The effect of anaesthesia on renal function.

Anaesthesia and surgical stress can affect renal function and body fluid regulation indirectly as well as directly. The indirect effects, through influences on haemodynamics, sympathetic activity and humoral regulation, are more pronounced than the direct ones. Inhalational anaesthetics generally reduce glomerular filtration rate and urine output, mainly by extra-renal effects that are attenuated by pre-operative hydration. Opioids, barbiturates and benzodiazepines also reduce glomerular filtration rate and urine output. The effects of regional anaesthesia seem to be less than those of general anaesthesia and are related to changes in systemic haemodynamics. These peri-operative alterations of renal function are usually transient and clinically insignificant. Mechanical ventilation decreases urine volume and sodium excretion to an extent that depends on the increase in intrathoracic pressure, though ADH release, unloading of baroreceptors and activation of the renin-angiotensin system may also be involved. The direct effects of anaesthesia which are dose- and agent-dependent include effects on autoregulation of renal blood flow, alteration in the effect of ADH, and effects on tubular transport of sodium and organic acids. The only proven direct toxic effect of any anaesthetic agent is the fluoride-related toxicity of methoxyflurane.