British Journal of Anaesthesia 1990; 65: 54-62 EFFECTS OF ANAESTHESIA ON RESPIRATION

Anaesthesia, with or without paralysis, causes profound changes in respiratory function. There is no question of an overall depression of every facet of the complex matrix of functions which contribute to the oxygenation of the arterial blood; instead, we find highly specific changes, with some aspects of functions markedly affected while others are unaffected. It would have been difficult, if not impossible, to have forecast what is and is not affected by anaesthesia. This has only been elucidated by painstaking research extending back over the past 40 years and with no end in sight. This work has been greatly limited by our lack of understanding of normal respiratory physi-ology in the conscious subject. A very important additional problem has been the effect of posture. Most studies of respiratory function in conscious subjects have been undertaken with the subjects in the upright (seated) position, while anaesthesia and intensive care are usually concerned with the supine, prone or lateral decubitus positions. These differences in posture have major effects on respiratory function which are sometimes greater than the effects of anaesthesia itself. Research in anaesthesia has usually followed closely behind advances in our understanding of respiratory function in the awake subject. Thus technological developments in research have, in general, been applied to anaesthetized patients 2 or 3 years after they have first been used in conscious subjects. This has been the pattern in the case of the measurement of deadspace, the polarographic determination of arterial Po2, analysis of the distribution of ventilation— perfusion relationships and computed tom-ography, to cite but a few examples. Either anaesthetists have learned the new technology or, less commonly, basic scientists have been lured into the study of anaesthesia. In both cases there have been formidable practical and ethical prob-lems in the application of new technology to the anaesthetized patient. In many cases ethical constraints have limited the expansion of knowl-edge, and extrapolation from animal experiments is fraught with difficulty