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Chapter 1
18
child’s REE can accurately be reflected by a measurement of at least 5 minutes
104
. However,
most measurements will take at least 30 minutes, taking into account the time to connect the
metabolic monitor and the time to reach steady state.
Within-day and between-day variations in REE from the acute phase to the stable phase are
small in the majority of critically ill children
1,94,105-107
, so a single measurement early during
admission may serve to guide nutritional therapy. Since REE remains stable, but requirements
are likely to change during the different phases of critical illness, the optimal caloric intake in
relation to REE is likely to vary as well.
Despite its superiority in predicting REE, only a minority of PICUs uses IC to determine
energy requirements
9
, because measurements are time consuming and limited to stabilised
mechanically ventilated children with mild ventilator settings or spontaneously breathing
children without need for oxygen. High purchase and maintenance expenses of metabolic
monitors further limit availability of IC. Alternatively, a simplified metabolic equation using
ventilator-derived VCO
2
measurements, could allow measurement of energy expenditure in
absence of a metabolic monitor
108
. However, this approach needs to be validated for use in
critically ill children.
Due to the limited availability and practice of IC, REE is predominantly predicted by age-
dependent equations based on weight and/or height. These equations, derived from
measurements in healthy children, do not predict energy requirements accurately in critically
ill children, resulting in an increased risk of malnutrition during PICU stay
105,109,110
. Several
factors, commonly present in the PICU, affect measured REE; fever is found to increase REE,
while sedatives and muscle relaxants have shown to decrease it
111
. An increase of REE is also
seen in children with burns
112
, septic neonates
30,113
and in children after major surgeries, but
only temporarily
31
. However, despite these established effects, the application of uniform
correction factors to REE for the whole PICU population is simplistic and likely to be inaccurate
4
.
Therefore, when IC is not possible, it is preferred to derive REE from Schofield’s formula for
weight, without the addition of stress or activity factors
4
.
Respiratory Quotient
The VCO
2
and VO
2
values obtained by IC are not solely used to calculate REE. Their ratio (VCO
2
/
VO
2
), known as the respiratory quotient (RQ), reflects the utilisation of different substrates. A
value >1.0 indicates lipogenesis and can be used to identify carbohydrate overfeeding
114-116
. A
high amount of carbohydrates will not always result in an RQ >1.0 because ongoing utilisation
of fat for energy, as seen in critical illness, will lower the measured RQ
117
. RQ is also affected by
hyperventilation and metabolic acidosis. Therefore, a cautious interpretation of this variable
is necessary before adjusting nutritional practices. The measured RQ value can also function
as an indicator of caloric overfeeding when it is compared to the predicted RQ based on the
macronutrients provided (RQ
macr
)
118,119
. Its adequacy to detect overfeeding is affected by the
presence of endogenous energy production, as seen in children with caloric intake below