Monitoring anaesthetic gas concentrations in the exhaust of the cardiopulmonary bypass oxygenator

in the exhaust of the cardiopulmonary bypass oxygenator Editor—Nitzschke and colleagues recently studied sevoflurane plasma concentrations during cardiopulmonary bypass (CPB). The authors found no relationship between sevoflurane plasma concentrations and either sevoflurane concentrations in the exhaust of the oxygenator or bispectral index (BIS) values, prompting them to conclude that ‘Measuring the concentration of sevoflurane in the exhaust from the oxygenator is not useful for monitoring sevoflurane administration during bypass’. However, the authors failed to take into account the consequences of Henry’s law: at aconstant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. Blood/gas partition coefficient changes for sevoflurane during CPB were not measured, and may have been considerable given the acute changes in blood temperature and haematocrit that routinely occur. For this reason, the partial pressure in the blood remains unknown. This is the important variable, because, like all gases, inhaled anaesthetics are transported down a partial pressure gradient (not a concentration gradient), and because their clinical effects correlate with the partial pressure. The appropriate technique to use is double headspace equilibration of blood samples, as described by many previous authors, which allows simultaneous measurement of partial pressure and solubility. – 6 To summarize, reporting plasma concentrations without blood solubility does not allow meaningful clinical recommendations to be made. By implication, trying to find a relationship between plasma sevoflurane concentration and BIS with these data is futile. Therefore we argue that the conclusions by Nitzschke and colleagues are premature: pending further evidence, it remains reasonable practice to monitor anaesthetic gas concentrations in the exhaust of the oxygenator.