Raised triglycerides and propofol infusion in H1N1 patients.

to enable them to use open TCI systems with the Schnider model for TCI in morbidly obese patients. Although we do not recommend these practices, we feel that it is important to mention this issue, to illustrate the inherent and significant dangers. One option for clinicians who wish to use the Schnider model in morbidly obese patients is to input a ‘corrected’ TBW value—typically the user will input the maximum TBW value that the system will allow for a patient of that height (i.e. a falsely low TBW is used). As illustrated by the dotted lines in Figure 2A and B, the result of this strategy is that all morbidly obese patients of a given height and gender will be assigned the same TBW, LBM, and k10 values, and all will thus receive the same amount of propofol for a given target concentration profile. By fixing k10, this strategy fixes the only Schnider model parameter that usually varies with weight (the only other parameters with co-variates are the fast re-distribution rate constants which vary with age). Thus, anaesthetists who use the Schnider model in morbidly obese patients should know that by inputting falsely low TBW values, they have generated a model where the infusion rates no longer scale according to the patient weight. Since propofol is a highly lipid-soluble agent, maintenance doses are likely to be related to TBW. There is thus a real danger that with this strategy insufficient maintenance doses will be administered to morbidly obese patients. At present, clinicians who wish to administer i.v. anaesthesia to morbidly obese patients are ‘between the devil and the deep blue sea’ (we promise not to re-invoke the satanic analogy!). Our personal view is that with current evidence and knowledge, the most prudent approach is to manually administer an induction bolus based on an estimate of LBM, and thereafter to administer a manually controlled infusion, carefully titrated to clinical effect. We have focused on obese patients, but there are several other groups who fall outside of the boundaries of the original research, such as the very young or old, and the critically ill, and in whom the utility of different models is uncertain. In his letter, Engbers has also touched on the issue of which model is best in ‘normal’ patients. The study of Glen and Servin which he mentions, is a valuable contribution, but cannot be regarded as conclusive evidence. A goal of the ‘WorldSIVA Open TCI’ initiative (www.opentci.org) is to attempt to answer precisely these sorts of questions, by harnessing the statistical benefits of combining data sets derived from different studies. A recent study resulting from this initiative, involving a very large data set, has shown results at odds with those of Glen and Servin. There is clearly some way to go yet.