Dynamics of middle cerebral artery blood flow velocity during moderate-intensity exercise

Abstract

The dynamic response to a stimulus such as exercise can reveal valuable insights into systems control in health and disease that are not evident from the steady-state perturbation. However, the dynamic response profile and kinetics of cerebrovascular function have not been determined to date. We tested the hypotheses that bilateral middle cerebral artery blood flow mean velocity (MCA_V) increases exponentially following the onset of moderate-intensity exercise in 10 healthy young subjects. The MCA_V response profiles were well fit to a delay (TD) + exponential (time constant, τ) model with substantial agreement for baseline [left (L): 69, right (R): 64 cm/s, coefficient of variation (CV) 11%], response amplitude (L: 16, R: 13 cm/s, CV 23%), TD (L: 54, R: 52 s, CV 9%), τ (L: 30, R: 30 s, CV 22%), and mean response time (MRT) (L: 83, R: 82 s, CV 8%) between left and right MCA_V as supported by the high correlations (e.g., MRT r = 0.82, P < 0.05) and low CVs. Test-retest reliability was high with CVs for the baseline, amplitude, and MRT of 3, 14, and 12%, respectively. These responses contrasted markedly with those of three healthy older subjects in whom the MCA_V baseline and exercise response amplitude were far lower and the kinetics slowed. A single older stroke patient showed baseline ipsilateral MCA_V that was lower still and devoid of any exercise response whatsoever. We conclude that kinetics analysis of MCA_V during exercise has significant potential to unveil novel aspects of cerebrovascular function in health and disease.NEW & NOTEWORTHY Resolution of the dynamic stimulus-response profile provides a greater understanding of the underlying the physiological control processes than steady-state measurements alone. We report a novel method of measuring cerebrovascular blood velocity (MCAv) kinetics under ecologically valid conditions from rest to moderate-intensity exercise. This technique reveals that brain blood flow increases exponentially following the onset of exercise with 1) a strong bilateral coherence in young healthy individuals, and 2) a potential for unique age- and disease-specific profiles.

Keywords: aging; blood flow velocity; brain blood flow; middle cerebral artery; stroke.

Fig. 1.

Typical middle cerebral artery velocity (MCA_V) at rest and response following the onset of moderate-intensity exercise (dashed vertical line, time 0). Notice the close fit (solid curve at top) to the time delay + exponential model as supported by the high correlation coefficient and residuals profile (solid line at bottom). Results from subject 2.

Fig. 2.

Middle cerebral artery velocity (MCA_V) at rest and response following the onset of moderate-intensity exercise (dashed vertical line, time 0). Solid symbols are left MCA, and hollow symbols are right MCA. Top: illustrates subject 10 in whom there was excellent agreement between left and right. Bottom: represents poorest agreement among the young participants, subject 7. MRT, mean response time. Overall left and right MCA_V correlation was 0.819 (P < 0.05) with a coefficient of variation of 7.6%.

Fig. 3.

Top: correlation between left and right middle cerebral artery velocity (MCA_V) mean response time (MRT) following the onset of moderate-intensity exercise (n = 8). Overall left and right MCA_V correlation was 0.819 (P < 0.05) with a coefficient of variation of 7.6%. Bottom: Bland-Altman plot demonstrating the generally close correspondence between left and right MRTs. Overall left and right MCA_V correlation was 0.819 (P < 0.05) with a coefficient of variation of 7.6%.

Fig. 4.

Typical middle cerebral artery velocity (MCA_V) at rest and response following the onset of moderate-intensity exercise (dashed vertical line, time 0). Solid circles are from a representative young healthy subject, subject 2. Hollow circles are from an older healthy subject (subject 20). Hollow squares are from stroke patient (subject 201) using the ipsilateral MCA. By comparison, note very slow mean response time (MRT) and low amplitude of response in the older subject and absence of any response in the stroke patient.