The relationship between cardiac output and dynamic cerebral autoregulation in humans

Abstract

Cerebral autoregulation adjusts cerebrovascular resistance in the face of changing perfusion pressures to maintain relatively constant flow. Results from several studies suggest that cardiac output may also play a role. We tested the hypothesis that cerebral blood flow would autoregulate independent of changes in cardiac output. Transient systemic hypotension was induced by thigh-cuff deflation in 19 healthy volunteers (7 women) in both supine and seated positions. Mean arterial pressure (Finapres), cerebral blood flow (transcranial Doppler) in the anterior (ACA) and middle cerebral artery (MCA), beat-by-beat cardiac output (echocardiography), and end-tidal Pco(2) were measured. Autoregulation was assessed using the autoregulatory index (ARI) defined by Tiecks et al. (Tiecks FP, Lam AM, Aaslid R, Newell DW. Stroke 26: 1014-1019, 1995). Cerebral autoregulation was better in the supine position in both the ACA [supine ARI: 5.0 ± 0.21 (mean ± SE), seated ARI: 3.9 ± 0.4, P = 0.01] and MCA (supine ARI: 5.0 ± 0.2, seated ARI: 3.8 ± 0.3, P = 0.004). In contrast, cardiac output responses were not different between positions and did not correlate with cerebral blood flow ARIs. In addition, women had better autoregulation in the ACA (P = 0.046), but not the MCA, despite having the same cardiac output response. These data demonstrate cardiac output does not appear to affect the dynamic cerebral autoregulatory response to sudden hypotension in healthy controls, regardless of posture. These results also highlight the importance of considering sex when studying cerebral autoregulation.

Fig. 1.

Group mean-averaged cardiovascular parameters for all subjects who completed both supine and seated (N = 11) thigh-cuff testing. Anterior (ACA) and middle cerebral artery (MCA) data are normalized to baseline supine levels. ACA, MCA, mean arterial pressure (MAP), heart rate (HR), and CO₂ data are the mean response for each subject; cardiac index (CI) is taken from one thigh-cuff release per subject. SI, stroke index; BFV, blood flow velocity; Pet_CO2, end-tidal Pco₂; bpm, beats/min. Values are means ± SE.

Fig. 2.

Plot of ACA and MCA autoregulatory index (ARI) vs. change (Δ) in CI for both supine and seated testing (N = 26, 17 supine, 9 seated). All data are taken from one thigh-cuff release per subject.

Fig. 3.

Cerebral blood flow velocity (CBFV), CI, and MAP group responses for low (N = 13) and high (N = 13) ΔCI. All data are taken from one thigh-cuff release per subject. Median ΔCI = +1.74 l/min. Values are means ± SE.

Fig. 4.

CBFV, CI, and MAP group responses for low (N = 14) and high (N = 12) ARI. All data are taken from one thigh-cuff release per subject. Median ARI = 4.5. Values are means ± SE.

Fig. 5.

Female vs. male ARIs in the supine (11 men, 6 women) and seated (6 men, 5 women) position for MCA and ACA. ARI values were significantly higher in both men and women during supine testing (P < 0.05). Women had significantly higher ARI values in the ACA than male subjects (P = 0.046). MCA ARIs were not significantly different in male and female subjects. Values are means ± SE.