Effects of Age and Coronary Artery Disease on Cerebrovascular Reactivity to Carbon Dioxide in Humans

Abstract

Alterations in cerebrovascular reactivity to CO₂, an index of cerebrovascular function, have been associated with increased risk of stroke. We hypothesised that cerebrovascular reactivity is impaired with increasing age and in patients with symptomatic coronary artery disease (CAD). Cerebrovascular and cardiovascular reactivity to CO₂ was assessed at rest and during hypercapnia (5% CO₂) and hypocapnia (hyperventilation) in subjects with symptomatic CAD (n=13) and age-matched old (n=9) and young (n=20) controls without CAD. Independent of CAD, reductions in middle cerebral artery blood velocity (transcranial Doppler) and cerebral oxygenation (near-infrared spectroscopy) were correlated with increasing age (r=-0.68, r=-0.51, respectively, P <0.01). In CAD patients, at rest and during hypercapnia, cerebral oxygenation was lower (P <0.05 vs young). Although middle cerebral artery blood velocity reactivity was unaltered in the hypercapnic range, middle cerebral artery blood velocity reactivity to hypocapnia was elevated in the CAD and age-matched controls (P <0.01 vs young), and was associated with age (r=0.62, P <0.01). Transient drops in arterial PCO₂ occur in a range of physiological and pathophysiological situations, therefore, the elevated middle cerebral artery blood velocity reactivity to hypocapnia combined with reductions in middle cerebral artery blood velocity may be important mechanisms underlying neurological risk with aging. In CAD patients, additional reductions in cerebral oxygenation may place them at additional risk of cerebral ischaemia.