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Ventilator-derived VCO
2
measurements to determine REE
65
3
The value of CO
2
depends on the technical performance and location of the CO
2
sensor. In the
Servo-I® the CO
2
fraction is measured mainstream simultaneously with the airway flow by an
infrared sensor attached to the endotracheal tube. Tidal volume CO
2
(TVCO
2
) is then calculated
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
.
REE could be accurately predicted based on ventilator-derived VCO
2
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