Comparison of different metrics of cerebral autoregulation in association with major morbidity and mortality after cardiac surgery

Abstract

Background: Cardiac surgery studies have established the clinical relevance of personalised arterial blood pressure management based on cerebral autoregulation. However, variabilities exist in autoregulation evaluation. We compared the association of several cerebral autoregulation metrics, calculated using different methods, with outcomes after cardiac surgery.

Methods: Autoregulation was measured during cardiac surgery in 240 patients. Mean flow index and cerebral oximetry index were calculated as Pearson's correlations between mean arterial pressure (MAP) and transcranial Doppler blood flow velocity or near-infrared spectroscopy signals. The lower limit of autoregulation and optimal mean arterial pressure were identified using mean flow index and cerebral oximetry index. Regression models were used to examine associations of area under curve and duration of mean arterial pressure below thresholds with stroke, acute kidney injury (AKI), and major morbidity and mortality.

Results: Both mean flow index and cerebral oximetry index identified the cerebral lower limit of autoregulation below which MAP was associated with a higher incidence of AKI and major morbidity and mortality. Based on magnitude and significance of the estimates in adjusted models, the area under curve of MAP < lower limit of autoregulation had the strongest association with AKI and major morbidity and mortality. The odds ratio for area under the curve of MAP < lower limit of autoregulation was 1.05 (95% confidence interval, 1.01-1.09), meaning every 1 mm Hg h increase of area under the curve was associated with an average increase in the odds of AKI by 5%.

Conclusions: For cardiac surgery patients, area under curve of MAP < lower limit of autoregulation using mean flow index or cerebral oximetry index had the strongest association with AKI and major morbidity and mortality. Trials are necessary to evaluate this target for MAP management.

Keywords: acute kidney injury; cardio pulmonary bypass; cerebral autoregulation; data visualisation; individualised blood pressure management; major morbidity, mortality; organ injury; postoperative outcome.

Fig 1

Identification of optimal arterial blood pressure (MAPopt) and lower limit of autoregulation (LLA), as indicated by the arrows. Mx, mean flow index; COx, cerebral oximetry index; CBFV, cerebral blood flow velocity; rSO₂, regional cortical oxygen saturation.

Fig 2

Patient flowchart.

Fig 3

Visualising the relationship between the magnitude of MAP below LLA (mm Hg), time duration (min) and occurrence of major morbidity and mortality or AKI. As an overview, each coordinate in the figures refers to a hypoperfusion episode of MAP below LLA at a certain magnitude (mm Hg) for at least a certain duration (min). For each pair of magnitude and duration thresholds, the probability of AKI or MMOM is calculated and then represented by colour codes (blue colour indicates low probability and red colour indicates high probability). The solid lines demarcate distinct transitions in probabilities for each outcome. For example, the colour coding and black line in panel (a) demonstrate that if the MAP was below LLA cerebral oximetry index by 13 mm Hg, and lasted for longer than 30 min, the probability of developing MMOM increased to 50%. Panels (a) and (d) demonstrate that the probability of both MMOM and AKI increase as both the magnitude and duration of MAP < LLA increase, with highest occurrence in the top-right corner of each panel. Panels (b) and (e) demonstrate the modifying effects of bypass duration on the probability of AKI (b) and MMOM (e). Panels (c) and (f) demonstrate the modifying effects of reduced cerebral oximetry values (regional cortical oxygen saturation [rSO₂]) on probability of AKI (c) and MMOM (f). These latter panels demonstrate that patients with longer duration of cardiopulmonary bypass and lower cerebral oximetry values (rSO₂) have an increased risk of AKI and MMOM even at similar intensities and duration of MAP below the LLA, as compared with patients who had shorter duration of bypass or higher cerebral oximetry values. MAP, mean arterial blood pressure; LLA, lower limit of autoregulation; AKI, acute kidney injury; MMOM, major morbidity and operative mortality.