British Journal of Anaesthesia 1997; 79: 517–520
Effect of temperature on the solubility of desflurane, sevoflurane,
enflurane and halothane in blood
G. G. LOCKWOOD, S. M. SAPSED-BYRNE AND M. A. SMITH
Summary
We have investigated the effect of temperature on
the blood-gas solubility of desflurane, sevoflurane,
enflurane and halothane. Blood was equilibrated
with gas mixtures of known composition in open
cuvette or closed flask tonometers over a temperature range of 29–39 ⬚C, and the concentration of
each anaesthetic in blood was measured at 37 ⬚C
by repeated headspace analysis using a gas
chromatograph. Solubility increased by 5.4 % of
the solubility at 37 ⬚C for each degree that
equilibration temperature was reduced. This result
was true for all anaesthetics in all blood samples,
and is in keeping with results for other volatile
anaesthetics. (Br. J. Anaesth. 1997; 79: 517–520).
Key words
Anaesthetics volatile, desflurane. Anaesthetics volatile,
enflurane. Anaesthetics volatile, halothane. Anaesthetics
volatile, sevoflurane. Temperature, solubility. Solubility,
temperature.
Measurement of anaesthetic partial pressure in
blood is affected by temperature because of a
concomitant change in solubility. The most direct
solution to this problem is to measure every sample
at the temperature at which it was sampled, but it is
inconvenient to measure several samples at different
temperatures and a correction factor is commonly
used instead. Measurements of solubility at different
temperatures, from which such correction factors
may be derived, have been published for halothane,
enflurane, isoflurane and older agents,1–4 but we
are not aware of similar publications for newer
anaesthetics. We have therefore measured the effect
of temperature on blood solubility for desflurane and
sevoflurane, but have included halothane to confirm
that the results of our method agree with standard
values, and enflurane to confirm the observation that
it is different from other volatile anaesthetics.4
Methods
After Ethics Committee approval, we obtained 25 ml
of blood from healthy volunteers and patients undergoing major surgery. Two methods were used to
prepare samples. In the first studies, 5-ml aliquots
were placed in the cuvette of an Instrumentation
Laboratory IL 237 tonometer and equilibrated in a
gas flow of 200 ml min91 of air containing a
single agent (approximately 3% desflurane or 1%
sevoflurane) at different temperatures over a range of
29–39 ⬚C. Blood temperature within the cuvette
was measured by a bead thermistor (Kanthal
Tempmaster). At the end of the equilibration period
(15 min at stable temperature) samples were
obtained from the tonometer gas outflow and
analysed in a gas chromatograph (GC, see below).
The tonometer was opened and two 2-ml samples of
blood were obtained consecutively into a 2.5-ml
gas-tight syringe (SGE, Milton Keynes, UK) and
transferred into 5-ml glass vials (actual volumes
4.77 (SD 0.03) ml) which were closed with screw
caps fitted with Teflon-faced septa. In the later
experiments we tonometered the entire blood
sample in a closed glass flask. The flask was flushed
with a gas mixture containing approximately 1%
each of sevoflurane, enflurane and halothane
(desflurane could not be separated easily and was
not included) and then sealed using a cap with a
Teflon-faced silicone septum. It was fitted into a
weighted jacket to lie submerged on rotating rollers
in a water bath. Every 15 min during equilibration
the septum was pierced to allow manometric
pressure measurement. If positive pressure was
found it was vented and the process repeated, but if
not samples were obtained. Two separate samples of
the flask headspace were first obtained by puncturing the septum, aspirating a gas sample and injecting
it into the GC. Blood was obtained by inverting the
flask and again sampling through the septum, and
2 ml were transferred into each of two 5-ml vials.
The flask septum was replaced with a new one, the
water bath thermostat adjusted and the process
repeated so that each sample was equilibrated at four
or five temperatures from 28 to 39 ⬚C.
Blood samples were analysed by a method which
determines the total content of each anaesthetic in
blood when it was originally placed in the vial.
Blood-gas solubility was derived from this because
G. G. LOCKWOOD, MB, BS, FRCA, BSC, S. M. SAPSED-BYRNE,
AIBIOL, BSC, M. A. SMITH*, BSC, MB, BS, FRCA, Department of
Anaesthesia, Hammersmith Hospital and Royal Postgraduate
Medical School, London W12 0HS. Accepted for publication:
May 20, 1997.
*Present address: Department of Anaesthesia, Royal National
Orthopaedic Hospital, Stanmore, Middlesex.
Correspondence to G. G. L.
518
British Journal of Anaesthesia
the initial anaesthetic partial pressure in the
tonometer was known. Blood-gas solubility at 37 ⬚C
was also derived in this analysis.5 The vials containing samples of blood from the tonometer were
agitated in a water bath at 37 ⬚C for 15 min to
equilibrate the anaesthetic between the blood and
gas phases. The headspace gas of each vial was
sampled through the septum and injected into the
GC. The vial was then opened, 1 ml of blood
aspirated into the 2.5-ml syringe (taking care to
prevent bubbling) and transferred to a clean vial and
capped. This second, new vial was agitated at 37 ⬚C
for 15 min and the headspace analysed in the same
way.
All gas analysis was performed in duplicate. Gas
was aspirated into a 100-␮l gas-tight syringe (SGE)
and flushed through the 20-␮l loop of a six-way valve
(Valco) on the inlet of the GC (Pye Series 204). The
carrier gas was nitrogen flowing at 30 ml min91. A
0.9-m glass column (2 mm id) packed with 3%
OV-17 on GasChrom Q and maintained at 125 ⬚C
was used when a single agent was being measured
(giving a retention time of approximately 30 s). A
1.5-m column, packed similarly, was maintained at
60 ⬚C with a carrier flow of 15 ml min91 to aid separation of multiple agents (retention time up to 2 min).
Anaesthetic was detected by a flame ionization
detector operating at 175 ⬚C. Output from the GC
was collected by a Perkin-Elmer LCI-100 integrator.
The blood-gas solubility coefficient at 37 ⬚C (␭37)
was derived by considering the amount of
anaesthetic in the second vial before and after
equilibration.
Content before equilibration:1 ml × P1 ×␭37
(1)
where P1:partial pressure (expressed as a fraction of
ambient pressure) of anaesthetic in the first vial after
equilibration and 1 ml:volume of blood.
Content after equilibration:
1 ml × P2 ×␭37 +3.77ml × P2
(2)
where P2:partial pressure of anaesthetic in the
second vial after equilibration and 3.77 ml:volume
of air in the headspace.
No anaesthetic is lost from the vial during
equilibration so these expressions are equal, and so
3.77P2
␭37:
P1 − P1
(3)
The amount of anaesthetic in the vial at the end of
the first equilibration can now be calculated.
Content of first vial after equilibration:
2 ml × P1 ×␭37 +2.77 ml × P1
(4)
This amount of anaesthetic was contained in 2 ml
of blood in equilibrium with the gas mixture in the
tonometer.
Content of first vial before equilibration
:2 ml × P0 ×␭tono
(5)
where P0:partial pressure of anaesthetic in the
tonometer gas flow and ␭tono:blood-gas solubility
coefficient at the temperature of equilibration.
Equating (4) and (5), and re-arranging we obtain:
2P ×␭ +2.77P1
␭tono: 1 37
(6)
2P0
If blood is equilibrated in the tonometer at 37 ⬚C
then ␭tono:␭37 and it is possible to rearrange
equation (6) to calculate P0:
2.77
Calculated P0:P1(1 +
)
(7)
2␭37
The difference between the calculated P0 and the
directly measured P0 is an indication of the overall
accuracy of our assay.
There is a final point to consider. P1 and P2 refer
to the partial pressures of anaesthetic within the vial.
During equilibration the total pressure within the
vial increases as the headspace gas is warmed from
room temperature to 37 ⬚C and saturated with water
vapour. (The tonometer gas is air carrying anaesthetic, so we can neglect any exchange of respiratory
gases between blood and headspace gas.) The
sample aspirated into the syringe is at ambient
pressure, and the anaesthetic it contains is “diluted”
by expansion to this pressure. The peak recorded
from the GC must therefore be increased:
310
Pvial:PGC
× 1.03
(8)
Tlab
where Pvial:partial pressure of anaesthetic in the
vial, PGC:partial pressure derived from the height of
the GC peak, Tlab:laboratory temperature (K) and
1.03:a factor to allow for humidity (although the
headspace gas contains 6% water vapour, the gas in
the syringe before injection into the GC is at room
temperature, so it cannot contain more than 3%
water vapour). Pvial is therefore approximately 8%
greater than PGC. Gas sampled from the flask
tonometer was assumed to be saturated, but at
ambient pressure, and therefore peak heights were
multiplied by 1.03. Gas from the open cuvette
tonometer was assumed to be dry and at ambient
pressure, so no correction was made to the GC
readings.
The temperature coefficient was given by the slope
of the regression line of solubility coefficient on
temperature, determined for each blood sample
using a Microsoft Excel v4 spreadsheet. The equivalence of the two tonometers was tested by comparing
the errors in measurement of sevoflurane partial
pressure of blood equilibrated at 37 ⬚C, using the
Student’s t test. Temperature coefficients of
different anaesthetics were compared by nonparametric analysis of variance (Kruskal–Wallis). All
statistical tests were calculated using Arcus
Biomedical v6.8 on an IBM-compatible personal
computer.
Results
In every sample we found a linear relationship
between temperature and blood-gas solubility so that
a temperature coefficient (defined as the slope of the
temperature–solubility graph) could be determined
Temperature and blood-gas solubility
519
Table 1 Total number of samples, mean error of measured anaesthetic blood partial pressures of the samples equilibrated at 37⬚C,
expressed as a percentage of the measured tonometer partial pressure and mean temperature coefficients
Total No. of samples (and No. at 37⬚C)
Mean (SEM) error at 37⬚C (% of tonometer partial pressure)
Mean (SD) temperature coefficient (⬚C91)
Mean (SEM) ␭B/G at 37⬚C
Mean (SEM) temperature coefficient as a percentage of ␭B/G
at 37⬚C (⬚C91)
Desflurane
Sevoflurane
Enflurane
Halothane
59 (14)
95.8 (1.1)
90.023 (0.003)
0.052 (0.02)
127 (26)
92.2 (1.4)
90.030 (0.001)
0.62 (0.02)
101 (20)
95.5 (1.3)
90.094 (0.008)
1.50 (0.07)
101 (20)
93.4 (1.4)
90.100 (0.008)
2.00 (0.11)
4.4 (0.2)
Figure 1 Typical results. Blood from one source was
equilibrated with desflurane in an open cuvette. A different
sample of blood was equilibrated with sevoflurane, enflurane and
halothane simultaneously in a closed flask. Paired samples were
obtained from the tonometers at different temperatures and
regression lines are shown for desflurane ( ), sevoflurane ( ),
enflurane ( ) and halothane (!): the slope of the regression line
is the temperature coefficient.
(fig. 1). Table 1 shows the mean error of measured
anaesthetic blood partial pressures of the samples
equilibrated at 37 ⬚C, expressed as a percentage of
the measured tonometer partial pressure, and
summarizes our results. There were no differences in
the sevoflurane results between blood equilibrated in
the cuvette or flask. The temperature coefficient of
enflurane, expressed as a fraction of blood-gas
solubility at 37 ⬚C, was significantly greater
(P:0.005) than those of desflurane, sevoflurane and
halothane, which were indistinguishable.
Discussion
We have used repeated headspace equilibration to
determine the blood-gas coefficient. An alternative
method is to vaporize a known amount of anaesthetic in a flask of known volume containing a
known volume of blood. This has the advantage of
being a single step technique, but the disadvantage
that it is difficult to handle small quantities of
liquid anaesthetic and there is no internal check for
accuracy within the method. In contrast, when blood
equilibrated in a tonometer at 37 ⬚C is analysed by
our method, we are able to calculate the initial
partial pressure of anaesthetic at the time it was first
sampled. If this calculation differs from the partial
pressure of anaesthetic in the tonometer gas, it is
known immediately that an error has occurred. The
small deviations from the tonometer gas in our 37 ⬚C
samples justify our confidence in our methods.
4.5 (0.2)
5.9 (0.5)
4.6 (0.4)
We started with a commercial tonometer in which
blood was equilibrated in an open cuvette. There are
no concerns about pressure effects with this system,
but we could not be sure of the humidity of the gas
in the tonometer outflow, and the temperature of
blood did not equilibrate with the water jacket and
had to be measured directly. These disadvantages
have been overcome in the simple, closed flask
tonometer and we believe that with care we can
ensure that the pressure within the flask is
atmospheric, and therefore we now prefer this
method.
Our two-stage headspace equilibration method
determined the blood-gas coefficient at 37 ⬚C and
deduced the coefficient at other temperatures.
Although it would have been simpler in principle to
undertake the analysis of each 2-ml sample at its
temperature of equilibration, this would have slowed
the measurements greatly. At first sight, it would
seem that this short-cut removes the internal check
for accuracy within the method, but because one
sample was equilibrated in the tonometer at 37 ⬚C,
it could be validated and its ␭37 value assumed
accurate. All samples have their blood-gas coefficient
measured at 37 ⬚C regardless of the temperature of
equilibration in the tonometer and that can be compared with the validated result for the same blood
sample. In practice no results were discarded on this
basis.
Our results support an essentially linear relationship between temperature and solubility. In a
comprehensive review of the subject, Allott and
colleagues made the empirical observation that for
any particular anaesthetic, the temperature
coefficient was itself proportional to the logarithm
of blood-gas solubility at 37 ⬚C for that anaesthetic.6 This relationship held over a very wide
range of gases and vapours from nitrogen
(␭:0.014) to diethyl ether (␭:13), but it was only
one of three that could be justified on theoretical
grounds. Our results suggested that, within the
range of temperatures encountered clinically, a
simpler relationship is adequate for modern,
halogenated volatile anaesthetics: the temperature
coefficient is proportional to blood-gas solubility at
37 ⬚C. Our results went a little further. We
obtained blood not only from starved, preoperative
patients but also from post-prandial volunteers and
patients during cardiopulmonary bypass so that for
each anaesthetic we had a wide range of values of
␭37. For each agent studied, the effect of temperature on solubility was greatest on blood samples
with the greatest solubility at 37 ⬚C, as shown in
figure 2. Logarithmic scales have been used to
520
Figure 2 Dependence of the temperature coefficient on
solubility at 37⬚C for desflurane ( ), sevoflurane ( ), enflurane
( ) and halothane (!). Logarithmic axes have been used to
expand the area of interest, but because the linear regression line
passes through the origin, it remains straight on the logarithmic
plot.
expand the region of greatest interest, and are
justified because the regression line drawn through
our data passes through the origin (the sense of the
temperature change has been reversed to allow a
logarithmic transformation). Although the correlation is not significant for any individual anaesthetic,
it is strongly significant when the results of all
agents are pooled and the resulting temperature
coefficient is 95.4% (95% CI 94.5%, 96.2%) of
the solubility at 37 ⬚C.
Figure 3 shows our mean results on the same plot
as group results from other studies; the proposed
linear correlation fits adequately and the significant
difference found with enflurane is of little
consequence. When the temperature coefficients
for halothane from the studies cited previously are
expressed in terms of solubility at 37 ⬚C (6.5%,1
5.7%,2 4.6%,3 4.8%4), the most recent measurements are seen to agree closely with ours. Eger and
Eger’s result of 5.8% for enflurane also agrees well,
giving us confidence in our method.
When measuring the partial pressure of an anaesthetic in blood, the temperature coefficient is used to
correct for differences between blood temperature at
the time of sampling and the temperature of the
analysis. The 95% confidence interval for such a
measurement is 5–10% of its value,5 so a little
uncertainty in the value of the temperature
coefficient is tolerable. We therefore advocate the
simple rule that the Ostwald solubility coefficient of
anaesthetics in blood increases (or decreases) by
5.4% of the coefficient at 37 ⬚C for every degree
British Journal of Anaesthesia
Figure 3 Dependence of the temperature coefficient on
solubility at 37⬚C: mean results from our study and previous
publications. Logarithmic axes have been used to expand the
area of interest. The linear regression line derived from the data
of the current study is shown: it passes through the origin so it
remains straight on the logarithmic plot. :Present study (from
left to right, desflurane, sevoflurane, enflurane and halothane);
:Han and Helrich1 (halothane); :Laasberg and HedleyWhyte2 (halothane); !:Stoelting and Longshore3 (from left to
right, fluroxene, halothane and methoxyflurane); and :Eger
and Eger4 (from left to right, isoflurane, enflurane, halothane,
methoxyflurane).
decrease (or increase) in temperature within the
clinical range.
Acknowledgement
We acknowledge the Association of Anaesthetists of Great
Britain and Ireland for their Project Grant supporting this work.
References
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