Intrathoracic and abdominal pressure variations as an efficient method for cardiopulmonary resuscitation: Studies in dogs compared with computer model results

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Abstract
Intrathoracic pressure variations are currently proposed as the main flow-generation mechanisms in standard and modified cardiopulmonary resuscitation (CPR) techniques. A method of changing pressure within the thorax and abdomen without any degree of heart compression was developed and tested in dogs. Intrathoracic and abdominal pressure waves were induced by cyclic inflation and deflation of the lungs and of perithoracic and periabdominal balloons. Various modes of CPR, depending on the rate of cycling, the use of a periabdominal balloon inflation, and the delay between the abdominal and thoracic pressure waves, were studied during ventricular fibrillation. During artificial systole (high intrathoracic pressure phase), the pressure which developed in the right ventricle (96.7 ± 20.5 mmHg) was higher than the pressure in the aorta (89.3 ± 20.5 mmHg, p−1, which was 18 ± 6% of the baseline carotid flow before CPR. Three different factors were found to improve the efficiency of CPR: 1) periabdominal balloon inflation simultaneous with the intrathoracic pressure waves; 2) increased frequency of the pressure waves from 60 to 100 cycles per minute; and 3) inflation of the periabdominal balloon 50 to 100 ms before the thoracic balloon. Blood-gas and acid-base balance analysis during CPR revealed well-oxygenated arterial blood with a marked respiratory alkalosis and a slowly developing metabolic acidosis. Oxygen extraction was close to the maximum values, and increased arteriovenous differences in CO2 content were measured. In summary, pressure variation within the thorax and abdomen with no direct heart compression caused carotid flow which was higher than the flow generated with most CPR techniques. Positive right atrial to aortic pressure gradients during artificial systole suggest positive flow through the heart chambers and lungs during this phase. An abdominal pressure wave preceding the thoracic pressure wave enhanced blood flow considerably. The acid-base balance in prolonged CPR reflected the combined effects of low perfusion state and hyperventilation.