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 Table of Contents  
ORIGINAL ARTICLE
Year : 2015  |  Volume : 28  |  Issue : 2  |  Page : 392-399

Fractional flow reserve and intravascular ultrasound in the assessment of moderate coronary lesions: a comparative study


1 Department of Cardiology, National Heart Institute, Cairo, Egypt
2 Cardiology Department of Cardiology, Faculty of Medicine, Menoufia University, Menoufia, Egypt
3 Department of Cardiology, Menoufia University, Shebin El-Kom, Nasser Institute Hospital, Egypt

Date of Submission31-Mar-2014
Date of Acceptance07-Jun-2014
Date of Web Publication31-Aug-2015

Correspondence Address:
Hazem Khamis
Waley El Ahd St., Hadayek El Koba, 11331 Cairo
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1110-2098.163891

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  Abstract 

Objective
The aim of this study was to assess intermediate coronary lesions as detected by visual estimation in coronary angiography using two invasive modalities: fractional flow reserve (FFR) and intravascular ultrasound.
Background
The significance of coronary artery stenosis of intermediate severity can be difficult to determine. The management of intermediate coronary lesions, defined by a diameter stenosis of more than 40% to less than 70%, continues to be a therapeutic dilemma for cardiologists. The two-dimensional representation of the arterial lesion provided by angiography is limited in distinguishing intermediate lesions that require stenting from those that simply need appropriate medical therapy.
Patients and methods
Thirty patients with intermediate coronary stenosis on coronary angiography were subjected to a noninvasive stress test, either exercise myocardial perfusion imaging or stress echocardiography, and the results were compared with those of intravascular ultrasound and FFR.
Results
In this study, 30 patients were subjected to FFR measurement. Twelve of 14 patients had an FFR of less than 0.80; reversible ischemia was detected on the stress test. In contrast, only two of 16 patients with an FFR of more than 0.80 had a negative stress test for reversible myocardial ischemia. FFR value has a sensitivity and a specificity of 85.7 and 87.5%, respectively, in predicting the results of a noninvasive stress test, with a positive predictive value and negative predictive value of 85.7 and 87.5%, respectively, with an accuracy of 86.7%. It is also showed that the best cutoff value for the minimal luminal area that is concordant with FFR values is less than 4.5 mm 2 , which has a sensitivity and a specificity of 57.1 and 56.3%, respectively, and positive predictive value of 53.3% and negative predictive value of 60%.
Conclusion
In patients with coronary stenosis of intermediate severity, FFR appears to be a useful index for assessment of the functional severity of coronary artery stenosis in comparison with a noninvasive stress test and can be used as a guide before coronary revascularization.

Keywords: fractional flow reserve, intermediate stenosis, intravascular ultrasonography


How to cite this article:
Khamis H, Reda A, Soliman MA, Yaseen R, Allah AS. Fractional flow reserve and intravascular ultrasound in the assessment of moderate coronary lesions: a comparative study. Menoufia Med J 2015;28:392-9

How to cite this URL:
Khamis H, Reda A, Soliman MA, Yaseen R, Allah AS. Fractional flow reserve and intravascular ultrasound in the assessment of moderate coronary lesions: a comparative study. Menoufia Med J [serial online] 2015 [cited 2020 Feb 27];28:392-9. Available from: http://www.mmj.eg.net/text.asp?2015/28/2/392/163891


  Introduction Top


An intermediate coronary lesion on coronary angiography is defined as a luminal narrowing with a diameter stenosis more than 40%, but less than 70%. Assessment of a coronary lesion with intermediate severity continues to be a challenge for cardiologists [1] .

In the current era of drug-eluting stents (DESs), when percutaneous coronary revascularization can be achieved with high success, a low complication rate, and excellent long-term patency, it might be tempting to treat all suspected lesions with implantation of a DES. However, there are still procedural complications associated with angioplasty, the inherent risk of restenosis, and late stent thrombosis [2] .

Selective coronary angiography is accepted as the standard for determining the presence and extent of epicardial coronary artery disease (CAD) [3] . However, there is significant interobserver and intraobserver variability in the interpretation of the severity of stenosis on coronary angiography. In addition, cross-sectional anatomic imaging obtained from histopathological specimens and intravascular ultrasound (IVUS) has highlighted the limitations of coronary angiography [4] .

Owing to the increased sensitivity of IVUS in identifying disease and its close correlation with pathology, IVUS has become the more accurate standard for defining the anatomy of atherosclerosis in vivo. IVUS is a catheter-based technique that provides tomographic images perpendicular to the length of the coronary arteries [5] .

Fractional flow reserve (FFR) is an invasive modality that is useful to overcome the limitations of coronary angiography in the assessment of the functional significance of a coronary stenosis. The FFR is defined as the ratio of maximal blood flow achievable in the stenotic coronary artery relative to the maximal flow in the same vessel if it were normal. The normal FFR value should be equal to 1. An FFR value of less than 0.75 correlates with ischemia on noninvasive stress tests. Several studies have shown that in patients with an intermediate coronary lesion and an FFR 0.80, percutaneous coronary intervention (PCI) can safely be deferred [6] .


  Patients and methods Top


This study included 30 patients with possibility to have CAD. Five of the patients were women and 25 were men, age range 38-74 years (mean ± SD 57.57 ± 10.30 years). They underwent both an invasive and a noninvasive assessment of intermediate coronary lesions from May 2011 to January 2014 in Wady El-Neel Hospital.

Inclusion criteria

This study included patients older than 18 years and younger than 75 years of age who had anginal symptoms, documented silent myocardial ischemia, and those who were eligible for coronary angiography and possible intracoronary stenting.

Exclusion criteria

Patients with in-stent restenosis and those with chronic total occlusion.

All patients were subjected to the following: complete and detailed assessment of medical history including risk factors of CAD (diabetes mellitus, hypertension, smoking, and dyslipidemia) and laboratory investigations including fasting blood sugar, glycated hemoglobin, complete lipid profile (LDL-C, HDL-C, and TG), and serum creatinine.

Resting standard 12-lead electrocardiogram with a special focus on ST-T wave changes resting transthoracic echocardiography to assess left ventricular ejection fraction and resting wall motion abnormalities [7] .

Noninvasive objective ischemia limited tests

These were carried out in symptom-free patients by either pharmacological stress echocardiography (by dobutamine) or exercise single-photon emission computed tomography (SPECT) myocardial perfusion imaging ( 99m TC sestamibi scintigraphy).

Coronary angiography

This was performed according to the standard techniques to detect angiographically intermediate coronary lesions, which are defined as luminal narrowing with a diameter stenosis more than 40% and less than 70% [8] .

Intravascular coronary ultrasound measurements

In this study, we used a rotational mechanical probe (Atlantis SR Pro; Boston Scientific, Boston Scientific Corporation, USA). The IVUS probe was advanced over a 0.014 guide wire and pushed beyond the lesion; the ultrasound catheter was then pulled back slowly at a constant speed either manually or motorized (usually 0.5 mm/s), which enables volumetric evaluation of the lesion and plaque dimensions after longitudinal or three-dimensional reconstruction. Manual or computer-assisted planimetry of these two borders allows precise measurements of the lumen area, intima-media area, and external elastic membrane (EEM) area [9] .

Measurement of coronary fractional flow reserve

The pressure wire system was used in this study to measure intracoronary pressure, namely, the Pressure Wire Certus (Saint Jude Medical Systems Inc., Uppsala, Sweden). It is a 0.014-inch wire that has handling characteristics similar to those of most standard angioplasty guide wires. The sensor is located 30 mm from the tip at the junction between the radiopaque and the radiolucent portions. In this study, an intracoronary injection of 150 μl adenosine for the left system and 120 μl for the right coronary artery was administered to achieve maximum hyperemia [10] .

Statistical analysis

Statistics were calculated by a computer using SPSS software (version 20; SPSS Inc., Chicago, Illinois, USA), a word processing database and statistics program. Descriptive statistics were calculated using mean (X) and SD to measure the central tendency of data and the distribution of data around their mean value. Student's t-test was used to determine the statistically significant difference between the mean values of two samples. χ2 -Tests were used to test for a statistically significant relation between different variable or grades in qualitative data. The Pearson correlation coefficient test (r) was used to test for the linear relation between two numeric variables. The Mann-Whitney test was used as the nonparametric test for the comparison of two groups of data not normally distributed or for a small sample size. The Fisher exact test was used to compare two independent proportions when the expected observation in any cell of the table was below 5. A result was considered significant if P value was less than 0.05. A result was considered highly significant if P value was less than 0.01.


  Results Top


This study included 30 patients with possibility to have CAD. They underwent an invasive and a noninvasive assessment of intermediate coronary lesions and a PCI if indicated. The age of the patients ranged from 38 to 74 years, with mean ± SD of 57.57 ± 10.30 years; 25 patients, representing 83.3% of the population, were men and five patients, representing 16.7% of the population, were women. There was no statistically significant difference among populations in terms of age, with P = 0.625 and 0.894, for the FFR value and the result of the noninvasive stress test, respectively [Table 2] and [Table 4]. Also, there was no statistically significant difference in traditional coronary risk factors - for example, hypertension, dyslipidemia, except for diabetes [Table 2], compared with the FFR value (P = 0.02).

[Figure 1] shows the demographic data as well as the risk factors for CAD in this study. Patients were divided according to the result of the noninvasive stress test; 14 patients (out of 30), representing 46.7% of all cases, were positive and 16 patients, representing 53.3%, were negative [Figure 2].
Figure 1: Demographic data and coronary risk factors of the stud y group

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Figure 2: Percentage of positive and negative cases for the results of a stress test

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[Table 1] shows the minimum, maximum, and mean ± SD of different measurements obtained from the cases studied including FFR value, reference luminal area (RLA) and minimal luminal area (MLA) obtained by IVUS as well as percentage of stenosis as detected by IVUS. The table showed that the minimum and maximum FFR value were 0.62 and 0.99, respectively, with mean ± SD of 0.806 ± 0.989; also, for the MLA obtained by IVUS, the minimal and maximal luminal areas were 2.8 and 8.5, respectively, with mean ± SD of 4.55 ± 1.31.
Table 1 Fractional flow reserve value and intravascular ultrasound measurements for the patients in the study

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Table 2 Difference between patients in relation to fractional flow reserve values

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[Table 2] classifies patients according to FFR values, whether lower than or higher than 0.80, into two categories and shows the relation between FFR values and different criteria of the population including demographic data and coronary risk factors as well as IVUS measurements obtained (RLA, MLA, and percent of stenosis) and whether they underwent a PCI or not [Figure 3].
Figure 3: Relation between the fractional fl ow reserve (FFR) value and both coronary risk factors and percutaneous coronary interventio n (PCI)

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According to the [Table 2], it is obvious that there was a statistically significant relation between the presence or the absence of diabetes mellitus and FFR value (P = 0.02) as in this study 11 patients (out of 14 patients with FFR<0.80) were diabetic and only six patients (out of 16 patients with FFR ≥ 0.80) were diabetic; also, there was a significant difference among the groups studied for ejection fraction compared with the FFR value (P = 0.010). There was also a statistically highly significant relation (P = 0.0001) between the FFR value and PCI as all cases with FFR value less than 0.80 had already undergone PCI and those with an FFR value 0.80 or more had not undergone PCI.

[Table 3] shows the correlation between the FFR value and different variables, showing that there was a highly significant positive correlation between the FFR value and fasting blood sugar (r = −0.65, P = 0.001).
Table 3 Correlation between fractional flow reserve values and other variables

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[Table 4] classifies patients according to the results of the noninvasive stress test, whether positive or negative, into two categories and shows the relation between the stress test and different criteria of the population including demographic data and coronary risk factors as well as IVUS measurements obtained (RLA, MLA, and percent of stenosis) and whether they underwent a PCI or not.
Table 4 Difference between patients in relation to a nonstress test

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According to the table, it is obvious that there was no statistically significant difference among the population in terms of demographic data and coronary risk factors compared with the results of the noninvasive stress test. There was also a highly statistically significant relation (P = 0.0001) between the results of the noninvasive stress test and whether patients underwent PCI as 12 patients (out of 14 patients with a positive stress test) had already undergone PCI, whereas only two patients (out of 16 patients with a negative stress test) underwent PCI.

[Table 5] shows the sensitivity, specificity, and predictive value of FFR in detecting the result of a noninvasive stress test. As shown in this table, patients were classified into two categories according to the results of the stress, whether positive or negative. It is noteworthy that 14 patients (out of 30) had a positive stress test; 12 of these patients (out of 14) had an FFR value less than 0.80 and only two patients had an FFR value 0.80 or more. However, 16 patients (out of 30 patients) had a negative stress test and 14 of these patients (out of 16 patients) had an FFR value 0.80 or more and only two patients had an FFR value less than 0.80.
Table 5 Sensitivity, specificity, and predictive values offractional flow reserve in relation to a stress test

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Accordingly, the FFR value has a sensitivity and specificity of 85.7 and 87.5%, respectively, in detecting the result of a noninvasive stress test with a positive predictive value (PPV) and negative predictive value (NPV) of 85.7 and 87.5%, respectively, with an accuracy of 86.7%.

[Table 6] shows the sensitivity, specificity, and predictive value of multiple cutoff values for MLA obtained by IVUS in relation to FFR values [Figure 4]. From the table, it is clear that the best cutoff value for MLA that is concordant with FFR values is 4.5 mm 2 , which has a sensitivity and a specificity of 57.1 and 56.3%, respectively, and PPV of 53.3% and NPV of 60% [Figure 4].
Figure 4: Receiver operating characteristic curve for the detection of the best cutoff value of minimal luminal area in relation to the fractional flow reserve value.

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Table 6 Best cutoff level of minimal luminal area in relation to a fractional flow reserve test

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[Table 7] shows the sensitivity, specificity, and predictive value of multiple cutoff values for MLA obtained by IVUS in relation to a noninvasive stress test. It is clear that the best cutoff value for MLA that is concordant with a noninvasive stress test is 4.5 mm 2 , which has a sensitivity and a specificity of 64.3 and 62.5%, respectively, and a PPV of 60% and NPV of 66.7% [Figure 5].
Figure 5: Receiver operating characteristic curve for the detection of the best cutoff level of minimal luminal area in relation to a stress test

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Table 7 Best cutoff level of minimal luminal area in relation to a stress test

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  Discussion Top


Decisions on performing a coronary intervention should be made on the basis of objective evidence of myocardial ischemia. Even in the DES era, every coronary intervention carries the risk of procedural-associated complications, late stent thrombosis, and restenosis [7] .

Angiography is inaccurate in assessing the functional significance of a coronary stenosis compared with FFR in intermediate lesions (4-70%) [8] . This study supports the concept that FFR reliably indicates functionally significant coronary stenosis compared with noninvasive stress tests. In the present study, 14 out of 30 patients had an FFR value less than 0.80; 12 of these patients already had a positive noninvasive stress test for myocardial ischemia and only two patients had a negative stress test despite the FFR value being less than 0.80. This may be attributed to the fact that a noninvasive stress test may be inconclusive in some patients as the sensitivity and specificity of SPECT for the diagnosis of coronary stenosis (defined as>50% stenosis) is 86 and 74%, respectively [9] . A false-negative study may be a feature of three-vessel and left main stem disease. This is because of the balanced ischemia phenomenon as in those with multivessel disease. False-positive tests because of attenuation artifacts lower the specificity [10] .

It was evident in our study that there was a statistically significant relation between the FFR value and the results of a noninvasive stress test as FFR has a sensitivity and specificity of 85.7 and 87.5%, respectively, in predicting the result of a noninvasive stress test, whether positive or negative, with a PPV and NPV of 85.7 and 87.5%, respectively, and a diagnostic accuracy of 86.7%.

The high PPV and NPV of FFR in relation to a noninvasive stress test made it a powerful diagnostic tool for the diagnosis of functionally significant coronary artery stenosis that can induce myocardial ischemia. In agreement with our study, Ramaiaha et al. [11] published a recent study carried out on 20 patients with angiographically intermediate coronary stenosis. Stress tests were compared with the results of FFR; in all 13 patients with an FFR of less than 0.75, reversible myocardial ischemia was found. In contrast, five out of seven patients with an FFR of more than 0.75 showed a negative noninvasive stress test for reversible myocardial ischemia. No revascularization procedures (PCI) were performed in seven (35%) patients who already had an FFR value of more than 0.75 [12] .

Our study showed a statistically highly significant relation (P = 0.0001) between the FFR value and PCI as all cases with FFR value less than 0.80 (14 cases) had already undergone PCI and those with an FFR value 0.80 or more had not undergone PCI; there was also a highly statistically significant relation (P = 0.0001) between the results of the noninvasive stress test and undergoing PCI as 12 patients (out of 14 patients with a positive stress test) had already undergone PCI, whereas only two patients (out of 16 patients with a negative stress test) underwent PCI; thus, in patients with coronary stenosis of intermediate severity, FFR appears to be a useful diagnostic index equivalent to objective noninvasive stress tests to assess the functional severity of coronary stenosis and thus the eligibility for coronary revascularization.

In terms of demographic data and coronary risk factors, there was no statistically significant difference among the population, except for diabetes mellitus, as 11 out of 14 patients with an FFR value less than 0.8 were diabetic; also, there was a highly significant negative correlation between the FFR value and diabetes mellitus (fasting blood sugar) (r = −0.65, P = 0.001), indicating the impact of glycemic state on the FFR value, and this is in agreement with a clinical research carried out by Tonino et al. [13] that showed that intermediate lesions from diabetics versus non diabetics differed significantly in both hemodynamic relevance (FFR ≤ 0.80, 37.7 vs. 24.2%, P = 0.018) and lesion length (10.91 ± 5.79 vs. 9.23 ± 3.85 mm, P = 0.005).

The use of IVUS to determine the functional significance of coronary artery lesions remains a matter of debate. IVUS measurements performed in this study included MLA and percent of stenosis; unlike the FFR value, there was no statistically significant difference among the population in the MLAs compared with the results of a noninvasive stress test. Our study showed that the best cutoff value for MLA that corresponds to the results of a noninvasive stress test and may enable discrimination of functionally significant coronary artery stenosis was an MLA less than 4.5 mm 2 with a sensitivity and a specificity of 64.3 and 62.5%, respectively, and PPV of 60% and NPV of 66.7%. In the current study, the suggested MLA cutoff value had a low PVP and NPV for clinical decision making. In addition, the agreement in the result between MLA and myocardial SPECT was poor. Thus, MLA less than 4.5 mm 2 is considered a weak diagnostic tool in relation to a noninvasive stress test for the diagnosis of functionally significant myocardial ischemia.

Despite the lack of data on this topic, Ahn et al. [14] published a paper that evaluated the ability of MLA measured by IVUS to assess the functional significance of coronary stenosis. The best cutoff value of MLA was 2.1 mm 2 or less with a sensitivity of 86.7%, a specificity of 50.4%, a PPV of 38.6%, and a NPV of 91.3% versus lesions with a positive SPECT [15] .

A previous study evaluated 300 patients with 357 de-novo intermediate native coronary lesions and confirmed the clinical usefulness of IVUS when an intervention was deferred on the basis of MLA more than 4 mm 2 , with an event rate of 4.4% and a target lesion revascularization rate of 2.8% during a mean follow-up of 13 months [16] .

Although MLA is an important factor to determine the coronary blood flow, other anatomical factors measured by IVUS also act as a resistance to the flow. Therefore, a single parameter such as IVUS MLA may have limitations in predicting the result of myocardial SPECT. In our study, it was found that an MLA less than 4.5 mm 2 has a sensitivity of 57.1% and a specificity of 56.3% in relation to an FFR value less than 0.80 (every 100 patients with FFR value<0.80 and 57 patients will have an MLA<4.5 mm 2 ); thus, MLA less than 4.5 mm 2 is considered a weak diagnostic tool in relation to FFR for the diagnosis of functional myocardial ischemia (sensitivity 57%).

The results of the FIRST study (Fractional Flow Reserve and Intravascular Ultrasound Relationship Study) were presented by Waksman et al. [17] to evaluate the correlation between FFR and IVUS parameters and attempt identification of IVUS predictors of ischemic FFR (defined as FFR < 0.80).

The major differences between our study and FIRST registry were related to, first, the number of patients included in both studies, being only 30 patients in our study and 350 patients (367 lesions) in the FIRST registry. Second, our study is a single-center research, whereas the FIRST trial was a multicenter registry. Third, the accuracy of the FIRST study was improved by a reference vessel-specific analysis unlike our study, which was carried out only on the large epicardial vessels; thus, the best cutoff value obtained in our study was larger than that obtained in the FIRST registry: 4.5 and 3.07 mm 2 , respectively. The results of the FIRST trial will certainly reinforce the view that IVUS is not a reliable alternative for FFR in the functional assessment of angiographically intermediate stenoses. FFR is superior for physiology and IVUS is superior for anatomy [11] .

Koo et al. [18] published a study that aimed to determine the optimal MLA obtained by IVUS and its diagnostic accuracy for defining the functional significance of intermediate coronary stenoses in different locations of the coronary tree among patients with an FFR value already less than 0.80. The best cutoff value of MLA to define the functional significance was 3.0 mm 2 for proximal left anterior descending artery lesions and 2.75 mm 2 for mid-left anterior descending artery lesions. Thus, when IVUS parameters are used to determine the functional significance of lesions in patients with intermediate coronary artery stenoses, different criteria should be used according to lesion location. In segments or vessels with anatomic variations, IVUS cannot be used for the functional assessment of a stenosis [19] .

The significance of an MLA less than 4.0 mm 2 should be considered in the context of reference vessel size, lesion length, area stenosis, plaque burden, and area of myocardium at risk, whereas FFR is the preferred tool for intermediate lesion assessment [14] .

Study limitations

First, the major limitation of this study is the number of patients included in this study, which is too small to make definitive conclusions, and this issue was obvious for the cutoff value of MLA corresponding to functional myocardial ischemia.

Second, the study was carried out mainly on large epicardial vessels and did not include small branches - for example, diagonal and obtuse marginal branches. It included only one case performed on the left main coronary artery. Thus, the cutoff value of MLA proposed by our study is not sufficient and larger than that in other studies.

Third, hyperemia induced by adenosine in the catheterization laboratory is not necessarily equivalent to exercise-induced maximal hyperemia in daily life. This mechanism could have played a role in patients with positive exercise tests, but FFR values of 0.80 or higher.

Fourth, the possibility of catheter-induced spasm during the introduction of an IVUS probe could not be completely excluded particularly in small-caliber vessels.

Fifth, the cutoff points of the minimum lumen diameter suggested in our study cannot be applied to lesions with multiple stenoses.

Sixth, the cutoff points of the MLA proposed in our study cannot be applied on its own as an accurate diagnostic tool for assessment of the severity of coronary artery stenosis in relation to the FFR value and a noninvasive stress test as other parameters such as lesion length and plaque burden should be incorporated to improve diagnostic accuracy.

Finally, clinical outcomes could not be evaluated in this study. The outcomes of an FFR-guided versus an IVUS-guided revascularization strategy need to be evaluated in future randomized trials.


  Conclusion Top


Measurement of FFR during coronary angiography is useful in determining whether intermediate stenosis is functionally important and may therefore be responsible for reversible myocardial ischemia. In this study, the accuracy of FFR for this purpose was equivalent to that of the information provided by the noninvasive tests. Therefore, it is believed that FFR may be useful in clinical decisions on revascularization in patients with intermediate stenosis when objective evidence of reversible ischemia is lacking.[21]


  Acknowledgements Top


Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
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  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]



 

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