The variability of cerebrovascular reactivity with posture and time.

Transcranial Doppler (TCD) ultrasonography has been used in a variety of clinical contexts to assess cerebrovascular reserve by measuring carbon dioxide reactivity. Reproducibility with time and altered position of the patient is examined in the present study. Carbon dioxide reactivity was determined in 10 healthy volunteers using TCD. Hypocarbia was produced by voluntary hyperventilation, and hypercarbia was produced by rebreathing from a circuit primed with 7% carbon dioxide. Each patient was studied in the supine position twice (1 week apart) and once in the seated position. Carbon dioxide reactivity was determined from linear regression analysis of paired middle cerebral artery flow velocity and end-tidal carbon dioxide values. Analysis of covariance for repeated measures was used for statistical analysis. Both the absolute slope and the relative slope (absolute slope expressed as a percentage of flow velocity at 40 mm Hg) were compared. In the supine position, flow velocity, absolute and relative slopes, and mean arterial pressure were similar from one week to the next at all carbon dioxide levels. In contrast, flow velocity, mean arterial pressure (adjusted for hydrostatic gradient), and absolute slope were decreased in the seated position (p < 0.05). No difference was observed when the relative slope was used for comparison. We conclude that absolute carbon dioxide reactivity is reproducible over time but may be influenced by position. Relative reactivity (relative slope), however, was both time and position independent.