How to Read an EKG Rhythm Strip
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William J. Kelly, MSN, FNP-C
Author | Nurse Practitioner
Learning how to read an EKG rhythm strip is an essential skill for nurses!
This skill becomes especially handy for nurses on Med-Surg, Telemetry, the Emergency Department, or Critical Care units.
If reading an EKG rhythm strip is new to you – this is the perfect place to start!
What is a Rhythm Strip?
An EKG or ECG stands for Electrocardiography, which is the electrical activity of the heart traced on paper (or a monitor).
A rhythm strip is at least a 6-second tracing printed out on graph paper which shows activity from one or two leads.
Leads are “views” of the heart. There are 12 leads that are traditionally obtained with a 12-lead EKG, but most portable and bedside monitors only monitor 3-5 leads at a time.
Luckily – interpreting a single rhythm strip is much easier than a 12-lead EKG. Most rhythm strips are interpreted from Lead II as this gives a great view of the heart.
The goal of reading an EKG rhythm strip is to determine the rate and rhythm of the patient. This is great for identifying baseline cardiac rhythm as well as any arrhythmias or ectopy that may occur (like a premature beat).
A 12-lead EKG also looks at the rate and rhythm, but additionally gives nearly a complete 360° view of the heart.
This means it can be used to assess for things like cardiac ischemia or infarction, conduction delays, and even enlarged chamber size.
The ECG Rhythm Strip Tracing
As I said earlier – an ECG Rhythm tracing is the electrical activity of the heart recorded on paper or a monitor.
This is traditionally printed out on a 6-second strip. This can make it easy to determine the rate of an irregular rhythm if it is not given to you (count the complexes and multiply by 10).
Thick black lines are printed every 3 seconds, so the distance between 3 black lines is equal to 6 seconds.
As you can see, a printed ECG rhythm strip is comprised of boxes – both small boxes and large boxes. 5 small boxes make up one large box.
Each small box is 1mm wide, signifying 0.04 seconds or 40 milliseconds (ms).
Each large box is 5 small boxes, signifying 0.20 seconds or 200ms.
This becomes important to remember when determining the rate of regular rhythms. The boxes and lines are also important in recognizing whether a rhythm is regular or irregular.
Okay so that covers the paper, but what about the actual tracings? That’s where the alphabet comes into play. By alphabet – I mean PQRST.
An electrical tracing of the heart is made up of waves, lines, complexes, and intervals, and each of these represents specific conduction within the heart. This is the key to interpreting a rhythm strip.
P waves represent atrial depolarization. This means that the electrical signal that starts in the SA node (the normal pacemaker of the heart) is traveling through both atria (top chambers of the heart) during the P wave.
A P wave should look smooth and upright in most leads including lead II.
The 3 things you’ll want to specifically look for in P waves in a rhythm strip are:
- Are there P waves before each QRS complex?
- Are there any P waves that do not have a QRS complex that follows?
- Do all the P waves look the same / have the same shape?
Keeping these 3 questions in mind will help you determine where the rhythm originates from (i.e. the sinus node), if there are any potential extra beats, or if there could be certain heart blocks present.
An inverted P wave means there is anterograde conduction to the atria (backwards direction). This means the electrical impulse originates from near, at, or below the AV node. Examples of this include Junctional rhythm, certain PACs, and PJCs.
The QRS complex represents ventricular depolarization. This means that the electrical signal is traveling through both ventricles (the bottom chambers of the heart). In a healthy heart – this should correlate with the pulse.
The QRS complex is actually made up of 1-3 waves, the Q wave, the R wave, and the S wave. Depending on which lead you look at and the specific heart, any combination of these waves may be present.
In lead II, usually all three waves are present. This includes an initial downward deflection (Q wave), an upward deflection (R wave), followed by a downward deflection (S wave).
The presence of a QRS complex indicates that the ventricles are receiving the electrical signal. These should follow shortly after a P wave in a sinus rhythm.
The main abnormality that can occur is a wide QRS complex. This either means that there is aberrant conduction (like a bundle branch block), or that the electrical signal starts in either the left or right ventricle (i.e. a PVC or Ventricular Tachycardia).
A bundle branch block just means there is a delay in the conduction tissue transmitting the signal to either the right or left ventricle. If the widened QRS is preceded by a P wave, it is probably a sinus rhythm with a BBB.
If there is no preceding P wave, you may have a PVC or even VTACH if it is sustained.
The T wave represents ventricular repolarization. This means that the myocardial cells within the ventricles are recovering and “getting ready for the next beat”.
This should be smooth and upright in most leads, including lead II.
Sometimes, the T wave can be inverted or flipped. This is nonspecific but can indicate cardiac ischemia or infarction, especially if it is in at least 2 contiguous leads (pertaining to the same anatomical area of the heart).
People may have flipped waves in certain leads at baseline after a heart attack, with a bundle branch block, or with a PVC, VTACH, or ventricular paced rhythms.
Tall or tented T waves are those that are > 1 large box in lead II and may be particularly pointed. This could be normal for the patient, but can also indicate hyperkalemia (high potassium).
The PR interval is from the beginning of the P wave to the beginning of the QRS complex. This represents the time it takes for the electrical signal to reach the ventricles from the SA node.
This should be 3-5 small boxes or 120-200ms. If longer, this is considered a first degree AV block.
A short PR interval could be from a a PAC, a junctional rhythm (associated with an inverted P wave), or Wolff-Parkinson-White syndrome.
The QT interval is the time between the start of the QRS complex to the end of the T wave. This will change depending on the heart rate, so a QTc (QT corrected) is calculated.
This should be 350-440ms in men, and 350-460ms in women. A QT interval >500ms predisposes the patient to deadly ventricular arrhythmias such as Torsades de Pointes.
QT prolongation can be caused by ischemia, electrolyte abnormalities, or from medications such as psych medications, Zofran, Azithromycin, Cipro, etc.
While you can calculate the QT interval from a single strip, a 12-lead EKG should be obtained and it will be listed on the EKG for you. Otherwise, there are online calculators which can be used to determine the corrected QT interval for the heart rate.
Arrhythmias on the ECG Rhythm Strip
An arrhythmia is any abnormal rhythm other than normal sinus rhythm – the baseline rhythm of the heart. This can be a benign variant (like sinus arrhythmia), or it could be deadly (like ventricular fibrillation).
In order to know how to read an EKG rhythm strip, you need to first be able to understand what normal sinus rhythm (NSR) looks like.
You should be comparing every rhythm strip to NSR. Recognizing where the rhythm differs from NSR will help you identify the rhythm.
Normal Sinus Rhythm (NSR)
Normal sinus rhythm is the gold standard. This is what a normal functioning heart beat should look like.
The “sinus” in the name indicates that the electrical signal is coming from the Sinoatrial node (SA node), the “normal” pacemaker of the heart.
The presence of sinus rhythm means the cardiac conduction system is functioning appropriately (although certain blocks may still be present).
The rate of NSR is 60-100 bpm. Slower is sinus bradycardia, and faster is sinus tachycardia. This just means that the heart is functioning at altered rates, possibly due to sleep, medications, infection, exercise, etc.
All sinus rhythms should be regular, meaning each of the QRS complexes are mapping out.
You can do this by measuring the R-R interval between any two beats, and then making sure the R-R interval stays constant throughout the strip. Some people use calipers, but I recommend a good old-fashioned alcohol pad or piece of paper and a pen.
Additionally, a P wave should precede each QRS complex.
The QRS complex should be narrow unless there is a bundle branch block present.
The ECG Rhythm Strip Interpretation
To read an EKG rhythm strip, you should do so in a systematic way, so that you don’t miss anything.
- Is the rhythm regular? Is every R-R interval equal?
- What’s the rate? This is usually printed for you
- P wave: Are there P waves before every QRS?
- PR interval: Is it wide >200ms?
- QRS: Is the QRS narrow or wide (>100-120ms)?
- T waves: Are the T waves upright and normal-appearing?
Using this systematic approach should help you interpret what each rhythm is. But you need to be familiar with most of the arrhythmias out there.
Other Sinus Rhythms
Other sinus rhythms are rhythms that may still “normal”. I include paced rhythms in this section as this replaces NSR once a pacemaker is placed.
Sinus Bradycardia (SB)
Sinus bradycardia is the same as NSR, but the HR is <60bpm.
This can be normal for well-conditioned individuals like athletes, can be normal if the patient is on a beta-blocker or similar medication, and can also be normal while sleeping.
The most important thing when the patient has SB is
- Is it new or severe (<40bpm or so)
- Are they symptomatic? (dizziness, lightheadedness, syncope, SOB, chest pain, etc)
Since this is often a normal variant – if the patient is asymptomatic there’s usually nothing that needs to be done.
Make sure a slow HR is actually SB and not a heart block!
Sinus Tachycardia (ST)
Sinus tachycardia is the same as NSR, but the HR is >100bpm and usually <150bpm, at least while at rest.
This can often be seen with exercise, but ST at rest often indicates anxiety, certain drugs, sepsis, dehydration, or volume loss. ST is usually a response to an underlying cause within the body.
You never treat the ST, but rather treat the underlying issue (i.e. give fluids with volume depletion).
Paced rhythms will look different depending on the location of the leads. If the lead is in the right atria, the rhythm will appear like NSR but with a pacer spike before the P wave.
If the lead is in the right ventricle, it will look like a slow VTACH with a pacer spike before the QRS. There can also be both of these at the same time.
Some monitors only show the pacer spike if you turn that function on – if you see a very slow VT – ask the patient if they have a pacemaker and adjust the monitors appropriately.
Other Cardiac Arrhythmias
Heart blocks are when there is significant delay or blockage in transmitting the signal from the atria to the ventricles. This is usually associated with a junctional or ventricular escape rhythm.
First degree AV block is generally “no big deal” and common in older age and with beta-blockers. The PR interval is consistently >200ms.
Second degree type 1 AV block or Wenckebach, is when there is a progressive lengthening of the PR interval which eventually leads to a dropped QRS complex.
Second degree type 2 AV block or Mobitz II is when there is a consistent PR interval but QRS complexes are randomly dropped.
Third degree AV block or complete heart block is when there is complete dissociation of the atria and the ventricles.
Atrial Fibrillation (AF)
Atrial Fibrillation is a very common type of arrhythmia that you will definitely run into in the hospital. AF could be new-onset, RVR (rapid ventricular response), could be intermittent (paroxysmal), or chronic/persistent.
AF is an irregularly irregular rhythm, meaning that there is no rhyme or reason for the regularity of each QRS complex.
This is usually from a structurally diseased heart where both atria are quivering rapidly, termed fibrillation. This leads to fast ventricular rates (AF RVR), as well as poor blood flow through the atria – predisposing the patient to blood clots.
This is why these patients are started on rate-control medications such as metoprolol or Cardizem, and usually anticoagulants like heparin, Eliquis, etc.
AF will not have p waves but instead, have a fibrillatory baseline. The QRS complexes will usually be narrow, and will not map out with each other in any way.
Rates >100bpm are considered AF RVR.
Atrial Flutter (Aflutter) is similar to Atrial fibrillation and is treated largely the same.
This is when the atria aren’t fibrillating but rather “fluttering”. This is usually from a reentrant loop near the AV node.
This will usually lead to a conduction ratio of 2:1, and a HR around 150bpm. Conduction ratios can be 3:1 (100bpm), 4:1 (75bpm) and variable as well.
You will see saw-tooth P waves termed “f waves”. Depending on the conduction ratio, you will see 2 (3 or 4) F waves per QRS complex. Aflutter is usually regular.
Supraventricular Tachycardia (SVT)
Supraventricular Tachycardia is an umbrella term referring to any fast tachycardia that originates above the ventricles. However, in clinical terms, this usually refers to AV Nodal Reentrant Tachycardia (AVNRT).
This occurs when there is an abnormal pathway of conduction tissue near/within the AV node, termed a “reentrant loop”.
If a PAC or PVC comes at the wrong time, this can send the electrical signal around and around this loop of conduction tissue, leading to very fast heart rates.
SVT can be as “slow” as 140bpm to as fast as 220bpm. The faster the heart rate, the more symptomatic the patient usually is.
In SVT, P waves are usually not present, there is usually ST depression, and the rhythm is regular with narrow QRS complexes.
Treatment for this involves vagal maneuvers and often adenosine or Cardizem.
Ventricular Tachycardia (VTACH or VT)
Ventricular Tachycardia is a fast tachyarrhythmia originating within the ventricles. This leads to very fast heart rates with or without a perfusing rhythm.
This means the patient may not have a pulse and may be a code blue. Either way, VT is a very serious arrhythmia.
VT is usually caused by Coronary heart disease, like a previous or current MI.
The rhythm is regular, and the rate is anywhere from 100-330bpm, and the QRS complex is wide (>140ms).
P waves are usually absent or undetectable, but 60% of cases can have AV dissociation present.
If there is no pulse, you use ACLS cardiac arrest algorithm.
If there is a pulse, you utilize the ACLS Adult tachycardia with a pulse algorithm.
Ventricular Fibrillation (VF or VFIB)
Ventricular Fibrillation is a deadly ventricular arrhythmia. There will not be a pulse, and the patient will be coding.
VF is a similar concept as AF, except the ventricles are the ones fibrillating. Coronary artery disease is again one of the main causes of VF. Severe electrolyte abnormalities can also cause VF.
VF is irregular and has no pattern. There is either coarse or fine fibrillation, eventually degenerating into asystole if not shocked back into a normal rhythm.
These patients need fast defibrillation, high-quality CPR, Epinephrine, antiarrhythmics, etc (Code blue algorithm).
Asystole is the absence of cardiac activity. This is essentially a straight wavy line but may have occasional p waves initially. The patient is dead. Follow ACLS algorithms as above.
Pulseless Electrical Activity (PEA)
PEA appears like a normal rhythm (Usually NSR or SB), but there is no actual mechanical contraction (no pulse). The patient will be unresponsive, pulseless, and this is a code blue as well (follow ACLS).
ECGs Made Easy (6th ed.) <<< this is my favorite ECG PDF
Want to learn more?
Hopefully this gave you a good idea about how to read an EKG rhythm strip. Unfortunately, I couldn’t include every single arrhythmia or detail, but this definitely should give you a good understanding of the basics!
If you want to learn more, I have a complete video course “ECG Rhythm Master”, made specifically for nurses which goes into so much more depth and detail.
With this course you will be able to:
- Identify all cardiac rhythms inside and out
- Understand the pathophysiology of why and how arrhythmias occur
- Learn how to manage arrhythmias like an expert nurse
- Become proficient with emergency procedures like transcutaneous pacing, defibrillation, synchronized shock, and more!
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