62 / 208
Chapter 3
60
Table 1b.
Ventilator settings and metabolic measurement data for all children and for subgroups based on
compatibility of the indirect calorimetry (IC) and ventilator method (defined as a difference ≤10% or >10%
between IC and ventilator-derived VCO
2
values)
All children
N=41
Difference
≤ 10%
N=18
Difference
>10%
N=23
P-value
Ventilation mode
Support
Control
%
34.1
65.9
33.3
66.7
34.8
65.2
0.94
c
FiO
2
Median (IQR) 0.26
(0.21-0.31)
0.28
(0.21-0.36)
0.25
(0.21-0.30)
0.57
b
Tidal volume (ml)
Inspiratory
Expiratory
Median (IQR)
89 (34-157)
91 (34-180)
142 (96-185)
143 (95-198)
36 (20-90)
35 (19-97)
0.002
b
0.001
b
Tube leak (%)
(inspiratory-expiratory tidal
volume)
Mean ± SD -1.9 ± 8.6 -4.8 ± 10.2 0.3 ± 6.4 0.07
a
Respiratory rate (/min)
Median (IQR) 38 (25-47)
26 (23-35)
43 (38-50)
0.02
b
Heart rate (/min)
Median (IQR) 116
(103-139)
113
(99-124)
128
(101-142)
0.27
b
LOS at moment of
measurement (days)
Median (IQR) 2 (1-4)
2 (1-4)
2 (1-4)
0.75
b
Length of measurement (min)
Mean ± SD
30 ± 9.5
34 ± 8
33 ± 11
0.71
a
VCO
2
Deltatrac (ml/min)
Median (IQR) 75.2
(41.6-116.1)
108.6
(94-132.9)
45.9
(28.3-78.6)
<0.001
b
VCO
2
Servo-I (ml/min)
Median (IQR) 74.0
(27.5-118.0)
111.6
(94.8-135.9)
33.4
(12.6-75.3)
<0.001
b
P-values are calculated for differences between the two subgroups using the following tests:
a
Independent
samples t-test,
b
Mann-Whitney test,
c
Chi-square test.
IQR= Interquartile Range, FiO
2
= Fraction inspired Oxygen, LOS= Length of Stay, SD= Standard Deviation
Correlationbetween Servo-I®-derivedVCO
2
values and IC-derived valueswas excellent [ρ=0.965
(95% CI: 0.935 to 0.981)]. However, despite being a necessary condition for agreement, high
correlation does not automatically imply good agreement. Figure 1a shows the Bland-Altman
plot for agreement between the two methods for measuring VCO
2
values. Measurements were
not comparable with a mean relative difference of 15.6% (p=0.002, one-sample t-test), due to
underestimation of VCO
2
values by the Servo-I®. The 95% limits of agreement were wide (-42.4
% and 73.6%). Linear regression of Servo-I®-derived VCO
2
values on IC-derived values showed
a regression line with a slope of 1.15 (95% CI: 1.07 to 1.23) and an intercept at -15.66 (95% CI:
-23.32 to -8.01). The slope of this regression line was significantly different from 1 (p<0.001),
and the intercept was significantly different from 0 (p<0.001), reflecting both proportional and
fixed bias.