 Introduction
Since the introduction of the Swan-Ganz
pulmonary artery catheter, thermodilution cardiac output (COtd) has become the accepted method
for monitoring patients with heart failure in the intensive care unit.[1] Although COtd is in theory the most direct
bedside measure of hemodynamic status, most authors agree that this
technique-dependent method estimates cardiac output at best to within 15% to
20% of true output.[2,3] It is widely known that thermodilution cardiac output measurements
are particularly inaccurate in low-flow states and in the presence of
tricuspid regurgitation or left-to-right shunt. In addition, COtd does not reflect the interaction
between oxygen delivery and tissue oxygen extraction.[4,5] Recent studies have focused on
the use of oxygen transport variables as therapeutic targets in critically
ill patients and as predictors of survival.[4-9] Mixed venous oxygen saturation
(SvO2), both
as an intermittent and continuous measure, has been proposed as an alternative
to conventional hemodynamic monitoring.
Several studies have attempted with varying success to show the
usefulness of monitoring mixed venous oxygen saturation in critically ill
patients. Some groups suggest that the ability of SvO2 to serve as a therapeutic
indicator is highly dependent on patient characteristics; others dismiss SvO2 in favor of variables more
representative of metabolic rate.[4-11] Although many of these studies
have evaluated the direct relation between thermodilution cardiac output and
mixed venous oxygen saturation in various patient populations, no one has
reported a comparison of COtd with SvO2 as a measure of hemodynamic
response during trials of inotropic therapy.
The purpose of this study was to determine whether SvO2 has added benefit over COtd for monitoring hemodynamic
response to inotropic therapy in patients with heart failure in whom
thermodilution technique is known to be less accurate. To this end we
compared the relation between COtd and increasing drug dose with the
relation between SvO2 and increasing drug dose during therapeutic trials of milrinone and
dobutamine in patients with severe heart failure.
 Discussion
Accurate and reliable measures of
hemodynamics are an essential part of the evaluation and treatment of
patients with heart failure in the intensive care unit. Although
thermodilution cardiac output has become the accepted measure of hemodynamic
status, it is least accurate in precisely those patients in whom invasive
monitoring is critical. Patients with heart failure not only have low cardiac
output but also frequently have secondary tricuspid regurgitation,[12] two variables that have been
associated with inaccurate COtd. Mixed venous oxygen saturation has emerged
as an attractive alternative for hemodynamic monitoring. Although many
studies have examined the relation between COtd and SvO2, little consensus has developed
as to which measurement should be used during evaluation and therapy. Our
study set out to show the usefulness of SvO2 monitoring compared with routine
thermodilution techniques during trials of two inotropic drugs, milrinone and
dobutamine, known to be of benefit in patients with congestive heart failure.
The relation between thermodilution cardiac output and mixed venous
oxygen saturation has been debated by several groups. Jain et al.[4] found a good linear correlation
coefficient in the subset of patients with cardiac index <2 L/min/m2 and SvO2 <55% in a group with
symptomatic heart failure and no evidence of tricuspid regurgitation;
however, in the same group of patients we found a nonlinear relation between
SvO2 and
cardiac output over a wide range of confidence intervals. Our study shows
that mixed venous oxygen saturation was more highly correlated with
increasing drug dose compared with thermodilution cardiac output in a group
of patients with severely depressed systolic function and some degree of
tricuspid regurgitation during trials of milrinone therapy; furthermore SvO2 was a more reproducible measure
of hemodynamic status compared with COtd in the same group of patients. In
addition, SvO2 was
comparable to thermodilution cardiac output as a measure of hemodynamic
status in a similar group of patients during trials of dobutamine.
Our results support the hypothesis that monitoring SvO2 has added benefit over COtd, particularly in patients with
severe heart failure (left ventricular ejection fraction <30%). One study
found SvO2 to be
a better predictor of survival than cardiac index in a group of patients with
acute myocardial infarction because of its ability to reflect peripheral
tissue oxygen demand and use.[9] In addition, several studies have suggested
that SvO2
monitoring may be the preferred method for early recognition of hemodynamic
change in the critically ill patient.[6,13,14]
We also found that SvO2 was a more reproducible measure of
hemodynamic status than COtd in the milrinone trials; in the dobutamine trials the differences
observed in reproducibility of measurements was not significant. These data
support previous studies that showed thermodilution cardiac output
measurements are less reproducible than other methods for determining cardiac
output because of the many sources of error in thermodilution technique.[3]
Limitations
This was a retrospective study in which there
was interobserver and intraobserver variability; however, the data acquired
were part of a prospective study on the efficacy of milrinone treatment in
heart failure. The assumption that changes in SvO2 represent actual changes in
cardiac function is not entirely correct, because the data did not include
arterial oxygen saturation, hemoglobin level, and total body oxygen
consumption; however, patients did receive supplemental oxygen, had no gross
bleeding, and were at bedrest generally afebrile and without seizures over
the short period of time encompassing the therapeutic trials. Finally,
although SvO2
correlates well with increasing dose of inotrope, the authors do not mean to
suggest that it is possible to predict SvO2 or the magnitude of improvement
in hemodynamic status at any particular dose of inotropic therapy.
Clinical Significance
We believe that insufficient attention is
paid to SvO2 and
that it is rarely used for the purpose of intensive care monitoring compared
with the widespread use of COtd. The knowledge that a significant relation
exists between SvO2 and increasing dose of inotropic drug in patients with severe heart
failure and tricuspid regurgitation has practical value for physicians
monitoring these patients in the intensive care unit. We believe this study
demonstrates the worth of SvO2 in detecting hemodynamic change during
trials of inotropic therapy and that this parameter may in fact be more
reproducible than traditional thermodilution methods. This appears to be
especially true in patients with very low levels of cardiac output and in
patients with significant tricuspid regurgitation, two cases in which
thermodilution measurements are notoriously unreliable.
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