Insensible fluid loss during cardiac surgery

Introduction

Insensible losses normally occur by diffusion through the skin and evaporation from the respiratory tract. Total losses per day at ambient temperature are 700 ml [1]. When a body cavity is breached during surgery evaporative losses increase and are difficult to measure. However underestimation of insensible losses has led to a reappraisal of fluid requirements perioperatively to prevent oliguria [2,3]. With invasive monitoring most clinicians prefer to titrate fluid replacement to measured variables. Cardiac patients leave theatre in a positive balance but as the early post bypass period is associated with a diuretic phase, relative fluid balance is usually achieved within a few hours. However we have observed a reluctance amongst trainee and nursing staff to give additional fluid when urine output is falling if the measured pressures are satisfactory and calculations continue to indicate a positive balance. Measurement of central venous pressure is insensitive and measurable changes only follow intravascular volume changes of 750 ml or more [3]. The prescription of a diuretic at this stage may be erroneous. This pilot study sought to quantify insensible losses during cardiac surgery.

Methods

Eight male and two female patients undergoing routine cardiac surgery (two mitral valve replacemenst, one aortic valve replacement + grafts, one mitral valve replacement + grafts) gave informed consent to the study. On arrival in the theatre suite patients were weighed using the Arjo Maximove (Arjo Ltd, Gloucester UK). This was calibrated according to manufacturer's recommendations and has an accuracy of ± 0.2 kg within the normal adult range of weight. Postoperatively, once stable, and within 30 min of arrival in the ICU patients were weighed a second time. Fluid gains and losses were charted accurately during surgery including priming volumes, residual pump volumes, irrigation fluids and infusion volumes from all sources. Blood loss was estimated by conventional weighing and suction with the addition of 25% to the total. All other additions including endotracheal tube, cannulae, catheter, drains and dressings were weighed separately and added where appropriate.

Results

Results are expressed as mean ± SD. Time on bypass was 1.5 ± 0.66 hours. Weight before operation was 78.97 ± 8.8 kg. Weight increased in all patients. The expected weight gain was 3.04 ± 0.75 kg. The observed weight gain was 1.86 ± 0.73 kg. The difference between the expected and observed weight gains was significant (P < 0.002, Student's t test). The estimated insensible loss in all patients was 1.18 ± 0.18 l equivalent to 15 ± 2.9 ml per kg.

Conclusions

We conclude that during routine cardiac surgery in adults, insensible losses of 1 l are to be expected. These losses should be taken into account in any subsequent estimation of fluid balance.