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 CO(2) (ET(CO(2)); 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 ET(CO(2)) (+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 Pco(2) 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 ET(CO(2)) (-7.2 +/- 4.1 mmHg, P < 0.001). Unexpectedly, CA efficiency dropped significantly with the inception of the metronome-controlled hyperventilation, but, after approximately 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.

Fig. 1.

Representative patterns of end-tidal CO₂ (ET_CO2; A), cerebral blood flow velocity (CBFV; B), blood pressure (BP; C), autoregulation index time-varying estimate [ARI(t); D], and unconstrained gain time-varying estimate [K′(t); E] during hyperventilation in a 22-yr-old subject.

Fig. 2.

Representative patterns of ET_CO2 (A), CBFV (B), BP (C), ARI(t) (D), and K′(t) (E) during breath hold in a 22-yr-old subject. The shaded area indicates the lack of ET_CO2 signal during breath hold.

Fig. 3.

Representative fluctuations in ET_CO2 (A), right middle cerebral artery (MCA) CBFV (B), BP (C), ARI(t) (D), and K′(t) (E) during baseline. ARI and K′(t) are for right (solid line) and left (dashed line) MCA. With the exception of CBFV, the maximum changes in the other parameters compare well with the mean values given in Table 2.

Fig. 4.

Maximum change in ET_CO2 (A), CBFV (B), ARI(t) (C), and K′(t) (D) during breath hold (B-H) and hyperventilation (HYP) maneuvers compared with corresponding values during baseline (BAS) for the right (solid bars) and left (shaded bars) MCA. Values are means ± SD.

Fig. 5.

Population coherent averages of breath-hold recordings. CBFV (A), ET_CO2 (B), ARI (C), K′ (D), BP (E), and heart rate (F) are plotted against time. G: critical closing pressure (CrCP). H: resistance-area product (RAP). The arrow indicates start of maneuver. In A, C, D, G, and H, the solid line represents right MCA, and dashed line represents left MCA. The shaded area indicates the lack of ET_CO2 signal during breath hold. The maximum SE bar is shown for each plot. ARMA, autoregressive moving average; bpm, beats/min.

Fig. 6.

Population coherent averages of hyperventilation recordings. CBFV (A), ET_CO2 (B), ARI (C), K′ (D), BP (E), and heart rate (F) are plotted against time. G: CrCP. H: RAP. Arrow indicates start of maneuver. In A, C, D, G, and H, the solid line represents right MCA, and dashed line represents left MCA. The maximum SE bar is shown for each plot.