The prehospital treatment of ST elevation acute myocardial infarction (STEMI) continues to evolve. Newer information has led to five major changes in prehospital STEMI care. In a 2009 State of the Science article, we discussed two of those changes in depth: the decreasing emphasis on morphine use in acute coronary syndromes (ACS), and the recommendation to stop utilizing IV beta blockade in initial STEMI care.
In this article, we briefly re-review the roles of morphine and beta blockers, and then address three more recent changes that will affect EMS: 1) minimizing D2B (door to balloon) times by also focusing on E2B times (EMS arrival to patient therapy with PCI); 2) the role of paramedics and emergency department (ED) physicians in prehospital activation of the cardiac cath lab; and 3) the changing role of EMS in patients treated with thrombolytics.
Morphine & Beta Blockers in STEMI Care
Although morphine (or other narcotics, such as fentanyl) can still be used in treating ischemic pain, patients should receive the minimum dose necessary to relieve their discomfort. Because there was increased mortality in the CRUSADE study of non-STEMI patients who were treated with morphine, most experts now recommend that pain relief focus on aggressive use of nitroglycerin and rapid reperfusion. Paramedics must be aware that the aggressive use of high doses of narcotics may be associated with a worse outcome.
The use of IV beta blockers has also changed based on the results of the COMMIT Trial, which showed a significant increase in complications in STEMI patients who received IV beta blockers within hours of STEMI onset. Intravenous beta blocker administration within the first 24 hours of a myocardial infarction (MI) should occur only after the adequacy of myocardial function has been determined. Thus, EMS administration of IV beta blockers, with drugs such as metoprolol, should no longer be considered in any ACS patient.
First D2B, Now E2B Matters Too
"Time is Muscle." This phrase was coined early in the thrombolytic era to emphasize that the best patient outcomes were seen in those individuals who received lytics most rapidly after ED arrival. This concept of early therapy led to the AHA-ACC recommendation of "door to drug" in less than 30 minutes, a standard that still exists today.
Studies have clearly shown that those patients who receive PCI in 90 minutes or less from arrival to the hospital (D2B) fair much better than those whose PCI is delayed. Thus, D2B < 90 min is now a national standard of care. In fact, every 15-minute delay in coronary intervention after 90 minutes increases mortality by 6.3 patients of 1,000 treated.
Most of us in emergency medicine have worked very hard to ensure a D2B of less than 90 minutes, but it has been unclear if trying to drive this time even lower makes any difference in patient outcome. Similarly, unlike major trauma, where EMS scene times are kept to a bare minimum, and a "scoop and run" mentality prevails, minimizing on-scene times for chest pain patients has not traditionally been a major focus for most services.
Recently, however, a large study comparing D2B times with patient survival was published. It reviewed more than 43,000 patients treated at 600 PCI centers throughout the United States. The study, as expected, showed that those patients who had a D2B time less of than 90 minutes did better than those who exceeded the ACC-AHA D2B goal time. Interestingly, the study also demonstrated that even for those patients who received their PCI in a timely fashion, every few minutes decrease in D2B made a significant difference.
Thus, patients who received coronary intervention in 60 minutes did better than those who were at 75 minutes post arrival, and those whose PCI was at 75 minutes did better than those who were just under 90 minutes. Even more impressive was that decreasing D2B times from a 45 minute D2B to an almost unheard of 30 minutes even further decreased mortality (see Figure 1).
E2B now matters. This in-hospital D2B study has very important implications for EMS. D2B is merely part of the time delay from the beginning of myocardial cell death to definitive coronary intervention. It is now imperative that we minimize on-scene and transport times and begin to track our E2B times—that is, EMS on-scene time to time of PCI balloon inflation of an occluded vessel by an interventional cardiologist.
EMS, like hospitals, should begin to focus on system enhancements to allow optimal E2B times with an ultimate goal of 90 minutes or less. It's clear that we must ensure expert and efficient EMS care is rendered to all potential STEMI patients, and on-scene time must be kept to an absolute minimum for this group of patients. Wasting time on scene for a complete and detailed history (such as obtaining risk factors and family history) should no longer occur.
Additional information can always be obtained en route, if at all. Similarly, hospital transport times should be as rapid and safe as possible. This study's message seems very clear: Paramedics are an essential part of the chain of survival as it applies to decreasing mortality from STEMI. It's also imperative that the hospital's ED and/or PCI team is notified as soon as possible that a possible STEMI has been identified.
Should Paramedics Directly Activate the PCI Lab?
Many studies now show that paramedics can directly activate the PCI lab with a high degree of accuracy and with what some authors call an "acceptable false positive rate." In fact, literature supports that paramedics can interpret ST elevation on a 12-lead ECG as well as ED physicians and cardiologists, respectively. That is, paramedics can identify most STEMIs (high sensitivity) and do not often falsely diagnose a STEMI when in fact the ECG does not show one (high specificity).
A recent study seemed to support this assumption but may actually call into question the blanket endorsement of PCI activation from the field. In this study, a paramedic's ability to read ST segment elevation on prehospital 12-lead ECG was evaluated. The author's conclusion stated that paramedics in an urban/suburban EMS system were very accurate and had an "acceptable" false positive (or over-call) rate. Paramedics were 92.6% sensitive for detecting true STEMIs and were 85.4% specific when STEMI was diagnosed.
Interpreting the data differently, paramedics missed 12% of STEMI activations and falsely activated the PCI team 8.1% of the time. Thus, one out of every 12 times the PCI team was activated from the field, it was a "false alarm." During a weekday, this might be a mere inconvenience. However, when cardiac PCI teams must emergently come to the hospital from home (usually nights, weekends or holidays), this has the potential to ruin their trust in EMS. Worse yet, it has resulted in some PCI teams refusing to activate until after an ED physician has reviewed the prehospital ECG.
Similarly, in another study from San Diego, 20% of activations were found to be inappropriate (not STEMI), involving instead patients having old left bundle branch blocks (LBBB) or left ventricular hypertrophy (LVH) with repolarization abnormalities.
Interestingly, of the false positive activations, the computer interpretation was for that of an acute MI (AMI). When possible, the addition of "another set of eyes," usually the ED physician after prehospital transmission, may significantly decrease false activation of the PCI lab.[10,11]
Although discussion about 12-lead transmission often revolves around limiting false activations, it would be remiss not to discuss the importance of minimizing missed opportunities for cardiac PCI lab activation based on inappropriate interpretation of the actual STEMI patient's ECG. Some literature demonstrates an EMS miss rate of close to 20%.7 Patients with a missed STEMI are less likely to receive aspirin, less likely to undergo prompt perfusion, and have up to a 40% increase in mortality.
Although paramedic interpretation of a 12-lead ECG is generally acceptable, rapid transmission of a prehospital ECG with concurrent ED physician evaluation whenever possible reduces error and appears preferable. Having a machine read, a paramedic interpret and the physician review decreases false activations and increases the likelihood of activation in cases where a true STEMI exists. For those systems that cannot transmit, efforts to decrease false activations must be undertaken.
Some possibilities to decrease false activation include activating the lab only if absolutely "classic" STEMI patterns are seen. Discussing the ECG online with a physician is also encouraged, as questions about reciprocal changes and paramedic certainty along with time of day can also be factored into the decision to activate before EMS arrival.
More Urgent STEMI Transfers
Balloon angioplasty (PCI) has now become the preferred method of opening and reperfusing occluded coronary arteries.4 There are numerous reasons for this, including a higher percentage of wide-open arteries after PCI as compared with lytics (95% versus 54–60%), lower number of complications, less bleeding and the ability to immediately see who might benefit from emergency coronary artery bypass grafting. However, many patients still receive thrombolytics because only about 25% of U.S. hospitals are PCI capable.
Lytic vs. Lab?
The current ACC-AHA recommendations on lytic versus lab for STEMI patients are as follows:
- PCI is the preferred therapy for STEMI patients if D2B will be 90 minutes or less.
- PCI is preferred if a patient's STEMI began more than three hours prior to ED arrival.
- Thrombolytics are preferred if a patient's STEMI began less than three hours prior to ED arrival and a D2B of less than 90 minutes cannot be achieved in a patient who has no contraindications to lytic therapy.
Paramedics should be aware that lytics have been shown to be actually more beneficial if given quickly to patients with hyper-acute AMIs (those less than two hours in duration). In one study (the CAPTIM Trial), mortality was lowered by about 50% in patients treated with prehospital full-dose lytic therapy versus patients taken urgently to PCI.
This trial has prompted others to see if half-dose lytics given in the prehospital setting might be successfully combined with PCI soon after arriving at the hospital. Preliminary results from one study involving almost 3,000 patients have yielded very encouraging results. Even though the prehospital half-dose lytic group had a slightly longer D2B time before they went to PCI (28 minutes longer), their incidence of death, re-infarction and stroke was significantly lower than those who went directly to PCI. If these results are replicated in future studies, more EMS systems will begin using prehospital half-dose lytics as part of routine STEMI treatment.
It was previously thought that stable patients who received thrombolytics did not require rapid transport to a PCI center for coronary angiography. The Canadian TRANSFER-AMI Study, however, showed a significant decrease in the combined end point of death, re-infarction and need for urgent revascularization when patients rapidly went on to PCI after lytic therapy (approximately 70 minutes later) versus waiting until the next day (about 21 hours later, 11.0% versus 17.2%; p = 0.04). The implication for many EMS agencies is that we may be seeing more urgent transfers of lytic-treated STEMI patients to PCI centers for immediate PCI.
The care of the patient with an acute coronary syndrome, specifically STEMI, continues to evolve. Although the goal of caring for any patient with STEMI is a D2B time of less than 90 minutes, research shows that every minute counts, and "time is muscle." Thus, even if a hospital has an acceptable D2B time, EMS on-scene and transport times must be minimized to ensure optimal treatment and recovery of a dying heart. All EMS systems should work with their destination hospitals to ensure E2B times are optimally lowered and aim for an E2B of under 90 minutes when possible.
Moreover, a team approach to PCI activation involving both the paramedic and ED physician whenever possible appears to be the best way to decrease false activations and increase recognition of STEMI patients. As research into myocardial infarction and treatment continues, EMS personnel should expect more critical care transports of patients who have already received lytic therapy. The in-hospital care of patients with acute coronary syndromes continues to evolve, and similarly EMS care for ACS patients continues to change at an ever-increasing rate.
Disclosure: The authors have no conflicts of interest with the sponsors of this supplement.
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- Chen ZM, Pan HC, Chen YP, et al. Early intravenous then oral metoprolol in 45,852 patients with acute myocardial infarction: Randomised placebo-controlled trial. Lancet. 2005;366:1622–1632.
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- McNamara RL, Wang Y, Herrin J, et al. Effect of door-to-balloon time on mortality in patients with ST-segment elevation myocardial infarction. J Am Coll Cardiol. 2006;47:2180–2186.
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