ECG interpretation: 10 steps for rhythm identification
Consistently following a process to analyze a patient's ECG will help you correctly identify their cardiac rhythm
ECG interpretation, using a step-by-step process, ensures we always provide the best patient care. Since no two emergency scenes or patients are the same, it’s imperative to be methodical about the elements of the call that we can control.
ECG tracings — the diagnostic tool that analyzes the electrical function of the heart and measure voltage (vertical measurement) versus time (horizontal measurement) — can be confusing, so here are the ten steps I follow on every ECG (or EKG) to ensure I correctly identify the rhythm.
1. Is the ECG rhythm regular or irregular?
As you look at the rhythm, locate the QRS segment which represents the depolarization (the electrical charging of cells) within the ventricles, the two lower chambers of the heart that gather and expel blood towards the body and lungs. Within the QRS, identify the R wave, the positive wave above the isoelectric line (baseline). Using a six second strip, measure the R to R intervals between QRS segments and determine if the rhythm is regular or irregular.
If you discover an abnormality or irregularity here — or in any of your subsequent findings on the ECG — ask your patient if this is normal for them and look for any associated symptoms such as C.H.A.P.S. — chest pain, hypotension, altered mental status, poor perfusion, or shortness of breath.
2. Calculate the heart rate
Take a radial pulse at the patient’s wrist, confirm it with the number displayed on the cardiac monitor or print a six-second strip of ECG paper and count the number of QRS complexes and multiply by 10 to arrive to a minute heart rate. From there, decide if the patient's heart rate is bradycardic (less than 60 beats per minute); within a normal range (60-100 bpm); tachycardic (100-150 bpm) or a potentially dangerous rhythm above 150 bpm such as supraventricular tachycardia or ventricular tachycardia with a pulse.
At this stage of ECG interpretation, be careful not to jump to a quick interpretation. Instead, note the information you find and continue with the subsequent steps.
3. Find the P waves
The P wave represents the depolarization of the atria, the two upper chambers of the heart, which receive blood from the vena cava and pulmonary veins. When searching for P waves: Ask yourself, are the P waves present? Are they upright in Lead II on the cardiac monitor? And are they followed by a QRS segment? If the answer is yes to all, it is likely the electrical impulse began in the sinoatrial (SA) node, the normal pacemaker of the heart.
4. Measure the PR interval
The PR interval is the time interval between the P wave (atrial depolarization) to the beginning of the QRS segment (ventricular depolarization). The normal PR interval is 0.12-0.20 seconds, or 3-5 small boxes on the ECG graph paper. A prolonged PR interval suggests a delay in getting through the atrioventricular (AV) node, the electrical relay system between the upper and lower chambers of the heart.
5. Measure the QRS segment
The normal QRS segment has three graphical deflections — the first negative wave (Q wave); the positive wave above the isoelectic line (R wave) and the negative wave after the positive wave (S wave) — and the normal time duration is 0.04-0.10 seconds. If you notice a prolonged QRS segment, it might be due to a bundle branch block which could be relatively benign or a sign of underlying heart disease.
6. Observe the T wave
The T wave represents repolarization (recovery) of the ventricles and should be upright in Lead II and appear after the QRS segment. Any variations in the T waves are important to note. Inverted T waves could be due to a lack of oxygen to the heart; too much potassium (hyperkalemia) could cause peaked T waves; flat T waves may be due to too little potassium and a raised ST segment — the end of the QRS segment to the beginning of the T wave — might be due to a heart attack.
7. Note any ectopic beats
An ectopic beat is a change in a heart rhythm caused by beats arising from fibers outside the SA node, the normal impulse-generating system of the heart. If you notice ectopic beats, determine if they are premature atrial contractions (PACs); premature junctional contractions (PJCs) or premature ventricular contractions (PVCs). Also, note how many ectopic beats are present in the ECG, the interval at which they are appearing, their shape, and if they arise singularly or in groups.
8. Determine the origin
The last step before correctly indentify your ECG is to determine where the rhythm is originating. Here are some key elements to look for:
- Sinus: 60-100 bpm; regular rhythm; P waves upright, round and present before each QRS segment; normal PR interval; normal QRS duration.
- Atrial: Rhythm may be regular or irregular; normal QRS segment, but P waves premature and different shapes — flattened notched, peaked, inverted or hidden.
- Junctional: Look for a junctional type P wave — inverted before, during or after the QRS segment that is normal in duration.
- Ventricular: Wide and bizarre QRS segment and no P waves since the impulse is originating below the SA node.
- Paced rhythm: Observe low voltage pacer spikes before the QRS.
9. Correctly identify the rhythm
Now that you’ve methodically analyzed the rhythm, you should be able to easily identify it. Once you do, consider your ECG interpretation in the context of the other information you’ve gleamed on the call — the patient’s chief complaint, mental status, OPQRST/SAMPLE histories, and vital signs — and then decide upon a correct treatment plan. When in doubt, always treat the problem you assess not the cardiac monitor.
10. Stay current on ECGs
If you’re still learning or want an additional reference on that 3 a.m. call when you’re mind is a bit foggy, don’t be afraid to create a job aid on a notecard, listing the key steps to analyzing an ECG rhythm.
Also, stay current on your ECG skills by using Skill Stat’s free, online ECG simulator, reading about clinical cases in Life in the Fast Lane’s informative ECG Library, and check and trying out these EKG challenges.