Distribution of cardiac output to the brain across the adult lifespan

A widely accepted dogma is that about 15–20% of cardiac output is received by the brain in healthy adults under resting conditions. However, it is unclear if the distribution of cardiac output directed to the brain alters across the adult lifespan and is modulated by sex or other hemodynamic variables. We measured cerebral blood flow/cardiac output ratio index in 139 subjects (88 women, age 21–80 years) using phase-contrast magnetic resonance imaging and echocardiography. Body mass index, cardiac systolic function (eject fraction), central arterial stiffness (carotid-femoral pulse wave velocity), arterial pressure, heart rate, physical fitness (VO₂ max), and total brain volume were measured to assess their effects on the cardiac output–cerebral blood flow relationship. Cerebral blood flow/cardiac output ratio index decreased by 1.3% per decade associated with decreases in cerebral blood flow (P < 0.001), while cardiac output remained unchanged. Women had higher cerebral blood flow, lower cardiac output, and thus higher cerebral blood flow/cardiac output ratio index than men across the adult lifespan. Age, body mass index, carotid-femoral pulse wave velocity, and arterial pressure all had negative correlations with cerebral blood flow and cerebral blood flow/cardiac output ratio index (P < 0.05). Multivariable analysis adjusted for sex, age showed that only body mass index was negatively associated with cerebral blood flow/cardiac output ratio index (β = −0.33, P < 0.001). These findings demonstrated that cardiac output distributed to the brain has sex differences and decreases across the adult lifespan and is inversely associated with body mass index.