Paradoxical dilation of the large cerebral arteries in hypocapnia in man.

Abstract

SUMMARY By grouping patients who had carotid angiograms under unusually carefully monitored conditions it has been shown that hypocapnia is associated with vasodilatation at low blood pressure but not at high blood pressure. The mechanism is discussed in general terms and it is suggested that the hypocapnic vasodilatation may be a response to cerebral hypoxia and may be transmitted via an intracerebral autonomic pathway. Clinical and angiographical diagnoses are given for SO patients.IN THE BABOON there is angiographical evidence that during hypocapnia the response of the larger cerebral arteries to a further drop in Paco 2 is vasodilatation. 1 This paradoxical reactivity was also observed in man, but the conditions under which diagnostic angiography had to be performed in the hospital could not be as carefully controlled as in the experiments with baboons, so that in man the effects of concomitant blood pressures (BP) were unknown.To investigate the phenomenon further we have measured the internal diameters of the intracranial carotid, the middle and the anterior cerebral arteries in a series of angiograms which were all carried out under unusually carefully monitored conditions. The arterial diameters have been related both to blood pressure and Paco 2 . The findings indicated that though paradoxical CO 2 reactivity is generally seen at low blood pressure in hypocapnic patients, it is not seen if a normal blood pressure is maintained. MethodsFrom patients at The National Hospital, Queen Square, London (England) who were anesthetized using a short-acting barbiturate for inductions and who were maintained on nitrous oxide and oxygen, supplemented by neuroleptanalgesia 2 in order to carry out intracarotid U3 Xe cerebral blood flow (CBF) studies and carotid angiography, 40 were chosen because their blood pressure levels fitted into our predetermined grouping. Ten further patients had had angiography at St. Bartholomew's Hospital also under very carefully monitored conditions, but anesthetized (after in-

Variations in middle cerebral artery blood flow investigated with noninvasive transcranial blood velocity measurements.

“…Changes of this magnitude have been observed angiographically when arterial blood pressure increased from < 7 6 to > 100 mm Hg in patients under general anesthesia and moderate hypocapnia. 19 The differences found in the present series might, therefore, reflect predictable individual variation. Although difficult to evaluate statistically, the linear relation between flow and blood velocity did not seem to be seriously offset by moderate therapeutic doses of metaraminol or thiopentone.…”

Functional transcranial Doppler sonography as a tool in psychophysiological research

“…The underlying regulatory mechanism is rather complex and involves different biochemical mediators like K + , H + , and adenosine (see Hamann & del Zoppo, 1994; Kuschinsky, 1991; Villringer & Dirnagl, 1995). In contrast to the small vessels, the diameter of the larger cerebral arteries primarily remains unchanged under varying stimulation conditions (Giller, Bowman, Dyer, Mootz, & Krippner, 1993; Huber & Handa, 1967; Kontos, 1989; Radu & du Boulay, 1976). From these results, it can be concluded that changes in flow velocities in the large basal arteries are not a result of their own vasomotor activity, but reflect changes in the blood demand in their perfusion territories, and thus changes in local neuronal activity patterns.…”

Impairment of Cerebrovascular Reactivity by Methionine-Induced Hyperhomocysteinemia and Amelioration by Quinapril Treatment

“…24 The diameter of large cerebral arteries such as the MCA, however, remains relatively constant during a reduction of blood pressure. 25,26 Therefore, the changes of flow velocity in the MCA might correlate with the changes of cerebral blood flow. 27 This noninvasive transient hyperemic response test is simple, safe, and reproducible at the bedside and could serve as a marker of CVR.…”