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Reduction in Myocardial Infarct Size by Postconditioning in
Patients after Percutaneous Coronary Intervention
Timely reperfusion is the most effective intervention to protect the heart from myocardial infarction resulting from coronary occlusion. However, experimental studies and clinical observations have provided strong evidence in support of the existence of reperfusion-induced myocardial injury. Reperfusion injury initiates a series of adverse events that offset the benefits intended by implementing early reperfusion.1–4 Protection elicited by pharmacological treatment applied briefly at the onset of reperfusion may be observed acutely, but may not be evident when the duration of reperfusion is extended.5,6 Therefore, it is valuable to explore some clinically feasible, applicable and effective therapeutic strategies that can address postischemic injury, particularly after long periods of reperfusion. Postconditioning, defined as a sequence of repetitive interruptions of coronary blood flow applied during the early moments of reperfusion after an index ischemia7 has been shown to attenuate reperfusion-induced myocardial tissue injury and to exert cardioprotection that is comparable to that observed with conventional preconditioning.8
Postconditioning has now been confirmed to reduce infarct size in the in vivo and in vitro models from different animal species after a short or prolonged period of reperfusion.9,10–22 A recent clinical study reported that postconditioning with 4 cycles of 1-minute reinflation followed by 1-minute deflation of the angioplasty balloon at the onset of reperfusion in patients reduces enzymatic infarct size estimated from the area under the plasma creatine-kinase curve during early reperfusion.23 However, this study did not provide direct evidence to show whether the reduction in infarct size by postconditioning is maintained after longer periods of reperfusion. In addition, no study has determined if postconditioning improves postischemic left ventricular performance. Therefore, the object of the present study was to determine whether application of postconditioning during PCI enhances global left ventricular function and reduces infarct size measured by nuclear imaging in patients after 7 days of reperfusion.
Methods
The study protocol was approved by the ethics committee of Beijing Chaoyang Hospital of Capital University of Medical Science. Informed consent was obtained before patient enrollment and participation.
Selection of patients for study enrollment. Patients whose ST-segment elevation was > 0.1 mV in at least two contiguous electrocardiographic leads from the onset of chest pain were accepted for enrollment in this study. Patients were excluded from enrollment based on the following criteria: (1) cardiogenic shock; (2) left main coronary artery occlusion or severe stenosis; (3) blood flow in the infarctrelated artery (IRA) ≥ Thrombolysis in Myocardial Infarction (TIMI) grade 1; (4) obvious coronary collaterals to therisk region evidenced by Rentrop grade ≥ 1; and (5) previous myocardial infarction; (6) treatment with glycoprotein IIb/IIIa receptor antagonists before the procedure.
Coronary angiography and experimental protocol. All patients were physically examined at admission and premedicated with clopidogrel (300–600 mg) and aspirin (300 mg) before catheterization. Coronary angiography was performed using the standard Seldinger technique to demonstrate the absence of coronary collaterals in the risk region and locate the site of occlusion in the IRA before the protocol was applied. The data were obtained in a prospective and randomized manner. The patients were assigned either to the Control group (n = 18) or the Postconditioning group (n = 23). In all patients, an angioplasty balloon without a stent was inserted into the coronary artery for one cycle of instant inflation and deflation with low pressure (2–4 atm) to avoid potential damage to the IRA. In the Control group, the balloon catheter was withdrawn immediately after the coronary artery was dilated, and no further intervention was applied during the first 3 minutes of reperfusion. The stent was then deployed using a different angioplasty balloon catheter. In the Postconditioning group, the artery was predilated as described, and immediately 3 cycles of 30-second deflation and 30-second inflation of the angioplasty balloon were applied for a total 3 minutes at the onset of reperfusion, as shown in Figure 1. The stent was deployed after the postconditioning protocol was completed using a different angioplasty balloon. In both groups, coronary angiography was performed after PCI to determine coronary TIMI flow and the blush grade to evaluate myocardial perfusion.
Analysis of electrocardiographic (ECG) data. A standard 12-lead ECG tracing was obtained at admission and 2 hours after PCI. A cardiologist who was blinded to the patient group assignment determined the ST-segment shift. The ST-segment change was measured 80 milliseconds after the J point. The resolution of maximal ST-segment elevation was calculated as a percentage using the equation: [(initial sum of ST-segment elevation – the sum of ST-segment elevation 2 hours after PCI) / initial sum of ST-segment elevation]. ST-segment resolution was classified as complete (≥ 70%), partial (< 70% to 30%) or none (< 30%).
Determination of cardiac function by echocardiography. Echocardiographic imaging was recorded during day 1 and day 7 after admission with commercially available systems equipped with harmonic power Doppler (Vivid 5, GE Medical Systems, United Kingdom). The left ventricle was visualized in apical, 4-chamber and long-axis views. Ejection fraction was measured using Simpson’s method.24 The observers quantifying the echocardiographic data were blinded to the patient’s treatment allocation.
Activity of serum creatine kinase (CK) activity during reperfusion. Venous blood samples were collected at baseline when PCI procedures were completed, every 4 hours on day 1, and every 8 hours on days 2 and 3. Serum CK activity was detected using an automated analyzer (Beckman Synchron LX 20) and results were expressed as international units per liter (IU/L). The area under theCK curve during the first 72 hours of reperfusion was plotted in every patient.
Determination of infarct size by single-photon emission computed tomography (SPECT). Myocardial perfusion was assessed in all patients using gated SPECT 1 week after completion of PTCA, as previously described.25 Briefly, patients were given 925 MBq 99mTc-MIBI (Atom Hightech Company, Beijing, China) intravenously at rest. SPECT was performed 30 minutes later with patients in the supine position, using AXIS dual-head rectangular detectors (Philips Medical Systems, the Netherlands) equipped with high-resolution collimators set at 140 Kev energy level, 20% window width, matrix 128 x 128 and amplifying factor 1.33. The heart was scanned over 180º, extending from 45º right anterior oblique to 45º left posterior oblique angles in step-and-shoot mode while gated SPECT images were collected at 16 frames (500 k counts/per frame) in each R-R interval. Short-axis, long-axis and vertical-axis images from every heart were analyzed for radionuclide distribution.26 The left ventricle was divided into 8 segments to calculate the total defect score using the following 4-grade assessment: 0 = normal; 1 = sparseness; 2 = apparent sparseness; 3 = defect. Two experienced radiologists who were blinded to the patients’ treatment allocation assessed all SPECT images and submitted a defect score. The SPECT perfusion defect regions with a score ≥ 2 were considered infarct area. Infarct size was calculated as a ratio of the pixels in the infarct region/pixels in the non-infarct left ventricle, and expressed as a percentage of the left ventricle.
Statistical analysis. The Pearson’s chi-square test was used to compare incidental demographic data between the two groups. The Student’s t-test was used to detect differences between the two groups in the area under the CK curve, coronary TIMI grade flow, blush grade and ST-segment shift. Data are presented as mean ± SEM. A p-value < 0.05 was considered statistically significant.
Results
Characteristics of selected patients. Demographic data for enrolled patients are summarized in Table 1. A total of 41 patients with a mean age of 61 ± 12 years were selected and divided into two groups. No differences in comorbidities, lesion site or culprit vessels were observed on admission between the two groups.
Hemodynamic data after PCI. Hemodynamic data before and after completion of PTCA in the two groups are shown in Figure 2. Heart rate (HR), systolic blood pressure (SBP) and diastolic blood pressure (DBP) 2 hours after completion of the PCI procedure were comparable to the baseline values before PCI.
PCI, ECG analysis and coronary angiography. Coronary dilatation and stent deployment were successfully performed in all patients. As shown in Table 2, there was no difference in the calculated ratio in ST-segment resolution 2 hours after PCI between the Control and Postconditioning groups in each category, i.e., complete resolution 39% versus 44%, partial resolution 44% versus 34%, and no resolution 16% versus 21%, respectively (all p > 0.05 for the Control and Postconditioning groups). At the time when the postconditioning protocol was completed, the number of patients with TIMI grade flow ≥ 2 in the culprit coronary artery was comparable in both groups. The majority of patients achieved TIMI 3 grade blood flow in both groups, however, no group differences were detected. In addition, the blush grade, a marker of myocardial perfusion, was comparable between the two groups. No adverse cardiac events occurred during hospitalization in either group including death, reinfarction, acute heart failure (Killip grade > 2) and target vessel repeat revascularization.
CK activity during the first 72 hours after PCI. As shown in Figure 3, compared to pre-PCI values, CK activity (IU/L) was significantly elevated after the first 8 hours of reperfusion from 1,604 ± 356 to 2,699 ± 634 in the Control group, and from 1,413 ± 288 to 2,229 ± 255 in the Postconditioning group, respectively. Although there was no statistical difference in the CK activity at any individual time point, the area under the curve of serum CK activity during the first 72 hours of reperfusion was significantly less in the Postconditioning group relative to those in the Control group (58,002 ± 593 versus 79,787 ± 681; p = 0.04).
Infarct size by SPECT imaging at 7 days after PCI. To demonstrate whether postconditioning reduces infarct size over time after reperfusion, SPECT scan imaging was performed 7 days after PCI in both groups. Representativeexamples of SPECT images are shown in Figure 4. Fixed SPECT perfusion defects were present in all patients, located in the anterior apical and the neighboring mid-anterior and infero-apical segments, respectively. The myocardial perfusion defect within infarct regions was significantly smaller in the Postconditioning group (22.8 ± 6.7%) versus the Control group (31.3 ± 8.6% of the left ventricle; p < 0.05). The 27% reduction in infarct size (Figure 5) with postconditioning estimated by nuclear imaging is consistent with a similar inhibition in CK activity in the first 72 hours of reperfusion. These data suggest that myocardial salvage achieved by an intervention applied during the early moments of reperfusion persists for at least 1 week.
Ejection fraction detected by echocardiography 7 days after PCI. As shown in Figure 6, no difference in ejection fraction was detected between two groups at day 1. There was a trend toward improvement in cardiac function evidenced by increased ejection fraction 7 days after PCI in the Postconditioning group. Ejection fraction averaged 54 ± 14.5% in the Postconditioning group relative to 44 ± 16.7% in the Control group. However, this did not reach statistical significance at this time point (p = 0.25).
Discussion
The present study is the first to demonstrate that a reduction in infarct size by postconditioning, evaluated by both enzymatic release and SPECT imaging, is still apparent after 7 days of reperfusion. These data are consistent with a previous clinical report23 showing that the application of postconditioning at the onset of reperfusion significantly reduced serum CK activity (area under the curve) after 72 hours of reperfusion. Postconditioning tended to increase postischemic left ventricular function several days after PCI.
To illustrate a reduction in infarct size after prolonged reperfusion in animal studies, Argaud et al have shown that postconditioning reduced infarct size by 58% in rabbits after 72 hours of reperfusion.12 Mykytenko et al also showed a 31% reduction in infarct size by postconditioning in the canine model after 24 hours of reperfusion.27 In the current patient study, postconditioning reduced infarct size by 27%, as detected by SPECT imaging. More importantly, the reduction in infarct size by postconditioning persisted for at least 7 days after reperfusion.
The degree of ST-segment resolution during reperfusion of acute myocardial infarction has been associated with the degree of myocardial perfusion and ultimately with postischemic cardiac function.28–31 In the current study, it is not clear why ST-segment shift and blush grade data in the Postconditioning group were not different relative to previous studies.23,32 It is possible that these variables may be affected by time from onset of reperfusion to data collection. In the current study, the ECG data were recorded 2 hours after PCI, and the blush grade data were collected immediately after PCI. In the study by Laskey,32 a significant improvement in the rate of ST-segment resolution and coronary flow velocity reserve detected by a Doppler-tipped wire were acquired during the “conditioning” treatment periods of balloon inflation and deflation. There may have been significant early hyperemic responses to the intermittent inflations.32 Staat et al demonstrated that postconditioning significantly increased angiographic blush grade immediately after PCI, but had no effect on ST-segment shift after 48 hours of reperfusion.23 Furthermore, the difference between our data and others may be related to differences in the periodicity of the postconditioning algorithm. To date, different cycles of postconditioning have been applied during PCI. In the study by Laskey et al,32 patients were postconditioned by 2 cycles of 90 seconds of uninterrupted balloon inflation separated by 3 to 5 minutes of reperfusion. However, in a study by Staat et al, patients were postconditioned by 4 cycles of 1-minute inflation and 1-minute deflation at the onset of reperfusion.23 In the current study, 30-second inflation and 30-second deflation of the angioplasty balloon was selected as a postconditioning protocol. However, we do not know whether the 30-second postconditioning algorithm is an optimal protocol for the human heart. In the porcine heart, the 30-second algorithm was not effective in reducing infarct size after 4 cycles, but was effective after 8 cycles.17 It is possible that the application of a longer-duration postconditioning algorithm shows a greater improvement in both ST-segment resolution and coronary blood velocity.23,32 Therefore, comparative studies are needed to correlate the outcomes of multiple endpoints (i.e., ST-segment shift, blush grade and infarct size) with different algorithms of postconditioning at fixed time periods of reflow (i.e., short versus longer period of reperfusion).
No significant improvement in ejection fraction between groups was observed at 7 days of reperfusion in both groups. Although a recent study reported that postconditioning does not protect the heart against myocardial stunning in conscious dogs and rabbits,33 the current study demonstrated a trend toward improved left ventricular function. This may be an issue of insufficient power, since ejection fraction was 23% greater in the postconditioning group. Additional patients in both study cohorts would be needed to resolve this issue.
Study Limitations
In the current study, the infarct size was reported as the percentage of left ventricle, not as the area of at-risk myocardium, a major factor that has been demonstrated to influence the infarct size after coronary occlusion in animal studies.34 However, patients were randomly assigned after admission, which may minimize the chance that there were differences in the area of at-risk myocardium between the groups. Another limitation of the current study is the small sample size, which likely limited the power of statistics. The trend toward greater blush grade, TIMI grade flow and complete ST-segment resolution observed in the Postconditioning group would require a larger number of patients in both groups to demonstrate potential significance. Large-scale trials are needed to further confirm the feasibility and efficacy of postconditioning in clinical practice.
Conclusions
Postconditioning by brief interruptions of coronary blood flow at the onset of reperfusion after PCI attenuated CK release and reduced infarct size for up to 7 days after reperfusion in patients with acute myocardial infarction. Since no adverse complications including severe ventricular arrhythmias, cardiac arrest, coronary artery dissection or subacute thrombus were observed during the procedure or in the post-interventional periods in this study, postconditioning represents a safe and effective intervention following PCI for the treatment of ischemic heart disease, and indeed may become an effective therapy for patients with acute myocardial infarction. In addition, this study added to the previous clinical studies23,32 supporting reperfusion as a clinical entity that is amenable to reperfusion therapies.
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