pla hearts (1.00+/-0.50 a.u.; P<0.05) and was significantly higher both in Met hearts (4.0+/-1.8 a.u.) and in Iva hearts (3.3+/-1.6 a.u.)., Conclusions: Met and Iva seem suited for the treatment of chronic myocardial infarction.

Background: Heart rate variability (HRV) describes the cyclic variations in heart rate and offers a non-invasive tool for investigating the modulatory effects of neural mechanisms elicited by the autonomic nervous system on intrinsic heart rate., Objective: To introduce the HRV concept to healthy volunteers under control conditions and during scuba diving. In contrast with more established manoeuvres, diving probably activates both the sympathetic and parasympathetic nervous system through various stimuli-for example, through cardiac stretch receptors, respiration pattern, psychological stress, and diving reflex. A further aim of the study was to introduce a measure for determining a candidate's ability to scuba dive by providing (a) standard values for HRV measures (three from the time domain and three from the frequency domain) and (b) physiological responses to a strenuous manoeuvre such as scuba diving., Methods: Twenty five trained scuba divers were investigated while diving under pool conditions (27 degrees C) after the effects of head out immersion and submersion on HRV had been studied., Results and Conclusions: (a) Immersion under pool conditions is a powerful stimulus for both the sympathetic and parasympathetic nervous system. (b) As neither the heart rate nor the HRV changed on going from immersion to submersion, the parasympathetic activation was probably due to haemodynamic alterations. (c) All HRV measures showed an increase in the parasympathetic activity. (d) If a physiological HRV is a mechanism for providing adaptability and flexibility, diving should not provoke circulatory problems in healthy subjects. (e) Either a lower than normal HRV under control conditions or a reduction in HRV induced by diving would be unphysiological, and a scuba diving candidate showing such characteristics should be further investigated.

Because heart rate (HR) is a major determinant of myocardial oxygen consumption (MVO2) a decrease in HR could prevent ischemia or reduce its consequences. We examined the effects of a novel bradycardic agent of the benzazepinone type, DK-AH 269 (DK), on ventricular function and perfusion in 12 isolated, blood-perfused rabbit hearts. HR was significantly reduced by 1mumol/L DK (160 +/- 28 vs. 124 +/- 23 min-1); diastole lengthened from 235 +/- 69 to 334 +/- 85 ms. Aortic flow tended to fall after DK (50.0 +/- 29.6 vs. 35.6 +/- 21.5 ml/min), but stroke volume remained unchanged (0.29 +/- 0.16 vs. 0.28 +/- 0.17 ml) following DK. Peak left-ventricular pressure (LVPmax) (106 +/- 29 vs. 92 +/- 35 mmHg) and dp/dtmax (1482 +/- 582 vs. 1247 +/- 644 mmHg/s) were decreased. dp/dtmin, as a measure of early relaxation, was also decreased (-1361 +/- 362 vs, -1125 +/- 488 mmHg/s), whereas the end-diastolic pressure (LVPed) was increased (20 +/- 12 vs. 25 +/- 17 mmHg). Coronary blood flow (CBF) per beat was not affected by DK: 0.07 +/- 0.02 vs. 0.07 +/- 0.02 ml. However, the coronary resistance increased with DK from 0.76 +/- 0.29 to 1.13 +/- 0.66 mmHg/(ml/min/100 g). The MVO2 was decreased (6.8 +/- 3.4 vs. 5.9 +/- 2.8 ml/min/100 g). The relation between subendocardial and subepicardial flow (colored microspheres) was unchanged after DK (1.12 +/- 0.22 vs. 1.13 +/- 0.16). Using electrical pacing to restore the control HR, dp/dtmax, LVPed, and MVO2 were nearly restored to predrug levels. In contrast, stroke volume, LVPmax, dp/dtmin and CBF per beat were less than control. In summary: DK effectively reduces heart rate and increases diastole. In parallel with the moderately reduced contractile function, MVO2 is reduced whereas CBF per beat is preserved. These results suggest that this novel bradycardic agent could be useful in treating unwanted tachycardia in the experimental setting, postoperative tachycardia in patients with heart disease or be useful even in treating coronary heart disease.

Beside wall tension and contractility, heart rate is a major determinant of myocardial oxygen consumption. Therefore, a decrease in heart rate could prevent ischemia or reduce its consequences. We examined the effect of a new bradycardic agent of the benzazepinone-type (DK-AH 269) on eight isolated, saline-perfused rabbit hearts, bradycardia resulted from a specific blockade of i(f)-channels in sinus node cells. After control measurements (C), the substance was added in three increasing concentrations (D1: 10(-8) M, D2: 10(-7) M, D3: 10(-6) M). We observed a dose-dependent reduction in heart rate (C: 206 +/- 25, D1: 195 +/- 30, D2: 77 +/- 41, D3: 154 +/- 48/min). In the highest dosage, the duration of diastole was increased by 100%. To characterize systolic function, we measured stroke volume (SV), peak left ventricular pressure (LVPmax) and its first derivative (dP/dtmax). Aortic flow was slightly decreased whereas SV increased to 108% of control after initial reduction at the two lower dosages. LVPmax remained unchanged, and dP/dtmax was dose-dependently reduced to 91, 81, and 70% of control (C: 1885 +/- 376, D1: 1721 +/- 525, D2: 1526 +/- 504, D3: 1327 +/- 337 mm Hg/s); dP/dtmin as a measure of early relaxation was also reduced. The coronary flow per beat did not change compared with control in the presence of the two lower doses of DK-AH 269, but was significantly increased with the highest dose (C: 0.29 +/- 0.06, D1: 0.28 +/- 0.07, D2: 0.29 +/- 0.09, D3: 0.34 +/- 0.11 ml). The myocardial oxygen demand was dose-dependently decreased (C: 10.4 +/- 2.5, D1: 9.6 +/- 2.5, D2: 8.8 +/- 2.6, D3: 7.9 +/- 2.4 ml/min/100 g). The relation between subendocardial and subepicardial flow, assessed with colored microspheres, exhibited no changes in the presence of the highest dose of DK-AH 269 (C: 1.28 +/- 0.09, D3: 1.27 +/- 0.08). DK-AH 269 reduced heart rate in isolated rabbit hearts and increased the duration of diastole. Whereas systolic function was primarily left unchanged, coronary flow per beat and oxygen consumption were decreased. According to our results, this new bradycardic agent could be useful in treating coronary heart disease.

Unlabelled: After heart rate variability (HRV) had been established in the clinic, the question about the minimum interval length for analyzing electrocardiograms emerged. Respiration rate, heart rate and heart rate variability were analyzed in 25 sport divers during 6 min intervals at control, immersion, submersion and while SCUBA diving. Thereafter, the interval length was systematically shortened to 1 min., Results: Respiration rate was significantly reduced during submersion and diving. Heart rate, in turn, remained essentially unchanged during the four experimental steps. The HRV measures in the time domain (standard deviation, coefficient of variation, RMSSD and pNN50) exhibited significant changes during immersion, submersion and diving compared to control conditions. The spectral density in the low frequency range was increased compared to control, the increase being significant during diving. Immersion, submersion and diving, thus, present strong stimuli for the autonomic nervous system. The length of the HRV measures of the time domain could be shortened to 3 min without significant loss of information, except for pNN50. Reduction of the respiration rate during diving considerably shifted the respiratory arrhythmia from the high to the low frequency range. Such shifts deserve special attention interpreting HRV measures from the frequency domain. The interval length for the measures in the frequency domain could only be shortened to 5 min., Conclusion: Measures from the time domain, in particular standard deviation and co-efficient of variation, seem to be superior to measures from the frequency domain in analyzing short-term HRV.