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 ca rbon dioxide reactivity. Reproducibility with time and altered positio n of the patient is examined in the present study. Carbon dioxide reac tivity was determined in 10 healthy volunteers using TCD. Hypocarbia w as produced by voluntary hyperventilation, and hypercarbia was produce d by rebreathing from a circuit primed with 7% carbon dioxide. Each pa tient 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 velo city and end-tidal carbon dioxide values. Analysis of covariance for r epeated 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 pressur e were similar from one week to the next at all carbon dioxide levels. In contrast, flow velocity, mean arterial pressure (adjusted for hydr ostatic gradient), and absolute slope were decreased in the seated pos ition (p < 0.05). No difference was observed when the relative slope w as used for comparison. We conclude that absolute carbon dioxide react ivity is reproducible over time but may be influenced by position. Rel ative reactivity (relative slope), however, was both time and position independent.