Coronary Flow Velocity Pattern and Coronary Flow Reserve by Contrast-Enhanced Transthoracic Echocardiography Predict Long-Term Outcome in Heart Transplantation
Background— We assessed coronary flow velocity pattern and coronary flow reserve (CFR) by contrast-enhanced transthoracic echocardiography (CE-TTE) as markers of major adverse cardiac events (MACE) related to cardiac allograft vasculopathy (CAV) after heart transplantation (HT).
Methods and Results— Deceleration time of diastolic flow velocity (DDT) and CFR were measured in the left anterior descending coronary artery (LAD) by CE-TTE in 66 consecutive HT patients (follow-up 19±5 months). CFR was calculated as the ratio of hyperemic to basal diastolic flow velocity. Angiographies were analyzed by a qualitative grading system; CAV was defined as changes grade II or higher. MACE were cardiac death, stent implantation, and heart failure. Patients with MACE had higher CAV incidence (P=0.004) and grade (P=0.008), shorter DDT (P=0.006), and lower CFR (P=0.008). A receiver-operating characteristic–derived DDT cutpoint ≤840 ms (area under the curve 0.793; P=0.01) was 75% specific and 86% sensitive for predicting MACE, with positive predictive value (PPV) and negative predictive value (NPV) of 33% and 97%, respectively (P=0.002). A CFR cutpoint of ≤2.6 (area under the curve 0.746; P=0.01) was 62% specific and 91% sensitive for predicting MACE (PPV =32%, NPV =97%) (P=0.001). Patients with CFR ≤2.6 and patients with DDT ≤840 ms had a lower survival free from MACE (P=0.006 and P=0.009, respectively). By Cox regression, only a lower CFR predicted the risk of MACE (relative risk 3.1; 95% CI, 1.26 to 7.9; P=0.01).
Conclusions— In HT patients, shorter DDT and lower CFR by CE-TTE are reliable markers for CAV-related MACE. CFR is the main independent predictor of MACE.
Cardiac allograft vasculopathy (CAV) is the main limiting factor of long-term survival in heart transplantation (HT).1 In CAV both epicardial coronary vessels and the microvasculature may be affected.2 CAV diagnosis is based on standard coronary angiography, which underestimates incidence and severity of the disease.3 Coronary flow reserve (CFR) measurements by intracoronary Doppler flow wire may provide functional assessment of the microvasculature in patients with CAV, and intravascular ultrasound (IVUS) appears to be a reliable marker for CAV-related major adverse cardiac events (MACE),4,5 but these techniques are invasive, expensive, and time-consuming.3 Recently, noninvasive evaluation of myocardial perfusion reserve and of transmural perfusion gradient by magnetic resonance perfusion imaging has been proposed to identify significant CAV;6 moreover, new multislice computed tomography technology allows direct noninvasive visualization of the anatomy of coronary vessels, including the vessel wall and lumen, in HT patients.7 However, prognostic implications of these new technologies are lacking. We have recently applied a new noninvasive technique based on contrast-enhanced transthoracic echocardiography (CE-TTE) for assessing CFR in the left anterior coronary descending artery (LAD) in HT patients.8,9 CFR by CE-TTE has been shown to correlate with angiographically detectable coronary artery lesion severity as well as intracoronary Doppler flow wire measurements.10 However, there is no clear noninvasive marker of outcome in HT. We assessed the validity of coronary flow velocity pattern and CFR by CE-TTE as potential noninvasive markers of CAV-related MACE.
Between January 2003 and March 2004, we studied 66 consecutive HT recipients (54 men, aged 49±12 years, range 16 to 70, mean ischemia time 174±41 minutes). Mean post-HT follow-up at study entry was 9±4 years (range, 13 months to 17 years). Mean post–CE-TTE evaluation follow-up was 19±5 months. Our immunosuppression protocol consisted of Cyclosporin A, Azathioprine, and steroids (triple therapy), as previously detailed.2,11 The study was approved by the institutional ethics committee, and all patients gave written informed consent.
Acute Rejection Scores
Acute graft rejection was monitored by endomyocardial biopsy after established protocols.12 No biopsies were performed after the first year unless clinically indicated to rule out acute rejection. A rejection score was assigned based on a modification of the ISHLT grading as detailed.2 The following scores were calculated for each patient: rejection score (RS) in the total follow-up; RS in the first year; RS including only severe grades (≥3A); first-year RS including only severe grades. All scores were normalized for the number of biopsies taken in each patient.
Angiography/Diagnosis of CAV
Cardiac catheterization was performed within 24 hours of CFR evaluation by CE-TTE. Angiograms were reviewed by a cardiologist who was unaware of clinical and echocardiographic findings. Data were analyzed using a qualitative grading system: grade I, normal angiogram; grade II, luminal irregularities and/or diameter reduction <30%; grade III, diameter reduction <50%; grade IV, diameter reduction ≥50% and/or diffuse narrowing of small vessels.13 CAV was defined as angiographic changes of grade II or greater, significant CAV was defined as grade IV angiographic changes.
In an attempt to better-quantify the extent of CAV, the coronary tree was subdivided in 17 segments as previously reported.2 We then calculated for each patient’s angiogram a total index of stenosis, summing up the scores of stenoses on all 17 segments: 10% stenosis=1; 20%=2; 30%=3; 40%=4; 50%=5; 60%=6; 70%=7; 80%=8; 90%=9; and 100%=10. The following indexes were computed and used as markers of CAV diffusion/severity: total index of stenosis and total index of stenosis normalized for the number of diseased segments.
Contrast-Enhanced Transthoracic Doppler Echocardiography
Echocardiography was performed for coronary flow evaluation using CE-TTE before and after adenosine infusion, with an ultrasound system (Sequoia C256; Acuson, Mountain View, Calif) connected to a broad-band transducer with second harmonic capability (3V2c). All studies were continuously recorded on 0.5-inch (1.27 cm) S-VHS videotape. Briefly, CFR was measured in the distal portion of the LAD, first obtaining a modified foreshortened 2-chamber view or, if a distal LAD flow recording was not feasible, using a low parasternal short-axis view of the base of the heart8 (Figure 1A). If the angle between color flow and the Doppler beam was >20°, angle correction was performed using the software package included in the software unit. Administration of the contrast agent (Levovist; Schering AG, Berlin, Germany) was performed both before and during adenosine intravenous administration.10
Coronary Flow Velocity Reserve Assessment
All patients had Doppler recordings of the LAD with adenosine infusion at a rate of 0.14 mg/kg per minute for 5 minutes.10 Cardiac drugs were not interrupted before testing, although all methylxanthine-containing substances or medications were withheld 48 hours before the study. The deceleration time of basal diastolic velocity (DDT) was measured from the peak diastolic velocity along the decline in initial velocity and extrapolated to the baseline (Figure 1B, left panel). CFR in the LAD was calculated as the ratio of hyperemic to basal diastolic flow velocity by one experienced echocardiographer blind to angiographic and clinical data (Figure 1B). For each variable in the CFR calculation, the highest 3 cycles were averaged.10
An independent investigator blinded to coronary flow measurements carefully reviewed MACE. MACE included death, myocardial infarction, need for percutaneous cardiac intervention, and congestive heart failure requiring hospital admission. The decision to perform interventions was based on angiography, without knowledge of CFR results.
Continuous variables are expressed as mean±standard deviation. Student t test for independent or paired samples, χ2 test, and Fisher exact test were used as appropriate. Sensitivity (Se), specificity (Sp), positive predictive value (PPV) and negative predictive value (NPV) were determined according to standard definitions. Evidence of MACE was taken as the positive reference standard and receiver-operating characteristics (ROC) curve analysis was generated. MACE-free survival curves were traced by use of the Kaplan-Meier method and compared by log-rank test. In the multivariate analysis we considered 2 blocks, echocardiographic/angiographic and clinical. Manual Cox regression model with backward elimination was performed on blocks of variables until regression models with only significant or marginally significant (P<0.1) variables were obtained; then, we evaluated the independent predictive value of selected covariates. Intraobserver and interobserver reproducibility of CFR were evaluated by linear regression analysis and expressed as correlation of coefficients (r) and standard error of estimates (SEE) by the Bland-Altman method and the intraclass correlation coefficient. Reproducibility is considered satisfactory if the intraclass correlation coefficient is between 0.81 and 1.0. Data were analyzed with SPSS software version 12.0 (Chicago; SPSS, Inc, Chicago, Ill). P<0.05 was considered to be significant. The authors had full access to the data and take full responsibility for its integrity. All authors have read and agree to the manuscript as written.
Baseline Clinical and Diagnostic Features
MACE occurred in 11 patients (17%) (1 death, 4 CHF, and 6 percutaneous cardiac interventions). The baseline features of recipients and donors in patients with and without events are shown in Table 1. A higher proportion of patients with MACE were on triple therapy. CAV was more frequent and CAV grade was higher in patients with MACE. The remaining features were similar in the 2 groups. Echocardiographic regional wall motion abnormalities were never detected. Thirty-four (51%) angiograms were classified as abnormal, of which 9 (26%) had grade II lesions, 4 (12%) grade III, and 21 (62%) grade IV. In the whole patient group the mean number of diseased traits, TSI, and TSI/nS were 3.6±2.3, 14.3±12, and 6.9±5, respectively.
Coronary Flow Velocity Pattern and CFR Evaluation
CE-TTE studies were always well tolerated. Overall, during adenosine infusion heart rate increased compared with baseline (91±12 bpm versus 84±10 bpm; P<0.0001), systolic blood pressure decreased (125±22 mm Hg versus 135±18 mm Hg; P<0.0001), as well as diastolic blood pressure (79±14 mm Hg versus 85±12 mm Hg; P<0.0001), whereas peak diastolic velocity in the LAD increased (76±24 cm/s versus 29±10 cm/s; P<0.0001). In the whole patient group, DDT was 947±251 ms and CFR 2.74±0.79. DDT was shorter and CFR lower in patients with MACE (Table 2).
Relation Between DDT and MACE
The ability of DDT to predict MACE was assessed by ROC curve analysis. As shown in Figure 1 C (left panel), a shorter DDT was associated with lower MACE-free survival (P=0.009). The area under the curve was 0.793 with a standard error of 0.075, yielding a 95% confidence interval (CI) of 0.645 to 0.941 (P=0.01). A DDT cutpoint of ≤840 ms, identified as optimal by ROC analysis, was 75% specific and 86% sensitive (PPV=33%, NPV=97%) (P=0.002); accuracy was 76%.
Relation Between CFR by CE-TTE and MACE
As shown in Figure 1C (right panel) lower CFR was associated with lower MACE-free survival (P=0.006). The area under the curve was 0.746 with a SE of 0.079, yielding a 95% CI of 0.613 to 0.901 (P=0.01). The CFR value of ≤2.6, identified as optimal by ROC analysis, was 62% specific and 91% sensitive, with PPV and NPV of 32% and 97%, respectively (P=0.001); accuracy was 69%.
Determinants of Event-Free Survival
CAV, CAV grade, basal diastolic peak velocity, DDT, and CFR were significant in the univariate analysis (Table 3). The relation of MACE to echocardiographic and angiographic variables was evaluated in the first block of multivariate analysis (Table 4). Echo ejection fraction (P=0.1), basal diastolic peak velocity (P=0.09), DDT (P=0.7), CAV (P=0.6), CAV grade (P=0.1), and total index of stenosis (P=0.5) dropped out, leaving CFR (P=0.02) as the only significant independent predictor of MACE. The relation of MACE to clinical variables was investigated in the second block of multivariate analysis (Table 5). The variables included in the analysis were those marginally significant in the univariate analysis (P<0.1); some variables were forced in the multivariate model because of their well known predictive value on clinical outcome in HT or their documented effect on CFR. Only triple therapy was an independent clinical predictor of MACE (P=0.02); donor age (P=0.08) and ischemic heart disease before HT (P=0.07) were marginally significant. In the final model, the relation of MACE to CFR, adjusted for clinical and echocardiographic covariates, was evaluated (Table 6). Only CFR remains independently related to MACE (RR, 3.1; 95% CI, 1.26 to 7.9; χ2 6.1; P=0.01).
Intraobserver and Interobserver Reproducibility
Intraobserver reproducibility was high (r=0.98, SEE =0.12); the mean difference was −0.02 and the upper and lower limits of agreement between the measurements were +0.14 (95% CI, +0.08 to +0.2) and −0.19 (95% CI, −0.26 to −0.13), respectively (Figure 2A); intraclass correlation coefficient was 0.986. Interobserver reproducibility was also high (r=0.96, SEE =0.18); the mean difference was 0.01 and the upper and lower limits of agreement between the 2 measurements were +0.36 (95% CI, +0.26 to +0.45) and −0.33 (95% CI, −0.43 to −0.23), respectively (Figure 2B); intraclass correlation coefficient was 0.966.
This study validates, for the first time to our knowledge, the use of coronary flow velocity pattern and CFR by CE-TTE as noninvasive markers for CAV related MACE in long-term clinically stable HT patients. The major finding of the present study was the ability of CFR to predict MACE-free survival. Multivariate analysis demonstrates that CFR evaluation is superior to standard angiography in CAV-related risk stratification.
Allograft heart disease is a diffuse coronary process involving the entire coronary circulation, including microvessels.14 IVUS has been shown to be more sensitive than angiography;15 in addition, a recent study suggests that progression of maximal intimal thickening ≥0.5 mm in the first year after HT appears to be a reliable marker for subsequent outcome.4 However IVUS is often technically difficult or not feasible in advanced disease and is certainly unsuited for long-term follow-up. Our patients had a mean follow-up of 8 years at study entry; at our Institution, as well as in the majority of Transplant Centers, IVUS is not routinely used for CAV monitoring because of its invasiveness, technical limitations, increased risks, and costs.16 In keeping with this, in the recent multicenter Everolimus study to which our Center participated, satisfactory IVUS measurements in the LAD, taken immediately after transplantation and at 1-year follow-up, could be obtained in only 33% of the 634 study patients.16 Moreover, standard coronary angiography and IVUS do not provide functional assessment of the microvasculature and represent invasive, expensive, and time-consuming procedures.14
Conversely, the new noninvasive technique for assessing CFR, applied here, provides a relatively simple, readily available, objective, rapid, and repeatable noninvasive diagnostic tool for CAV detection.9 Thus, our data provide a rationale for including coronary flow velocity pattern and CFR by CE-TTE as noninvasive tools in future clinical trials aimed at assessing new immunosuppressive and anti-atherosclerotic agents for CAV prevention or stabilization.15
In keeping with our study, Hollenberg et al, using invasive Doppler flow wire, reported the relationship between abnormal microvascular response and the clinical end points of angiographically significant CAV, ischemic events, and death. In their study, comparable to the present work in terms of patients’ number, incidence of events, and follow-up, the group with an end point had decreased epicardial and microvascular endothelium-dependent responses to acetylcholine.17
Another finding of the present study was that DDT cutpoint of ≤840 ms was 74% specific and 86% sensitive in identifying future MACE. CE-TTE allows the measurement of the coronary blood flow velocity pattern with high reliability.18 These are the first data to our knowledge on DDT in HT recipients and no comparison can be made with previous studies. However, rapid DDT (≤600 ms) after successful angioplasty, related to the no-reflow phenomenon and microvascular damage, predicted the risk of long-term cardiac events after acute myocardial infarction19.
The measurement of coronary flow velocity by CE-TTE is only applicable to the distal part of the LAD, although a recent study reported the feasibility of measuring CFR in the posterior descending coronary artery.20 Good correlations with invasive intracoronary Doppler flow wire measurements have been shown both in LAD21 and right coronary artery.22 However, the feasibility of CE-TTE–derived CFR is higher in the LAD (80% to 98%) than in the right coronary (50% to 87%) or in the circumflex (43% to 72%) artery.21 In addition CE-TTE–derived CFR measurements relate to the microcirculatory function, thus the choice of the sample vessel does not affect results.21 Thus, in this noninvasive study we, as other groups who used invasive Doppler flow wire measurements in HT patients,12 sampled the LAD. In keeping with this, CFR measurements were successfully achieved in the absence of contrast agents in 80% of our patients. With further advances in ultrasound technology, measurements of DDT and CFR in other coronary arteries may become possible.
The prognostic value may be limited by the fact that many MACE were coronary interventions. The number of hard events was low, as in other studies.13 Moreover, the limited number of events available for analysis, and the fact that measured and/or unmeasured confounders that cannot be adjusted for simultaneously in a multivariable model may alter the results obtained. Nevertheless, our data represent the largest CE-TTE study after HT reported to date.
Shorter DDT and lower CFR were noninvasive markers for CAV-related MACE. Lower CFR was the main independent predictor of poor outcome in long-term clinically stable HT patients. These results are sufficiently encouraging. However, further evaluation is warranted to provide a definitive answer regarding the robustness of the thresholds found using CE-TTE measures. It would be useful to apply these thresholds to other datasets to understand the external validity of our findings.
Presented at the American Heart Association Scientific Sessions, Dallas, Tex, November 13–16, 2005.
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