Patients were attached to either an INVOS (Somenetics, Inc., Troy, MI) or Foresight (CAS Medical Systems, Branford, CT) NIRS monitor, depending on availability. Electrodes for monitoring NIRS were placed on the right and left forehead using the respective manufacturer’s recommendations and after first cleaning the skin with an alcohol swab. Transcranial Doppler monitoring (Doppler Box, DWL, Compumedics, USA, Charlotte, NC) of the middle cerebral arteries was with two 2.5-MHz transducers fitted on a headband. The depth of insonation varied between 35 and 52 mm until representative spectral artery flow was identified.
Analog arterial pressure data from the operating room hemodynamic monitor, TCD, and NIRS signals were sampled with an analog-to-digital converter at 60 Hz and then processed with ICM+ software version 6.1 (University of Cambridge, Cambridge, UK). These signals were time-integrated as non-overlapping 10-second mean values, which is equivalent to applying a moving average filter with a 10-second time window and re-sampling at 0.1 Hz. This operation was used to eliminate high-frequency noise from the respiratory and pulse frequencies, while allowing detection of oscillations and transients that occur below 0.05 Hz. Doppler, oximetry, and arterial blood pressure waveforms were further high pass filtered with a DC cutoff set at 0.003 Hz. This step removed slow drifts associated with hemodilution at the onset of bypass, blood transfusions, cooling, and rewarming. A continuous, moving Pearson’s correlation coefficient was calculated between the MAP and TCD blood flow velocities and between MAP and NIRS data, rendering the variables Mx (mean velocity index) and COx (cerebral oximetry index), respectively. Of note, MAP is used in this calculation and not cerebral perfusion pressure since intracranial pressure data are not available and since central venous pressure is often negative as a result of suction assisted venous drainage to the CPB reservoir. Consecutive, paired, 10-second averaged values from 300 seconds duration were used for each calculation, incorporating 30 data points for each index. Intact CBF autoregulation is indicated by an Mx value of approximately zero (CBF and MAP are not correlated), and CBF dysautoregulation is indicated by an Mx value approaching +1 (CBF and MAP correlated). Similar findings occur experimentally with COx.13 (link)
Analog arterial pressure data from the operating room hemodynamic monitor, TCD, and NIRS signals were sampled with an analog-to-digital converter at 60 Hz and then processed with ICM+ software version 6.1 (University of Cambridge, Cambridge, UK). These signals were time-integrated as non-overlapping 10-second mean values, which is equivalent to applying a moving average filter with a 10-second time window and re-sampling at 0.1 Hz. This operation was used to eliminate high-frequency noise from the respiratory and pulse frequencies, while allowing detection of oscillations and transients that occur below 0.05 Hz. Doppler, oximetry, and arterial blood pressure waveforms were further high pass filtered with a DC cutoff set at 0.003 Hz. This step removed slow drifts associated with hemodilution at the onset of bypass, blood transfusions, cooling, and rewarming. A continuous, moving Pearson’s correlation coefficient was calculated between the MAP and TCD blood flow velocities and between MAP and NIRS data, rendering the variables Mx (mean velocity index) and COx (cerebral oximetry index), respectively. Of note, MAP is used in this calculation and not cerebral perfusion pressure since intracranial pressure data are not available and since central venous pressure is often negative as a result of suction assisted venous drainage to the CPB reservoir. Consecutive, paired, 10-second averaged values from 300 seconds duration were used for each calculation, incorporating 30 data points for each index. Intact CBF autoregulation is indicated by an Mx value of approximately zero (CBF and MAP are not correlated), and CBF dysautoregulation is indicated by an Mx value approaching +1 (CBF and MAP correlated). Similar findings occur experimentally with COx.13 (link)
