Clinical Characteristics and Reperfusion Times Among Patients With an Isolated Posterior Myocardial Infarction

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Author(s): 

Stephen W. Waldo, MD1, Ehrin J. Armstrong, MD, MSc2, Ameya Kulkarni, MD3, Kurt S. Hoffmayer, MD1, Priscilla Hsue, MD1, Peter Ganz, MD1, James M. McCabe, MD4

Abstract: Background. An isolated posterior myocardial infarction (PMI) is associated with significant morbidity and mortality. Because physicians often fail to recognize this diagnosis, there may be delays in the timely revascularization of these patients. The present study sought to identify the clinical characteristics and reperfusion times among patients presenting with isolated PMI. Methods. We identified subjects with isolated PMI within a registry of all catheterization laboratory activations for ST-elevation myocardial infarction (STEMI) from 2008 to 2012. Association between PMI and revascularization within 90 minutes was evaluated by logistic regression. Results. Among 318 patients who underwent revascularization for STEMI, a total of 20 (6%) had electrocardiographic evidence of an isolated PMI. Compared to non-PMI STEMI, subjects with PMI were more often female (45% vs 22%; P=.02) and less likely to have chest pain (40% vs 75%; P<.01). The median door-to-activation (25.5 min vs 12 min; P<.01), activation-to-laboratory (36.5 min vs 29 min; P<.01) and door-to-balloon times (107 min vs 72 min; P<.01) were longer among subjects with PMI, with fewer patients achieving reperfusion within 90 minutes (30% vs 71%; P<.01). After multivariable adjustment, individuals with PMI had 82% lower odds (adjusted odds ratio, 0.18; 95% confidence interval, 0.06-0.50) of achieving coronary reperfusion within 90 minutes. Door-to-activation time accounted for 96% of variation in the total revascularization time (R2=0.96; P<.0001). Conclusions. Door-to-activation time was prolonged for those with PMI, resulting in longer door-to-balloon times and fewer patients revascularized within the recommended time. An isolated PMI should be considered among individuals presenting with symptoms consistent with myocardial infarction.

J INVASIVE CARDIOL 2013;25(8):371-375

Key words: acute coronary syndromes, posterior myocardial infarction, reperfusion time

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Contemporary guidelines indicate that a 12-lead electrocardiogram (ECG) should be performed in all patients that present with chest discomfort.1 Patients with ECGs that demonstrate evidence of contiguous ST-elevation suggestive of an ST-elevation myocardial infarction (STEMI) should subsequently undergo immediate revascularization with intravenous thrombolytics or percutaneous coronary intervention. Consistent with professional society guidelines, patients without ST elevation on ECG are initially managed with medications and do not necessarily undergo immediate revascularization.2 The current guidelines make two exceptions to this treatment algorithm: the presence of a new left bundle branch block or anterior ST depression consistent with an isolated posterior myocardial infarction (PMI).1,3 Previous research has demonstrated that a new left bundle branch block is associated with a significant delay in revascularization when compared to others with STEMI.4 Similar studies evaluating patients with an isolated PMI have not yet been performed.

An isolated PMI is a relatively common entity accounting for up to 21% of all patients presenting with transmural myocardial infarctions.5-8 Previous research has demonstrated that an isolated PMI may be associated with a high mortality rate, approaching that seen with an anterior STEMI.9,10 The morbidity and mortality associated with posterior myocardial ischemia significantly improves with prompt reperfusion therapies.11 Because of its anatomic location, though, the lack of typical ST elevation makes an isolated PMI difficult to diagnose on a standard 12-lead electrocardiogram.12 Studies have thus demonstrated that physicians fail to recognize this condition and obtain the appropriate confirmatory testing in over 50% of cases.12 Because of this, there may be significant delays in the timely revascularization of these patients. With this in mind, the present study sought to identify the clinical characteristics and reperfusion times among patients presenting with an isolated PMI.

Methods

Study population. All patients presenting to an urban trauma center (San Francisco General Hospital) or tertiary-care center (University of California, San Francisco Medical Center) referred for emergent angiography for a potential STEMI were enrolled in the ACTIVATE-SF Registry. As described previously, this registry includes all emergency-physician initiated cardiac catheterization laboratory activations from October 2008 through July 2012.13 The current analysis focused on patients within this registry who had an angiographic culprit lesion and underwent percutaneous coronary intervention. The present project has been reviewed and approved with a waiver of consent by the institutional review board at the University of California, San Francisco.

Clinical data. Clinical information was collected from ambulance and emergency department records obtained upon presentation. ECGs from the initial presentation were de-identified and independently evaluated by two cardiologists (EJA and KSH) blinded to the clinical outcome. In cases of disagreement, a third blinded cardiologist (JMM) adjudicated the ECG findings. An isolated PMI was defined as 1.0 mm of horizontal ST depression in two contiguous precordial leads (V1-3) associated with prominent R-waves, in accordance with professional society guidelines.14 Reperfusion times were ascertained through emergency department and cardiac catheterization laboratory records. Laboratory values and echocardiographic data obtained later in the hospital course were retrieved from the electronic medical record. All study data were collected and managed using the Research Electronic Data Capture (REDCap) reporting system hosted at the University of California, San Francisco.15

Statistical analysis. Summary statistics are reported as mean with standard deviation (SD) or median and interquartile range (IQR) for normally and non-normally distributed continuous data, respectively. Simple comparisons were performed with t-tests and Kruskal-Wallis ANOVA by ranks for continuous variables or Fisher’s exact and chi-square tests for differences in proportions. Directed acyclic graphs were created to identify covariates that could confound the relationship between a PMI and reperfusion time as described previously.16 Logistic regression models were subsequently created to assess the odds of achieving the target reperfusion time after adjusting for the presence of an anginal chief complaint (chest pain, chest pressure, dyspnea), emergency medical services utilization and ECG evidence of an isolated PMI. Linear regression was performed to determine the amount of variability in the overall revascularization time that was accounted for by each component of door-to-balloon time (door-to-activation, activation-to-laboratory). All statistical analyses were performed using STATA 12 (STATA Corporation). A P-value <.05 was considered statistically significant. All authors take full responsibility for the integrity of the data and agree to the manuscript as written.

Results

Population. A total of 318 patients underwent percutaneous coronary intervention for STEMI between October 2008 and July 2012. Among them, twenty (6%) had an isolated PMI based on typical ECG changes. The clinical characteristics of patients with an isolated PMI and those with another STEMI are presented in Table 1. As shown, subjects with ECG evidence of an isolated PMI were more likely to be female (P=.02) and were less likely to present with chest pain (P<.01). As shown in Table 2, the magnitude of the maximum ST deviation was similar between the two groups (P=.61). 

Reperfusion times. The median reperfusion times for both isolated PMIs and non-PMI STEMIs are presented in Table 3. The median door-to-balloon time was longer for those with an isolated PMI (P<.01), resulting in fewer patients achieving reperfusion within the 90-minute target (P<.01). Among the components of door-to-balloon time, the time from arrival to the emergency department to catheterization laboratory activation (P<.01) and laboratory activation to catheterization laboratory arrival (P<.01) were significantly longer among patients with a PMI (P<.01). Figure 1, which demonstrates the cumulative percentage of patients receiving reperfusion per unit time stratified by PMI, demonstrates consistently longer reperfusion times among isolated PMI patients. After multivariable adjustment for an anginal chief complaint (chest pain, chest pressure, dyspnea) or utilization of emergency medical services, ECG evidence of an isolated PMI was associated with an 82% lower odds (adjusted odds ratio [AOR], 0.18; 95% confidence interval [CI], 0.06-0.50) of achieving restoration of coronary blood flow in the guideline-recommended 90 minutes when compared to those with other STEMI. The variability in the door-to-activation time accounted for 96% of the variation in the overall revascularization time (R2 = 0.96; P<.0001) among patients with PMI. In comparison, the addition of activation-to-laboratory time to the model accounted for only an additional 1% (R2 = 0.97; P<.0001) of the variation in the overall revascularization time for the same population. 

Clinical outcomes. As shown in Table 4, patients with an isolated PMI had a higher proportion of culprit lesions in the left circumflex artery (P<.01). The in-hospital outcomes for the two populations are presented in Table 5. As shown, the length of stay was longer for patients with a PMI (P=.05). The in-hospital mortality, however, was similar between the two groups (P=.62), though the modest overall numbers of isolated PMI likely limits our ability to detect differences in mortality.

Discussion

The present study evaluates the clinical characteristics and reperfusion times among patients presenting with an isolated PMI. To our knowledge, this is the first time that reperfusion time and its components have been reported in this population. As the data demonstrate, subjects with ECG evidence of an isolated PMI are more likely to be female and less likely to present with chest pain in our cohort. The door-to-activation and door-to-balloon times among those with a PMI are significantly longer than for those with STEMI in other anatomic locations and thus fewer patients reach the guideline-recommended door-to-balloon time of 90 minutes. These data collectively highlight the need to rapidly recognize an isolated PMI in order to achieve prompt coronary reperfusion.

Posterolateral myocardial ischemia is a common clinical condition that remains challenging to diagnose. Previous research has demonstrated that isolated PMI constitutes 7%-21% of all STEMIs in the modern era and is primarily attributed to occlusion of the left circumflex coronary artery.5-8 The present study reinforces these findings, since 6% of subjects warranting emergent percutaneous coronary intervention for STEMI presented with ECG evidence of an isolated PMI. Unfortunately, the sensitivity of the standard ECG for the diagnosis of an isolated PMI in clinical practice remains poor, primarily due to the absence of ST-segment elevations on a standard 12-lead ECG.17 When physicians were formally tested, only 44 of 117 patients (38%) properly identified PMI as a potential diagnosis.12 The addition of posterior ECG leads improves the sensitivity of diagnosing this condition, but is not routinely used in clinical practice.8,17-19 Our study shows that the difficulties in diagnosis result in significant delays in the appropriate and immediate revascularization of these patients. 

The current professional society guidelines suggest that patients presenting with an isolated PMI should achieve rapid (<90 minutes) restoration of coronary blood flow.1,3 Systemic thrombolysis (class IIA) or primary percutaneous coronary intervention (class I) can be used to achieve this goal. Previous research has demonstrated that patients presenting with anterior ST depressions are often treated non-emergently, with the median time from electrocardiogram to revascularization reported as 29 hours, raising the possibility that some PMIs may be managed inappropriately as non-ST elevation myocardial infarctions.20 The present study demonstrates that delays in revascularization for patients with an isolated PMI persist even when they are all recognized as STEMIs. The door-to-balloon times in our cohort, for example, were significantly longer among those with PMI. Furthermore, fewer patients with a PMI were reperfused within the guideline-recommended 90 minutes when compared to those with other STEMIs.

The delays in coronary revascularization observed among those with an isolated PMI may be attributed to challenges in recognizing this condition. Previous research has demonstrated that a large proportion of the variation in reperfusion times could be attributed to variation in the time from presentation to activation of the cardiac catheterization laboratory.21 More specifically, the vast majority (>90%) of the variation in door-to-balloon time among all subjects with STEMI in our cohort was due to variability in door-to-activation time. In an analysis of only those with PMI, the variability in the door-to-activation time accounted for 96% of the variation in the door-to-balloon time. In contrast, the variability in the time from catheterization laboratory activation to laboratory arrival accounts for a much smaller amount of the variability in overall revascularization time. This likely reflects delayed recognition of PMI among diagnosing and treating clinicians. Once the infarction is recognized, however, the time from arrival in the catheterization laboratory to percutaneous coronary intervention is similar for those with isolated PMI and those with other STEMI diagnoses. The delayed recognition of PMI has serious clinical consequences and may worsen the morbidity and mortality associated with this condition.22,23

Additional efforts to educate clinicians about the morbidity and mortality associated with an isolated PMI may improve the clinical care of these patients. Previous research has demonstrated that PMIs are associated with significant bradycardia and mitral papillary muscle dysfunction.9,24 Others have shown that the presence of posterolateral ischemia on the ECG is associated with increased mortality among patients presenting with an inferior myocardial infarction, possibly reflecting the larger myocardial territory at risk.10 The present study demonstrates that subjects with an isolated PMI had a longer length of hospital stay, possibly reflecting the potential severity of this illness. The in-hospital mortality among those with an isolated PMI in our cohort was similar to all other subjects with STEMI, including those with anterior infarctions. It is possible that the worse outcomes observed in patients with PMI reflect selection bias for the sickest and most clinically severe subjects with STEMI. Regardless, the potential morbidity and mortality associated with an isolated PMI reinforce the importance of recognizing this condition and ensuring rapid coronary revascularization.

Study limitations. The present study should be interpreted within the context of several limitations. An isolated PMI remains challenging to diagnose on the standard ECG. Thus, it is possible that some subjects with posterolateral injury were not classified as having PMI. Furthermore, some patients with PMI may have been managed as non-ST elevation myocardial infarctions and precluded from the analysis. It is important to note that the inclusion of these misdiagnosed PMI patients would only magnify the observed differences in reperfusion times. Likewise, the modest sample size in the present study may also have limited our power to detect the clinical consequences, such as in-hospital mortality, of delayed reperfusion among PMI patients. Finally, subjects were drawn from two urban tertiary-care medical centers, and thus the findings may not be generalizable to other institutions. Additional studies incorporating larger patient populations should be designed to address these limitations.

Conclusion

In summary, the present study demonstrates that patients presenting with an isolated PMI have longer door-to-activation times, leading to prolonged door-to-balloon times and a lower proportion of patients achieving coronary revascularization within the guideline-recommended time period. These findings highlight the importance of considering and recognizing the typical ECG findings of PMI among patients presenting with symptoms consistent with myocardial ischemia. 

References

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From the 1Department of Medicine, Division of Cardiology, University of California, San Francisco, California, 2Department of Medicine, Division of Cardiovascular Medicine, University of California, Davis, California, 3Department of Medicine, Division of Cardiovascular Medicine, Massachusetts General Hospital, Boston, Massachusetts, and 4Department of Medicine, Division of Cardiovascular Medicine, Brigham and Women’s Hospital, Boston, Massachusetts.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. Dr Hsue reports honoraria from Gilead and Pfizer; Dr Kulkarni reports royalties from UpToDate. The authors report no conflicts of interest regarding the content herein.

Manuscript submitted March 18, 2013 and accepted March 26, 2013.

Address for correspondence: Stephen W. Waldo, MD, Department of Medicine, Division of Cardiology, University of California, 505 Parnassus Ave, Box 0124, San Francisco, CA 94134. Email: [email protected]

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