Sweet 16

     There's a growing demand for early prehospital determination of acute ST-segment elevation myocardial infarctions (STEMIs) for fast and efficient transport of patients to hospitals with cardiac catheterization and angioplasty capabilities. Accordingly, EMS responders must be well attuned to the true capabilities of electrocardiography machines. Currently, STEMIs require accurate EKG interpretation in the field to save precious heart muscle and decrease the risk of morbidity and mortality.

     From the advent of EKG interpretation, with Alexander Muirhead recording wrist-obtained heartbeats in 1872, through Williem Einthoven assigning the letters P, Q, R, S and T to its now-well-known components, advances in our understanding of the intricacies and abilities of the electrocardiogram have changed patients' lives. Today, with a stronger understanding of these EKG capabilities, we can, through the addition of four simple leads to those commonly recorded, use standard 12-lead EKG machines to evaluate the complete heart.

     Will these four additional leads cost more? Do we need to buy new machines? Fortunately, no. Instead, simple training in using current 12-lead machines with new lead placement can help you develop the ability to form 16-lead EKGs. With these new skills, an experienced EMT can quickly and effectively evaluate the right ventricle as well as the posterior heart. These locations are neglected with traditional 12-leads.

     Twelve-lead EKGs consist of three limb leads (I, II and III), three augmented leads (aVR, aVF and aVL) and six precordial leads (V1–V6). These lead placements allow evaluation of the inferior, anterior, septal and lateral heart for STEMI. Lead placement is essential to proper EKG interpretation (Figure 1).

     Further, the printout of the 12-lead EKG is standardized to allow for universal interpretation. These leads are quickly combined to correlate to locations throughout the heart (Figure 2). What's lacking, though, is the ability to evaluate the right ventricle and posterior wall of the heart, where STEMIs may occur alone or in concert with other heart locations.

     The key to understanding 12-lead interpretation is to understand the criteria for considering STEMI. Conventionally, ST elevation is defined as the ST segment of a particular lead being at least one small EKG box (1 mm or 0.1 mV) elevated compared to the corresponding TP segment. For a portion of the heart to be considered electrographically as having an acute ST-elevation myocardial infarction, this significant ST elevation must be in two or more contiguous leads. In addition, concurrent ST segment depression in other lead distributions increases the likelihood of an acute STEMI. ST segment depression is defined as greater than one small EKG box (1 mm or 0.1 mV) depressed compared to the TP segment in two or more contiguous leads.

     In addition, early STEMIs may not present with ST elevation at all, but may instead present with hyperacute, or peaked, T waves alone before further progression to the ST segment elevation commonly seen. New left bundle branch blocks must also be considered an acute myocardial infarction until proven otherwise. Other cardiac and electrolyte abnormalities besides STEMIs can cause peaked T-waves, left bundle branch blocks or ST segment elevation or depression. It is beyond the scope of this article to discuss these numerous causes.

     Right ventricular STEMIs are particularly worrisome because they drastically affect the prehospital management of the patient. The old adage that inferior-wall MIs should not receive nitroglycerin due to the possibility of a subsequent rapid fall in blood pressure is false. Rather, the concern over nitroglycerin administration lies solely with right ventricular myocardial infarctions.

     Right ventricular STEMIs are preload-dependent—i.e., dependent on the volume of blood return to the right atrium and ultimately the right ventricle. Nitroglycerin, pharmacodynamically, causes relaxation of the vessels of the heart and body, thereby causing decreased blood return to the heart and thus decreased preload. This decreased preload, along with the already decompensated right ventricle, causes a rapid and dangerous drop in blood pressure.

     Why, then, the adage of no nitroglycerin for inferior STEMIs? Inferior-wall STEMIs have associated right ventricular STEMIs approximately 50% of the time. It is not the inferior wall causing the hypotension with nitroglycerin administration, but rather the associated right ventricular STEMI. Nitroglycerin is safe to administer once the associated right ventricular STEMI is ruled out.

     Either isolated right ventricular STEMIs or ones associated with inferior- or posterior-wall STEMIs can be hinted at by evaluating lead V1. V1 is the only precordial lead to sit to the right of the sternum, peering down toward the right ventricle. With possible right ventricular wall STEMIs, lead V1 will show isolated ST segment elevation (Figure 3). This finding cues the provider to place further leads to evaluate the right ventricle.

     The right ventricle can be rapidly and effectively evaluated by moving leads V5 and V6 from the conventional 12-lead placement to the positions of V3R and V4R respectively. These two unique positions are mirror images of the placement of V3 and V4 on the standard 12-lead EKG (Figure 4). V3R and V4R are considered contiguous leads, and if the standard criteria are met for STEMI, then the patient is considered to have either an isolated right ventricular STEMI or an associated right ventricular STEMI, depending on the other 12-lead findings. These patients should not be administered nitroglycerin and are at increased risk of cardiogenic shock if preload is reduced.

     Some 10%–15% of right ventricular STEMIs associated with inferior STEMIs present with hemodynamic instability. These patients may require fluid boluses and possibly vasopressors to maintain preload and thus blood pressure during transport. Care must be taken to prevent fluid overload during volume resuscitation; it can easily occur, leading to right septal deviation impinging on the function of the left ventricle. This impingement will cause significantly decreased afterload, cardiac failure and pulmonary edema. Typically, fluid resuscitation is limited to less than a liter of crystalloid before progressing to a vasopressor. Vasopressors are not without risk, as they increase myocardial oxygen demand in an already hypoxic myocardium.

     In addition, patients with inferior-wall STEMIs with associated right ventricular STEMIs have a 25%–30% increased risk of mortality compared to those with solitary inferior-wall STEMIs. In documenting your printed EKGs, be certain to prominently write Right-sided EKG, along with changing the V5 and V6 leads to respectively read V3R and V4R to avoid confusion upon arrival at the hospital.

     Another elusive infarction is that of the posterior wall of the heart. With all of the standard 12 EKG leads placed on the anterior chest, this major portion of the heart is left without evaluation. Though not appreciated through standard electrocardiography techniques, these STEMIs are rare as isolated attacks, but contribute to 15%–20% of all STEMIs. Though management of posterior-wall STEMIs is similar to conventional prehospital STEMI care, including aspirin, oxygen, nitroglycerin and morphine, recognition of this infarction is imperative to prompt cardiac care and reducing morbidity and mortality. A patient could have a seemingly normal 12-lead EKG, but still be infarcting this critical portion of the heart!

     Conventional leads V1 and V2 can once again be helpful in hinting at an easily overlooked posterior-wall STEMI. Posterior-wall STEMIs present on conventional 12-lead EKGs with what appear to be large R waves and depressed ST segments in either V1, V2 or a combination of both (Figure 5). The large R wave is actually a large posterior Q wave, and the depressed ST segment is actually an elevated posterior-wall ST segment. Imagine the findings in V1 and V2 from the anterior-oriented conventional 12-lead EKG as reflections of what is really occurring on the posterior wall of the heart. These findings should cue the provider to place further leads to evaluate the posterior wall.

     The posterior wall can be rapidly and effectively evaluated by moving leads V5 and V6 from the conventional placement to the positions of V8 and V9, respectively. V8 placement is just inferior to the left scapular tip, and V9 is midway between V8 and the lateral edge of the bony spine (Figure 6).

     V8 and V9 are considered contiguous leads, and if the standard criteria are met for STEMI, then the patient is considered to have either an isolated or associated posterior-wall STEMI, depending on the other 12-lead findings. Though prehospital management of the patient does not change when finding a posterior-wall STEMI, the reward is in thoroughly evaluating patients with cardiac complaints to find this all-too-often hidden infarction. In documenting your printed EKGs, be certain to prominently write Posterior-wall EKG, along with changing the V5 and V6 leads to respectively read V8 and V9 to avoid confusion at the hospital.

     Thorough and prompt evaluation of patients with chest pain, shortness of breath or other signs and symptoms of myocardial infarction is vital to initiating prehospital care and rapid transport to facilities capable of managing STEMIs. Furthermore, prehospital evaluation and recognition of all types of STEMIs, including right ventricular and posterior-wall STEMIs, is critical to providing patients with the best outcomes possible. Together, the emergency cardiac care system—from prehospital EMS on through the cardiac catheterization lab—must be a well-oiled machine with complete integration to decrease the morbidity and mortality of these devastating infarctions. The concepts described here allow for safer, more thorough prehospital care and communication for patients with specific types of STEMIs, based on the interpretation of 16-lead EKGs.

     Jeremy DeWall, MD, NREMT-P, is a resident physician practicing emergency medicine at the Medical College of Wisconsin's Department of Emergency Medicine in Milwaukee. He has been a nationally registered EMT-Paramedic for 10 years, working actively in Sheboygan, WI, for the first eight. In addition, he is a volunteer ski patroller and Outdoor Emergency Care instructor. He is also a flight physician for Flight For Life in Milwaukee.