Abstract
Reduced cerebrovascular reactivity to a vasoactive stimulus is associated with age-related diseases such as stroke and cognitive decline. Habitual exercise is protective against cognitive decline and is associated with reduced stiffness of the large central arteries that perfuse the brain. In this context, we evaluated the age-related differences in cerebrovascular reactivity in healthy adults who habitually exercise. In addition, we sought to determine the association between central arterial stiffness and cerebrovascular reactivity. We recruited 22 young (YA: age = 27 ± 5 years, range 18–35 years) and 21 older (OA: age = 60 ± 4 years, range 56–68 years) habitual exercisers who partake in at least 150 min of structured aerobic exercise each week. Middle cerebral artery velocity (MCAv) was recorded using transcranial Doppler ultrasound. In order to assess cerebrovascular reactivity, MCAv, end-tidal carbon dioxide (ETCO2), and mean arterial pressure (MAP) were continuously recorded at rest and during stepwise elevations of 2, 4, and 6% inhaled CO2. Cerebrovascular conductance index (CVCi) was calculated as MCAv/MAP. Central arterial stiffness was assessed using carotid–femoral pulse wave velocity (PWV). Older adults had higher PWV (YA: 6.2 ± 1.2 m/s; OA: 7.5 ± 1.3 m/s; p < 0.05) compared with young adults. MCAv and CVCi reactivity to hypercapnia were not different between young and older adults (MCAv reactivity, YA: 2.0 ± 0.2 cm/s/mmHg; OA: 2.0 ± 0.2 cm/s/mmHg; p = 0.77, CVCi reactivity, YA: 0.018 ± 0.002 cm/s/mmHg2; OA: 0.015 ± 0.001 cm/s/mmHg2; p = 0.27); however, older adults demonstrated higher MAP reactivity to hypercapnia (YA: 0.4 ± 0.1 mmHg/mmHg; OA: 0.7 ± 0.1 mmHg/mmHg; p < 0.05). There were no associations between PWV and cerebrovascular reactivity (range: r = 0.00–0.39; p = 0.07–0.99). Our results demonstrate that cerebrovascular reactivity was not different between young and older adults who habitually exercise; however, MAP reactivity was augmented in older adults. This suggests an age-associated difference in the reliance on MAP to increase cerebral blood flow during hypercapnia.
Highlights
A healthy brain is highly sensitive to changes in arterial carbon dioxide (CO2) such that elevations in the arterial partial pressure of carbon dioxide (PaCO2) can cause profound vasodilation of cerebral vasculature and hypercapnia is associated with augmentation of global cerebral blood flow (CBF) (Kety and Schmidt, 1948)
Young and older adults had a similar body mass index (BMI) and supine systolic blood pressures at baseline, yet older adults had higher supine diastolic blood pressures and mean arterial pressure (MAP) when compared with young adults
In our population of adults who habitually exercised, we did not find a relationship between central arterial stiffness and cerebrovascular reactivity, nor did we report an age-related difference in cerebrovascular reactivity, providing one potential explanation as to why habitual exercise is protective against certain cerebral pathologies and cognitive decline
Summary
A healthy brain is highly sensitive to changes in arterial carbon dioxide (CO2) such that elevations in the arterial partial pressure of carbon dioxide (PaCO2) can cause profound vasodilation of cerebral vasculature and hypercapnia is associated with augmentation of global cerebral blood flow (CBF) (Kety and Schmidt, 1948). There are conflicting reports regarding the changes in cerebrovascular reactivity to hypercapnia related to primary aging with some studies demonstrating ageassociated declines (Rogers et al, 1985; Reich and Rusinek, 1989; Tsuda and Hartmann, 1989; Matteis et al, 1998; Barnes et al, 2012), some demonstrating age-associated increases (Zhu et al, 2013) while others show no age-associated declines (Schieve and Wilson, 1953; Schwertfeger et al, 2006; Galvin et al, 2010; Oudegeest-Sander et al, 2014; Coverdale et al, 2017) These discrepancies in the literature may be due to methodological differences as well as participants’ underlying comorbidities, biological sex, medication use, or habitual exercise status. Previous studies have shown a positive relationship between cardiorespiratory fitness and cerebrovascular reactivity to hypercapnia (Bailey et al, 2013; Barnes et al, 2013; Murrell et al, 2013); it is important to consider activity status of participants as the reported effects of aging may have been amplified by sedentary behavior
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