New Findings What is the central question of this study?We sought to establish between‐day reproducibility in estimates of middle cerebral artery blood velocity (MCAv) and cerebrovascular reactivity (CVR) in young, healthy male and female adults in tightly controlled experimental conditions. What is the main finding and its importance?Measures of MCAv assessed during morning, afternoon and evening hours are reproducible between days. There is diurnal variation in CVR, with values being highest during the evening compared with the morning. Greater diurnal variation in CVR is associated with more efficient sleep and greater nocturnal blood pressure dipping. These data enhance our understanding of modulators of MCAv and CVR. Abstract Transcranial Doppler (TCD) is used to assess cerebral blood velocity (CBV) and cerebrovascular reactivity (CVR). Assessments of TCD reproducibility are limited, and few include multiple within‐day measurements. We sought to establish reproducibility of CBV and CVR in healthy adults during three time periods (morning, afternoon and evening). We hypothesized that CBV and CVR measured at the same time of day are reproducible between days. We also hypothesized that CBV and CVR exhibit diurnal variation, with measurements being higher in the evening compared with morning/afternoon hours. Twelve adults [six male and six female, 27 years (95% CI, 22–31 years)] completed three measurements (morning, afternoon and evening) on two separate days in controlled conditions (e.g., meals, activity and sleep). Middle cerebral artery blood velocity (MCAv, TCD) was measured continuously at rest and during two CVR tests (end‐expiratory apnoea and carbogen inhalation). Intraclass correlation coefficients for resting MCAv showed moderate to good reproducibility, which did not differ between morning, afternoon and evening (0.87, 0.56 and 0.67, respectively; P > 0.05). Intraclass correlation coefficients for peak MCAv during apnoea (0.80, 0.46 and 0.65, respectively; P > 0.05) and minute 2 of carbogen inhalation (0.81, 0.74 and 0.73, respectively; P > 0.05) were also not different from morning compared with afternoon/evening. Time of day had no effect on resting MCAv (F = 0.69, P = 0.51, ƞp2 = 0.06) or the peak response to apnoea (F = 1.00, P = 0.39, ƞp2 = 0.08); however, peak MCAv during carbogen breathing exhibited diurnal variation, with highest values in the evening (F = 3.41, P = 0.05, ƞp2 = 0.24). Measures of CBV and CVR assessed via TCD during morning, afternoon and evening hours are reproducible between days. There is diurnal variation in the MCAv response to carbogen exposure, with CVR being highest during evening compared with morning hours.

Background Insulin‐stimulated peripheral and cerebrovascular vasodilation are consistently observed in preclinical animal models. Hyperinsulinemia also has profound vasodilatory effects within the skeletal muscle vasculature in healthy young adults; however, the magnitude of insulin‐stimulated cerebrovascular vasodilation in humans remains unclear. We hypothesized insulin‐stimulated vasodilation would be observed in both the peripheral and cerebral circulations in healthy young adults. Methods Middle cerebral artery velocity (MCAv) was measured with transcranial Doppler ultrasound in 9 healthy young adults (4M/5F, 28±2 yrs, 24±1 kg/m2) at baseline and following a 60‐min hyperinsulinemic (40 mU/m2 body surface area/min), euglycemic infusion. Femoral artery diameter and mean blood velocity were measured by Doppler ultrasound to determine femoral artery blood flow (FBF). Mean arterial blood pressure (MABP) was measured continuously by finger photoplethysmography. Cerebrovascular conductance index (CVCi = MCAv/MABP x 100) and femoral vascular conductance (FVC = FBF/MABP x 100) were calculated. Results Intravenous insulin infusion resulted in an increase in plasma insulin (5±1 to 41±2 µIU/mL, p<0.01) with blood glucose maintained at baseline levels (75±2 to 72±4 mg/dL, p=0.48). MABP was maintained throughout the infusion protocol (85±3 to 88±3 mmHg, p=0.21). There was a significant increase in FBF (134±24 to 184±30 mL/min, p=0.05) and FVC (153±22 to 211±31 mL/min/100 mmHg, p=0.04) during hyperinsulinemia. In contrast, there was no effect of hyperinsulinemia on MCAv (54±3 to 51±3 cm/s, p=0.28) or CVCi (65±5 to 58±3 AU, p=0.14). There was no association between insulin‐stimulated peripheral and cerebrovascular vasodilation (R=‐0.421, p=0.26). Conclusions Hyperinsulinemia in healthy young adults elicits peripheral vasodilation. Contrary to our hypothesis, following a 60‐minute systemic intravenous insulin infusion there is no change in MCAv or CVCi in healthy young adults. These data indicate the cerebral and peripheral vasculature are differentially regulated during hyperinsulinemia.