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1 From the Cardiac Arrhythmia Center, Cardiovascular Division, University of Minnesota, Minneapolis, University of Ulm, Ulm, Germany
2 From the Department of Anesthesiology and Critical Care Medicine, University of Ulm, Ulm, Germany
Objectives: This study was designed to assess whether intermittent impedance of inspiratory gas exchange improves the efficiency of standard cardiopulmonary resuscitation (CPR).
Background: Standard CPR relies on the natural elastic recoil of the chest to transiently decrease intrathoracic pressures and thereby promote venous blood return to the heart. To further enhance the negative intrathoracic pressures during the "relaxation" phase of CPR, we tested the hypothesis that intermittent impedance to inspiratory gases during standard CPR increases coronary perfusion pressures and vital organ perfusion.
Methods: CPR was performed with a pneumatically driven automated device in a porcine model of ventricular fibrillation. Eight pigs were randomized to initially receive standard CPR alone, while seven pigs initially received standard CPR plus intermittent impedance to inspiratory gas exchange with a threshold valve set to 
40 cm H2O. The compression:ventilation ratio was 5:1 and the compression rate was 80/min. At 7-min intervals the impedance threshold valve (ITV) was either added or removed from the ventilation circuit such that during the 28 min of CPR, each animal received two 7-min periods of CPR with the ITV and two 7-min periods without the valve.
Results: Vital organ blood flow was significantly higher during CPR performed with the ITV than during CPR performed without the valve. Total left ventricular blood flow (mean±SEM) (mL/min/g) was 0.32±0.04 vs 0.23±0.03 without the ITV (p<0.05). Cerebral blood flow (mL/min/g) was 20% higher with the ITV (+ITV, 0.23±0.02; ITV, 0.19±0.02; p<0.05). Each time the ITV was removed, there was a statistically significant decrease in the vital organ blood flow and coronary perfusion pressure.
Conclusions: Intermittent impedance to inspiratory flow of respiratory gases during standard CPR significantly improves CPR efficiency during ventricular fibrillation. These studies underscore the importance of lowering intrathoracic pressures during the relaxation phase of CPR.
Key Words: active compression-decompression CPR • cardiac arrest • cardiopulmonary resuscitation • heart • impedance threshold valve • ventricular fibrillation
Submitted on April 16, 1997
Accepted on September 10, 1997
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