Am. J. Respir. Crit. Care Med.,
Volume 160, Number 4, October 1999, 1429-1429
EFFECT OF BODY POSITION CHANGES
ON PULMONARY GAS EXCHANGE IN
EISENMENGER'S SYNDROME
To the Editor:
Sandoval and colleagues have recently reported that the supine
position was responsible for a decrease in arterial oxygenation in Eisenmenger's syndrome (1). Their study clearly indicated that both PaO2 and SaO2 decrease during room air breathing.
In this study the authors also measured arterial oxygenation during 100% oxygen breathing. They compared the changes
in PaO2 from sitting to supine during room air breathing and
the changes in PaO2 from sitting to supine during 100% oxygen.
They found no correlation between these two changes in PaO2
and conclude that an increase in pulmonary shunting was not
the mechanism of the decrease in PaO2.
I am reluctant to accept this conclusion. Indeed, in Table 2 of this article, it appears that during 100% oxygen breathing there is a significant decrease in PaO2 in the supine posture in
comparison to sitting. Provided that the cardiac output remains constant, this clearly indicates an increase in shunting.
Furthermore, the absence of correlation between the changes
in PaO2 from sitting to supine during room air and 100% oxygen breathing can easily be explained by the shape of the oxygen dissociation curve (ODC). During room air breathing the
slope of the ODC is very high for the lower PO2 observed in
this study (42 mm Hg), whereas it progressively decrease in
the range of observed PaO2 (maximal value from Figure 1: 68 mm Hg). By contrast, during 100% oxygen breathing the
slope of the ODC is low and constant for the range of observed values (150-200 mm Hg). It is therefore not surprising
that large changes in 100% oxygen PaO2 could be accompanied by different changes during room air breathing, either
minimal changes for the lower values of PaO2, or significant
changes for the higher PaO2 values.
It is always quite difficult to evaluate the mechanisms of
hypoxemia with the conventional measurement of blood
gases, and I feel that the authors have overinterpreted their
results. The fact remains that the observed decrease in arterial
oxygenation supports the conclusion of Sandoval and colleagues that a trial with long-term nocturnal oxygen therapy is
warranted in these patients.
ALAIN
HARF
Service de Physiologie
Explorations FonctionnellesHôpital Henri MondorCréteil, France
1.
Sandoval, J.,
P. Alvarado,
M. L. Martinez-Guerra,
A. Gómez,
A. Palomar,
S. Meza,
E. Santos, and
M. Rosas.
1999.
Effect of body position
changes on pulmonary gas exchange in Eisenmenger's syndrome.
Am.
J. Respir. Crit. Care Med.
159:
1070-1073
[Abstract/Free Full Text].
From the Authors:
The main objective of our article was simply to describe the
finding of position-related hypoxemia in patients with Eisenmenger's syndrome (1). In the DISCUSSION section of our article
we speculated on the possible mechanisms to explain this phenomenon, but we also acknowledged the limitations of our
study, leaving open the question as to the precise pathophysiologic mechanism(s) involved.
As noted by Dr. Harf, along with the position-related
changes in PaO2 and saturation during room air breathing,
there was also a statistically significant difference in PaO2 going from sitting to supine during 100% oxygen breathing. The
response to this maneuver among patients, however, was quite
variable, as noted by the large standard deviation (from 190 ± 131 to 168 ± 110 mm Hg). Moreover, except in 6 of the patients,
the alveolar-arterial oxygen tension differences during 100%
oxygen breathing were not significantly modified by changing
position, and the magnitude of the changes in this parameter
were not paralleled or correlated with the observed changes in
PaO2 during room air, as one would expect if an increase in
right-to-left shunt were the only mechanism responsible.
We would certainly agree with Dr. Harf that it is quite difficult to evaluate the mechanisms of hypoxemia with the conventional measurement of blood gases. We also believe that in
some patients there should be an increase in shunt with changing position. For most other patients, however, the precise
definition of the mechanism involved in this position-related
hypoxemia awaits a better study methodology, perhaps multiple inert gas elimination technique (2).
JULIO
SANDOVAL
Cardiopulmonary DepartmentNational Institute of CardiologyMexico City, Mexico
1.
Sandoval, J.,
P. Alvarado,
M. L. Martinez-Guerra,
A. Gómez,
A. Palomar,
S. Meza,
E. Santos, and
M. Rosas.
1999.
Effect of body position
changes on pulmonary gas exchange in Eisenmenger's syndrome.
Am.
J. Respir. Crit. Care Med.
159:
1070-1073
.
2.
Wagner, P. D., and
R. Rodriguez-Roisin.
1991.
Clinical advances in pulmonary gas exchange.
Am. Rev. Respir. Dis.
143:
883-888
[Medline].
