Reading ECG: Made Simple for Nurses

An electrocardiograph (EKG or ECG) records the electrical signals from a heart and captures them on a computer monitor or printer. For instance, Cardiologists often run this test to decide whether a heart is healthy. They are also often used to check for different heart conditions. 

ECG Basics

1. ECG (Electrocardiogram) measures heart’s electrical activity.
2. P-wave: Atrial depolarization.
3. QRS complex: Ventricular depolarization.
4. T-wave: Ventricular repolarization.
5. PR interval: Time between P-wave and QRS complex.
6. QT interval: Time between QRS complex and T-wave.

Normal ECG Values

1. Heart rate: 60-100 bpm.
2. PR interval: 0.12-0.20 seconds.
3. QRS duration: 0.06-0.10 seconds.
4. QT interval: 0.36-0.44 seconds.

Regular heart rhythm using ECG

If a patient has a regular heart rhythm, their heart rate can be calculated using the following method:

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

Heart rate calculation example

• 4 large squares in an R-R interval
• 300/4 = 75 beats per minute

Irregular heart rhythm

If a patient’s heart rhythm is irregular, the first heart rate calculation method doesn’t work (as the R-R interval differs significantly throughout the ECG). As a result, you need to apply a different method:

• Count the number of complexes on the rhythm strip (each rhythm strip is typically 10 seconds long).
• Multiply the number of complexes by 6 (giving you the average number of complexes in 1 minute).

Heart rate calculation example

• 10 complexes on a rhythm strip
• 10 x 6 = 60 beats per minute

Heart rhythm

A patient’s heart rhythm can be regular or irregular.

Irregular rhythms can be either:

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

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 similar.

Common ECG Abnormalities

1. Bradycardia (slow heart rate): <60 bpm.
2. Tachycardia (fast heart rate): >100 bpm.
3. Arrhythmias (irregular heartbeat):
   • Atrial fibrillation (AFib).
   • Ventricular tachycardia (VT).
4. Blockages:
   • First-degree AV block.
   • Second-degree AV block.
   • Third-degree AV block.
5. Ischemia/Infarction:
   • ST-elevation myocardial infarction (STEMI).
   • Non-ST-elevation myocardial infarction (NSTEMI).

Cardiac axis

Cardiac axis describes the overall direction of electrical spread within the heart.

In a healthy individual, the axis should spread from 11 o’clock to 5 o’clock.

To determine the cardiac axis, you must look at leads I, II and III.

Normal cardiac axis

Typical ECG findings for normal cardiac axis:

• Lead II has the most positive deflection compared to leads I and III.

Right axis deviation

Typical ECG findings for right axis deviation:

• Lead III has the most positive deflection, and lead I should be negative.
• Right axis deviation is associated with right ventricular hypertrophy.

Left axis deviation

Typical ECG findings for left axis deviation:

• Lead I has the most positive deflection.
• Leads II and III are negative.
• Left axis deviation is associated with heart conduction abnormalities.

ECG Interpretation Steps

1. Determine heart rate.
2. Identify rhythm (regular/irregular).
3. Measure PR interval.
4. Examine QRS complex.
5. Evaluate T-wave.
6. Check for ST-segment changes.

Nurse’s Role in ECG Interpretation

1. Recognize normal/abnormal ECG patterns.
2. Identify potential cardiac conditions.
3. Communicate findings to healthcare team.
4. Monitor patient’s condition.
5. Provide appropriate care.

Tips for Nurses

1. Practice ECG interpretation regularly.
2. Use ECG interpretation guides.
3. Correlate ECG findings with patient symptoms.
4. Seek guidance from experienced healthcare professionals.
5. Stay updated on latest ECG guidelines.

ECG Interpretation for Specific Conditions:

1.Myocardial Infarction (MI)

• ST-elevation MI (STEMI):
  • ST elevation >2mm in 2+ contiguous leads
  • Q-waves indicate necrosis
• Non-ST-elevation MI (NSTEMI):
  • ST depression, T-wave inversion
  • No Q-waves

2.Arrhythmias

• Atrial Fibrillation (AFib):
  • Irregularly irregular rhythm
  • No P-waves
• Ventricular Tachycardia (VT):
  • Wide QRS complex
  • Rate >100 bpm

3.Heart Block

• First-degree AV block:
  • PR interval >0.20 seconds
• Second-degree AV block:
  • Intermittent dropped beats
• Third-degree AV block:
  • Complete dissociation between P-waves and QRS

4.Cardiac Conduction Disorders

• Left Bundle Branch Block (LBBB):
  • Wide QRS complex
  • Left axis deviation
• Right Bundle Branch Block (RBBB):
  • Wide QRS complex
  • Right axis deviation

5.Electrolyte Imbalances

• Hyperkalemia:
  • Tall, peaked T-waves
  • Wide QRS complex
• Hypokalemia:
  • Flat, inverted T-waves
  • U-waves

6.Other Conditions

• Hypertrophic Cardiomyopathy (HCM):
  • Left ventricular hypertrophy
  • Abnormal Q-waves
• Wolff-Parkinson-White (WPW) Syndrome:
  • Short PR interval
  • Wide QRS complex

Advanced ECG: Bundle Branch Blocks

Bundle Branch Blocks (BBB):

1. Left Bundle Branch Block (LBBB)
2. Right Bundle Branch Block (RBBB)
3. Bifascicular Block
4. Trifascicular Block

1.LBBB:

1. Characterized by:
   • Wide QRS complex (>0.12 seconds)
   • Notched or slurred R-wave in lateral leads (I, aVL, V5-6)
   • Absent S-wave in lateral leads
2. Causes:
   • Cardiomyopathy
   • Hypertension
   • Coronary artery disease
   • Cardiac surgery
3. Clinical significance:
   • May indicate underlying cardiac disease
   • Can lead to complete heart block

2.RBBB:

1. Characterized by:
   • Wide QRS complex (>0.12 seconds)
   • RSR’ pattern in V1-2
   • S-wave in lateral leads
2. Causes:
   • Pulmonary embolism
   • Chronic obstructive pulmonary disease (COPD)
   • Cardiac surgery
   • Congenital heart disease
3. Clinical significance:
   • May indicate pulmonary or cardiac disease
   • Can lead to complete heart block

3.Bifascicular Block:

1. Characterized by:
   • RBBB + left anterior or posterior fascicular block
2. Causes:
   • Cardiomyopathy
   • Coronary artery disease
   • Cardiac surgery
3. Clinical significance:
   • Increased risk of complete heart block

4.Trifascicular Block:

1. Characterized by:
   • RBBB + left anterior and posterior fascicular block
2. Causes:
   • Cardiomyopathy
   • Coronary artery disease
   • Cardiac surgery
3. Clinical significance:
   • High risk of complete heart block

REFERENCES

1. ECG interpretation. Medical training and simulation. (2019) accessed 02/19/2019. https://www.practicalclinicalskills.com/ecginterpretation
2. Jackson, Matthew. (Cardiology Data interpretations) How to read an ECG. Accessed 02/19/2019 from https://geekymedics.com/how-to-read-an-ecg/
3. Wetherell, Heather. My top 10 tips for ECG interpretation. Accessed 02/19/2019 from https://bjcardio.co.uk/2014/03/my-top-10-tips-for-ecginterpretation/
4. American Heart Association (AHA) – ECG Guidelines

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