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OBJECTIVE:: An inspiratory impedance threshold device was evaluated in spontaneously breathing animals with hypotension to determine whether it could help improve systemic arterial pressures when fluid replacement was not immediately available. DESIGN:: Prospective, randomized. SETTING:: Animal laboratory. SUBJECTS:: Thirty-nine female farm pigs (weight, 28 –33 kg). INTERVENTIONS:: A total of 39 anesthetized spontaneously breathing pigs were treated with an impedance threshold device, with cracking pressures from 0 to −20 cm H2O. Four separate experimental protocols were performed: protocol A, in which the hemodynamics of seven pigs were examined during application of an impedance threshold device at various levels of inspiratory impedance (−5, −10, −15, and −20 cm H2O), both before and after a severe, controlled hemorrhage to a systolic blood pressure of 50 –55 mm Hg; protocol B, in which nine pigs bled to systolic blood pressure of 50 –55 mm Hg were treated with an impedance threshold device set at −12 cm H2O and were compared with nine others treated with a sham device; protocol C, in which the effects of the impedance threshold device on mixed venous gases were measured in seven hemorrhaged pigs; and protocol D, in which the effects of the impedance threshold device on cardiac output in seven hemorrhaged pigs were measured. METHODS AND MAIN RESULTS:: During initial studies with both normovolemic and hypovolemic pigs, sequential increases in inspiratory impedance resulted in a significant increase in systolic blood pressure, whereas diastolic left ventricular and right atrial pressures decreased significantly and proportionally to the level of impedance. When comparing the sham vs. active impedance threshold device (−12 cm H2O) in hypotensive pigs, systolic blood pressure (mean ± sem) with active impedance threshold device treatment increased from 70 ± 2 mm Hg to 105 ± 4 mm Hg (p < .01). Pressures in the control group remained at 70 ± 4 mm Hg (p < .01). Cardiac output increased by nearly 25% (p < .01) with the active impedance threshold device when calculated using the mixed gas equation and when determined by thermodilution. CONCLUSIONS:: These studies demonstrate that it is feasible to use a device that creates inspiratory impedance in spontaneously breathing normotensive and hypotensive pigs to increase blood pressure and enhance cardiopulmonary circulation in the absence of immediate fluid resuscitation. Further studies are needed to evaluate the potential long-term effects and limitations of this new approach to treat hypovolemic hypotension. [ABSTRACT FROM AUTHOR]

OBJECTIVE:: To determine whether an impedance threshold device, designed to enhance circulation, would increase acute resuscitation rates for patients in cardiac arrest receiving conventional manual cardiopulmonary resuscitation. DESIGN:: Prospective, randomized, double-blind, intention-to-treat. SETTING:: Out-of-hospital trial conducted in the Milwaukee, WI, emergency medical services system. PATIENTS:: Adults in cardiac arrest of presumed cardiac etiology. INTERVENTIONS:: On arrival of advanced life support, patients were treated with standard cardiopulmonary resuscitation combined with either an active or a sham impedance threshold device. MEASUREMENTS AND MAIN RESULTS:: We measured safety and efficacy of the impedance threshold device; the primary end point was intensive care unit admission. Statistical analyses performed included the chi-square test and multivariate regression analysis. One hundred sixteen patients were treated with a sham impedance threshold device, and 114 patients were treated with an active impedance threshold device. Overall intensive care unit admission rates were 17% with the sham device vs. 25% in the active impedance threshold device (p = .13; odds ratio, 1.64; 95% confidence interval, 0.87, 3.10). Patients in the subgroup presenting with pulseless electrical activity had intensive care unit admission and 24-hr survival rates of 20% and 12% in sham (n = 25) vs. 52% and 30% in active impedance threshold device groups (n = 27) (p = .018, odds ratio, 4.31; 95% confidence interval, 1.28, 14.5, and p = .12, odds ratio, 3.09; 95% confidence interval, 0.74, 13.0, respectively). A post hoc analysis of patients with pulseless electrical activity at any time during the cardiac arrest revealed that intensive care unit and 24-hr survival rates were 20% and 11% in the sham (n = 56) vs. 41% and 27% in the active impedance threshold device groups (n = 49) (p = .018, odds ratio, 2.82; 95% confidence interval, 1.19, 6.67, and p = .037, odds ratio, 3.01; 95% confidence interval, 1.07, 8.96, respectively). There were no statistically significant differences in outcomes for patients presenting in ventricular fibrillation and asystole. Adverse event and complication rates were also similar. CONCLUSIONS:: During this first clinical trial of the impedance threshold device during standard cardiopulmonary resuscitation, use of the new device more than doubled short-term survival rates in patients presenting with pulseless electrical activity. A larger clinical trial is underway to determine the potential longer term benefits of the impedance threshold device in cardiac arrest. [ABSTRACT FROM AUTHOR]

OBJECTIVEIntrathoracic pressure changes are of particular importance under hypovolemic conditions, especially when central venous blood pressure is critically low. Accordingly, the purpose of this study was to assess the feasibility of transcutaneous phrenic nerve stimulation, used in conjunction with an inspiratory impedance threshold, on hemodynamic variables during hemorrhagic shock.DESIGNProspective, randomized laboratory investigation using a porcine model for measurement of hemodynamic variables, left and right ventricular diameter, and transmitral, transpulmonary, and transaortic blood flow employing transesophageal echo-Doppler technique.SETTINGUniversity hospital laboratory.SUBJECTSThirteen female pigs weighing 28-36 kg.INTERVENTIONSThe anesthetized pigs were subjected to profound hemorrhagic shock by withdrawal of 55% of estimated blood volume over 20 mins. After a 10-min recovery period, the diaphragm was stimulated with a prototype transcutaneous phrenic nerve stimulator at a rate of ten per minute while the airway was intermittently occluded with an inspiratory threshold valve between positive pressure ventilations. Hemodynamic variables were monitored for 30 mins.MEASUREMENTS AND MAIN RESULTSPhrenic nerve stimulation in combination with the inspiratory threshold valve significantly (p < .001) improved right and left ventricular diameter compared with hypovolemic shock values by 34 ± 2.5% and 20 ± 2.5%, respectively. Moreover, phrenic nerve stimulation together with the inspiratory threshold valve also increased transaortic, transpulmonary, and transmitral valve blood flow by 48 ± 6.6%, 67 ± 13.3, and 43 ± 8.2%, respectively (p < .001 for comparisons within group). Mean ± sem coronary perfusion and systolic aortic blood pressures were also significantly (p < .001) higher compared with values before stimulation (30 ± 2 vs. 20 ± 2 mm Hg, and 37 ± 2 vs. 32 ± 3 mm Hg, respectively).CONCLUSIONSThis feasibility study suggests that phrenic nerve stimulation with the inspiratory threshold valve may improve cardiac preload and, subsequently, key hemodynamic variables in porcine model of severe hemorrhagic shock. [ABSTRACT FROM AUTHOR]