Cerebral Autoregulation Fails to Maintain Cerebral Blood Flow During Arterial Blood Pressure Oscillations at Respiratory Frequency

Abstract

IntroductionCerebral autoregulation (CA) maintains cerebral blood flow (CBF) despite changes in arterial blood pressure (ABP). However, CA has been shown to be frequency‐dependent: Slow‐frequency ABP oscillations are buffered, while rapid changes in ABP may be transmitted to the cerebral circulation. This may be clinically relevant, as mechanical ventilation during hypovolemia can induce oscillations in ABP. Our objective was to investigate whether CBF is maintained when ABP oscillates at respiratory frequency.MethodsIn eight healthy subjects (3 males), median age 21 yrs (range 19–29), rapid cyclical variations in ABP were generated by oscillatory lower body negative pressure (LBNPosc). The LBNP was changed from 0 to −30 mmHg within 0.5 s, oscillating at the individual subject's respiratory frequency, which was maintained by metronome breathing. Start of inspiration coincided with onset of LBNP, while chamber pressure was normalized during expiration. Beat‐by‐beat blood velocity in the right internal carotid artery (ICA) was measured by Doppler ultrasound (5 MHz, Vingmed SD‐100, GE). ICA beat volume (ICA BV) was calculated using right internal carotid artery diameter measured at normovolemia by B‐mode ultrasound. Non‐invasive finger ABP was recorded continuously from the right middle finger (Finometer), providing beat‐to‐beat mean arterial pressure (MAP) and estimated beat‐to‐beat cardiac stroke volume (SV). Heart rate (HR, ECG), respiratory frequency (respiration belt), and end□tidal CO2 (ETCO2, capnography) were also recorded. The study was approved by the regional ethics committee and conformed to the Declaration of Helsinki. MAP variability (MAPV) and ICA beat volume variability (ICA BVV) were calculated by spectral analysis. Results are medians and 95% confidence intervals calculated by Hodges‐Lehmann's estimates. Wilcoxon signed rank sum test for paired samples was used to test differences between conditions. P<0.05 was considered significant.Preliminary resultsMAPV increased significantly from 1.18 mmHg2 (0.55; 1.59) at baseline to 4.81 mmHg2 (2.8; 7.4) during LBNPosc (p=0.01). Median MAP barely decreased [baseline: 70 mmHg (62; 83), LBNPosc: 68 mmHg (58; 82), p=0.02], while SV, HR, and ETCO2 remained unchanged. Median ICA BV decreased by 10% (−19.7; −2.1) from baseline [6.7 ml (5.3; 8.2)] to LBNPosc [6 ml (4.8; 6.9), p=0.04]. ICA BVV was not significantly changed [from 0.07 ml2 (0.04; 0.18) to 0.14 ml2 (0.05; 0.29)], however coherence between MAPV and ICA BVV was high [0.6 (0.43; 0.76)] at LBNPosc, and significantly increased from baseline [0.32 (0.2; 0.56), p=0.01].ConclusionsOscillatory LBNP at respiratory frequency increased MAPV markedly, while the change in MAP was very small. ICA BV decreased by 10% during oscillations in MAP. Coherence between MAPV and ICA BVV was high during LBNPosc. Thus, the buffering capacity of the CA may be outpaced during rapid variations in ABP, resulting in CBF not being maintained. This might have clinical implications for cerebral perfusion in hypovolemic patients undergoing mechanical ventilation.Support or Funding InformationRCN and UiOThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.