Fluid and electrolyte balance after major thoracic surgery by bioimpedance and endocrine evaluation

Weight gain with oedema development is a complication of major surgical procedures with an incidence as high as 40%. Fluid retention is not always clinically evident and it is reported despite fluid-restriction regime. The causes are several and not totally clear. We performed a prospective study to assess the amount of fluid accumulation and redistribution observed after major thoracic surgery.

In 49 patients submitted to lobectomy with systematic lymph node dissection for lung cancer, we measured preoperatively and on the postoperative days 1, 2, 4 and 7, body weight, fluid balance, brain natriuretic peptide (BNP) and bioimpedance analysis (BIA)-derived parameters resistance (R) and reactance (X(c)).

The postoperative course was characterised by significant changes. Mean increase in body weight was 2.7 kg ((1.9-3.4); p<0.001) on postoperative day 2. Most of the patients had a negative basal fluid balance (-244 ml (-520 to -50)), whereas, on postoperative day 2, we observed a positive and significant change (+968 ml (646-1456), p<0.001)). Total body R and X(c) fell on the first day (p<0.001), anticipating the changes in weight and fluid balance. BNP increased on day 1, immediately after surgery, and remained significantly above basal values for the entire observation period (p<0.001), in the absence of clinical signs of heart failure.

The three methods used consistently showed a significant fluid retention over the course of the study. BIA was an easy, reproducible and non-invasive method for the estimation and early detection of fluid retention. Increase in BNP may be related to the systemic reaction to stress and to the decreased pulmonary vascular bed. We found no correlation between fluid retention and length of anaesthesia, sex, age, blood loss and body mass index. The clinical and prognostic implication of weight gain may be relevant to patient's health.