ORIGINAL ARTICLE
Interaction of impaired myocardial flow reserveand extent of myocardial ischemia assessed using
13
N-ammonia positron emission tomography
imaging on adverse cardiovascular outcomes
Shiro Miura, MD, MSc,a Atsutaka Okizaki, MD, PhD,b Hiraku Kumamaru, MD,
PhD,c Osamu Manabe, MD, PhD,d Masanao Naya, MD, PhD,e Chihoko Miyazaki,
MD, PhD,f and Takehiro Yamashita, MD, PhDaa

Department of Cardiology, Hokkaido Ohno Memorial Hospital, Sapporo , Japan
Department of Radiology, Asahikawa Medical University, Asahikawa, Hokkaido, Japanc
Department of Healthcare Quality Assessment, Graduate School of Medicine, The Universityof Tokyo, Tokyo, Japand
Department of Radiology, Saitama Medical Center, Jichi Medical University, Saitama-Shi, Japane
Department of Cardiovascular Medicine, Hokkaido, University Graduate School of Medicine,
Sapporo, Japanf
Department of Diagnostic Radiology, Hokkaido Ohno Memorial Hospital, Sapporo, Japanb

Received Apr 8, 2022; accepted Feb 14, 2023
doi:10.1007/s12350-023-03255-x

Background. Myocardial flow reserve (MFR) and the extent of myocardial ischemia identify
patients at high risk of major adverse cardiovascular events (MACEs). Associations between
positron emission tomography (PET)-assessed extent of ischemia, MFR, and MACEs is unclear.
Method. Overall, 640 consecutive patients with suspected or known coronary artery disease
undergoing 13N-ammonia myocardial perfusion PET were followed-up for MACEs. Patients werecategorized into three groups based on myocardial ischemia severity: Group I (n = 335), minimal(myocardial ischemia < 5%); Group II (n = 150), mild (5–10%); and Group III (n = 155),
moderate-to-severe (> 10%).
Results. Cardiovascular death and MACEs occurred in 17 (3%) and 93 (15%) patients,
respectively. Following statistical adjustment for confounding factors, impaired MFR (global
MFR < 2.0) was revealed as an independent predictor of MACEs in Groups I (hazard ratio [HR],2.89; 95% confidence interval [CI], 1.48–5.64; P = 0.002) and II (HR, 3.40; 95% CI 1.37–8.41;
P = 0.008) but was not significant in Group III (HR, 1.15; 95% CI 0.59–2.26; P = 0.67), with asignificant interaction (P < 0.0001) between the extent of myocardial ischemia and MFR.
Conclusion. Impaired MFR was significantly associated with increased risk of MACEs inpatients with £ 10% myocardial ischemia but not with those having > 10% ischemia, allowing aclinically effective risk stratification. (J Nucl Cardiol 2023)
Key Words: Myocardial flow reserve Æ Myocardial ischemia Æ Coronary artery disease
Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s12350-02303255-x.
The authors of this article have provided a PowerPoint file, available
for download at SpringerLink, which summarises the contents of thepaper and is free for re-use at meetings and presentations. Search forthe article DOI on SpringerLink.com.
The authors have also provided an audio summary of the article, whichis available to download as ESM, or to listen to via the JNC/ASNC
Podcast.

Reprint requests: Shiro Miura, MD, MSc, Department of Cardiology,
Hokkaido Ohno Memorial Hospital, 2-1-16-1 Miyanosawa, NishiKu, Sapporo 063-0052, Japan; shirotan1027m@yahoo.co.jp
J Nucl Cardiol
1071-3581/$34.00
Copyright Ó 2023 The Author(s) under exclusive licence to American
Society of Nuclear Cardiology

Miura et al
Interaction of impaired myocardial flow

Journal of Nuclear CardiologyÒ

Abbreviations
CAD
Coronary artery disease
HF
Heart failure
MACE
Major adverse cardiac event
MBF
Myocardial blood flow
MFR
Myocardial flow reserve
MPI
Myocardial perfusion imaging
PCI
Percutaneous coronary intervention
PET
Positron emission tomography
SDS
Summed difference score

INTRODUCTION
The diagnostic and prognostic benefits of myocardial perfusion imaging (MPI) using single-photon
emission computed tomography (SPECT) and positronemission tomography (PET) are important. The magnitude of ischemia on SPECT-MPI scans could be agatekeeper to identify ideal revascularization candidates
with improved long-term major adverse cardiac event(MACE) outcomes.1 Patients with 10–12.5% myocardialischemia may have a survival benefit with early revascularization.1,2 The diagnostic accuracy of MPI using
82
Rb- or 13N-ammonia PET appears to be superior tothat of SPECT-MPI, although its disadvantages include
lower patient access, higher costs, and technical/logistical difficulties related to radiotracer use.3 Reportedly,
patients with higher PET-MPI-measured ischemia levels
had a greater survival benefit from early revascularization with a potential ischemia threshold of 5%, lower
than that for SPECT-MPI.4 Thus, PET-MPI allows
accurate obstructive coronary artery disease (CAD)
detection, myocardial ischemia assessment, and discrimination between low- and high-risk patients for
MACEs, helping to identify ideal revascularization
candidates with improved long-term cardiovascularoutcomes.5
PET can provide quantitative myocardial perfusion
measurement, offering information on macro- andmicrocirculation, leading to more accurate detection ofearly and advanced CAD. Quantitative myocardial flow
reserve (MFR), calculated as the ratio of hyperemic toresting myocardial blood flow (MBF), integrates thehemodynamic effects of epicardial coronary stenosis,
diffuse atherosclerosis, and microvascular dysfunction,
contributing to risk estimation for future MACEs.6,7Moreover, the combined MFR and myocardial ischemia
measurements could identify at-risk patients.8 However,
the relationships among PET-assessed extent of myocardial ischemia, MFR, and MACEs are unknown. MFR is
associated with MACEs independently of angiographic
score and modifies the effect of revascularization with

Figure 1. Schematic showing the selection of the study
population. CAD, coronary artery disease; PET, positronemission tomography; SDS, summed difference score; MFR,
myocardial flow reserve; MACEs, major adverse cardiovascular events.

an interaction between myocardial ischemia and earlyrevascularization.4,9 Therefore, we investigated theclinical impact of impaired MFR in predicting future
MACEs when combined with the extent of myocardialischemia, assessed using 13N-ammonia PET-MPI, considering the effect of early revascularization. We
examined the annualized MACE rates for each ischemia
level and the prognostic value of MFR over the semiquantitative assessment of myocardial ischemia.
METHODS
Study population
Myocardial perfusion and MFR were assessed in
675 consecutively admitted patients with suspected or
known CAD from January 2017 to April 2021 at our
institution (Figure 1). Patients with cardiomyopathy(n = 26), severe valvular disease (n = 3), and congenital
heart disease (n = 6) were excluded. The remaining 640
patients were categorized into three groups based on theextent of total myocardial ischemia indicated by thesummed difference score (SDS) assigned segmentallyby software: Group I (n = 335), minimal (SDS = 0–3, \ 5% myocardium ischemia); Group II (n = 150),
mild (SDS = 4–7, 5–10%); and Group III (n = 155),
moderate-to-severe (SDS C 8, [ 10%).8,10 MFR C 2.0
was defined as MFRpreserved, and MFR \ 2.0 was
defined as MFRimpaired.6,11,12 Early revascularizations

Journal of Nuclear CardiologyÒ

with PCI or coronary artery bypass grafting (CABG)
were defined as those occurring within 90 days of PET
imaging results.8,9,12 Pretest CAD probability scores
integrating age, gender, chest pain type, diabetes mellitus, hyperlipidemia, and smoking status were
calculated.13 Prognostic scores (five-year death risk)
were also calculated using the CALIBER model (www.caliberresearch.org/model), and this included the following baseline predictors as validated previously14:
age, gender, CAD diagnosis, deprivation, smoking,hypertension, diabetes, lipids, heart failure (HF), peripheral arterial disease, atrial fibrillation, stroke, chronic
kidney disease, chronic pulmonary disease, liver disease, cancer, depression, anxiety, heart rate, creatinine,
white cell count, and hemoglobin level. Demographic
and risk factors and medication use data were obtained
from medical records. This study complied with the
Declaration of Helsinki and ethical principles in the
Belmont Report and was approved by the Institutional
Review Board of Hokkaido Ohno Memorial Hospital(approval number: 20197). The requirement for
informed consent was waived because the study was
retrospective.
PET acquisition protocol and imaging
analysis
Patients underwent single-day stress (pharmacological)/rest 13N-ammonia PET with a PET/CT scanner(Biograph mCT Flow 64-4R PET/CT system; Siemens
Healthcare, Germany) in the 3D list mode. PET images
were reconstructed using the vendor-recommended
blob-based ordered-subset expectation maximization
time of flight algorithm with the default setting of 2
iterations and 21 subsets and a 256 9 256 matrix,
acquiring a field of view (FOV) of 407 mm.13 CT wasused for attenuation correction, and the acquisition was
obtained in a helical mode, using 120 kV, 20 mAs, and a
512 9 512 matrix size, acquiring a FOV of 500 mm.
Patients fasted for [ 6 h; caffeinated beverages andfoods were avoided for at least 12 h prior. The pharmacological stress scan was performed during adenosine
triphosphate (ATP)-induced hyperemia for 5 min, at a
rate of 160 lgkg - 1min - 1; 13N-ammonia dose of
3 MBq/kg was injected for 30 s, 3 min after ATP
infusion commencement. The rest scan was performed
with 3 MBq/kg of 13N-ammonia for 30 s, 1 h after the
stress scan. Heart rate, blood pressure, and electrocardiograms were recorded every minute during ATP
infusion and image acquisition. PET images were

Miura et al
Interaction of impaired myocardial flow

quantitatively analyzed using a dedicated software
package (Syngo MBF, Siemens Healthcare, Germany).
Segmental perfusion scores based on a 17-segment,
multi-point scale with corresponding summed scores
were automatically calculated as summed rest score(SRS), summed stress score (SSS), and SDS (SSS-SRS)
with the QPET software from Cedars-Sinai.15 These
indices (SSS, SDS, and SRS) were converted to thepercentage of the myocardium having stress, ischemia(% ischemia), or fixed defects by normalizing to the
maximum potential score (4 9 17). Absolute MBF wasquantified at rest and peak hyperemia with automatedfactor analysis and a validated 2-compartment kinetic
model.16
Outcome assessment
Follow-up data were obtained via phone questionnaires from patients and/or general practitioners or
attending cardiologists and from medical charts. The
overall follow-up period was 2.0 years (interquartile
range [IQR]: 1.2–3.0), with a 99% follow-up rate
(Online Resource 1). The primary endpoint was
MACEs, including cardiovascular death, non-fatal
myocardial infarction (MI), hospitalization for HF, andlate coronary revascularization ([ 90 days post-PET)
with PCI/CABG. The secondary outcome was defined as
hard events (all-cause death, non-fatal MI, admission for
HF, or unstable angina). Cardiovascular deaths resulted
from MI, sudden cardiac arrest, HF, stroke, cardiovascular hemorrhage, or other cardiovascular causes.
Statistical analysis
Baseline characteristics are expressed as number(percentage) for categorical variables and median (IQR)
for continuous variables. We used the Fisher exact andWilcoxon rank-sum tests to compare categorical andcontinuous baseline characteristics. Enrollment year,
follow-up period, and detailed outcomes for patientswith and without MACEs stratified by group (I-III) andMFR status were compared. Box plots demonstrated the
distribution of pretest CAD probability scores andprognostic scores comparing patients with MFRpreservedand MFRimpaired according to the ischemia level. Cox
proportional-hazards models were used to examine theassociation between MFRimpaired and MACEs in Groups
I-III, adjusting for pretest CAD probability scores plus
early revascularization. Interaction between % ischemia
and MFR (value) was also tested. At each level, theMiura et al
Interaction of impaired myocardial flow

models were examined for the validity of the proportional-hazard assumption. Adjusted survival curves for
MACEs were plotted from the Cox model, including
early revascularization and pretest CAD probability
scores, stratified by MFR status in Groups I-III. Similarly, adjusted survival curves for cardiovascular deathand MACEs were plotted by replacing pretest CAD
probability scores with prognostic scores. We performed
additional analysis to examine the association between
MFRimpaired and hard events in Cox regression models
with first revascularization as a time-dependent variable,
testing the two-way interaction of % ischemia and
MFR.17 The codes for the adjusted survival curves areshown in Online Resource 2. Starting with the pretest
score (Model 1), % ischemia (Model 2) and MFR(Model 3) were sequentially added as continuous variables. The models’ discriminatory power was assessedusing the Harrell C-index with 95% confidence intervals(CI), and the Akaike information criterion and computed
models were compared by the likelihood ratio test. We
estimated the annualized event rates of MACEs inGroups I-III by fitting Poisson regression models inpatients with MFRpreserved and MFRimpaired. Sensitivity
analyses assessed the prognostic value of MFRimpaired byre-defining early revascularization (B 60 days postPET) in the Cox model for future MACEs, including
late revascularization ([ 60 days post-PET), after
adjustment for pretest CAD probability scores plus
early revascularization.6,18 The correlation between
global MFR and % ischemia was calculated using
Spearman’s rank correlation coefficient in patients withand without early revascularization. P \ 0.05 was considered statistically significant. All statistical analyses
were performed using Stata 14.0 (StataCorp, College
Station, TX, USA) and R software, version 4.1.1 (RProject for Statistical Computing).
RESULTS
Patient and imaging characteristics
Baseline and PET imaging characteristics forGroups I-III are presented in Tables 1 and 2. Overall,
the patients were predominantly male (n = 451, 71%),
with a median age of 72 years; 17% had a history of MI,
and early revascularization was performed in 33%. The
incidence of early revascularization was 18%, 35%, and
63% in Groups I-III, respectively (P \ 0.001). There
was a higher proportion of males (P = 0.002), pretest
CAD probability scores (P \ 0.001), and incidence ofprevious MI (P \ 0.001), with a greater extent of

Journal of Nuclear CardiologyÒ

myocardial ischemia from Groups I-III. Pretest CAD
probability and prognostic scores significantly differed
between patients with MFRpreserved and MFRimpaired
regardless of the ischemia level (Group I-III) except forthe pretest CAD probability score in Group II (P = 0.08)(Online Resource 3). Regarding PET findings, rest MBF
was 0.88 (IQR 0.74–1.04) mL/g/min; stress MBF, 2.10
(1.60–2.56) mL/g/min; and global MFR, 2.41 (1.87–
2.86). Global MFR and % ischemia were inversely
correlated (r = – 0.27; P \ 0.0001) (Figure 2). An
MFR scatterplot illustrated a wide range of values, even
within Group I, and early revascularization was often
deferred even in Group III patients with MFRimpaired.
Outcome events
The annualized rate of cardiovascular death andMACEs in Group I-III patients with MFRpreserved andMFRimpaired is shown in Figure 3. Annualized cardiovascular death rates were 1.3% (95% CI 0.8–2.0)
overall, 1.2% (0.6–2.3) in Group I, 0.9% (0.3–2.8) inGroup II, and 1.8% (0.8–4.0) in Group III, while thosefor MACEs were 10.8% (9.2–12.7), 7.9% (6.0–10.3),
9.4% (6.7–13.4), and 18.1% (14.1–23.3), respectively.
The annualized MACEs rate significantly differed
between patients with MFRpreserved and MFRimpaired inGroup I (P \ 0.0001) and II (P \ 0.001), but not inGroup III (P = 0.05). Table 3 summarizes clinicaloutcomes in Groups I-III by MFR status. The percentage
of MACEs (P = 0.001), non-fatal MI (P = 0.03), andlate revascularization (P = 0.001) significantly differedamong Group I-III patients.
Survival analysis
Figure 4 displays survival curves for MACEs forGroups I-III patients according to MFR status, adjusted
for pretest CAD probability scores and early revascularization. A significant difference in MACEs between
patients with MFRpreserved and MFRimpaired was observed
overall (risk-adjusted P \ 0.001), in Groups I (riskadjusted P = 0.002) and II (risk-adjusted P = 0.008), but
not in Group III (risk-adjusted P = 0.67). These associations remained evident even after adjusting for
prognostic scores and early revascularization, as demonstrated in the risk-adjusted survival curves and Cox
proportional-hazards analysis (Online Resource 4 and 5,
respectively), but a significantly higher rate of cardiovascular death was observed only in Group I (riskadjusted P = 0.02).

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Interaction of impaired myocardial flow

Table 1. Baseline characteristics of study participants

Characteristic
Demographic characteristics
Age, years
Male gender, n (%)
Body mass index, kg/m2
PET indication
Chest pain
Dyspnea
Preoperative
OthersPretest CAD probability
score, %Prognostic score, %Medical history, n (%)
Hypertension
Dyslipidemia
Diabetes mellitus
Prior PCI
Prior CABG
Prior myocardial infarction
Prior heart failure
Atrial fibrillation
Hemodialysis
Current smoker
Chronic lung disease
Malignancy
Medications, n (%)
Antiplatelet therapy
Calcium channel blockers
b-blockers
Cholesterol-lowering
agents
ACEIs or ARBs
Nitrates
Diuretics
Oral hypoglycemic agents
Insulin
Early revascularization, n (%)
PCI
CABG

Overall(n = 640)

I (n = 335)

II (n = 150)

III (n = 155)

72 [65, 77]451 (71)
24 [22, 27]

72 [64, 77]
218 (65)
24 [22, 26]

73 [66, 77]
108 (72)
24 [22, 27]

71 [64, 77]
125 (81)25 [23, 27]

0.35
0.002
0.06

394 (62)55 (9)
31 (5)
160 (25)
48 [29, 68]

211 (63)
27 (8)
18 (5)
79 (24)43 [25, 62]

90 (60)18 (12)7 (5)35 (23)48 [30, 69]

93 (60)10 (7)6 (4)
46 (30)56 [39, 76]

0.48\ 0.001

12 [7, 20]

12 [7, 20]

13 [7, 23]

12 [7, 19]0.39

420 (66)391 (61)239 (37)224 (35)
53 (8)108 (17)76 (12)
83 (13)17 (3)
129 (20)44 (7)
47 (7)

205 (61)196 (59)
103 (31)100 (30)
14 (4)
37 (11)34 (10)41 (12)7 (2)68 (20)28 (8)
30 (9)

105 (70)
93 (62)64 (43)56 (37)
12 (8)
33 (22)21 (14)22 (15)4 (3)29 (20)10 (7)
11 (7)

110 (71)102 (66)
72 (47)68 (44)27 (17)38 (25)21 (14)20 (13)6 (4)32 (21)
6 (4)
6 (4)

0.04
0.29
0.001
0.008
\ 0.001\ 0.001
0.36
0.76
0.52
0.95
0.18
0.13

355 (56)257 (40)224 (35)367 (57)

151 (45)147 (44)96 (29)182 (54)

85 (57)56 (37)64 (43)86 (57)119 (77)
54 (35)64 (41)99 (64)\ 0.001
0.11
0.002
0.13

280 (44)42 (7)
70 (11)170 (27)
32 (5)
210 (33)
168 (26)
43 (7)

130 (39)
17 (5)
35 (10)77 (23)
12 (4)
60 (18)55 (16)6 (2)74 (49)12 (8)
20 (13)41 (27)9 (6)52 (35)43 (29)8 (5)76 (49)13 (8)
15 (10)52 (34)
11 (7)
98 (63)70 (45)29 (19)

0.03
0.27
0.54
0.04
0.20
\ 0.001\ 0.001\ 0.001

P Value

Data are presented as medians [interquartile range] or n (%) of patients. PET, positron emission tomography; CAD, coronary arterydisease; PCI, percutaneous coronary intervention; CABG, coronary artery bypass grafting; ACEI, angiotensin converting enzyme
inhibitors; ARB, angiotensin II receptor blockers

Following adjustments for pretest CAD probability
scores and early revascularization, MFRimpaired was asignificant predictor of MACEs in Groups I (hazard ratio[HR], 2.89; 95% CI 1.48–5.64; P = 0.002) and II (HR,3.40; 95% CI 1.37–8.41; P = 0.008) but not in III (HR,1.15; 95% CI 0.59–2.26; P = 0.67) (Table 4). A significant interaction was found between MFR and %ischemia (P \ 0.0001) in the adjusted model. We foundsimilar results with hard events, and the Cox proportional-hazards model using first revascularization as aMiura et al
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Table 2. PET findings

Parameter

Overall(n = 640)

I (n = 335)

II (n = 150)

III (n = 155) P value

Rest EDV, mL
Rest LVEF, %Stress LVEF, %Myocardium fixed, %Myocardium stress, %Myocardium ischemia,%
Rest MBF, mL/g/min

91 [75, 115]
67 [57, 74]
65 [55, 72]
3 [1, 9]10 [4, 19]
4 [1, 10]88 [73, 110]
69 [61, 76]
67 [61, 73]
3 [0, 6]
4 [1, 9]1 [0, 3]89 [73, 117]
68 [54, 75]
66 [53, 73]
4 [1, 10]
12 [9, 18]
7 [6, 9]100 [83, 128]62 [53, 68]
58 [49, 65]
7 [3, 15]25 [18, 35]
16 [13, 19]\ 0.001\ 0.001\ 0.001\ 0.001\ 0.001\ 0.001

0.88 [0.74, 1.04]
2.10 [1.60, 2.56]

MFR

2.41 [1.87, 2.86]

MFRimpaired, n (%)

192 (30)

0.89 [0.73,
1.08]
1.99 [1.56,
2.48]
2.33 [1.78,
2.76]
50 (33)0.81 [0.68,
0.97]
1.63 [1.33,
2.07]
2.07 [1.70,
2.51]
71 (46)\ 0.001

Stress MBF, mL/g/min

0.91 [0.77,
1.08]
2.33 [1.90,
2.73]
2.59 [2.11,
3.09]
71 (21)\ 0.001\ 0.001\ 0.001

Data are presented as medians [interquartile range] or n (%) of patients. PET, positron emission tomography; EDV, end-diastolicvolume; LVEF, left ventricular ejection fraction; MBF, myocardial blood flow; MFR, myocardial flow reserve

Figure 2. Association between MFR and % myocardialischemia with and without early revascularization (B 90 days
post-PET). Abbreviations as in Figure 1.

time-dependent variable revealed that MFRimpaired was asignificant predictor of MACEs in Groups I (HR, 5.09;
95% CI 2.17–11.9; P \ 0.0001) and II (HR, 4.16; 95%
CI 1.49–11.6; P = 0.006), but not in III (HR, 1.20; 95%
CI 0.48–3.01; P = 0.68), with a significant interaction
between them (P \ 0.0001). Additionally, sensitivity

Figure 3. Annualized event rate of cardiovascular death andMACEs by MFR in Groups I-III. Abbreviations as in Figure 1.

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Table 3. Detailed clinical outcomes in patients with MFRpreserved and MFRimpaired in Groups I-III

I(n = 335)II(n = 150)

III(n = 155)

P
value

448
192
25 (4)
10 (2)
15 (8)
17 (3)4 (1)
13 (7)
93 (15)46 (10)47 (25)13 (2)

26471
12 (4)4 (2)
8 (11)
8 (2)
2 (1)
6 (9)
36 (11)20 (8)
16 (23)
3 (1)

100
50
6 (4)
4 (4)
2 (4)
3 (2)
1 (1)
2 (4)
21 (14)8 (8)13 (26)
3 (2)

84
71
7 (5)
2 (2)
5 (7)
6 (4)
1 (1)
5 (7)
36 (23)18 (21)18 (25)
7 (5)

0.88
0.35
0.32
0.54
0.92
0.62
0.001
0.001
0.88
0.03

7 (2)
6 (3)
53 (8)
34 (8)
19 (10)41 (6)
13 (3)
28 (15)

2 (1)
1 (1)
20 (6)
15 (6)5 (7)
15 (5)
4 (2)
11 (16)

1 (1)
2 (4)
9 (6)
5 (5)
4 (8)
11 (7)2 (2)9 (18)4 (5)3 (4)24 (16)14 (17)10 (14)15 (10)7 (8)8 (11)

0.03
0.57
0.001
0.002
0.32
0.08
0.004
0.56

Overall(n = 640)

Outcomes
MFRpreserved, n
MFRimpaired, nAll-cause death, n (%)
MFRpreserved
MFRimpaired
Cardiovascular death, n (%)
MFRpreserved
MFRimpaired
MACEs, n (%)
MFRpreserved
MFRimpaired
Non-fatal myocardial infarction, n(%)
MFRpreserved
MFRimpaired
Late revascularization, n (%)
MFRpreserved
MFRimpaired
Heart failure admission, n (%)
MFRpreserved
MFRimpaired

MFR, myocardial flow reserve; MACEs, major adverse cardiovascular events

Figure 4. MACE-free survival adjusted for pretest CAD probability scores and early revascularization (B 90 days post-PET) in each group. Abbreviations as in Figure 1.

analysis with early revascularization re-defined to B 60
days post-PET revealed that MFRimpaired was a significant prognostic indicator in Groups I and II, but not inIII, with significant interaction found between %ischemia and MFR (P \ 0.0001) (Online Resource 6).
A series of multivariable models were constructed toassess the incremental value of MFR (Figure 5). The
addition of MFR to Model 2 resulted in a significant
improvement in the model fit (P \ 0.0001), and the

addition of % ischemia to Model 1 also improved risk
discrimination (P = 0.009).
DISCUSSION
This study is the first to demonstrate the added andindependent prognostic value of MFRimpaired and thedifferential effect of MFRimpaired based on the myocardial ischemia level assessed by PET-MPI. In Groups I

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Table 4. Association of MFR with MACEs and hard events in Cox proportional-hazards analysis

Outcome
MACEs

Group
OverallGroup IGroup IIGroup III

Overall(Interaction)*Hard
eventsOverallGroup IGroup IIGroup III

Overall(Interaction)*

Variable
Pretest CAD probability
score
Early revascularization
MFRimpaired
Pretest CAD probability
score
Early revascularization
MFRimpaired
Pretest CAD probability
score
Early revascularization
MFRimpaired
Pretest CAD probability
score
Early revascularization
MFRimpaired
Pretest CAD probability
score
Early revascularization
MFR% ischemia
MFR x %ischemia*
Pretest CAD probability
score
MFRimpaired
Pretest CAD probability
score
MFRimpaired
Pretest CAD probability
score
MFRimpaired
Pretest CAD probability
score
MFRimpaired
Pretest CAD probability
score
MFR
% ischemia
MFR x %ischemia*

Beta
coefficient

HR (95% CI)

P value

0.01

1.01 (1.00–1.02)

0.001- 0.170.82
0.02

0.84 (0.54–1.31)
2.27 (1.49–3.47)
1.01 (1.00–1.03)

0.44\ 0.001
0.01

0.27
1.06
0.01

1.30 (0.62–2.75)
2.89 (1.48–5.64)
1.01 (0.99–1.03)

0.47
0.002
0.27

- 0.601.22
0.01

0.54 (0.21–1.45)
3.40 (1.38–8.41)
1.01 (0.99–1.03)

0.22
0.008
0.22

- 0.600.15
0.01

0.54 (0.28–1.06)
1.15 (0.59–2.26)
1.01 (1.00–1.02)

0.07
0.67
0.002

- 0.44- 1.40- 0.150.09
0.01

0.64 (0.40–1.03)
0.24 (0.15–0.39)
0.85 (0.77–0.94)
1.09 (1.04–1.14)
1.01 (1.00–1.03)

0.07\ 0.0001
0.001\ 0.0001
0.01

1.15
0.03

3.17 (1.86–5.41)
1.03 (1.02–1.06)\ 0.0001\ 0.001

1.63
0.01

5.09 (2.17–11.9)
1.00 (0.98–1.03)\ 0.0001
0.54

1.43- 0.01

4.16 (1.49–11.6)
0.99 (0.97–1.01)

0.006
0.53

0.19
0.02

1.20 (0.48–3.01)
1.01 (1.00–1.02)

0.68
0.01

- 2.010.13 (0.070.24)
0.80 (0.71–0.91)
1.12 (1.06–1.19)\ 0.0001- 0.210.12\ 0.001\ 0.0001

MFR, myocardial flow reserve; MACEs, major adverse cardiovascular events; HR, hazard ratio; CI, confidence interval; and CAD,
coronary artery disease*Statistics for the interaction were obtained from different models including the main effects and the interaction term
Cox regression models for hard events were used with first revascularization as time-dependent variable

and II, MFRimpaired allowed for further risk stratification(high and low) for MACEs. Conversely, MFRimpaired
was not associated with reduced event-free survival for

MACEs in Group III. These results remained unchanged
when adjusted for prognostic scores instead of pretest
CAD probability scores or when early revascularization

Journal of Nuclear CardiologyÒ

Figure 5. Comparative analysis of C-statistics and Akaike’s
information criterion (AIC) with 95% confidence interval topredict future MACEs. Abbreviations as in Figure 1.

was re-defined from 90 days post-PET to 60 days postPET. We also obtained similar results for hard eventswith the first revascularization as a time-dependent
variable. Additionally, patients with MFRpreserved had
consistently low cardiovascular death rates (B 0.5%)
regardless of the ischemia level. An interaction term
between the severity of myocardial ischemia and MFR
was significant for both MACEs and hard events.
In patients with minimal-to-mild ischemia, MFRimpaired was significantly associated with reduced event-freesurvival from MACEs. These patients (18% in Group I;
35% in Group II) underwent early revascularization, withpotentially little impact on MACEs as previously discussed.4,17 The annualized MACE rate was less than 5%in Group I-MFRpreserved and Group II-MFRpreserved compared with 20% and 17% in Group I-MFRimpaired andGroup II-MFRimpaired, respectively, allowing for effective
risk stratification. These findings suggest that we shouldcarefully manage patients with MFRimpaired, including
those with low levels of myocardial ischemia. In line withprior studies,9,19 we speculated that microvascular dysfunction might be associated with increased
cardiovascular risk via multifactorial mechanisms; however, these discussions are beyond the scope of this study.
Significant variability in the relationship between global
MFR and myocardial ischemia level was found, particularly in Group I, where 21% had microvascular
dysfunction reflected by MFRimpaired. The annualized
cardiovascular death rate in patients with MFRimpaired
differed between Group I (4.5%) and II (1.6%); however,

Miura et al
Interaction of impaired myocardial flow

Table 3 shows no statistical difference between patientswith MFRimpaired among Groups I-III for both cardiovascular and all-cause deaths. No meaningful difference wasobserved in the cardiovascular death rates between
Groups I and II. However, we have not analyzed thesepoints further because of the relatively low event rate andsample size in both groups.
In patients with moderate-to-severe ischemia(Group III), MFRimpaired did not stratify cardiovascularrisk, although patients with MFRimpiared tended to havehigher annualized cardiovascular death (P = 0.08) and
MACE rates (P = 0.05) than those with MFRpreserved,
suggesting that patients with MFRpreserved could be at
relatively high risk for MACEs and low risk for
cardiovascular death. Our findings are consistent withthose of a large-scale study8 in which the 3-year survival
rate for patients with normal MFR ([ 1.8) was muchhigher ([ 90%) than that of those with lower MFR(B 1.8) regardless of the ischemia level (0%, 1–10%,
and [ 10%). The annualized MACEs rate was muchhigher in Group III than in other groups. Survival
analysis showed no statistical difference in MACE-freesurvival between Group III patients with MFRpreservedand MFRimpaired after adjusting for pretest CAD probability scores and early revascularization. These uniquefindings require careful interpretation because survival
benefits can alter with early revascularization based on
the extent of ischemia and MFR status, as previously
mentioned.4,8 Furthermore, ischemia reduction may nothave been fully achieved through early revascularizationin patients with greater ischemia. In recent studies, earlyrevascularization in CAD patients with moderate-tosevere ischemia resulted in no clear survival or MACEs
outcome benefits, though early revascularization more
effectively reduced ischemia10 and anginal symptoms
than optimal medical therapy alone.20
We performed additional analyses to ensure therobustness and generalizability of our results. Instead ofpretest CAD probability scores, prognostic scores wereused to conduct risk-adjusted survival analysis, thoughthese prognostic models may require further external
validation.21 Pretest CAD probability scores increased inpatients with both MFRpreserved and MFRimpaired withincreased ischemia levels; in contrast, prognostic scores
were comparable in patients with MFRpreserved andMFRimpaired. These findings imply that MFRimapired might
have greater prognostic impact than the extent of myocardial ischemia. We obtained similar results with prognostic
scores: MFRimpaired was significantly associated with ahigher MACE risk in Group I and II patients but not inGroup III patients, confirming the prognostic impact ofMiura et al
Interaction of impaired myocardial flow

MFRimpaired in the lower level of ischemia with different
approaches. We also revealed the interaction between
MFR and % ischemia for two outcomes: MACEs and hard
events. Analyzing the effect of early revascularization
was challenging because PET-triggered revascularization
showed survival benefits in patients with greater ischemia.8 Thus, early revascularization was excluded from
MACEs as a frequently used method,4,6,18,21 though there
were limited data for comparison of defining earlyrevascularization between B 90 days,4,6,21 and B 60
days.1,18 Nevertheless, the associations between MFRimpaired and MACEs were similar for both definitions at eachischemia level. An alternative approach was implemented
to examine the relationship between MFRimpaired andclinical outcomes by considering first revascularization
after PET as a time-dependent variable with hard events as
the main outcome to prevent misclassification.8 We foundsimilar results with hard events; thus, these findings
should reinforce our conclusions.

Journal of Nuclear CardiologyÒ

perfusion as positron radiopharmaceuticals, and 82Rb,
not commercially available in Japan, has somewhat
worse spatial resolution and reduced first-pass extraction, resulting in a potential underestimation of flow at
maximal hyperemia compared to 13N-ammonia.3
NEW KNOWLEDGE GAINEDOur study clarified an interaction between myocardialischemia severity and PET-measured MFR to predict
future MACEs in patients with suspected or known CAD.
A similar interaction was observed for hard events,
highlighting the robustness of the association. Quantitation
with MFR had significant prognostic power in addition tosemi-quantitative findings and pretest CAD probability
scores. Accordingly, our study provided evidence tosupport the combination of myocardial ischemia and global
MFR utilizing perfusion 13N-ammonia PET as a novel risk
stratification method in these patients.
CONCLUSIONS

Study limitations and strengths
First, the retrospective nature, limited subgroupsample size, and overfitting models for events may beconsidered limitations of this study. The evidence wasinsufficient for analyzing differential effects of MFRimpaired based on the myocardial ischemia level assessed by
PET-MPI. Therefore, larger prospective studies are
required to validate the prognostic value of quantitative
PET imaging. Second, this was a single-center observational study; there may be a selection bias in patientsreferred for PET-MPI. For instance, relatively high-riskpatients may have been included, such as those withsignificant coronary calcium or plaque burden or undiagnosed silent subendocardial MI, because myocardial
perfusion PET is often applied in Japan when othermethods are inconclusive. This may have resulted inrelatively high event rates. Third, we conducted all
myocardial perfusion PET examinations with pharmacological stress testing, not exercise stress testing thatcan be a more physiological procedure with the added
prognostic value of heart rate reserve.22 Nevertheless,myocardial perfusion PET with exercise stress testing is
not commercially available in Japan. Fourth, we adopted
the CALIBER model in the survival analysis as a wellvalidated prognostic indicator, although the model might
be difficult to implement in a retrospective study.14,23
However, we were able to calculate the scores byintegrating our comprehensive electronic medical record
systems. Finally, we emphasize the accuracy and effectiveness of 13N-ammonia PET for qualitative andquantitative analyses. 82Rb and 13N-ammonia are theonly FDA-approved radiotracers to assess myocardial

MFRimpaired was independently associated with ahigher MACE risk in patients with B 10% myocardialischemia but not in those with [ 10% myocardialischemia. The robustness of this result was supportedby the similar association observed after statisticallyadjusting for prognostic scores instead of pretest CAD
probability scores or after replacing MACEs with hard
events with first revascularization after PET as a timedependent variable. Our findings depict the significanceof risk stratification using PET-measured MFR and theextent of ischemic burden, especially in patients withlower ischemic burden, providing additional evidence tosupport physiologically based revascularization strategies. Quantitative PET assessments could help
understand potential mechanisms of impaired MFR
despite the level of myocardial ischemia. Prospective
evidence from a PET-based revascularization strategy isrequired to confirm our results.
DISCLOSURES
Shiro Miura, Atsutaka Okizaki, Hiraku Kumamaru,
Osamu Manabe, Masanao Naya, Chihoko Miyazaki, andTakehiro Yamashita have no conflicts of interest to
declare.

Funding
None.

Journal of Nuclear CardiologyÒ

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