Anesthesiology
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Anesthetic induction and maintenance with propofol are associated with decreased blood pressure that is, in part, due to decreased peripheral resistance. Several possible mechanisms whereby propofol could reduce peripheral resistance include a direct action of propofol on vascular smooth muscle, an inhibition of sympathetic activity to the vasculature, or both. This study examined these two possibilities in humans by measuring the forearm vascular responses to infusions of propofol into the brachial artery (study 1) and by determining the forearm arterial and venous responses to systemic (intravenous) infusions of propofol after sympathetic denervation of the forearm by stellate blockade (study 2). ⋯ In contrast to SNP infusions, propofol infusions into the brachial artery of conscious persons caused no significant vascular responses, despite the presence of therapeutic plasma concentrations of propofol within the forearm. The effects of propofol anesthesia on FVR and FVC are similar to the effects of sympathetic denervation by stellate ganglion blockade. Thus the peripheral vascular actions of propofol appear to be due primarily to an inhibition of sympathetic vasoconstrictor nerve activity.
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Cricoid cartilage pressure induced to prevent pulmonary aspiration from regurgitation of gastric contents has been recommended, and its efficacy requires a force greater than 40 Newtons. For regurgitation to occur, both an increase in gastric pressure and relaxation of the lower esophageal sphincter (LES) are necessary. However, the effect of cricoid cartilage pressure on the LES is unknown. This study evaluated the effects of cricoid cartilage pressure on LES in human volunteers. ⋯ These findings may explain the occurrence of pulmonary aspiration before tracheal intubation despite application of cricoid cartilage pressure.
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Acute respiratory failure may develop in patients with chronic obstructive pulmonary disease because of intrinsic positive end-expiratory pressure (PEEPi) and increased resistive and elastic loads. Proportional assist ventilation is an experimental mode of partial ventilatory support in which the ventilator generates flow to unload the resistive burden (flow assistance: FA) and volume to unload the elastic burden (volume assistance: VA) proportionally to inspiratory muscle effort, and PEEPi can be counterbalanced by application of external PEEP. The authors assessed effects of proportional assist ventilation and optimal ventilatory settings in patients with chronic obstructive pulmonary disease and acute respiratory failure. ⋯ Application of PEEP to counterbalance PEEPi and FA to unload the resistive burden provided the optimal conditions in such patients. Ventilator over-assistance and patient-ventilator asynchrony was observed when VA was added to this setting. The clinical use of proportional assist ventilation should be based on continuous measurements of respiratory mechanics.
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Halothane induces negative inotropic and lusitropic effects in myocardium. It has been suggested that halothane potentiates beta-adrenoceptor stimulation. However, its effects on the inotropic response to alpha-adrenoceptor stimulation and its effects on the lusitropic effects of alpha- and beta-adrenoceptor stimulation are unknown. ⋯ At clinically relevant concentrations, halothane potentiated the positive inotropic effects of both alpha- and beta-adrenoceptor stimulation. Furthermore, halothane alters the positive lusitropic-effect of beta-adrenoceptor stimulation under low load.