Proefschrift Kerklaan

Chapter 3

DISCUSSION

In this study we compared the VCO 2

values by IC (Deltatrac®) and ventilator (Servo-I®) in 41

mechanically ventilated critically ill children; VCO 2 comparable due to underestimation of VCO 2 values by the Servo-I®. 95% limits of agreement in the Bland-Altman analysiswerewide, showingpoor agreement. Clinically useful measurements (difference ≤10% between VCO 2 values of the Servo-I® and those of IC) were seen in children with higher weight. In the 20 children weighing ≥15 kg, VCO 2 measurements were comparable between IC and Servo-I® and the derived REE values were more precise than predominantly used predictive equations with a smaller difference and narrower limits of agreement. In 81% of the children weighing <15 kg, measurements by the Servo-I® deviated >10% from those of IC, which made the use of measurements in these children very limited. The wide limits of agreement may be due to the technical specifications of the sampling methods; especially the underestimation by the Servo-I® in children <15 kg may be affected by the characteristics of the sensor. VCO 2 is the volume of eliminated CO 2 calculated over one minute. CO 2 is mainly measured in the exhaled breath of alveoli (phase 2 and 3 of the capnogram, Fig 4), while breath from the upper airways is void of CO 2 (dead space, phase 1 of capnogram). values were highly correlated, but not

Figure 4. Capnogram divided in 4 phases. Phase I represents airway dead space. It is the CO 2 -free portion of the exhaled breath from the conducting airways. Phase II (expiratory upstroke) represents the mixing of airway dead space gas with alveolar gas, and is characterised by a significant rise in CO 2 . The steep slope is due to fast-emptying alveoli. Phase III is the alveolar plateau; it reflects the level of effective ventilation of the alveoli. The gradual rise in the slope is due to late-emptying alveoli. Phase IV is the inspiratory down stroke, the beginning of the next inspiration

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