Continuous estimates of dynamic cerebral autoregulation during transient hypocapnia and hypercapnia

Abstract

Dynamic cerebral autoregulation (CA) is the transient response of cerebral blood flow (CBF) to rapid blood pressure changes: it improves in hypocapnia and becomes impaired during hypercapnia. Batch-processing techniques have mostly been used to measure CA, providing a single estimate for an entire recording. A new approach to increase the temporal resolution of dynamic CA parameters was applied to transient hypercapnia and hypocapnia to describe the time-varying properties of dynamic CA during these conditions. Thirty healthy subjects (mean ± SD: 25 ± 6 yr, 9 men) were recruited. CBF velocity was recorded in both middle cerebral arteries (MCAs) with transcranial Doppler ultrasound. Arterial blood pressure (Finapres), end-tidal CO2 (ETCO2; infrared capnograph), and a three-lead ECG were also measured at rest and during repeated breath hold and hyperventilation. A moving window autoregressive moving average model provided continuous values of the dynamic CA index [autoregulation index (ARI)] and unconstrained gain. Breath hold led to significant increase in ETCO2 (+5.4 ± 6.1 mmHg), with concomitant increase in CBF velocity in both MCAs. Continuous dynamic CA parameters showed highly significant changes ( P < 0.001), with a temporal pattern reflecting a delayed dynamic response of CA to changes in arterial Pco2 and a maximal reduction in ARI of −5.1 ± 2.4 and −5.1 ± 2.3 for the right and left MCA, respectively. Hyperventilation led to a marked decrease in ETCO2 (−7.2 ± 4.1 mmHg, P < 0.001). Unexpectedly, CA efficiency dropped significantly with the inception of the metronome-controlled hyperventilation, but, after ∼30 s, the ARI increased gradually to show a maximum change of 5.7 ± 2.9 and 5.3 ± 3.0 for the right and left MCA, respectively ( P < 0.001). These results confirm the potential of continuous estimates of dynamic CA to improve our understanding of human cerebrovascular physiology and represent a promising new approach to improve the sensitivity of clinical applications of dynamic CA modeling.