Changes of aortic dimensions as evidence of cardiac pump mechanism during cardiopulmonary resuscitation in humans.

The mechanism of forward blood flow during cardiopulmonary resuscitation (CPR) remains controversial. We hypothesized that, if the heart acts as a pump, the proximal descending thoracic aorta would be distended during compression by forward blood flow, and would be constricted or remained unchanged if blood flow is generated by increased intrathoracic pressure. Fourteen patients with nontraumatic cardiac arrest underwent transesophageal echocardiography to verify changes in the descending thoracic aorta during standard manual CPR. The aortic dimensions, including cross-sectional area and diameter at the end of compression and relaxation, were measured proximal to, and at the maximal compression site of the descending thoracic aorta. At the maximal compression site, deformation of the descending thoracic aorta was observed during compression in all patients and the ratio of maximal to minimal diameter of the aorta (deformation ratio) decreased during compression compared with relaxation (0.58+/-0.15 vs. 0.81+/-0.11, P=0.001). This suggests eccentric compression of the descending thoracic aorta by external chest compression. The deformation ratio of the descending thoracic aorta proximal to the maximal compression site remained unchanged during compression and relaxation (1.0+/-0.88 vs. 1.0+/-0.9, P=0.345). The cross-sectional area of the descending thoracic aorta proximal to the maximal compression site increased 15% on average during compression compared with relaxation in 12 of 14 patients. In conclusion, deformation of the aorta at the maximal compression site and increase in the cross-sectional area of the proximal aorta suggests that cardiac pumping is the dominant mechanism in generating forward blood flow during CPR in humans.