British journal of anaesthesia
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Comparative Study
The auditory evoked response as an indicator of awareness.
The latency of the early cortical wave Nb of the auditory evoked response (AER) was compared with responses to Tunstall's isolated forearm test, while the concentration of nitrous oxide was progressively reduced during light anaesthesia in seven patients. A threshold Nb latency of 44.5 ms was chosen to discriminate between an early cortical AER containing three waves and that with two waves of longer latency. ⋯ The addition of a volatile anaesthetic abolished any response, and increased Nb latency to more than 44.5 ms. The three wave AER pattern, therefore, is associated with a depth of anaesthesia at which awareness occurs.
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This study was designed to investigate if propofol produced cardiovascular effects by direct actions or by indirect actions secondary to depression of the central nervous system. Experiments were performed on chloralose anaesthetized dogs in which all neurogenic cardiovascular reflexes were abolished by bilateral vagotomy and common carotid ligatures, in combination with i.v. bretylium and propranolol. Bolus doses of propofol followed by infusions at rates up to 160 mg kg-1 h-1 produced blood concentrations of propofol from 1.99 to 112 micrograms ml-1. ⋯ Blood concentrations of propofol less than 10 micrograms ml-1 caused an increase in mean capacitance of 8.0 (SEM 1) ml kg-1 with no significant changes in systemic vascular resistance, pulmonary vascular resistance or inotropic state of the heart. We conclude that anaesthesia with propofol may be accompanied by decreased cardiac output secondary to reduction in preload by a direct venodilator effect. Our experiments indicate that cardiac output and arterial pressure are preserved well at normal anaesthetic blood concentrations of propofol if the preload is maintained.