Tubular proteinuria and enzymuria following open heart surgery

Objective We investigated the effect of cardiac surgery on a marker of tubular damage, an enzyme called neutral endopeptidase (NEP), and on a marker of tubular function, retinol binding protein (RBP). Markers of tubular damage or function are useful in the early detection of acute renal failure and help identify the risk factors for this disease. We also examined if colloid interfered with NEP measurement.Design A controlled prospective cohort study.Setting A teaching cardio-thoracic unit in London, England.Patients and participants Thirty-four patients underwent cardiac surgery. Eight patients waiting for cardiac surgery acted as controls.Interventions Twenty-five patients had coronary artery bypass graft, four patients had valve replacements, one patient had a coronary artery bypass graft with a valve replacement and one patient had a left ventricular aneurysm repair.Measurements and Results Neutral endopeptidase was measured in all the patients and controls. In separate subgroups RBP (n=5) and Gelofusine use (n=12) were recorded. Urine samples were collected pre-operatively, 3 h, 1 and 4 days post-operatively. NEP rose significantly (p<0.05) after cardiac surgery compared with the control population. RBP also rose significantly (p<0.05) after cardiac surgery. NEP correlated with RBP 3 h post-operatively (p<0.05, r²=0.97). There was no correlation between the amount of Gelofusine given and NEP excretion.Conclusion Excretion of NEP and RBP were both increased after cardiac surgery. Colloid did not affect the excretion of NEP, although in other studies it has affected the excretion of RBP. This may make NEP excretion a better index of acute and impending renal damage following cardiac surgery.Partial funding was obtained via a research vacation scholarship from the Wellcome Trust. (JB)Work was carried out at The Middlesex Hospital, London.