April 6, 2026

This month, we discuss the importance of EKGs and why they remain one of the most essential tools we use in the prehospital setting—and why continued practice and review are so important for all of us.

EKGs are simple, non-invasive, but incredibly powerful. They save lives by helping us identify critical and life-threatening conditions early, thereby helping us reduce morbidity and mortality. Having knowledge of important EKG findings can help guide key decisions, such as transport destination and early activation of resources, including the cath lab. It can even help us prevent cardiac arrests if we can identify concerning rhythms early on, before our patients decompensate. It’s no surprise that this topic is consistently requested for continuing education — mastery requires constant review, and the stakes are high.

To better understand what we’re seeing, it’s important to revisit some fundamentals of cardiac anatomy and physiology. The heart functions through a coordinated electrical conduction system that drives rhythmic contraction and ensures effective perfusion of both the coronary arteries and the body as a whole. At rest, myocardial cells maintain a polarized state through ion exchange — pumping sodium out and potassium in. When stimulated, sodium rushes back into the cell, causing depolarization and triggering calcium release, which leads to muscle contraction. Afterward, the cell resets through repolarization, entering a refractory period that allows the heart chambers to refill. This is a well-coordinated dance that requires the right balance of electrolytes, healthy, well-perfused tissue, and impeccable timing. 

This electrical activity is what we capture on the EKG. As a positive wave of depolarization moves through the heart toward a lead, it produces a positive deflection on the tracing. Each component of the waveform corresponds to a specific event: the P wave represents atrial contraction, the PR interval reflects conduction time from atria to ventricles, the QRS complex indicates ventricular depolarization, and the T wave shows ventricular repolarization. The ST segment is particularly important, as elevation or depression may indicate myocardial ischemia, and abnormalities in the QT interval can predispose patients to dangerous arrhythmias. Remembering what each portion of the EKG tracing looks like and how long it takes is crucial to being able to identify defective areas and help arrive at a diagnosis.

Electrical activity is not the only important information that can be discerned by looking at a tracing. Knowing coronary perfusion territories can help identify the area of infarct by examining the affected leads. Coronary arteries deliver oxygenated blood to the myocardium, and different vessels supply specific regions of the heart. The left coronary artery primarily supplies the left atrium and most of the left ventricle, with its major branches — the left anterior descending artery (LAD) supplying the anterior wall, septum, and apex, and the left circumflex (LCX) supplying the lateral and posterior walls. The right coronary artery (RCA) supplies the right atrium and ventricle, much of the conduction system (including the SA and AV nodes in most individuals), and portions of the posterior heart. Additional branches, such as the posterior descending artery and right marginal artery, further supply the ventricles. Understanding these territories helps us correlate EKG changes with the affected area of the heart.

Since the heart is a three-dimensional structure, we rely on multiple leads to give us different perspectives. The 12-lead EKG allows us to view specific regions: inferior (II, III, aVF), lateral (I, aVL, V5, V6), septal (V1, V2), and anterior (V3, V4). Contiguous leads provide insight into localized areas of the heart, helping us pinpoint where pathology may be occurring. Proper lead placement and interpretation are key, and sometimes initial findings will prompt us to obtain additional or serial EKGs — especially when clinical suspicion remains high or when early changes are subtle. Remember, we are not just putting stickers on a patient; we are choosing the most appropriate vantage point to get the clearest picture of what is going on. 

It’s also important to remember that STEMIs and obvious ischemic changes are not the only time-sensitive findings. Subtle abnormalities, evolving patterns, and early warning signs can be just as critical and must be recognized promptly.

Ultimately, a well-functioning heart is central to a healthy patient, and the EKG is one of our best tools for assessing that function in real time. As we will see, however, cardiac pathology is not the only thing we can discern from a timely performed EKG. The more comfortable and confident we are in interpreting these tracings, the better equipped we are to make rapid, informed decisions that truly impact outcomes.

Thank you all for your continued dedication to learning and excellence in patient care,

Cristina M. Carpintero-Ramirez, MD