Echocardiographic assessment of mitral valve function during mechanical cardiopulmonary resuscitation in pigs

Using two-dimensional and M-mode echocardiography, mitral valve function was assessed during mechanical cardiopulmonary resuscitation (CPR). In 10 anesthetized pigs CPR began 1 min after induction of ventricular fibrillation; in all pigs three different compressive forces (200, 350, and 500 Newton [N]) were applied in randomized sequence for 3 min each, thus resulting in a reduction of anterior-posterior chest diameter by 15%, 20%, and 25%, respectively. Echocardiographic recordings of adequate technical quality were obtained from seven animals. During ventricular fibrillation, mitral valve leaflets showed fibrillatory movements without closure. During CPR systole with a compressive force of 200 N, the mitral valve closed in 114 of 700 CPR cycles (16%). This rate increased significantly (P less than 0.001) to 399 of 584 cycles (68%) with 350 N, and further to 470 of 494 cycles (95%) with 500 N. The higher mitral valve closure incidence was linked to statistically significant increases in systolic cerebral (+125%) and diastolic myocardial perfusion pressures (+136%) and cardiac output. We thus conclude that with low compressive force blood flow during mechanical CPR is due mainly to the chest pump mechanism, whereas the cardiac pump mechanism is effective with high compressive forces.