Pressures generated by rib cage and abdominal compressions during cardiopulmonary resuscitation.

When the rib cage and abdomen are compressed during cardiopulmonary resuscitation (CPR), the effect on intrathoracic pressure, and therefore on haemodynamics, cannot be quantitatively predicted without a physiologically based mathematical model of chest wall dynamics. Using such a model, we compared model simulations of pleural Ppl and abdominal Pab pressures with those from dog experiments in which the compression of the rib cage was delayed from 0 to 500 ms after compression of the abdomen. Integrals of Ppl and transdiaphragmatic pressure, Pdi = Pab-ppl, over their positive and negative values during a cycle were chosen as indices of driving pressures for cardiac output. Both from the model output and experimental data, we found that the positive ppl integral PPI tends to increase with a longer delay between rib cage and abdominal compressions. The negative ppl integral NPI, however, tends to decrease according to the model predictions and data. Furthermore, the positive and negative integrals of Pdi also tend to change with delay time in the opposite way, as shown by both the model simulations and the experiments. Our results show that chest wall tissues modify the externally applied pressures, thereby not allowing us to use the externally applied pressure sources directly as the driving pressure of the cardiovascular system under study. The optimal conditions for haemodynamics during CPR require a compromise between the positive and negative integral indices. Prediction of the optimal haemodynamics from externally applied pressures requires the coupling of appropriate physiological models of chest wall dynamics and haemodynamics.