Effect of intrathoracic pressure on diastolic function of the heart during cardiopulmonary resuscitation in an animal model of cardiac arrest

Abstract

Background and Purpose: : The mechanism controlling blood flow during standard cardiopulmonary resuscitation (CPR) remains controversial. The two most likely explanations at present for generation of blood flow by external chest compression are the thoracic pump and cardiac pump theories. However, diastolic phase of the heart during CPR has never been investigated. Cardiac arrest is not associated with the occurrence of either active diastolic relaxations or cardiac value mechanisms that prevent retrograde blood flow and atrial contraction during ventricular diastole. Investigation of the diastolic filling mechanism during cardiac arrest is important for understanding one of the determinants of cardiac performance. The aim of this study was to investigate the effect of changes in intrathoracic pressure (ITP) on diastolic function of the heart during CPR in a swine model of cardiac arrest. Subjects and Methods: : Twelve domestic male pigs weighing 39.6±8.4 kg were acquired from a single-source breeder for use in this study. The animals were fasted overnight with only access to water. After anesthesia, the right carotid vein, right carotid artery, and both femoral arteries were cannulated with an introducer sheath. Micromanometer-tipped catheters for measuring hemodynamic data were inserted through the sheaths. Carotid blood flow was measured at the left carotid artery with an ultrasonic flow measurement system. Chest tubes were inserted into both thoracic cavities to manipulate and measure the intrathoracic pressure. After 2 minutes of ventricular fibrillation induced by AC current to the right ventricle, the animals received a total of 12 minutes of standard manual cardiopulmonary resuscitation (CPR) comprising two 6-minute courses of CPR consisting of chest tube opening (CTO) for 3 minutes and chest tube closure (CTC) for 3 minutes. A sequence of CPR with CTO or CTC was performed alternatively, according to the randomization schedule. Chest compressors were rotated every 2 minutes. Pressures were measured from the aorta, left ventricle (LV), right ventricle (RV), and right atrium (RA). Carotid blood flow (CBF) and end-tidal carbon dioxide concentration (EtCO2) were also measured. Coronary perfusion pressure (CPP), maximal and minimal dp/dt, and slopes of dp/dt of the ventricles were calculated. Statistical analysis: Normality tests were conducted on the results. A paired t-test or a Wilcoxon test, as appropriate, was used to compare outcomes of CTO and CTC. Results: Ten animals were included in the final analysis. Maximal dp/dt and upslope dp/dt of the RV and LV were higher during CPR than during spontaneous circulation (p<0.001). Minimal dp/dt and downslope dp/dt of the RV and LV were lower during CPR than during spontaneous circulation (p<0.001). Maximal dp/dt, upslope dp/dt, minimal dp/dt, and downslope dp/dt of the RV and LV, CBF, CPP, and EtCO2 did not differ between CPR with CTC and CPR with CTO. Conclusion: Intrathoracic pressure did not affect diastolic filling of the heart during CPR in an animal model of cardiac arrest.