Blood pressure excursions below the cerebral autoregulation threshold during cardiac surgery are associated with acute kidney injury

Abstract

Objectives: To determine whether mean arterial blood pressure excursions below the lower limit of cerebral blood flow autoregulation during cardiopulmonary bypass are associated with acute kidney injury after surgery.

Setting: Tertiary care medical center.

Patients: Four hundred ten patients undergoing cardiac surgery with cardiopulmonary bypass.

Design: Prospective observational study.

Interventions: None.

Measurements and main results: Autoregulation was monitored during cardiopulmonary bypass by calculating a continuous, moving Pearson's correlation coefficient between mean arterial blood pressure and processed near-infrared spectroscopy signals to generate the variable cerebral oximetry index. When mean arterial blood pressure is below the lower limit of autoregulation, cerebral oximetry index approaches 1, because cerebral blood flow is pressure passive. An identifiable lower limit of autoregulation was ascertained in 348 patients. Based on the RIFLE criteria (Risk, Injury, Failure, Loss of kidney function, End-stage renal disease), acute kidney injury developed within 7 days of surgery in 121 (34.8%) of these patients. Although the average mean arterial blood pressure during cardiopulmonary bypass did not differ, the mean arterial blood pressure at the limit of autoregulation and the duration and degree to which mean arterial blood pressure was below the autoregulation threshold (mm Hg × min/hr of cardiopulmonary bypass) were both higher in patients with acute kidney injury than in those without acute kidney injury. Excursions of mean arterial blood pressure below the lower limit of autoregulation (relative risk 1.02; 95% confidence interval 1.01 to 1.03; p < 0.0001) and diabetes (relative risk 1.78; 95% confidence interval 1.27 to 2.50; p = 0.001) were independently associated with for acute kidney injury.

Conclusions: Excursions of mean arterial blood pressure below the limit of autoregulation and not absolute mean arterial blood pressure are independently associated with for acute kidney injury. Monitoring cerebral oximetry index may provide a novel method for precisely guiding mean arterial blood pressure targets during cardiopulmonary bypass.

Figure 1

Clinical cerebral oximetry index (COx) recordings during CPB illustrating a lower limit of cerebral autoregulation at a mean arterial blood pressure (ABP) of 80 mmHg (red asterisk). The top channel represents raw ABP and regional cerebral oxygen saturation (rSO2) for each cerebral hemisphere. Right and left COx recordings are shown in the other graphs. The lower limit of autoregulation is defined as the blood pressure where COx increases from < 0.3 to ≥0.3 (see text).

Figure 2

The percentage of patients with each classification of RIFLE acute kidney injury (AKI) after surgery. The number of patients in each class is noted above each column.

Figure 3

Whisker plot of duration and magnitude of blood pressure below the limit of cerebral autoregulation during cardiopulmonary bypass (mmHg x min/h) for patients with and without acute kidney injury (AKI) after surgery (p=0.014 between groups). The horizontal line represents the median value while upper and lower boarders of the shaded area represent the inter-quartile range. The error bars represent the boundaries of ±1.5xinterquartile range and the points beyond represent outliers.