Rational approach to clinical fluid balance

Clinical assessment of intravascular volume and laboratory investigations are essential to guide perioperative fluid therapy. Thirst, skin turgor, hydrationofmucousmembranes, core– peripheral temperature gradient, pulse rate and volume, changes in blood pressure in the upright position and urine output are invaluable clues to the state of hydration. However, haemodynamic variables are affected by factors other than volume status such as drugs and the physiological effects of surgical stress. Therefore, it is useful to look at trends rather than a single reading. Thirst occurs in response to hypovolaemia (via baroreceptors) and to changes of as little as 1% in osmolarity. In normal life, thirst plays a crucial role as a sensitive indicator of fluid deficit; it must be distinguished from dry mouth, which can result from oxygen therapy or drugs. Fluid balance is rarely a major problem in those patients who are able to drink. Loss of skin turgor indicates an intravascular deficit of about 10%.Orthostatic and supine hypotension implies deficits of 20% and 30%, respectively. Healthy individuals may sustain 20% loss of circulating volume and only exhibit postural tachycardia. With autonomic dysfunction, postural hypotension may occur in normovolaemia. Signs of hypervolaemia are suspected in the presence of pitting oedema and increased urinary flow in patients with normal cardiac, hepatic and renal function. Pulmonary oedema is a late sign of hypervolaemia. Laboratory signs of dehydration include a rising haematocrit, progressive metabolic acidosis, hypernatraemia and urinary sodium >20 mmol litre . Changes in plasma urea and creatinine should be interpreted in the light of other factors such as age and lean body mass. Central venous pressure readings must be interpreted in light of the clinical setting. Low values (<5 mm Hg) may be normal unless associated with other signs of hypovolaemia. The principle behind a fluid challenge is to assess the compliance of the circulation and to re-evaluate intravascular volume status by observing the haemodynamic and clinical response.