Cerebral perfusion, oxygenation and metabolism during exercise in young and elderly individuals

Abstract

We evaluated cerebral perfusion, oxygenation and metabolism in 11 young (22 ± 1 years) and nine older (66 ± 2 years) individuals at rest and during cycling exercise at low (25% W(max)), moderate (50% Wmax), high (75% W(max)) and exhaustive (100% W(max)) workloads. Mean middle cerebral artery blood velocity (MCA V(mean)), mean arterial pressure (MAP), cardiac output (CO) and partial pressure of arterial carbon dioxide (P(aCO2)) were measured. Blood samples were obtained from the right internal jugular vein and brachial artery to determine concentration differences for oxygen (O2), glucose and lactate across the brain. The molar ratio between cerebral uptake of O2 versus carbohydrate (O2-carbohydrate index; O2/[glucose + 1/2 lactate]; OCI), the cerebral metabolic rate of O2 (CMRO2) and changes in mitochondrial O2 tension ( P(mitoO2)) were calculated. 100% W(max) was ~33% lower in the older group. Exercise increased MAP and CO in both groups (P < 0.05 vs. rest), but at each intensity MAP was higher and CO lower in the older group (P < 0.05). MCA V(mean), P(aCO2) and cerebral vascular conductance index (MCA V(mean)/MAP) were lower in the older group at each exercise intensity (P < 0.05). In contrast, young and older individuals exhibited similar increases in CMRO2 (by ~30 μmol (100 g(-1)) min(-1)), and decreases in OCI (by ~1.5) and (by ~10 mmHg) during exercise at 75% W(max). Thus, despite the older group having reduced cerebral perfusion and maximal exercise capacity, cerebral oxygenation and uptake of lactate and glucose are similar during exercise in young and older individuals.

Figure 1. Mean arterial pressure, heart rate, systemic VC and cardiac output at rest and during submaximal and maximal exercise in young and older individuals

A, mean arterial pressure; B, heart rate; C, systemic VC; D, cardiac output. Young and older subjects are compared at the same relative exercise intensities, representing low (25%W_max), moderate (50%W_max), high (75%W_max) and exhaustive (100%W_max) exercise workloads. Systemic VC, systemic vascular conductance. Values are mean ± SEM. P values represent ANOVA results. *P < 0.05 vs. rest, †P < 0.05 vs. young.

Figure 2. MCA V_mean, cerebral VC index and at rest and during submaximal and maximal exercise in young and older individuals

A, MCA V_mean; B, cerebral VC index; C, . V_mean, middle cerebral artery mean blood velocity; cerebral VC, cerebral vascular conductance; , partial pressure of arterial carbon dioxide. Values are mean ± SEM. P values represent ANOVA results. *P < 0.05 vs. rest, †P < 0.05 vs. young.

Figure 3. OCI, OGI, ΔCMRO₂ and at rest and during submaximal and maximal exercise in young and older individuals

A, OCI; B, OGI; C, ΔCMRO₂; D, . OCI represents the molar ratio between the O₂ and carbohydrate (glucose and lactate) taken up by the brain; OGI, molar ratio between the cerebral uptake of O₂ versus glucose; ΔCMRO₂, change from rest in cerebral metabolic rate for O₂; , change from rest in mitochondrial oxygen tension. Values are mean ± SEM. P values represent ANOVA results. *P < 0.05 vs. rest, †P < 0.05 vs. young.