To determine if the timing of interposed abdominal compressions (IAC) affects the augmented blood flow during this form of cardiopulmonary resuscitation (CPR), we performed early-onset or late-onset abdominal compressions at three vascular volumes in nine dogs. Early-onset IAC began immediately following chest compression; we predicted that this would act primarily by emptying the aorta and sustaining the elevated intrathoracic pressure. Late-onset IAC began one-fourth to one-third of the time into diastole; this would have primarily increased venous return. We measured carotid blood flow (electromagnetic flow probe) and right atrial (Pra), thoracic aortic (Pta), abdominal aortic (Paa), and intra-abdominal pressures. The IAC-CPR increased carotid blood flow compared with conventional CPR (22.8 +/- 13.1 percent vs 8.7 +/- 5.8 percent of control; p less than 0.003), but there was no difference between the early and late modes of IAC (22.7 +/- 11.6 percent vs 22.9 +/- 14.7 percent of control). The increase in carotid blood flow was present with the first abdominal compression and was constant over the 40 to 60 seconds of CPR. Peak Pra, Pta, and Paa were similar during abdominal compression (91.8 +/- 16.9 mm Hg, 96.1 +/- 16.0 mm Hg, and 102.4 +/- 15.2 mm Hg, respectively; p less than 0.001). The Pta-Pra diastolic gradient was 18.0 +/- 8.2 mm Hg for early-onset and 20.6 +/- 7.5 mm Hg for late-onset compression (not significant). We conclude that increased carotid blood flow in IAC-CPR in the dog is principally due to the increased pressure in a common thoracoabdominal unit.