Optimal blood pressure during cardiopulmonary bypass defined by cerebral autoregulation monitoring

Abstract

Objectives: We sought to define the lower and upper limits of cerebral blood flow autoregulation and the optimal blood pressure during cardiopulmonary bypass. We further sought to identify variables predictive of these autoregulation end points.

Methods: Cerebral autoregulation was monitored continuously with transcranial Doppler in 614 patients during cardiopulmonary bypass enrolled in 3 investigations. A moving Pearson's correlation coefficient was calculated between cerebral blood flow velocity and mean arterial pressure to generate the variable mean velocity index. Optimal mean arterial pressure was defined as the mean arterial pressure with the lowest mean velocity index indicating the best autoregulation. The lower and upper limits of cerebral blood flow autoregulation were defined as the mean arterial pressure at which mean velocity index was increasingly pressure passive (ie, mean velocity index ≥0.4) with declining or increasing blood pressure, respectively.

Results: The mean (± standard deviation) lower and upper limits of cerebral blood flow autoregulation, and optimal mean arterial pressure were 65 ± 12 mm Hg, 84 ± 11 mm Hg, and 78 ± 11 mm Hg, respectively, after adjusting for study enrollment. In 17% of patients, though, the lower limit of cerebral autoregulation was above this optimal mean arterial pressure, whereas in 29% of patients the upper limit of autoregulation was below the population optimal mean arterial pressure. Variables associated with optimal mean arterial pressure based on multivariate regression analysis were nonwhite race (increased 2.7 mm Hg; P = .034), diuretics use (decreased 1.9 mm Hg; P = .049), prior carotid endarterectomy (decreased 5.5 mm Hg; P = .019), and duration of cardiopulmonary bypass (decreased 1.28 per 60 minutes of cardiopulmonary bypass). The product of the duration and magnitude that mean arterial pressure during cardiopulmonary bypass was below the lower limit of cerebral autoregulation was associated with the risk for stroke (P = .02).

Conclusions: Real-time monitoring of autoregulation may improve individualizing mean arterial pressure during cardiopulmonary bypass and improving patient outcomes.

Keywords: blood pressure; cardiopulmonary bypass; cerebral autoregulation.

Figure 1

Flow diagram of the patients enrolled in the study and included in this analysis.

Figure 2

Representative graph of cerebral autoregulation monitoring by mean velocity index (Mx) during cardiopulmonary bypass. The top graph represents the time-series of arterial blood pressure (ABP) while the bottom bar-graph the percentage of the time of the recording spent at 5 mmHg bin. Optimal mean arterial pressure (MAP, ABP) for the left and right side of the brain was defined as that MAP with the lowest Mx. Lower limit of cerebral autoregulation (LLA) and upper limit of cerebral autoregulation (ULA) were defined as the MAP at which Mx reached 0.4. In this example, the optimal MAP is 85 mmHg (black arrow) and the LLA and ULA are 70 mmHg and 95 mmHg, respectively (black-outlined arrow).

Figure 3

Distribution of optimal mean arterial pressure (MAP) for the entire cohort.