Real-Time Intraoperative Determination and Reporting of Cerebral Autoregulation State Using Near-Infrared Spectroscopy

Abstract

Background: Cerebral blood flow (CBF) is maintained over a range of blood pressures through cerebral autoregulation (CA). Blood pressure outside the range of CA, or impaired autoregulation, is associated with adverse patient outcomes. Regional oxygen saturation (rSO2) derived from near-infrared spectroscopy (NIRS) can be used as a surrogate CBF for determining CA, but existing methods require a long period of time to calculate CA metrics. We have developed a novel method to determine CA using cotrending of mean arterial pressure (MAP) with rSO2that aims to provide an indication of CA state within 1 minute. We sought to determine the performance of the cotrending method by comparing its CA metrics to data derived from transcranial Doppler (TCD) methods.

Methods: Retrospective data collected from 69 patients undergoing cardiac surgery with cardiopulmonary bypass were used to develop a reference lower limit of CA. TCD-MAP data were plotted to determine the reference lower limit of CA. The investigated method to evaluate CA state is based on the assessment of the instantaneous cotrending relationship between MAP and rSO2 signals. The lower limit of autoregulation (LLA) from the cotrending method was compared to the manual reference derived from TCD. Reliability of the cotrending method was assessed as uptime (defined as the percentage of time that the state of autoregulation could be measured) and time to first post.

Results: The proposed method demonstrated minimal mean bias (0.22 mmHg) when compared to the TCD reference. The corresponding limits of agreement were found to be 10.79 mmHg (95% confidence interval [CI], 10.09-11.49) and -10.35 mmHg (95% CI, -9.65 to -11.05). Mean uptime was 99.40% (95% CI, 99.34-99.46) and the mean time to first post was 63 seconds (95% CI, 58-71).

Conclusions: The reported cotrending method rapidly provides metrics associated with CA state for patients undergoing cardiac surgery. A major strength of the proposed method is its near real-time feedback on patient CA state, thus allowing for prompt corrective action to be taken by the clinician.

Conflict of interest statement

Conflicts of Interest: See Disclosures at the end of the article.

Figures

Figure 1.

A, Schematic showing the buildup of state indication (intact: green; impaired: red) over time for the MAP signal determined through comparison of the cotrending of MAP with the rSo2 signal (not shown). The LLA is defined as the transition between the red and green sections. B, Display output showing MAP signal over time with state determination (green: intact; red: impaired) superimposed as MAP values are visited. The state has been determined from the cotrending of MAP and rSo2 using our method. The cyan line is the reference LLA, determined from the TCD signal. Note that the proposed method does not post a result during periods when the MAP or rSo2 are missing (indicated as the periods displayed as vertical black bands). LLA indicates lower limit of autoregulation; MAP, mean arterial pressure; rSo2, regional oxygen saturation; TCD, transcranial Doppler.

Figure 2.

Manual LLA assessment tool. An example usage of the tool used to build the reference LLAs from the TCD signals—a single case has been loaded. Top 2 plots: MAP and TCD flow time series. Bottom left plot: TCD flow-MAP plot; bottom middle plot: Mx-MAP plot (the black dots are the raw points and the blue overlay the corresponding binned values). The user is able to interrogate the signals in different time windows and is able to integrate the information from all 4 plots to help set a reference value. In this example, the user has selected the whole case and assigned a single LLA at 52 mmHg. LLA indicates lower limit of autoregulation; MAP, mean arterial pressure; Mx, mean flow index; TCD, transcranial Doppler.

Figure 3.

NIRS-based cotrending method output for all 69 cases showing the MAP signal (in white). The intact and impaired states are depicted in green and red, respectively. The rSo2 signals are also included in magenta. The cyan line is the combined manual LLA assessment; the black dashed line is the LLA produced by the cotrending method. LLA indicates lower limit of autoregulation; MAP, mean arterial pressure; NIRS, near-infrared spectroscopy; rSo2, regional oxygen saturation.

Figure 4.

A, Histogram of the LLA references taken from TCD signal for the 69 patients in this study. Many guidelines indicate a blood pressure management of 50 or 60 mmHg, but approximately 50% of patients have an LLA higher than 60 mmHg and 95% have an LLA >50 mmHg. B, Histogram of the LLAs calculated with our method. The distribution strongly resembles the reference LLA distribution. LLA indicates lower limit of autoregulation; NIRS, near-infrared spectroscopy; TCD, transcranial Doppler.

Figure 5.

Distributions of LLAs–cotrending algorithm and TCD reference. A, Bland-Altman plot of the data displaying mean bias = 0.22 mmHg, an upper LOA = 10.79 mmHg (95% CI, 10.09–11.49) and a lower LOA = −10.35 mmHg (95% CI, −9.65 to −11.05). B, Scatter plot of individual LLAs over time over all cases (bubble sizes proportional to number of colocated data points). The dashed line in the middle represents the unity line, which indicates a perfect agreement between the method and the reference. CI indicates confidence interval; LLA, lower limit of autoregulation; LOA, limit of agreement.