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To test whether the radial artery-to-aorta pressure gradient seen in some patients after cardiopulmonary bypass (CPB) is due to reduction in hand vascular resistance, the authors compared pressures in the ascending aorta with pressures in the radial artery before and after CPB in 12 patients. They increased hand vascular resistance by briefly occluding the radial and ulnar arteries at the wrist and recorded that effect on the radial artery-to-aorta pressure relationship. They also recorded the effect of wrist compression on radial artery pressures before and after CPB in 38 patients not having aortic pressure measurements. Before CPB in the first 12 patients, the radial systolic arterial pressure (SAP) was significantly higher (P < 0.05) than the ascending aortic SAP, and wrist compression did not significantly affect that difference (P > 0.05). After CPB, the radial artery and aortic SAPs were not statistically different (P > 0.05), but wrist compression restored the higher radial artery SAP. The mean arterial pressure (MAP) was equal in four patients and 1–3 mmHg higher or lower in eight patients before CPB, and wrist compression did not alter those relationships. After CPB, MAP was equal in four patients; radial MAP was 1–3 mmHg higher or lower in six patients, and 7 and 10 mmHg lower in the last two patients. Wrist compression did not affect the dispersion around the zero mark in the first ten patients, but it increased radial MAP by 7 mmHg in the last two patients. In the final 38 patients, wrist compression did not affect radial MAP before CPB, but increased it by 7–12 mmHg in six patients after CPB. This increase was statistically significant (P < 0.05). These findings strongly suggest that a radial artery pressure lower than the aortic pressure after CPB is due mainly to a marked decrease in hand vascular resistance.