BACKGROUND Recent data suggest that generation of negative intrathoracic pressure during the decompression phase of CPR improves hemodynamics, organ perfusion and survival. OBJECTIVE Incomplete chest wall recoil during the decompression phase of standard CPR increases intrathoracic pressure and right atrial pressure, impedes venous return, decreases compression-induced aortic pressures and results in a decrease of mean arterial pressure, coronary and cerebral perfusion pressure. METHODS Nine pigs in ventricular fibrillation (VF) for 6 min, were treated with an automated compression/decompression device with a compression rate of 100 min(-1), a depth of 25% of the anterior-posterior diameter, and a compression to ventilation ratio of 15:2 with 100% decompression (standard CPR) for 3 min. Compression was then reduced to 75% of complete decompression for 1 min of CPR and then restored for another 1 min of CPR to 100% full decompression. Coronary perfusion pressure (CPP) was calculated as the diastolic (aortic (Ao)-right atrial (RA) pressure). Cerebral perfusion pressure (CerPP) was calculated multiple ways: (1) the positive area (in mmHg s) between aortic pressure and intracranial pressure (ICP) waveforms, (2) the coincident difference in systolic and diastolic aortic and intracranial pressures (mmHg), and (3) CerPP = MAP--ICP. ANOVA was used for statistical analysis and all values were expressed as mean +/- S.E.M. The power of the study for an alpha level of significance set at 0.05 was >0.90. RESULTS With CPR performed with 100%-75%-100% of complete chest wall recoil, respectively, the CPP was 23.3 +/- 1.9, 15.1 +/- 1.6, 16.6 +/- 1.9, p = 0.003; CerPP was: (1) area: 313.8 +/- 104, 89.2 +/- 39, 170.5 +/- 42.9, p = 0.03, (2) systolic aortic minus intracranial pressure difference: 22.8 +/- 3.6, 16.5 +/- 4, 23.7 +/- 4.5, p = n.s., and diastolic pressure difference: 5.7 +/- 3, -2.4 +/- 2.4, 3.2 +/- 2.5, p = 0.04 and (3) mean: 14.3 +/- 3, 7 +/- 2.9, 12.4 +/- 2.9, p = 0.03, diastolic aortic pressure was 28.1 +/- 2.5, 20.7 +/- 1.9, 20.9 +/- 2.1, p = 0.0125; ICP during decompression was 22.8 +/- 1.7, 23 +/- 1.5, 19.7 +/- 1.7, p = n.s. and mean ICP was 37.1 +/- 2.3, 35.5 +/- 2.2, 35.2 +/- 2.4, p = n.s.; RA diastolic pressure 4.8 +/- 1.3, 5.6 +/- 1.2, 4.3 +/- 1.2 p = 0.1; MAP was 52 +/- 2.9, 43.3 +/- 3, 48.3 +/- 2.9, p = 0.04; decompression endotracheal pressure, -0.7 +/- 0.1, -0.3 +/- 0.1, -0.75 +/- 0.1, p = 0.045. CONCLUSIONS Incomplete chest wall recoil during the decompression phase of CPR increases endotracheal pressure, impedes venous return and decreases mean arterial pressure, and coronary and cerebral perfusion pressures.