Proefschrift Kerklaan

Chapter 3

Measurements VCO 2

values were simultaneously collected over 1 min intervals from IC (Deltatrac II® Datex- Ohmeda, Finland) and ventilator (Servo-I® with the Capnostat-III sensor, Maquet, Rastatt, Germany) over at least a 10-min period during steady state (less than 10% fluctuation in VCO 2 and VO 2 by IC). Before each study, the calorimeter was calibrated with a reference gas mixture (95% O 2 , 5% CO 2 , Datex Division Instrumentarium Corp.) The properties of the Deltatrac® metabolic monitor have been described before 11 . Per-minute measurements from IC with an RQ < 0.67 or >1.3 or ventilator-derived VCO 2 values of 0 were discarded, since these values are physiologically impossible. REE by IC was calculated using the modified Weir formula (REE (kcal/day)=[3.941*VO 2 + 1.106*VCO 2 ]*1440 8 ). For the Servo-I®-derived VCO 2 values REE was calculated using the following formula: REE=5.5* VCO 2 (L/min)*1440 10 . REE was calculated using the following predictive equations: Schofield-weight, Schofield- weight/height 3 and the WHO (based on weight) 4 . The following clinical data were recorded from the Patient Data Management System (PDMS) for all patients: sex, age, weight and height, diagnosis category, ventilation mode and settings, ICU stay and survival, FiO 2 , temperature, PRISM score on admission, use of catecholamines/sedatives/muscle relaxants and beta blockers and length of stay at moment of measurement. Statistical analysis Descriptive statistics were expressed as means ± standard deviations (SD) in case of normally distributed data; otherwise data are expressed as medians with interquartile ranges (IQR). Relative differences between IC and other methods were calculated as follows: ((value IC - value other method)/ value IC)*100%. Paired samples t-tests were performed to check if there was a difference in mean values between methods. Spearman’s correlation coefficient (ρ) was used to describe the association between methods of measurement in case of non-normality. This correlation coefficient was also used to describe the association between patient weight and the absolute value of the relative difference between methods. Linear regression analysis was performed to detect proportional and fixed bias between Servo-I®-derived VCO 2 values and IC-derived VCO 2 values. This method was chosen because the predictor (IC-derived VCO 2 values) is expected to be free of error (due to steady state measurements). Bland-Altman analysis was used to assess the agreement 1) between Servo-I® and IC-derived VCO 2 values and 2) between IC-derived REE values and calculated REE values (Servo-I® and predictive equations) 12 . Accuracy was also quantified by the proportion of comparable measurements, defined as a relative difference ≤10%between values derived from the Servo-I® or predictive equations, and those of IC, to be clinically useful. Inaccuracy was quantified by the proportion of measurements with a relative difference >30%, to determine the prevalence of large errors 13 . Differences between the children with and without comparable measurements were analysed using independent samples t-tests, Mann-Whitney tests or chi-square tests, depending on the outcome used.

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