Modelling of hypoxaemia after gynaecological laparotomy

Background: Late postoperative arterial hypoxaemia is common after major su rgery, and may contribute to cardiovascular, cerebral or wound complication s. This study investigates the time course of hypoxaemia following gynaecol ogical laparotomy, and estimates parameters of mathematical models of pulmo nary gas exchange to describe hypoxaemia. Methods: Twelve patients were studied on four occasions; preoperatively, 2, 8 and 48 h after surgery. On each occasion inspired oxygen fraction (FIO2) was varied, changing end-expired oxygen fraction (FEO2) to achieve arteria l oxygen saturations (SaO(2)) ranging from 90% to 100%. Measurements of ven tilation and blood gases were taken. Oxygenation was characterized plotting FEO2 against SaO(2). The shape and position of the FEO2/SaO(2) curve was d escribed using two mathematical models including parameters describing gas exchange: either shunt and resistance to oxygen diffusion (Rdiff); or shunt and asymmetry of ventilation-perfusion (fA2). Results: Two hours after surgery SaO(2) was reduced from 97.5%+/-1.2% (mean +/-SD) to 93.8%+/-2.7% (mean+/-SD) (P<0.001). Values of shunt, Rdiff and fA 2 were significantly changed at 2 and 8 h postoperatively. Forty-eight hour s postoperatively Rdiff and fA2 were still significantly changed. Conclusion: Oxygenation in 12 patients preoperatively, 2, 8 and 48 h after gynaecological laparotomy is described. Two patients were hypoxaemic (SaO(2 ) <92%) 48 h postoperatively. When two different models of oxygen transport are fitted to patient data, high values of Rdiff or low values of fA2 desc ribe the right shift in the FEO2/SaO(2) curve seen in patients with oxygena tion problems. These models fit patient data identically, and may be useful in quantifying postoperative hypoxaemia.