The rhythm is either sinus rhythm or not sinus rhythm. Sinus rhythm refers to the origination of the electrical activity coming from the sinus node — also known as the sinoatrial node, or SA node. This results in an upright P wave in lead II on the ECG. If there is a P wave before every QRS complex, and it has a sinus morphology, then normal sinus rhythm, or NSR, is said to be present. A sinus morphology is an upright P wave in lead II and biphasic (up and down) P wave in lead V1. The first ECG strip below shows a P wave with sinus morphology, thus normal sinus rhythm. If the P wave has a morphology different from the typical sinus morphology, it is termed ectopic, meaning coming from somewhere other than the sinus node. The second ECG strip below shows an ectopic atrial rhythm. Note that the P wave is down in lead II and only up (not biphasic) in lead V1. Ectopic atrial rhythms including atrial tachycardia, multifocal atrial tachycardia and junctional rhythms all have P waves that are not of sinus morphology and will be reviewed in detail later. Enlarge  If there is sinus rhythm, and the heart rate is less than 60 beats per minute, then sinus bradycardia is present. If there is sinus rhythm, and the heart rate is greater than 100 bpm, then sinus tachycardia is present. The links below take you to examples of each of these.
If there are no P waves present, or the P wave morphology is not normal, then the exact rhythm must be determined. Various arrhythmias — including atrial fibrillation, atrial flutter, and ventricular rhythms such as ventricular tachycardia or ventricular fibrillation — are discussed in detail in their respective sections in ECG Reviews and Criteria. Below are three more examples of rhythms other than sinus rhythm: atrial fibrillation, atrial flutter and multifocal atrial tachycardia.
Enlarge Note that when atrioventricular, or AV, dissociation is present (complete heart block or VT), as previously described, there may not be a P wave before every QRS complex. However, as long as the P wave is upright in lead II, sinus rhythm is still said to be present.
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:
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.
The rhythm is best analyzed by looking at a rhythm strip. On a 12 lead ECG this is usually a 10 second recording from Lead II.
7 step approach to ECG rhythm analysis1. Rate
2. Pattern of QRS complexes
3. QRS morphology
4. P waves
5. Relationship between P waves and QRS complexes
6. Onset and termination
7. Response to vagal manoeuvres
Differential DiagnosisFollow links below for examples of individual rhythms. Narrow Complex (Supraventricular) TachycardiaATRIAL – REGULAR ATRIAL – IRREGULAR ATRIOVENTRICULAR Broad Complex Tachycardia (BCT)
REGULAR BCT
Note: All regular BCTs should be considered to be VT until proven otherwise. IRREGULAR BradycardiaP WAVES PRESENT1. Every P wave is followed by a QRS complex (= sinus node dysfunction)
2. Not every P wave is followed by a QRS complex (= AV node dysfunction)P WAVES ABSENTFor escape rhythms to occur there must be a failure of sinus node impulse generation or transmission by the AV node. Advanced ReadingOnline Textbooks
LITFL Further Reading
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education. He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference. His one great achievement is being the father of three amazing children. On Twitter, he is @precordialthump. | INTENSIVE | RAGE | Resuscitology | SMACC |
What does a rhythm strip show?
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