Aanesthetic Agents for Day Surgery - NIHR Health Technology ...

Ethical approval
This was covered by the ethical approval obtained
for the empirical study.
Analysis
In 1986, Bland and Altman 247 presented a statistical
method for assessing the agreement between two
methods of clinical measurement. This approach
was used to determine the ‘limits of agreement’
between the Dion estimate and the weight of
volatile anaesthetic. The limits of agreement are
defined by the formula:
Lower limit of agreement = (mean of the
difference) – (2 SD of the difference)
Upper limit of agreement = (mean of the
difference) + (2 SD of the difference)
where
Difference = true weight – Dion weight (g)
The limits of agreement were used to determine
how closely the two methods of quantifying the
weight of volatile anaesthetic agreed. The acceptable
range of the limits of agreement (within 10%)
was predefined to establish if the two methods of
quantifying the weight of volatile anaesthetic are
similar enough to use the two methods
interchangeably.
The limits of agreement for isoflurane and
sevoflurane used in the anaesthetic room and
theatre were calculated separately. There were
no data on the use of halothane and sevoflurane
in paediatric patients.
The Bland and Altman method of analysis does not
provide any information on the correction factor
that must be applied to the Dion estimate for it
to match the true weight of volatile anaesthetic.
Results
The quantity of volatile anaesthetic used was
weighed for 42 patients who were recruited for the
empirical study (3 for halothane, 10 for isoflurane,
29 for sevoflurane). It was not feasible to analyse
data for halothane as the number of cases weighed
was small. The analysis focused on the data for
isoflurane and sevoflurane.
The Dion formula generally underestimated the
weight of isoflurane and sevoflurane in the anaesthetic
room and operating theatre (Figures 28 to
© Queen’s Printer and Controller of HMSO 2002. All rights reserved.
Health Technology Assessment 2002; Vol. 6: No. 30
31). The limits of agreement for isoflurane and
sevoflurane, which are also shown on the graphs,
are given in Table 89. The limits of agreement for
these data were not sufficient (not within 10%) to
assume that the Dion estimate and true weight
could be used interchangeably.
This analysis confirmed that the Dion formula
tended consistently to underestimate the quantity
of volatile anaesthetic used. It was not possible to
estimate formally a correction factor for the Dion
formula because of the small sample size in this
substudy. However, at this stage it is possible to
calculate the mean difference between the Dion
estimate and the true weight of volatile anaesthetic
used (Table 90). The percentage mean difference
(shown in the last column of Table 90) was used to
indicate the average magnitude of the difference
between the Dion estimate and the actual weight
of anaesthetic.
The time over which the volatile anaesthetic was
administered to the patient was recorded for each
procedure in the anaesthetic room and theatre
(Table 91). Figures 32 and 33 show that the difference
between the true weight and the estimated
weight was not constant over the passage of time
for each volatile anaesthetic. It was not possible
formally to estimate the influence of time on the
difference between the true weight and the weight
estimated by means of the Dion formula because
of the small sample size in this substudy.
Implications for the
empirical study
The Dion formula was used in the baseline analysis
of the empirical study because the published
literature suggested this was the most valid means
to estimate the quantity of volatile anaesthetic used
without weighing each vaporiser used to administer
the volatile anaesthetic to a patient. However, this
substudy has shown that the Dion formula
consistently underestimated the quantity of volatile
anaesthetic used. There were insufficient data in
this substudy to create an adjusted Dion estimate.
A larger study with more patients was required.
Results from this study suggested that the actual
amounts of isoflurane and sevoflurane were
between 6% and 27% higher than estimated,
respectively. A sensitivity analysis of the Dion
formula was used in the empirical study (see
chapter 5) to illustrate the extent to which
changing the quantity of volatile anaesthetic used
would have an effect on the rank order of the
ICERs for each anaesthetic agent used.
217