(Circulation. 1995;91:1697-1705.)
© 1995 American Heart Association, Inc.
Articles |
Prediction of ReversibleIschemia After Revascularization
Perfusion and Metabolic Studies WithPositron Emission Tomography
From the Department of Radiology and Nuclear Medicine (N.T., M.K., E.T., Y.M., Y.Y., J.K.); The Third Division (R.N., S.S.), Department of Internal Medicine; and Department of Cardiovascular Surgery (K.N., T.B.), Kyoto University Faculty of Medicine, Kyoto, Japan.
Correspondence to Nagara Tamaki, MD, Department of Nuclear Medicine, Kyoto University Faculty of Medicine, Shogoin, Sakyo-ku, Kyoto, 606 Japan.
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Abstract |
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Background Accurate noninvasive determination of myocardial viability is of paramount importance for the clinical identification of patients who will benefit most from revascularization. The preserved metabolic activity in the myocardium, as studied with positron emission tomography (PET), has been considered a gold standard for this purpose. However, recent reports show that moderate hypoperfusion or stress-induced ischemia may represent reversible ischemia. The present study was undertaken to compare the value of perfusion and metabolic studies with PET for predicting improvement in wall motion after revascularization.
Methods and Results Of 61 patients who had regional asynergy and
underwent PET before revascularization, 43 patients who had successful
revascularization were included in the study. Each patient underwent
rest-stress 13N-ammonia perfusion scans and
18F-fluorodeoxyglucose (FDG) scan at rest while in a
fasting state. Reversible ischemia was considered to be present
when the resting perfusion was 
50% of the peak value, stress-induced
hypoperfusion was present, or an increase in FDG uptake was
observed. Of 130 asynergy segments, 51 segments had improved wall
motion after revascularization. The positive and negative predictive
values for improvement in asynergy were 48% and 87% by the rest
perfusion study, 63% (P=.05 versus the rest value) and 87%
by the rest-stress perfusion study, and 76% (P<.01 versus
the rest value) and 92% by the FDG study.
Conclusions FDG PET provided the best predictive value for improvement in wall motion after revascularization. On the other hand, 13N-ammonia PET is useful for predicting nonreversible myocardial scarring when it shows severe hypoperfusion at rest or hypoperfusion without stress-induced ischemia.
Key Words: tomography • ischemia • heart diseases
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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It is well known that myocardial areas with impaired ventricular function often show improved function after revascularization. Therefore, accurate identification of such reversible ischemic myocardium, "viability assessment," has important clinical implications, especially in patients being considered for interventional therapy.1 2
Many studies have focused on the assessment of tissue viability.3 4 5 6 7 8 9 10 11 Among them, positron emission tomography (PET) using 18F-fluorodeoxyglucose (FDG) has been proved to be the most accurate method of differentiating reversible ischemic myocardium from irreversible scar tissue.10 11 On the other hand, recent investigations have indicated that the 201Tl reinjection method or resting thallium imaging has potential for providing accuracy similar to that obtained with PET.12 13 Areas with relatively preserved 201Tl uptake on the delayed or resting scan may indicate viable myocardium. However, the low-energy photons from 201Tl may not provide accurate distribution of the tracer because of great photon attenuation. Because of the accurate photon attenuation of PET, PET study has the potential for more accurate assessment of myocardial perfusion than 201Tl single-photon emission computed tomography. We hypothesized that a residual perfusion of >50% or the presence of stress-induced ischemia on 13N-ammonia PET scans may predict reversible ischemia as accurately as the preserved glucose metabolism on FDG PET scans. Accordingly, the present study was undertaken to compare the clinical value of 13N-ammonia perfusion studies with that of FDG metabolic studies for prediction of reversible ischemia that is likely to show improved regional wall motion after revascularization.
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Methods |
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TopAbstractIntroductionMethods Results Discussion References |
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Patient Population
A consecutive sample of 61 patients with chronic myocardial infarction who underwent myocardial perfusion and metabolic studies with PET before revascularization were evaluated. Forty-three of these patients demonstrated successful revascularization on the follow-up angiogram and were included in the study. The remaining 18 patients had inadequate revascularization and were excluded from the study. The 43 patients consisted of 41 men and 2 women (age range, 36 to 72 years; mean, 58.1 years). The left ventricular ejection fraction on contrast ventriculography ranged from 21% to 64%, with a mean of 41%. All of the 43 patients had regional wall motion abnormalities on contrast ventriculography and a history of myocardial infarction with a duration of more than 1 month from onset. Twenty-four patients underwent coronary artery bypass graft surgery (CABG) and 19 patients underwent percutaneous transluminal coronary angioplasty (PTCA) within 4 weeks after the PET study. Two patients had non–insulin-dependent diabetes mellitus. Patients with insulin-dependent diabetes were excluded from the study. All 43 patients gave written, informed consent, and the study was approved by the Kyoto University Ethical Committee.
PET Studies
PET studies were performed using a whole-body PET
camera
(Positologica III [n=30] or PCT 3600W
[n=13], Hitachi Medico Co).
The Positologica III has four rings that provide seven tomographic
slices at 16-mm intervals. The intrinsic spatial resolution in the
tomographic plane was 7.6 mm full-width half-maximum (FWHM) at the
center, and the axial resolution was 12 mm FWHM.14 The PCT
3600W has eight rings that provide 15 tomographic slices at 7-mm
intervals, and the intrinsic resolution was 4.6 mm FWHM. The effective
resolution of these PET cameras after reconstruction was 11 mm and 10
mm FWHM, respectively.
Each patient was kept in a fasting condition for at least 5 hours during the study. After accurate positioning of the patient under the PET camera using the ultrasound technique, we performed transmission scanning for 15 to 20 minutes to obtain accurate correction of photon attenuation. Then, 80 to 300 MBq (2.2 to 8.1 mCi) of FDG was injected with the patient at rest. Approximately 60 minutes later, a glucose metabolic scan was performed for 10 to 15 minutes.
The perfusion study was performed separately within 1 week of the FDG study. Approximately 400 to 600 MBq (10.8 to 16.2 mCi) of 13N-ammonia was injected at rest, and resting perfusion scanning was started 3 minutes later.
Two hours later, patients performed graded exercise using a supine ergometer; they started at 25 W with 25-W increments every 3 minutes. The exercise continued until the patient experienced fatigue, severe chest pain, dyspnea, more than 0.2 mV of ST-segment depression, or 85% of the age-predicted maximal heart rate. Another dose of 13N-ammonia was injected at peak exercise, and the exercise was continued for an additional 30 to 60 seconds. The stress perfusion scan was performed 3 minutes after tracer administration.15 16 17
From a series of transverse slices, oblique tomograms perpendicular to the long and short axes of the left ventricular myocardium were also reconstructed for three-dimensional analysis of the tracer distribution.18
Image Analysis
In each patient, resting and stress perfusion
images were
normalized to the normal reference regions. Coronary angiography was
reviewed to determine that the reference regions corresponded to a
myocardial segment supplied by a vessel without significant stenosis.
The left ventricular myocardium was divided into five segments
(anterior, septal, apical, inferior, and lateral) to calculate the mean
regional perfusion as percent activity of the maximal counts in the
normal reference regions by using the circumferential profile
analysis of four to six short-axis slices of the
13N-ammonia distribution. Segments with perfusion above the
lower limit of the normal values were considered to be normal. The
lower limit was determined as the mean -2 SD of the perfusion profile
for each segment from 12 healthy subjects who had less than 5%
likelihood of coronary artery disease.16 17 Segments
with
perfusion below the lower limit were defined as hypoperfused segments.
In addition, stress and resting perfusions were compared using the
circumferential profile analysis based on the corresponding
short-axis slices. Segments with a stress perfusion of more than 10%
lower than the resting perfusion on the profile analysis were
considered as stress-induced hypoperfusion.17
The FDG uptake was quantitatively measured as the body weight (BW)–corrected percent of the injected dose per 100 g of tissue (% ID/100 g) in each segment19 according to the following equation:
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where CT is the myocardial tissue activity of FDG (cpm/mL), BW is the patient's body weight, and CF is the calibration factor between millicuries on the curie meter and counts per minute per milliliter on the PET images.19 FDG images were shown as a parametric display as FDG uptake index (not normalized uptake) to define the ischemic myocardium as increased FDG uptake. Because of heterogeneity of FDG uptake in normal myocardium, the normal range of FDG uptake was defined as the mean±2 SD of the FDG uptake in eight healthy subjects.13 Hypoperfused segments with an increase in FDG uptake above the normal range were defined as PET ischemia, whereas those with no increase in FDG uptake were defined as PET scarring.11 13 19
Wall Motion Analysis
Analysis of the regional wall motion was
performed within 4
weeks of the PET study before revascularization and repeated at 4 to 8
weeks after revascularization. Each patient underwent biplane contrast
ventriculography in right anterior oblique and left lateral projections
to assess regional wall motion before and after revascularization. In
three patients who could not undergo the contrast ventriculography,
radionuclide ventriculography was performed in the anterior and left
anterior oblique projections after intravenous injection of 740 MBq (20
mCi) of 99mTc red blood cells. The left ventricle was
divided into anterior, apical, inferior, septal, and lateral segments.
Wall motion was visually assessed in a blinded fashion by three
experienced observers and scored using a five-point grading system
(normal, mild hypokinesis, severe hypokinesis, akinesis, and
dyskinesis).17 When the wall motion score improved by one
or more points after intervention, the segment was considered to show
improved wall motion.11 17 In the study of patients
who
underwent CABG, the septal segment was excluded from the wall motion
analysis because of frequent paradoxical motion after surgery.
Statistical Analysis
Comparisons of the predictive value of
each study were performed
with 
2 analysis or Fisher's exact test. A
value of P<.05 was considered statistically
significant.
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Results |
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There were a total of 130 asynergic segments before revascularization. After revascularization, 51 of the segments showed improvement in the wall motion abnormality, whereas the remaining 79 segments did not show improved wall motion.
Among the 130 asynergic segments, the resting 13N-ammonia
PET study showed normoperfusion with 80% or more of the maximal value
in the whole myocardium in 1 segment, mild hypoperfusion with 70% to
80% of the maximal value in 21 segments, hypoperfusion with 60% to
70% of the maximal value in 38 segments, hypoperfusion with 50% to
60% of the maximal value in 38 segments, hypoperfusion with 40% to
50% of the maximal value in 21 segments, and severe hypoperfusion with
less than 40% of the maximal value in 11 segments (Table 1
). To
compare the improvement in wall motion after
revascularization (Table 2), these segments were divided
into two groups: those with perfusion of more than 50% and those with
perfusion of less than 50% of the peak value. Of 98 segments with
mild-to-moderate hypoperfusion with at least 50% of the peak value, 47
segments showed improved wall motion (Figs 1 and
2) and 51 segments did not show improved wall motion
after revascularization. On the other hand, of 32 segments with severe
hypoperfusion with less than 50% of the normal value, only 4 segments
showed improved wall motion and the remaining 28 segments did not show
improved wall motion after revascularization (Fig 3)
(2=12.7, P<.001). The positive and
negative predictive values of the resting perfusion study for
improvement in asynergy were 48% and 87%, respectively.
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The stress perfusion study was performed approximately 2 hours
after the resting perfusion study. Stress-induced hypoperfusion was
observed in 68 of the 130 asynergic segments (Table 3).
Forty-three of them showed improved wall motion (Figs 1
and
2), yielding a positive predictive value of 63%. On the
other hand, no stress-induced hypoperfusion was observed in the
remaining 62 asynergic segments. Only 8 segments showed improved wall
motion after revascularization, whereas the remaining 54 segments did
not (Fig 3) (2=34.5,
P<.001). Thus,
the negative predictive value was 87%. When the segments with
mild-to-moderate hypoperfusion (at least 50% of the peak value) were
selected for analysis, improvement in the asynergy was observed in
40 of the 62 segments with stress-induced hypoperfusion, whereas
improvement was seen in only 7 of the 36 segments without
stress-induced hypoperfusion (2=18.5,
P<.001). Thus, the positive and negative predictive values
were similar (65% and 81%, respectively).
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Among the 130 asynergic segments, FDG PET showed an increase in FDG
uptake in 59 segments but no increase in 71 segments (Table 4).
Forty-five of the 59 segments exhibiting an increase
in FDG uptake showed improved regional wall motion after
revascularization (Figs 1 and 2), whereas only 6
of the 71 segments
showing no increase in FDG uptake improved (Fig 3)
(2=62.2, P<.001). Thus, the positive
and negative predictive values of the FDG findings were 76% and 92%,
respectively.
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The overall accuracy of the prediction of improvement in asynergy was 58% (65 of 130) in the resting perfusion study, 75% (97 of 130) in the rest-stress perfusion study, and 85% (110 of 130) in the FDG study (P<.05 within each study).
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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The present data indicate the importance of myocardial perfusion and metabolic PET imaging in differentiating reversible ischemia from nonreversible myocardial scarring. Among the various available parameters of PET perfusion and metabolism, FDG PET may provide the best accuracy for predicting reversibility of regional wall motion abnormalities after revascularization. On the other hand, severe hypoperfusion (less than 50% of the peak value) and hypoperfusion with no stress-induced ischemia on 13N-ammonia PET scans are likely to represent irreversible myocardial damage that shows no improvement in asynergy after revascularization. In contrast, PET patterns with mild reduction of perfusion showed poor predictive value for functional recovery after revascularization.
Although a number of prior clinical studies have shown that glucose metabolic imaging is superior to the assessment of myocardial perfusion alone in identifying reversible ischemia, to our knowledge this is the first study that addressed the usefulness of PET evaluation of the relative reduction in myocardial perfusion in identifying viable tissue.
Value of Perfusion Imaging
Previous experimental studies
indicated that a certain threshold
of myocardial perfusion is required to maintain tissue
viability.21 22 23 24 25
Segments with very low flow below this
threshold may show decreased exogenous glucose use, which may
represent irreversible myocardial scarring. The present
data indicate that none of the segments with perfusion of less than
40% of the maximal value at rest recovered regional function after
revascularization. The present data are in agreement with the
results of a 201Tl study of stress-delayed
scanning,26 rest-redistribution scanning,27
or reinjection scanning.28 However, PET perfusion studies
should be more accurate in assessing regional perfusion than
201Tl studies because of accurate correction of photon
attenuation and scattering. Therefore, 13N-ammonia PET
might be more suitable for defining the threshold of perfusion in
vivo.
Our results are in agreement with those reported by Duvernov et al,29 who showed that severely hypoperfused segments with less than 40% of the normal value did not show improved regional wall motion. In addition, our previous PET study demonstrated that none of the segments with severe hypoperfusion (less than 45% of the peak value) had regional myocardial glucose use of more than 0.3 µmol · min-1 · g-1, suggesting that a certain minimal threshold of residual perfusion is required to maintain glucose metabolism.25
The threshold determined in these studies appears to be much higher than the experimental value. Because myocardial perfusion images in vivo may contain a significant amount of noise and background activity compared with in vitro tissue counting, the perfusion in ischemic tissue in vivo is likely to be overestimated. Although PET images contained much less background activity than the 201Tl single-photon emission computed tomography or planar images, there appears to be no definite method of accurately correcting for background activity on PET. In addition, ischemic tissue in human studies should be quite different from that in experimental infarct models, considering the collateral circulation and duration of ischemic insult.
On the other hand, there was much variation in the changes in wall motion after revascularization in the areas with moderate hypoperfusion. Improvement in wall motion was observed in only 50% of the segments with perfusion of at least 50% of the peak value. It is important to note that detection of residual viable myocardium may not be equal to detection of reversible ischemia. A myocardial segment with a mixture of healthy and scar tissues without ischemia may not show improved regional function after revascularization, although the residual perfusion in such areas may be relatively well maintained. In such circumstances, other indicators, such as the 201Tl redistribution or perfusion-metabolic mismatch, may be required for accurate identification of reversible ischemia.
Although resting perfusion studies may be useful for identifying viable myocardium on the basis of the residual myocardial perfusion, stress perfusion studies may add important clinical data for detecting stress-induced ischemia. Detection of stress-induced ischemia is one method of detecting reversible ischemia. Data from our previous study30 indicated that stress-rest 13N-ammonia PET scans identified stress-induced ischemia more frequently than did stress-delayed 201Tl scans. In addition, reversible ischemic myocardium after CABG was more accurately identified by PET perfusion studies than by the 201Tl studies.17 Furthermore, segments showing stress-induced ischemia are often associated with an increase in FDG uptake in PET studies.31 32
Although supine ergometer exercise was applied, adequate exercise levels were obtained, as described previously.30 PET imaging started 3 minutes after the tracer injection, and there was no motion artifact in any of the patients. Our results indicated that segments without stress-induced ischemia were least likely to show improved regional function, and therefore this was considered to be a highly specific sign of irreversible damage. On the other hand, hypoperfused segments associated with stress-induced ischemia may not always show improved regional function. The positive predictive value (60%) was not very satisfactory. Similar predictive values for stress-perfusion studies were obtained when segments with mild-to-moderate hypoperfusion at rest were selected for analysis.
Because 13N-ammonia images represent the relative myocardial perfusion, areas with stress-induced hypoperfusion indicate a decrease in the flow reserve compared with other, normal areas. It is well known that ischemic tissues distal to stenotic coronary arteries show a decrease in flow reserve.33 However, areas with decreased flow reserve may not necessarily represent ischemic myocardium. A recent preliminary report showed that changes in the perfusion reserve may be unrelated to residual glucose metabolism as a marker of tissue viability.34 A prospective study may be warranted to evaluate whether a decrease in the flow reserve is a good indicator for identifying reversible ischemia in patients undergoing revascularization.
Value of Metabolic Imaging
PET has emerged as an excellent
technique for assessing myocardial
viability in patients with left ventricular dysfunction since it can
demonstrate preserved glucose metabolism in regions with reduced
perfusion.10 11 20 In ischemic
myocardium, ß-oxidation
of fatty acids in the mitochondria is suppressed, and the myocytes
compensate for the loss of oxidative potential by shifting toward
greater glucose use to generate high-energy
phosphates.35 36 Thus, analysis of the glycolytic
process could play an important role in assessing myocyte
viability.
Previous preliminary reports showed that a perfusion-metabolic mismatch may be an excellent indicator for predicting recovery of myocardial function after revascularization.10 11 37 38 The present data are consistent with these findings. In particular, areas with no increase in FDG uptake were least likely to improve, yielding a high negative predictive value (78% to 92%). On the other hand, the positive predictive value may not be as high (72% to 82%). One of the major reasons is that recovery of regional function may not be completed within 4 to 8 weeks after revascularization. Prolonged postischemic ventricular dysfunction is often observed after revascularization.39 40 Such prolonged dysfunction may be associated with a sustained increase in FDG uptake in postischemic areas.11 37
The present study provided slightly better predictive values than did a previous study.11 Several reasons may be considered. First, the number of patients enrolled in the present study was greater. Second, those who had inadequate revascularization on the follow-up angiogram were excluded from the present study. The present study included 19 previously reported cases and 24 new cases. Third, the FDG uptake was quantitatively measured in each segment.19 Therefore, an increase in FDG uptake can be defined more accurately than in the previous report. In addition, the present study includes a comparison of the predictive value obtained with FDG PET with those obtained with rest perfusion and rest-stress perfusion studies.
The prediction of reversible ischemia by metabolic analysis was slightly better than that by perfusion analysis. In areas with moderate hypoperfusion, it will be important to differentiate ischemic and compromised myocardium from a mixture of healthy and scar tissues. This differentiation may be more accurately performed by combined perfusion-metabolic imaging than by only perfusion imaging at rest. In addition, an increase in tracer uptake, as in the case of FDG uptake, may be easier to interpret than a decrease in tracer distribution as perfusion imaging.
On the other hand, FDG uptake is known to differ depending on the dietary condition.41 42 In the postprandial condition, increased use of glucose by the healthy myocardium would result in an increase in FDG uptake with a relative decrease in its uptake in the ischemic myocardium. As a result, the extent of tissue viability may be underestimated. In contrast, in the fasting condition, as was used in the present study, preferential use of fatty acids by the healthy myocardium occurred. Therefore, extensive necrosis with a small amount of ischemic tissue may show intense uptake of FDG compared with the healthy myocardium; thus, it may overestimate the viable tissue. In addition, a recent report suggests heterogeneous uptake of FDG by the healthy myocardium in the fasting state.43 To resolve these limitations, we quantitatively measured FDG uptake for parametric display of FDG uptake to identify segments with FDG uptake above the normal range as PET ischemia.19 The current criteria may overcome the limitation of using the FDG distribution in the fasting state.
In the present study, we did not correct for a partial volume effect, which may lead to underestimation of the 13N-ammonia and FDG uptake in the asynergic segments. An accurate and feasible method for making this correction may further improve the accuracy of the prediction. Echocardiography has been commonly used to correct the partial volume effect. However, this technique may not be reproducible, and the measured segment may not correspond well to the PET segment.
Furthermore, background activity was not subtracted, which may affect observed PET perfusion defect severity measurement. An accurate correction of such noise may permit more definite distinction to be made between viable and nonviable myocardium.
Another study limitation was the use of ungated PET imaging, which may underestimate regional tracer distribution in asynergic segments compared with those with normal function.44 However, ungated scanning remains a common PET acquisition method at most clinical PET centers.
Clinical Implications
Accurate noninvasive determination of
myocardial viability is of
paramount importance for the clinical identification of patients who
will benefit most from revascularization. This investigation confirmed
that a perfusion-metabolism mismatch on PET can accurately identify
reversible dysfunction in the ischemically compromised myocardium.
Furthermore, this investigation addressed the usefulness of severity of
myocardial hypoperfusion in identifying such reversibility. In
particular, areas of severe hypoperfusion at rest or hypoperfusion
without stress-induced ischemia on PET may represent
irreversible damage. However, such findings were seen less frequently
than no FDG uptake. In this respect, FDG PET may provide the best
accuracy in predicting the reversibility of regional wall motion
abnormalities. However, when 13N-ammonia PET shows severe
myocardial hypoperfusion at rest or no stress-induced ischemia,
revascularization therapy will be of no use.
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Acknowledgments |
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This study was supported in part by a grant-in-aid for General Scientific Research from the Ministry of Education, Science and Culture, Tokyo, Japan. We gratefully acknowledge the valuable comments of Susumu Shindo, MD; Norio Takahashi, MD; Shinji Ono, MD; Kazumi Okuda, MD; and Tatsuo Torizuka, MD. We also thank Emiko Komori, Toru Fujita, and the Cyclotron Staff for their technical assistance.
Received August 12, 1994; revision received August 16, 1994; accepted September 28, 1994.
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