To evaluate hemodynamics during human CPR, 32 patients with witnessed cardiac arrest were studied during manual and mechanical conventional CPR. In eight patients during manual conventional CPR, peak systolic radial artery, right atrial (RAP), and pulmonary artery pressures were found to be similar (59.8 ± 4.3, 70.8 ± 4.7, 71.2 ± 8.2 mm Hg) and higher than external jugular venous pressure (33.8 ± 1.9 mm Hg, p <.0001). The diastolic radial artery to RAP gradient was found to be only 10.7 ± 2.1 mm Hg. In four patients, maneuvers that avoid chest compression and thus obviate cardiac compression, i.e., rhythmic compression of the abdomen, yielded arterial pressures comparable to those generated by conventional CPR (48 ± 6.4 vs. 52.8 ± 3.4 mm Hg, NS). In 13 other patients during mechanical conventional CPR at constant chest compression force, radial artery pressure was higher during the rust compression after ventilation as compared with subsequent chest compressions (67.5 ± 5.0 vs. 61.9 ± 4.8 mm Hg, p <.007). These hemodynamic observations are similar to those reported in large dogs during CPR and support the generation of vascular pressures during CPR by an increase in intrathoracic pressure. They also suggest that despite anatomic differences, the similarity of hemodynamics in dogs and humans justifies the use of large dogs as a human model during acute resuscitation studies.