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(Circulation. 1999;100:1035-1042.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Division of Nuclear Medicine (S.L., I.C., H.C.L., J.D.F., D.S.B.), Department of Imaging, and the Division of Cardiology (T.S., H.C.L., J.D.F., M.J.Z., D.S.B.), Department of Medicine, and AIM Program (G.G., P.B.K.), Cedars-Sinai Medical Center, the CSMC Burns and Allen Research Institute, and the Departments of Medicine (D.S.B.) and Radiological Sciences (G.G.), University of California at Los Angeles School of Medicine, Los Angeles. Dr Sharir is a research fellow funded by the Save a Heart Foundation, Los Angeles, Calif.
Correspondence to Daniel S. Berman, MD, Department of Imaging, Cedars-Sinai Medical Center, 8700 Beverly Boulevard, Room A042, Los Angeles, CA 90048. E-mail bermand{at}cshs.org
| Abstract |
|---|
|
|
|---|
Methods and ResultsWe identified 1680 consecutive patients who
underwent rest Tl-201/stress Tc-99m sestamibi gated single photon
emission computed tomography (SPECT) and who were followed-up for
569±106 days. Receiver-operator characteristics analysis
defined an EF<45%, an end-systolic volume (ESV) >70 mL, and
an end-diastolic volume >120 mL as optimal thresholds,
yielding moderate sensitivity and high specificity in the prediction of
cardiac death. Patients with an EF
45% had mortality rates
<1%/year, despite severe perfusion abnormalities, whereas patients
with an EF<45% had high mortality rates, even with only mild/moderate
perfusion abnormalities (9.2%/year; P<0.00001).
Similarly, an ESV
70 mL was related to a low cardiac death rate
(<1.2%/year), even for patients with severe perfusion abnormalities,
whereas patients with an ESV>70 mL and only mild/moderate perfusion
abnormalities had high death rates (8.2%/year;
P<0.00001). Patients with an EF<45% and an ESV
70 mL
had low cardiac death rates (1.7%/year); those with an EF<45% but an
ESV>70 mL had high death rates (7.9%/year; P<0.02).
Multivariate Cox proportional hazards regression showed
that perfusion variables and ESV were independent predictors of
overall coronary events, whereas EF and ESV demonstrated
incremental prognostic values over prescan and perfusion information in
predicting cardiac death and cardiac death or myocardial
infarction.
ConclusionsPost-stress EF and ESV by gated-SPECT have incremental prognostic values over prescan and perfusion information in predicting cardiac death, and they provide clinically useful risk stratification.
Key Words: myocardial perfusion imaging cardiac volume prognosis
| Introduction |
|---|
|
|
|---|
The addition of gating to routine myocardial perfusion SPECT provides accurate and reproducible information on left ventricular function11 12 and volume.13 Early post-stress acquisition of gated SPECT, after the injection of Tc-99m sestamibi at peak stress, provides information regarding peak stress perfusion and myocardial function at the time of acquisition.12 14 The incremental prognostic value of post-stress left ventricular function and volume measurements over perfusion parameters has not been evaluated.
The goal of the present study was to evaluate whether post-stress EF and left ventricular volume, measured by gated myocardial perfusion SPECT, had incremental prognostic value over clinical, exercise, and perfusion data in predicting cardiac death in a large, unselected population of patients referred for nuclear testing.
| Methods |
|---|
|
|
|---|
1 year for cardiac events. Patients
with nonischemic cardiomyopathy (EF<45%
and prescan likelihood of CAD<0.40) were excluded (n=15), and patients
revascularized within 60 days after nuclear testing (n=229) were
censored from the prognostic portion of the
analysis,15 16 which left 1680 patients (1029
underwent treadmill exercise and 651 adenosine stress
testing). Normal limits of post-stress left ventricular volume and EF were determined in 44 patients (22 men and 22 women aged 53±10 years) with a low prescan likelihood (<5%) of CAD.17
Acquisition Protocol
All patients underwent separate dual-isotope myocardial
perfusion SPECT as previously described.18 Tl-201 (3 to
4.5 mCi) was injected intravenously at rest, and SPECT
imaging was initiated 10 minutes later. Tc-99m sestamibi (25 to 40 mCi)
was then injected during stress, and 8-frame gated SPECT imaging (100%
acceptance window) was initiated 15 to 30 minutes after
exercise or 30 to 60 minutes after adenosine stress.
Acquisitions were performed using a 2-detector (Vertex, ADAC),
3-detector (Prism, Picker), or single detector (Orbiter, Siemens)
camera to get 60 to 64 projections over 180° for 35 (Tl-201) or
25 seconds (Tc-99m sestamibi) per projection. The 8 projection
sets were also summed to generate an "ungated" set. Projection
images were filtered using a 2D Butterwarth filter, order of 5 (Tl-201)
or 2.5 (Tc-99m sestamibi), and a cutoff frequency of 0.25 cycles/pixel
(Tl-201) or 0.3 cycles/pixel (Tc-99m); images were reconstructed into
transaxial images using filtered backprojection with a ramp filter.
No scatter or attenuation correction was applied.
Exercise Protocol
Patients were instructed to discontinue ß-blockers and calcium
antagonists 48 hours before testing and nitrates 6 hours
before testing whenever possible. A symptom-limited treadmill exercise
test (Bruce protocol) was performed. Patients received an injection of
Tc-99m sestamibi at peak stress; they then exercised at the same level
for an additional 60 seconds and for 2 minutes more at 1 level lower.
Horizontal or downsloping ST segment depression
1 mm or
upsloping
1.5 mm at 80 ms after the J point was considered
positive. Failure to achieve 85% of maximal predicted heart rate or
ischemic ECG response during exercise was followed by
conversion to an adenosine stress test.
Adenosine Protocol
Patients were instructed to discontinue caffeine-containing
products for 24 hours before the test. Adenosine (140
µg · kg-1 ·
min-1) was infused over 6 minutes, and Tc-99m
sestamibi was injected at the end of the third minute. Whenever
possible, patients performed a low-level treadmill exercise during the
adenosine infusion.19 ECG response was evaluated
by the same criteria mentioned for exercise testing.
Visual Analysis of Perfusion SPECT
Perfusion images were scored semiquantitatively using a
20-segment, 5-point (0=normal uptake, 1=mildly reduced uptake,
2=moderately reduced uptake, 3=severely reduced uptake, and 4=no
uptake) model for the left ventricle.18 Summed stress
score (SSS) and summed rest score (SRS) were calculated by adding the
scores of 20 segments in the stress and rest images, respectively.
Summed difference score (SDS) was derived as the difference between
stress and rest scores. SSS<4 was considered normal, SSS=4 to 13,
mildly/moderately abnormal, and >13, severely
abnormal.5
Quantitative Analysis of Gated Tc-99m Sestamibi
SPECT
After automatic reorientation,20 gated short-axis
images were processed using the quantitative gated SPECT
algorithm,11 and left ventricular
end-diastolic volume (EDV), ESV, and EF were automatically
calculated.
Patient Follow-up
Patient follow-up consisted of scripted and blinded telephone
interviews corroborated by objective methods.3 The mean
follow-up interval was 569±106 days (range, 365 to 968 days).
Prescan Likelihood of Coronary Artery Disease
Prescan likelihood of CAD was calculated using
CADENZA,17 which was based on a Bayesian
analysis of prescan patient data. For patients undergoing
exercise testing, the prescan likelihood of CAD included clinical
history and exercise information, whereas for patients undergoing
pharmacological stress testing, prescan likelihood was based on
historical data only.
Statistical Analysis
Comparisons between patient groups were performed using 1-way
ANOVA for continuous variables and the
2
test for categorical variables. Continuous variables were
described by mean±SD. A value of P<0.05 was considered
statistically significant.
Cox proportional hazards regression analysis was applied to determine the independent predictors of cardiac death, cardiac death or MI, and overall cardiac events (cardiac death, MI, or late revascularization) as separate end points. The variables tested were prescan likelihood of CAD; prior MI, coronary angioplasty, and/or coronary artery bypass surgery; type of stress; perfusion variables (SSS, SRS, and SDS); and gated SPECT variables (EF, EDV, and ESV). A value of P<0.05 in univariate analysis was required for inclusion in the multivariate analysis. Multivariate analysis was performed in a stepwise fashion, evaluating prescan, perfusion, and function data. At each step, variables were removed from the model, until all remaining variables were significantly independent (P<0.05).
Receiver-operator characteristics (ROC) analysis was performed to define thresholds for EF, EDV, and ESV; this analysis provided optimal sensitivity and specificity in predicting cardiac death. Thresholds were obtained by minimizing the expression (1-sensitivity)2+(1-specificity)2.
Kaplan-Meier cumulative survival analysis with stratification by EF and ESV was performed, and survival curves were compared by the Wilcoxon test. Statistical significance was defined as P<0.05.
| Results |
|---|
|
|
|---|
Patient Characteristics
Of the 1680 patients included in the prognostic analysis,
480 (25%) had a history of MI, 305 (18%) had a prior coronary
angioplasty, and 336 (20%) had prior bypass surgery. Compared with
patients with no cardiac events (Table 1
), patients experiencing cardiac
death and those experiencing cardiac death or MI were older and had a
higher prescan likelihood of CAD; frequently had a history of MI and
coronary angioplasty; and had higher SSS, SRS, and SDS values,
lower EF, and higher ESV and EDV. Patients who had late
revascularizations had higher SSS and SDS values,
lower EF, and higher left ventricular volumes compared with
patients with no events. However, compared with patients experiencing
cardiac death, these patients had less severe perfusion abnormalities
and better cardiac function. Patients who underwent early
revascularization had higher SSS and SRS values,
lower EF, and larger volumes compared with patients with no events.
However, compared with patients who experienced cardiac death, these
patients had better cardiac function.
|
Compared with patients who underwent exercise stress testing, those
undergoing adenosine stress testing were older, more frequently
had a history of MI or bypass surgery, and had a higher likelihood of
CAD, more severe and extensive perfusion defects, worse cardiac
function, and a higher frequency of cardiac death (Table 2
).
|
Cox Proportional Hazards Regression for Prediction of Cardiac
Events
The final models of multivariate
analysis for prediction of cardiac death, cardiac death or MI,
and cardiac death, MI, or late revascularization
are summarized in Table 3
. The
independent variables for prediction of cardiac death were EF, ESV,
and type of stress used; for prediction of cardiac death or MI, the
variables were EF and ESV. The independent predictors of cardiac
death, MI, or late revascularization were prescan
likelihood of CAD, history of MI, type of stress, SSS and SRS, and ESV.
The addition of EF and ESV to perfusion data resulted in a significant
improvement in global
2 in the prediction of
cardiac death compared with the model that contained perfusion data
only (
2=72.13 versus 31.1, respectively;
P<0.0001).
|
Normal Values of EF, ESV, and EDV and Thresholds for Prediction of
Cardiac Death
Table 4
summarizes the results
of normal values of EF, ESV, and EDV in the 44 patients with <5%
likelihood of CAD and the results of ROC analysis for the
prediction of cardiac death in the 1680 patients included in the
prognostic evaluation. Based on ROC curves, optimal thresholds for the
prediction of cardiac death were EF<45%, ESV>70 mL, and EDV>120 mL
(-2.2, 2.9, and 2.0 SDs from the mean normal values, respectively).
These thresholds yielded moderate sensitivities and high specificities
for the prediction of cardiac death, and they were used in risk
stratification in the following analyses.
|
EF and Cardiac Death
The annual cardiac death rate increased with the amount of
perfusion abnormality. Normal perfusion (SSS
3) was related to a very
low mortality rate (0.3%/year), whereas mild/moderate and severe
perfusion abnormalities (SSS=4 to 13 and >13, respectively) resulted
in higher mortality rates (2.4%/year and 3.7%/year, respectively;
P<0.0001 versus normal perfusion). Further stratification
of these groups by EF (Figure 1
) demonstrated
that patients with an EF<45% and mild/moderate or severe perfusion
abnormalities had high mortality rates (9.2% and 5.7%, respectively),
whereas patients with an EF
45% had a cardiac death rate <1%/year,
regardless of the degree of perfusion abnormality. EF<45% was not
related to a higher cardiac death rate in patients with normal
perfusion.
|
ESV and Cardiac Death
Figure 2
illustrates cardiac death rate
as a function of perfusion abnormality and ESV. An ESV
70 mL was
related to a low mortality rate, even in patients with severe perfusion
abnormalities (0.4%/year), whereas an ESV>70 mL was related to a high
death rate in patients with mild/moderate or severe perfusion
abnormalities (8.2%/year and 7.5%/year, respectively;
P<0.00001 versus ESV
70 mL in the same perfusion
category). Patients with normal perfusion had low (<1%/year) death
rates, regardless of their ESV.
|
Incremental Value of ESV over EF
Multivariate analysis showed that ESV was
an independent predictor of cardiac death after adjustment for EF
(Table 3
). The incremental prognostic value of ESV over EF is
demonstrated in Figure 3
. An ESV>70 mL
identified patients at a significantly higher risk than patients with a
similar EF but an ESV
70 mL. Patients with an EF<45% and an ESV>70
mL had a high cardiac death rate (7.9%/year), compared with a death
rate of only 1.7%/year for patients with an EF<45% but an ESV
70 mL
(P<0.017). Even in patients with preserved global left
ventricular function (EF
45%), those with an ESV>70 mL
had a relatively high death rate (2.6% versus 0.5%;
P<0.02).
|
Exercise Versus Adenosine Stress
Figure 4
shows cardiac mortality rates
in patients undergoing exercise and pharmacologic stress as a function
of EF (Figure 4A
) and ESV (Figure 4B
). Patients with an
EF<45% or an ESV>70 mL had significantly higher cardiac death rates,
both in the exercise and the adenosine groups. Patients who
underwent adenosine stress had higher mortality rates compared
with patients undergoing exercise.
|
Kaplan-Meier Survival Analysis: Stratification by EF
and ESV
Figures 5
and
6 show cumulative survival curves in patients
with mild/moderate perfusion abnormalities and those with severe
perfusion abnormalities, stratified by EF (Figure 5
) and ESV
(Figure 6
). An EF
45% and an ESV
70 mL identified patients
with low risk, even in the severe perfusion abnormality category.
Patients with an EF<45% or with an ESV>70 mL were at a high risk for
cardiac death. Cumulative survival curves of patients with an EF
45%
and an EF<45%, stratified by ESV, are shown in Figure 7
. Cumulative survival was progressively
lower with increasing ESV.
|
|
|
| Discussion |
|---|
|
|
|---|
Prognostic Value of Post-Stress EF: Incremental Value
Multiple studies have shown that resting and exercise left
ventricular EF, measured by radionuclide angiography, are
powerful predictors of cardiac events.6 7 8 21 22 The Duke
investigators showed that exercise radionuclide angiography provides
incremental prognostic value over clinical, exercise, and angiographic
data in patients with documented CAD.22
Few studies have evaluated the incremental prognostic value of perfusion and function, and those that did reported conflicting results.23 24 These studies used either gated radionuclide angiography23 or first-pass angiography24 to assess resting EF. Whereas Marie et al23 showed the incremental prognostic value of perfusion and function, Nallamothu et al24 found that exercise SPECT perfusion imaging had a much stronger prognostic power than resting EF. Small and selected patient populations likely accounted for these differences.
Our data indicate that post-stress left ventricular EF has
significant incremental value over prescan and perfusion variables
in predicting cardiac death. As previously shown, cardiac death rate
was a function of the extent and severity of perfusion defects at
stress.5 However, further patient stratification by EF
showed that patients with an EF
45% had low cardiac death rates
(<1%/year), despite severe perfusion abnormalities.
Prognostic Value of Left Ventricular Volume
No previous study has assessed the incremental value of ESV over
perfusion variables in predicting cardiac death. Angiographically
measured ESV has been previously reported as a better predictor of
survival compared with EF, EDV, and coronary angiographic data
in patients after MI9 and in patients with left
ventricular dysfunction who underwent coronary
bypass surgery.10
In the present study, univariate Cox proportional
hazards regression identified both EDV and ESV as highly significant
predictors of cardiac death. However, multivariate
analysis identified only ESV as an independent predictor of
cardiac death after adjustment for prescan, perfusion, and EF data. The
threshold of ESV=70 mL, derived by ROC analysis, provided
significant stratification of patients into high-risk and low-risk
groups. Patients with severe perfusion abnormalities but an ESV
70 mL
had very low cardiac death rates (0.4%/year), whereas patients with
only mild/moderate perfusion defects but an ESV>70 mL had high cardiac
death rates (8.2%/year).
EF and ESV data provided significant stratification of patients
undergoing exercise or adenosine stress. The higher mortality
rates in the adenosine group are related to the worse clinical,
perfusion, and function characteristics of these patients (Table 2
).
Post-Stress Left Ventricular Function and
Volume
Ventricular size and function assessed during the
first hour after stress incorporates information on baseline and
post-stress cardiac function. Johnson et al12 showed that
patients with reversible stress perfusion defects frequently had
post-stress (exercise or adenosine) stunning. We recently
showed that post-stress stunning is a marker of severe angiographic
CAD.25 Our group has also demonstrated the
diagnostic value of transient, post-stress dilatation of
the left ventricle as a marker of severe and extensive
CAD.26 Therefore, the prognostic value of post-stress ESV
and EF may not be solely due to baseline ventricular
dilatation and dysfunction; it may also be attributed to transient
worsening of ventricular function in patients with
stress-induced ischemia.
Referral for Catheterization
Hachamovitch et al4 showed that referral for cardiac
catheterization increased as a function of worsening
scan results. Lewin et al27 preliminarily reported that EF
had less impact on referral for catheterization than
perfusion information. In the present study, patients referred for
early revascularization (
60 days) had perfusion
abnormalities comparable to those of patients experiencing cardiac
death, but they had significantly better cardiac function. Patients
referred for late (>60 days) revascularization
also had similar perfusion abnormalities but better function compared
with patients experiencing cardiac death. Multivariate
analysis showed that perfusion data were superior to function
in predicting total events, 64.7% of which were late
revascularization, whereas function data were
superior to perfusion in predicting cardiac death. The greater impact
of perfusion compared with function information on early referral for
revascularization and on the crossover of patients
from medical to revascularization treatment (late
revascularization) may account for the higher
prognostic power of EF and ESV in the prediction of cardiac death
compared with perfusion information, because patients with significant
perfusion abnormalities are referred for aggressive treatment. Further
incorporation of gated SPECT data (EF and ESV) into the decision
process of referral for revascularization may
result in a reduction of the cardiac death rate.
Study Significance
In contrast to previous studies, which assessed the incremental
prognostic value of EF over perfusion23 24 or ESV over
clinical and angiographic information9 10 in selected
patient populations, this study is the first to evaluate the prognostic
value of gated SPECT in a large, consecutive patient population who
were referred for nuclear testing and the first to incorporate
perfusion, EF, and volume. Therefore, our results may be applied to the
general population of patients referred for nuclear testing.
Limitations
This study did not evaluate the prognostic value of perfusion and
function variables in predicting MI as a separate end-point because
of the small number of MIs during the follow-up period. Further studies
with larger patient populations are required to investigate this issue.
The frequency of cardiac death was higher than that of nonfatal MI in
this population. This unexpected finding my be due to our higher
success rate in defining cardiac death.
Conclusions
Post-stress Tc-99m sestamibi gated SPECT provides incremental
prognostic information in patients with known or suspected CAD that is
better than perfusion data alone. Although perfusion variables are
powerful in predicting worsening of coronary disease,
post-stress EF and ESV provide incremental value in the prediction of
cardiac death. Therefore, the information provided by gated SPECT
should be considered in the referral of patients for coronary
angiography and revascularization.
| Acknowledgments |
|---|
Received February 25, 1999; revision received June 1, 1999; accepted June 14, 1999.
| References |
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