Real-time continuous monitoring of cerebral blood flow autoregulation using near-infrared spectroscopy in patients undergoing cardiopulmonary bypass

Abstract

Background and purpose: Individualizing mean arterial blood pressure targets to a patient's cerebral blood flow autoregulatory range might prevent brain ischemia for patients undergoing cardiopulmonary bypass (CPB). This study compares the accuracy of real-time cerebral blood flow autoregulation monitoring using near-infrared spectroscopy with that of transcranial Doppler.

Methods: Sixty adult patients undergoing CPB had transcranial Doppler monitoring of middle cerebral artery blood flow velocity and near-infrared spectroscopy monitoring. The mean velocity index (Mx) was calculated as a moving, linear correlation coefficient between slow waves of middle cerebral artery blood flow velocity and mean arterial blood pressure. The cerebral oximetry index was calculated as a similar coefficient between slow waves of cerebral oximetry and mean arterial blood pressure. When cerebral blood flow is autoregulated, Mx and cerebral oximetry index vary around zero. Loss of autoregulation results in progressively more positive Mx and cerebral oximetry index.

Results: Mx and cerebral oximetry index showed significant correlation (r=0.55, P<0.0001) and good agreement (bias, 0.08+/-0.18, 95% limits of agreement: -0.27 to 0.43) during CPB. Autoregulation was disturbed in this cohort during CPB (average Mx 0.38, 95% CI 0.34 to 0.43). The lower cerebral blood flow autoregulatory threshold (defined as incremental increase in Mx >0.45) during CPB ranged from 45 to 80 mm Hg.

Conclusions: Cerebral blood flow autoregulation can be monitored continuously with near-infrared spectroscopy in adult patients undergoing CPB. Real-time autoregulation monitoring may have a role in preventing injurious hypotension during CPB. Clinical Trials Registration- at www.clinicaltrials.gov (NCT00769691).

Conflict of interest statement

Under a licensing agreement with Somanetics, Dr. Brady is entitled to a share of fees and royalty received by The Johns Hopkins University on the monitoring technology described in this manuscript. The terms of this arrangement are being managed by the Johns Hopkins University in accordance with its conflict of interest policies.

Figures

Figure 1

Coherence analysis between middle cerebral artery flow velocity (MCA-FV) and cerebral oximetry. The top panel shows low-pass filtered MCA-FV across a 12-minute window. The middle panel shows cerebral oximetry across the same time window. The bottom panel shows coherence between MCA-FV and cerebral oximetry as a function of frequency from the 12-minute window shown. The maximum coherence at the frequency of slow waves (0.4 to 4 beats per minute) was averaged across the entire monitoring period to give the results reported.

Figure 2

Average Mx and COx values obtained during CPB were compared by linear correlation (A) and the Bland-Altman method (B). The dashed lines represent the 95% confidence band of the regression line and the 95% limits of agreement (−0.27 to 0.43) for the bias analysis.

Figure 3

Distribution of COx and Mx over mean arterial blood pressure (MAP) during CPB for the study cohort. A, COx values (mean + SD) are binned to the corresponding MAP at the time of the measurement. B, Simultaneous reading of Mx (mean + SD) are also binned to the corresponding MAP. Note that a wide range of MAP is covered during CPB, with a significant increase (impaired autoregulation) in both the COx and Mx at lower MAP.

Figure 4

Histogram showing the number of subjects versus the mean arterial pressure during cardiopulmonary bypass where the mean velocity index (Mx) increased to > 0.45 indicating a putative lower cerebral blood flow autoregulatory limit.

Figure 5

Representative graph of cerebral blood flow autoregulation monitoring during cardiopulmonary bypass in a single patient. A) The top panel shows mean arterial pressure (MAP), the middle panel shows the middle cerebral artery flow velocity (MCA FV), and the lower panel shows the near infrared spectroscopy (NIRS) measurements of frontal lobe oximetry, all synchronously measured on the same time scale. B) Values of mean velocity index (Mx) and C), values of cerebral oximetry index (COx) obtained from the time window in A are sorted according to MAP and show a similar putative lower limit of autoregulation at 75 mmHg, where Mx and COx values increase precipitously. A horizontal line at Mx and COx values of 0.4 is shown for reference.