Predictive performance of a physiological model for enflurane closed-circuit anaesthesia: effects of continuous cardiac output measurements and age-related solubility data  P.M. Vermeulen, C.J. Kalkman, R. Dirksen, J.T.A. Knape, K.G.M. Moons, G.F. Borm  British Journal of Anaesthesia  Volume 88, Issue 1, Pages 38-45 (January 2002) DOI: 10.1093/bja/88.1.38 Copyright © 2002 British Journal of Anaesthesia Terms and Conditions

Fig 1 Four extended model versions of the basic physiological model (model version A) were formulated. The formerly validated version C uses a fixed value of the cardiac output calculated per patient (Brody's formula Q=0.2 BW0.75).1–3 In the present study the model version E, which uses the continuous cardiac output measurements tracked from the individual, was validated. Both versions C and E use the fixed solubility coefficients of a standard human.4 To evaluate the influence of the factor solubility, two extended versions (C’ and E’) which adopt age-related partition coefficients according to the studies of Lerman and Malviya (Table 1) were also tested.5–7 The four model versions tested all account for a constant fraction of non-pulmonary elimination (fNPE) for enflurane. The small difference between the fNPE for the versions C/E vs C’/E’ (that is 0.131 vs 0.126) was effected by the different solubility data (fixed vs age-related) used while determining the appropriate size of non-pulmonary elimination for enflurane (the procedure was described in a previous study).1 British Journal of Anaesthesia 2002 88, 38-45DOI: (10.1093/bja/88.1.38) Copyright © 2002 British Journal of Anaesthesia Terms and Conditions

Fig 2 For the data processing, the patient characteristics (age, gender, body weight, and height) were collected for each patient. The enflurane administration schedule (injection time and volume) was added as input to the model. The amount of liquid enflurane per injection was converted into millilitres of vapour, and supplied to the model as if added to the anaesthetic system over a 60-s interval (i.e. the average evaporation time). The continuous cardiac output measurements per patient were entered to the model as an independent variable. Next, the appropriate version of the model was selected and activated to generate the predicted time courses of the end-expired enflurane concentrations by running a special purpose simulation programme (TUTSIM®, Meerman Automation, Neede, The Netherlands).12 For each patient and anaesthetic procedure the end-expired concentrations were predicted by applying the appropriate version of the model. Finally the model's predictive performance measures were calculated by comparing the predicted with the measured end-expired enflurane concentrations quantitatively. British Journal of Anaesthesia 2002 88, 38-45DOI: (10.1093/bja/88.1.38) Copyright © 2002 British Journal of Anaesthesia Terms and Conditions

Fig 3 Scattergram of the cardiac output values: that is the calculated vs the mean measured cardiac output (l min−1) per patient. The triangles and thick lines on the abscissas and ordinates, represent the group mean cardiac output and sd, respectively. The coefficient of variation was 13% for the calculated vs 24% for the measured cardiac output value. Both methods correlate moderately well (R2=0.064). British Journal of Anaesthesia 2002 88, 38-45DOI: (10.1093/bja/88.1.38) Copyright © 2002 British Journal of Anaesthesia Terms and Conditions

Fig 4 The group means cardiac output values (litre min−1) are plotted over time (in blocks of 5 min). The mean calculated (□, sd dotted line) values of the cardiac output prove to be a good average of the mean measured (•, sd bars) cardiac output values throughout the different stages of routine surgical anaesthesia. British Journal of Anaesthesia 2002 88, 38-45DOI: (10.1093/bja/88.1.38) Copyright © 2002 British Journal of Anaesthesia Terms and Conditions