How to Read an Electrocardiogram (EKG/ECG)

Being able to read an EKG or ECG is an important skill for nurses. But looking at a 12-lead EKG/ECG can put your own heart into a lethal rhythm. Nurses are usually the first ones to read that “hot off the press” EKG tracing. This article will explain everything you need to know about EKG readings.

EKG vs ECG

It’s important to know that there is no difference between an ECG and an EKG. Both refer to the same procedure; however, one is in English (electrocardiogram – ECG) and the other is based on the German spelling (elektrokardiogramm – EKG).

What is an EKG?

Before interpreting an EKG, it is important to know what an EKG is and its importance. An EKG/ECG is a representation of the electrical activity of the heart muscle as it changes with time, usually printed on paper for easier analysis. The EKG/ECG is a printed capture of a brief moment in time. 

A 12-lead EKG is considered the gold standard; however, a 4-lead EKG can also diagnose different heart conditions.

EKGs can be used to diagnose heart attacks, heart problems including electrical malfunctioning, and other heart problems. They are often used to diagnose heart problems in combination with an echocardiogram or echo.

How to Read an EKG/ECG

How do you know when you need to act immediately or can wait for expert consultation? Here are seven tips to help you gain confidence in interpreting what you see.

1. Assess Your Patient

This must come first! There are many clues you can learn when obtaining the EKG that will help you analyze and act on what you see.

Looking at a sheet of paper with a tracing on it does not provide enough information. A heart rate of 38 can be normal in an athlete.¹ But it also can require an immediate pacemaker insertion if accompanied by chest pain, shortness of breath, and an EKG/ECG interpretation of third-degree heart block.²

2. Know Your Normals

Don’t sweat all the complex details when you are first beginning to read and interpret EKGs. A normal heart rhythm contains a P wave, a QRS, and a T wave.³ Knowing the normal amplitude, deflection, and duration of each component is essential to accurate rhythm and EKG/ECG interpretation.

• Amplitude: This measures the voltage of the beat and is determined by how high the wave reaches, as measured by each square vertically on the chart. 10 mm = 1 mv. 5 squares = .5 mV and 2.5 squares = .25 mV
• Deflection: Which lead on the patient it’s coming from
• Duration: How long it is, as measured by squares going horizontal⁴

What’s a normal adult heart rate?

• Normal = 60 – 100 bpm
• Tachycardia > 100 bpm
• Bradycardia < 60 bpm

The twelve leads show the electrical current through the heart from different planes.  Think of each lead as a different snapshot of the heart you are trying to interpret.  

There are six limb (I, II, III, AVR, AVL, AVF) leads and six precordial (V1-V6) leads.  The limb leads look at the heart from a vertical perspective; the V leads show a horizontal perspective.  

Lead placement: 

• V1: at the 4th intercostal space (ICS), on the right sternal border
• V2: 4th ICS, along the left sternal border
• V4: 5th ICS, at the mid-clavicular line
• V6: 5th ICS, mid-axillary line (same level as V4)
• V5: 5th ICS, at the anterior axillary line (same level as V4)
• V3: midway between V2 and V4

Keeping this in mind will help you interpret what you see and identify which areas of the heart may be “hurting” or damaged.  

3. Use a Systematic Approach

Approach your analysis to a 12-lead EKG/ECG the same way every time. 

First, determine the rate, and if any tachycardia (more than 100 beats/minute) or bradycardia is present (less than 60 beats/minute).  

Next, determine whether your rhythm is regular or irregular; you can use calipers if you have them or use a simple piece of paper and track your P waves and QRS complexes with a pencil mark and see if they march along or have gaps. 

If a patient has a regular heart rhythm, determine their heart rate by,

• Count the number of large squares present within one R-R interval.
• Divide 300 by this number to calculate heart rate.

If the heart rhythm is irregular, then you will not be able to use the aforementioned method. Instead, a different method will need to be used,

• Count the number of complexes on the rhythm strip
• Multiply the number of complexes by 6

This will identify the average number of complexes in one minute. 

After determining this, next decide if your rhythm is fast or slow, irregular or regular (more on this in the next section).  

Lastly, examine the ST segments for any elevation or depression; again, you can use a sheet of paper to help you evaluate this.  If you can tell elevation or depression without the help of paper, be prepared to act and inform the provider immediately. 

4. Determine Your Heart Rate 

Look at the EKG/ECG to see if the rate is regular and how fast the heart is beating; both are important for rhythm interpretation. The pace at which a rhythm is conducting can help determine the stability of the rhythm.  A stable rhythm often correlates with a stable patient.  Slow or fast can be “good” or “bad” depending on the patient presentation and corresponding rhythm.  

Rate is usually determined by which electrical circuit is “conducting” the heart.  Rhythms conducted above the atria are usually above 60 and tend to be abnormal when the rate is fast (atrial flutter, atrial fibrillation, supraventricular tachycardia).  Rhythms conducted below the atria are slower and tend to be unstable when the rate is irregular (heart blocks). 

Another comment about rate: know what medications your patient is taking. Many heart medications have beta-adrenergic effects, which correlate to slower heart rates, such as beta-blockers.    

It is important to determine if a heart rate is regular or irregular. A regular heart rhythm has all of the aspects previously discussed. 

Irregular rhythms can be either:

• Regularly irregular (i.e. a recurrent pattern of irregularity)
• Irregularly irregular (i.e. completely disorganized)

In order to determine if a rhythm is regular, mark out several consecutive R-R intervals on a piece of paper, then move them along the rhythm strip to check if the subsequent intervals are the same.

5. Identify the Cardiac Axis

The axis of an ECG is the major direction of the overall electrical activity of the heart. The QRS complex is used to identify this. There are several different findings related to the axis including, 

• Normal - QRS complex is upright in both lead I and lead aVF
• Leftward (left axis deviation, or LAD) - QRS is upright in lead I (positive) and downward in lead aVF (negative)
• Rightward (right axis deviation, or RAD) - negative in lead I and positive in lead aVF
• Indeterminate (northwest axis) - downward (negative) in lead I and downward (negative) in lead aVF

Causes related to left axis deviation include, 

• Normal variant
• Left anterior fascicular block
• Left ventricular hypertrophy (rarely with LVH; usually axis is normal)
• Left bundle branch block (rarely with LBBB)
• Mechanical shift of heart in the chest (lung disease, prior chest surgery, etc.)
• Inferior myocardial infarction
• Wolff-Parkinson-White syndrome with “pseudoinfarct” pattern
• Ventricular rhythms (accelerated idioventricular or ventricular tachycardia)
• Ostium primum atrial septal defect

Causes related to right axis deviation include, 

• Normal variant
• Right bundle branch block
• Right ventricular hypertrophy
• Left posterior fascicular block
• Dextrocardia
• Ventricular rhythms (accelerated idioventricular or ventricular tachycardia)
• Lateral wall myocardial infarction
• Wolff-Parkinson-White syndrome
• Acute right heart strain/pressure overload — also known as McGinn-White Sign or S1Q3T3 that occurs in pulmonary embolus

6. Identify Lethal Rhythms

When evaluating lethal rhythms on a 12 lead EKG, it is important to remember the rhythm alone can be lethal as well as what the EKG/ECG is showing you in terms of heart function.  

A rhythm that does not perfuse well can lead to impending heart failure quickly if not addressed.  

Some dangerous heart rhythms are: 

• Mobitz Type II (Type 2 Heart Block)
  • Consistent PR interval with intermittently dropped QRS complexes
• Third Degree Heart Block
  • No electrical communication between the atria and ventricles due to a complete failure of conduction.
  • Presence of P waves and QRS complex with no association
• Ventricular Tachycardia
  • Widened QRS complex with a rate greater than 100 beats per minute

• Idioventricular Rhythms
  • Absence of P waves, prolonged QRS interval, and rate of less than 50 beats per minute

Other potentially concerning heart rhythms are:

When it comes to heart function, the view (lead) you are looking at will determine which part of the heart you are trying to interpret. This is especially important when analyzing ST segment abnormalities. The location of the infarct determines what treatment should be used to improve oxygenation to the heart to minimize damage.  

7. Access Your Resources

There are many awesome resources available for review, as well as practice EKG tracings to perfect your skills.  

The more familiar you are with different rhythms, the easier interpretation becomes.  Don’t forget your colleagues are great resources as well; let them know you are working on your 12 lead EKG interpretation skills and ask them to save interesting tracings for your review.  

A favorite EKG interpretation resource is ECG Interpretation Made Incredibly Easy. I also really like websites that let you practice rhythm strips and EKG interpretation for free like PracticalClinicalSkills.com.

8. Look at Your Patient

Yes, this is a repeat, but it is an important repetition. The most stable-looking rhythm can be lethal if it doesn’t match what your patient is telling you.  If your EKG findings are in complete disconnect from what you are seeing with your patient, you should also double-check your leads to make sure they are on correctly, have not come loose, or have any disturbance, as correct lead placement is critical to accurate interpretation.  

Trust your gut. Nurses have great intuition skills. Don’t be afraid to ask questions and seek more information when you feel something isn’t right. 

FAQs

• How do you read an EKG/ECG for beginners?
  • Determine the heart rate by counting the number of large squares present on the EKG within one R-R interval and dividing by 300. Identify the axis. Know abnormal and lethal rhythm findings
• What does a normal EKG/ECG look like?
  • A normal EKG is one that shows what is known as sinus rhythm. There will be P Waves, T Waves, and the QRS Complexes. 
• What are good numbers for an EKG/ECG?
  • Good numbers on an EKG depend on several factors including the age, sex, race, and health condition of the patient. For example, a patient with an underlying heart defect or disease will have a different “normal” than a healthy middle-age individual. However, the normal heartbeat of a healthy adult is 60 to 100 beats per minute (bpm)
• What is an abnormal EKG/ECG?
  • An abnormal EKG can mean many things. It’s important to know if the patient has any underlying heart disease or known defects. In this case, an abnormal finding would be “normal” for this patient. However, typically an abnormal EKG would be any rhythm that is NOT sinus rhythm and an abnormality in the P Waves, T Waves, and/or  QRS Complexes. 

Sources

• Heart Rhythm Society
• Third-Degree Atrioventricular Block - Vinicius Knabben; Lovely Chhabra; Matthew Slane
• The University of Toledo Medical Center
• How to Read an ECG Strip
• Cleveland Clinic 12 Lead ECG Interpretation

*This website is provided for educational and informational purposes only and does not constitute providing medical advice or professional services. The information provided should not be used for diagnosing or treating a health problem or disease.