Proefschrift Kerklaan

Ventilator-derived VCO 2

measurements to determine REE

The value of CO 2 Servo-I® the CO 2

depends on the technical performance and location of the CO 2

sensor. In the

fraction is measured mainstream simultaneously with the airway flow by an

(TVCO

2 ) is then calculated

infrared sensor attached to the endotracheal tube. Tidal volume CO 2

based on the fraction and the instantaneous flow over a single breath. When the transition from phase 2 to phase 3 of the capnogram cannot be clearly identified by the sensor, or if there is no alveolar plateau in phase 3, this detection method fails and CO 2 values may be underestimated or even absent. This is predominantly a problem in small lungs 14 , and is also affected by the exhalation time of the child. If the exhalation time is too short as compared to the rise time of the CO 2 analyser, the alveolar plateau is not reached and CO 2 is underestimated. This might have been a problem in our study; we found significantly higher respiratory rates in children with non-comparable measurements leading to decreased exhalation time and therefore worse detection of the alveolar plateau. Next to that, adding the Capnostat® airway adapter to the ventilation circuit leads to an increase in dead space and even to extra turbulence due to the difference in diameter. In smaller children with smaller airways, this increase is relatively large, resulting in blending of inspired and expired gasses leading to inaccurate measurements. A last explanation for the underestimation of VCO 2 values in smaller children, is the difficulty of distinguishing the inspiratory and expiratory phase in children with higher respiratory rates. Since the CO 2 in inspired gas is approximately 0, false interpretation of this gas for expired gas, will underestimate the true CO 2 values. The method of measurement is different for IC by the Deltatrac® device, which uses an air-dilution method, which is independent of the tidal volume and exhalation time of the patient measured 15 . This might explain the wide limits of agreement between the two methods. In theory, the ventilation mode might also influence the technical performance of the two methods, mainly since respiratory rates vary between the different modes. We did not find a significant difference in ventilator mode between children with and without clinically comparable measurements; this is in accordance with findings of previous studies where no significant influence of ventilator mode on VCO 2 measurements were found in critically ill children and adults 16,17 . values (mean relative difference of 11.3% with narrow limits of agreement), but only in children with a weight ≥15 kg (n=20). This prediction was more precise than those by the frequently used predictive equations to determine REE in critically ill children. The use of weight ≥15 kg could therefore be a clinically acceptable threshold. Our study is limited by its specific study population, due to the restriction of Deltatrac® usage to mechanically ventilated children with an FiO 2 <0.6 and tube leak <10% and without additional nitric oxide therapy. Secondly, in the smallest children many VCO 2 values needed to REE could be accurately predicted based on ventilator-derived VCO 2

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