Hyperventilation, which commonly occurs during orthostatic stress both in patients with tendencies to syncope and in normal volunteers, leads to cerebral vasoconstriction and systemic vasodilatation. In the accompanying communication (Norcliffe et al. 2002) we present evidence that in individuals with a high sensitivity of vascular resistance to changes in CO₂ there is a high incidence of syncope during orthostatic stress. This demonstration shows the method used for assessing CO₂ reactivity in both the cerebral circulation and in a forearm.

Blood flow in the cerebral circulation is assessed in the middle cerebral artery, determined using the technique of transcranial ultrasonography (Multi-Dop¿ X4, TCD-8.01, DWL Elektronische System GmbH, Sipplingen, Germany) whereby a pulsed ultrasound beam insonates the middle cerebral artery through a thinning of the temporal bone, the transcranial window. Forearm blood velocity is assessed by a second ultrasound beam along the direction of the brachial artery. Blood pressure is determined using a Finapres photo-plethysmograph (Finapres, Ohmeda, Winconsin, USA), calibrated frequently using an automated sphygmomanometer (Hewlett Packard 78352C, Boebringen, Germany). Cerebral perfusion pressure is estimated from brachial pressure, corrected if necessary for height differences. Vascular resistance in the two beds is calculated as mean arterial pressure divided by mean blood velocity. As the Doppler device does not give absolute values of blood flow, vascular resistance can only be assessed in relative terms.

Arterial blood P_CO2 is assumed to be equal to the end-tidal value and this is determined using an infrared analyser (model Binos 1, Leybold-Haraeus Limited, Koln, Germany). The levels of CO₂ are increased by hypoventilation, aided by the use of increased dead space, and decreased by hyperventilation. CO₂ sensitivity in the two vascular beds is expressed as the percentage change in vascular resistance divided by the change in end-tidal CO₂ concentration.

This technique is used to assess the reactivity in the two vascular beds to CO₂ and results of research using this method are given as a communication (Norcliffe et al. 2002).

All procedures accord with current local guidelines.