Nitrous oxide inhalation does not influence plasma concentrations of beta-endorphin or Met-enkephalin-like immunoreactivity.

The possibility that nitrous oxide releases endogenous opioid peptides into the circulation has been tested in 10 pain-free, unstressed volunteers breathing 30% nitrous oxide in oxygen. Despite achieving plateau concentrations in venous blood, accompanied by subjective effects, there were no significant changes in plasma concentrations of immunoreactive beta-endorphin, methionine-enkephalin or ACTH. These results indicate that, in the absence of nociceptive input, the effects of the inhalation of nitrous oxide are unrelated to alterations in peripheral concentrations of these endogenous opioid peptides.

Venous pressures in the isolated upper limb during saline injection.

Venous pressure changes were assessed in the antecubital vein of an isolated arm during saline injection via an intravenous site on the dorsum of the hand. Although leak of contrast medium has been radiologically demonstrated in these circumstances, the compliance of the venous system of the isolated and exsanguinated limb has not been investigated. In five male and five female volunteers, after exsanguination and isolation of the upper limb, volumes of 40 ml or 60 ml of physiological saline were injected manually. The resultant pressure changes in the antecubital vein were continuously recorded during and after injection, until a steady state was reached. Although peak pressures as high as 190 mmHg were noted early in the injection, venous pressures tended to plateau before the injection was complete and remained constant until the end of the injection. Resting venous pressures of 20 to 50 mmHg at the end of infusion occurred in all subjects. At no time did venous pressure approach cuff pressure. The mechanism of leak with an intact tourniquet is not explained, in our study, by the development of venous pressure in excess of tourniquet pressure in the isolated limb.