The Trip Report: Time to PCI vs. STEMI Mortality

Reviewed This Month

Decreased Time From 9-1-1 Call to PCI Among Patients Experiencing STEMI Results in a Decreased One Year Mortality.

Authors: Studnek JR, Infinger A, Wilson H, et al. 

Published in: Prehospital Emergency Care, 2018 Nov–Dec; 22(6): 669–75.  

When discussing ST-segment elevation myocardial infarction (STEMI), we’ve all heard that a strategy to reduce mortality is to reduce door-to-balloon times. This has been demonstrated through multiple studies.

However, door-to-balloon time is defined as the time from patient contact at a hospital to primary percutaneous intervention (PCI). In the study we review this month, Mecklenburg EMS deputy director Jon Studnek and his coauthors astutely recognized that door-to-balloon time does not fully account for the impact EMS recognition of STEMI can have on mortality. 

This and a 2013 study that found a 16-minute reduction in door-to-balloon time was not associated with a decrease in mortality were identified as evidence that the prehospital phase of patient care needs to be incorporated and assessed to fully evaluate the influence of total ischemic time on STEMI patient mortality. Further, there is limited research available evaluating the impact of prehospital STEMI times on mortality. There was a clear need for this interesting and important study. 

While there are multiple time periods the authors could have chosen that incorporate the prehospital phase of care, they decided to evaluate the time between when the 9-1-1 call was received and PCI. This is likely the most relevant measure in this EMS system because dispatchers utilize priority dispatch protocols and provide prearrival instructions for patients with suspected cardiac etiologies. Therefore, this time metric truly encompasses first medical contact. It’s also likely a metric that closely captures total ischemic time. 

The study objective was “to determine if differences in mortality at one year in STEMI patients exist based on variations in the time interval [from] 9-1-1 call pickup to PCI.” The authors also stated their hypothesis, which was “As the time interval 9-1-1 call pickup to PCI decreases, mortality will also decrease.”

To meet the study objective and test their hypothesis, Studnek and coauthors conducted a retrospective observational cohort study. Data were obtained from Mecklenburg County, N.C. This county has a longstanding and highly integrated STEMI system of care. Since 2007 it has maintained a registry of all prehospital STEMI activations in the county. By utilizing data from this registry, the authors were able to evaluate prehospital and in-hospital patient care records. 

The study period ran from March 2008 to May 2014. The study specifically evaluated patients who presented with an acute STEMI as identified by a prehospital ECG interpretation. Patients were included if they were at least 18 years of age, were transported to a PCI center in Mecklenburg County, and met the local protocol for prehospital cardiac catheterization lab activation.

Patients were excluded from this analysis if they were initially transported to a hospital that did not perform PCI and were subsequently transported to a PCI center; if they met STEMI criteria after resuscitation from out-of-hospital cardiac arrest; or if they were deemed to not be a STEMI patient following in-hospital assessment. 

The primary outcome of interest was mortality a year post-PCI. The authors also evaluated a secondary outcome of mortality at one month post-PCI. Studnek and his team took multiple steps to confirm whether the patient survived or died.

To verify survival the authors depended on a documented follow-up visit or paramedic contact “at or beyond one year post-PCI.” For patients who didn’t have a documented follow-up, hospital records as well as records from local and national offices of vital statistics were reviewed to confirm deaths. If a patient did not have a hospital record post-PCI and no death certificate was found, they were classified as having survived. 

The main independent variable of interest was the time interval between the initial 9-1-1 call and PCI. The authors’ statistical analysis was rigorous in that they not only evaluated the impact of this time interval on mortality, but they adjusted their results for potential confounding variables.

Specifically they used multivariable logistic regression modeling to adjust for patient age, sex, initial blood pressure, presence of cardiogenic shock, prior history of MI, and risk level. Risk level was categorized as high or low. High-risk patients were at least 70 years of age, presented in cardiogenic shock, and had an initial blood pressure of less than 100 mmHg or a prior history of MI. 

Results

There were 551 patients enrolled in this study. There were 8.0% of patients with confirmed mortality at one year and 5.1% at one month. Average time from 9-1-1 call to PCI was 81.8 minutes. Patients who died had a significantly longer time to PCI when compared to those who survived.

For those who died the time from 9-1-1 call to PCI was 93.6 minutes, with a 95% confidence interval of 88.1–99.7 minutes. Those who survived had a 9-1-1-to-PCI time of 80.8 minutes, with a 95% confidence interval of 79.1–82.5 minutes. This difference was statistically significant, with a p-value less than 0.001. Patients who died were also older (67.8 vs. 59.1 years) and more likely to be classified in the high-risk category (11.0% vs. 4.4%). 

In the multivariable logistic regression model, the only confounding variable that remained after adjustment was age. The model revealed that for every one-minute increase in the time between the 9-1-1 call and PCI, the odds of survival at one year deceased by 3% (OR 0.97; 95% CI, 0.96–0.99). Results were similar for patient mortality at one month (OR 0.97; 95% CI, 0.95–0.98). 

This study identified an important metric that can be used as an effective predictor of mortality. But while this metric can be consistently and reliably recorded and assessed, simply measuring the time from 9-1-1 to PCI is not sufficient. The authors importantly emphasized the need for coordination in a STEMI system of care. Specifically they said, “PCI centers interested in decreasing mortality must engage their EMS systems to develop a comprehensive approach to STEMI care. EMS systems interested in achieving their mission of providing emergency care to patients when and where they are needed should engage with their PCI centers and measure their system performance as it relates to STEMI patients.”

Limitations

The authors here did a good job in detailing their study’s limitations. Again, this is a mature system with a highly integrated STEMI system of care, so these results may not be generalizable.

The authors also noted that patients may have been excluded from the study because they were not classified as STEMI patients in the prehospital setting. They also may have misclassified patients as survivors if they didn’t have a follow-up visit or a death certificate was not identified.

Finally, the authors noted they may have introduced selection bias by only including patients who received primary PCI. These limitations are common with this research design. The authors took multiple steps to reduce bias, and the system design certainly reduced the impact of these limitations.

Antonio R. Fernandez, PhD, NRP, FAHA, is research director at the EMS Performance Improvement Center and an assistant professor in the Department of Emergency Medicine at the University of North Carolina–Chapel Hill.