Extracorporeal Shunt

Abstract

To the Editor: During cardiopulmonary bypass (CPB), membrane oxygenators are ventilated with a sweep gas, which may be 100% oxygen (O2) or an air-O2 mixture. The use of 100% O2 reduces the risk of nitrogen (N2) microbubble formation but results in arterial hyperoxia, which increases the risk of O2 microbubble formation. Arterial hyperoxia could be avoided by creating a venous to arterial shunt in the bypass circuit. This would decrease the PaO2 despite using 100% O2 and would increase the gradient for the absorption of both O2 and N2 emboli. The magnitude of the shunt fraction required for any desired PaO2 can be predicted using the virtual shunt equation: Equation 1 and the Fick equation: Equation 2 where C = content (mL/L), c = ideal alveolar capillary, a = arterial, v = venous, VO2 = O2 consumption (mL/min), Q = blood flow, s = shunt, and t = total. By replacing CCO2 with the oxygenator output O2 content (CoxyO (2)), the following expression for extracorporeal shunt can be derived: Equation 3 This formula can be used to calculate the shunt fractions over a range of conditions. For example, at 28 degrees C, given hemoglobin = 8 g/dL, pump flow = 1.5 L/m2, oxygenator output PO2 = 550 mm Hg (temperature corrected), and VO2 between 20 and 60 mL centered dot min-1 centered dot m-2, the shunt fraction required to produce a PaO2 of 150 mm Hg would range from 0.49 to 0.25. Similarly, at 37 degrees C, with pump flow = 2.4 L/m2 and VO2 between 60 and 120 mL centered dot min-1 centered dot m-2, the shunt fraction required would range from 0.29 to 0.17. It should be feasible to introduce shunt fractions of this size into CPB design. Gaseous emboli continue to be a source of morbidity during CPB. The use of a deliberate, adjustable, extracorporeal shunt would theoretically reduce this risk while avoiding hyperoxia. However, further studies are required to determine whether such shunts are safe during CPB and whether the theoretical advantages lead to improvements in outcome. William M. Weightman, FANZCA Neville M. Gibbs, FANZCA Department of Anaesthesia Sir Charles Gairdner Hospital Perth, Western Australia 6009 Australia