Biomedical subjects
Publications and source records attributed to D R Bevan.
The measurement of the osmolality of body fluids is used increasingly in clinical practice. This paper discusses the terminology and describes the methods available for its measurement.
The maintenance of osmolar constancy of the body fluids is dependent upon the recognition of osmolar disequilibrium and its correction by modifying the ingestion and excretion of fluid and solute. Osmolar changes are sensed by the hypothalamus which regulates the secretion of antidiuretic hormone to modify the renal excretion of water. The integrity of the system depends upon the renal ability to vary the solute concentration of urine.
Measurement of plasma and urine osmolality is quick, easy and accurate. The recognition of the interdependence of urine volume and osmolality on the excretion of the daily obligatory solute load assists in the diagnosis and management of fluid balance and renal excretory problems in the acutely ill. In addition, syndromes of osmotic disequilibrium present a challenge in metabolic care.
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Rational intraoperative fluid therapy is based on an understanding of the pathophysiology of severe trauma and surgery. Fluids of suitable compositions are administered in sufficient quantities to form part of the daily maintenance requirement and also to replace blood and ECF lost during surgery.
The effects of positive and negative end expiratory pressure on renal function have been studied in adult greyhounds with a normal blood volume and without evidence of cardiorespiratory disease. Positive expiratory pressure (+ 10 cm H2O) resulted in a decrease of urine volume, glomerular filtration rate and renal blood flow which returned towards normal following removal of the positive pressure. Negative expiratory pressure (-6 cm H2O) resulted in opposite changes in renal function.
During pregnancy there is a fall in functional residual capacity and expiratory reserve volume. The point of airway closure in relation to vital capacity and the expiratory reserve volume was determined in 11 post-partum patients who had previously been studied during pregnancy. The lung volume at which airway closure occurred in pregnancy and the post-partum period did not change in relation to the vital capacity whereas the expiratory reserve volume did, so that airway closure became imminent in the pregnant women during tidal ventilation, especially whilst in the suping position. The significance of this alteration in pulmonary function to obstetric anaesthesia is discussed.
A retrospective study of postoperative respiratory morbidity in 247 patients requiring renal transplantation between 1955 and 1973 showed that 7 patients required postoperative controlled ventilation for up to 6 days. The nondepolarising relaxants tubocurarine and pancuronium were used in only 65 patients, but all 7 cases of respiratory failure occurred in this group. This suggests that the use of these drugsin anephric patients is potentially hazardous so far as postoperative respiratory insufficiency is concerned.
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Measurements of closing volume have been made in 20 women between the 36th and 40th weeks of pregnancy. The patients were studied in the erect and supine positions and the point of airway closure was related to functional residual capacity. The results show that airway closure occurred during tidal ventilation in 10 patients in the erect position and in six patients in the supine position. These results may explain the variation found in maternal arterial oxygen tension during pregnancy.
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