On the preservation and regulation of vascular tone in arteriovenous anastomoses during anesthesia

Abstract

In conscious pigs, arteriovenous anastomoses (AVAs) are in a constricted state so that < 5% of intra-atrially injected radioactive (15-microns-diam) microspheres are shunted to the lungs. Many of the anesthetic regimens frequently used in cardiovascular research dilate AVAs, thereby greatly increasing the percentage of microspheres reaching the lungs. This may seriously limit extrapolation of results obtained under anesthesia to the conscious state. We now describe that anesthesia with a combination of fentanyl and thiopental preserves the tone of AVAs, maintaining shunting under 4% of cardiac output. Furthermore, we studied in the carotid circulation of this model whether norepinephrine or 5-hydroxy-tryptamine (5-HT), both contained in perivascular nerves, is responsible for this tone. Consecutive antagonism of alpha 1-, alpha 2-, 5-HT1, and 5-HT2 receptors was obtained by sequential injection of prazosin, phentolamine, ketanserin, and methiothepin. Prazosin increased AVA blood flow, partly at the expense of extracerebral tissue blood flow, but preserved cerebral blood flow. None of the other antagonists had any additional significant effect. Therefore, in this model the tone in AVAs seems to be maintained by sympathetic norepinephrine-containing nerves via alpha 1-adrenoceptors.

Effects of the CGRP receptor antagonist BIBN4096BS on capsaicin‐induced carotid haemodynamic changes in anaesthetised pigs

“…This anaesthetic regimen, together with bilateral vagosympathectomy (see below), increases the heart rate and markedly dilates carotid arterioles and arteriovenous anastomoses due to a loss of parasympathetic and sympathetic tone, respectively. Consequently, carotid blood flow, particularly its arteriovenous anastomotic fraction, is considerably higher in these pigs than in conscious or thiopental‐anaesthetised pigs ( Den Boer et al , 1993 ).…”

Effects of Donitriptan on Carotid Haemodynamics and Cardiac Output Distribution in Anaesthetized Pigs

“…infusion of pentobarbitone sodium at 12–20 mg kg −1 h −1 . It may be noted that with this anaesthetic regimen, arteriovenous anastomotic blood flow is considerably higher than that in pigs in a conscious state or under thiopentone anaesthesia (26), thereby producing one of the putative features of migraine, i.e. dilatation of carotid arteriovenous anastomoses (25, 27).…”

Sumatriptan Does Not Affect Arteriovenous Oxygen Differences in Jugular and Cubital Veins in Normal Human Subjects

“…The most likely reason for the lack of effect is that AVA blood flow is only minor in humans, as in conscious pigs (15). The tone in AVAs in pigs is maintained by sympathetic norepinephrine‐containing nerves via α 1 ‐adrenoceptors (18). Lack of tone can explain the effect of sumatriptan in anaesthetized and bilaterally vagosympatectomized pigs.…”