Influence of variations in systemic blood flow and pressure on cerebral and systemic oxygen saturation in cardiopulmonary bypass patients.

Although both pressure and flow are considered important determinants of regional organ perfusion, the relative importance of each is less established. The aim of the present study was to evaluate the impact of variations in flow, pressure, or both on cerebral and whole-body oxygen saturation. Thirty-four consenting patients undergoing elective cardiac surgery on cardiopulmonary bypass were included. Using a randomized cross-over design, four different haemodynamic states were simulated: (i) 20% flow decrease, (ii) 20% flow decrease with phenylephrine to restore baseline pressure, (iii) 20% pressure decrease with sodium nitroprusside (SNP) under baseline flow, and (iv) increased flow with baseline pressure. The effect of these changes was evaluated on cerebral (Sc(O₂)) and systemic (Sv(O₂)) oxygen saturation, and on systemic oxygen extraction ratio (OER). Data were assessed by within- and between-group comparisons. Decrease in flow was associated with a decrease in [from 63.5 (7.4) to 62.0 (8.5) %, P<0.001]. When arterial pressure was restored with phenylephrine during low flow, Sc(O₂) further decreased from 61.0 (9.7) to 59.2 (10.2) %, P<0.001. Increase in flow was associated with an increase in Sc(O₂) from 62.6 (7.7) to 63.6 (8.9) %, P=0.03, while decreases in pressure with the use of SNP did not affect Sc(O₂). Sv(O₂) was significantly lower (P<0.001) and OER was significantly higher (P<0.001) in the low flow arms. In the present elective cardiac surgery population, Sc(O₂) and Sv(O₂) were significantly lower with lower flow, regardless of systemic arterial pressure. Moreover, phenylephrine administration was associated with a reduced cerebral and systemic oxygen saturation.