Basic ECG Interpretation

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Bare Bones Basics of ECG Interpretation from a First Year Medical Student Perspective

Medical Student Clinical Pearl – October 2018

Victoria Kulesza – Med I Class of 2021, Dalhousie Medicine New Brunswick 

Reviewed and Edited by Dr. David Lewis

Physiology

Electrical Events and Corresponding Waves and Lines on a Standard ECG

Basic Interpretation

Common Arrhythmias

Summary

Suggested Resources

References

Physiology

Cardiac cells are electrically polarized in their resting state, with the inside holding a negative charge in comparison to the outside.1,3 Membrane pumps maintain this electrical polarity through the regulation of ions including potassium, sodium, chloride and calcium.1 Depolarization is the key electrical event of the heart that occurs spontaneously in some cells and is initiated by the arrival of an electrical impulse carrying positively charged ions in other cells.1 There are 3 key cells involved in the electrical and mechanical activities that occur within the heart:

 

The sequential depolarization of cells creates a wave of depolarization that transmits across the entire heart, representing a flow of electricity that can be detected by the electrodes placed on the surface of that patient’s body. The waveforms visible on the ECG represent the electrical activity of the myocardial cells, the cells making up the vast majority of the heart.1 At the end of the depolarization process, cardiac cells are repolarized through membrane pumps reversing the flow of ions. Both the depolarization and repolarization are represented as the wave forms on the ECG.1

Electrical Events and Corresponding Waves and Lines on a Standard ECG

P Wave

The heartbeat is initiated in the sinoatrial node located in the posterior wall of the right atrium.4 After the sinus node fires, the atrial myocardium is depolarized in a wave-like fashion causing the atrial contraction. This depolarization and contraction of the atrial myocardial cells results in the first P wave.1 The wave of depolarization does not immediately pass through to the ventricles, the atrioventricular node located at the floor of the right atrium, slows the conduction of the electrical impulse to allow the atria to fully complete their contraction. 1,4 The contraction of the atria forces blood from the atria through the atrio-ventricular valves, known as the tricuspid and mitral valves, into the ventricles.3

PR Interval

This interval is the time that is required for the electrical impulse to travel from the atria, through the AV node, bundle of His, bundle branches and Purkinje fibers to the point where the ventricular myocardium begins its depolarization.5 As blood flows through the AV valves the physiologic pause in electrical conduction is represented on the EKG as the flat line following the initial P wave. The ventricular conduction system is composed of 3 parts including the Bundle of His, Bundle Branches and the Terminal Purkinje Fibers.1 The ventricular depolarization is rapidly transmitted through the Bundle of His which emerges from the AV node and subsequently bifurcates into the left and right bundle branches which carry the impulse down the interventricular septum to their terminating fascicles in multiple Purkinje fibers.1,3 Once this current is delivered to the ventricular myocardium the depolarization causes ventricular contraction visible on the ECG as the QRS complex.1

PR Segment

A straight line between the end of the P wave and the start of the QRS complex reflects the time between the end of atrial depolarization and the start of ventricular depolarization.1

QRS Complex

The QRS complex consists of 3 individual waves in a normal conduction1,3:

  • Q Wave: first deflection downward
  • R Wave: first upward deflection
  • S Wave: first downward deflection subsequent to an upward deflection

A complete QRS complex represents ventricular depolarization as well as the initiation of ventricular contraction.1,3 The use of the term QRS Interval describes the duration of the QRS complex alone indicating the duration of ventricular depolarization specifically.1

ST Segment

A straight line between the end of the QRS complex and the beginning of the T wave known as the ST segment measures the time from the end of ventricular depolarization to the beginning of repolarization.1

T Wave

Following the depolarization of the myocardial cells, there is a short refractory period and subsequent recovery phase identified as the T wave on the ECG.1,3,5 This is phase of ventricular repolarization that begins after the QRS and is completed at the end of the T wave.3,5 Repolarization is a slower process than the depolarization which is illustrated by the broader nature of the T wave in comparison to the QRS.1,5

QT Interval

This interval includes the QRS complex, ST segment as well as the T wave which allows for the measurement of time between the beginning of ventricular depolarization to the end of ventricular repolarization.

 

Basic Interpretation

The most effective way to ensure clinically significant abnormalities are not missed on ECG is to develop a consistent order of analysis. One suggested order is as follows:

 

A. Determine Rate:

  1. Sinus Tachycardia = >100 BPM
  2. Sinus Bradycardia = <60 BPM
  3. Three Ways to Determine Rate:
    • Identify an R wave that falls on or near one of the heavy lines of the ECG strip, count the number of large squares between this first R wave and the beginning of the subsequent wave. Divide 300 by the number of large squares between the R waves to determine the number of cardiac cycles per minute. Counting the number of small squares between R waves and dividing 1500 by this number would identify with greater accuracy the heart rate.1
    • Identify the series of small pink indicators above the rhythm strip that identify 3 second intervals and count the number of cycles between two 3 second intervals – multiply this number by 10 to identify the number of beats per minute.1
    • In the event of an irregular heartbeat identify the number of QRS complexes and multiply this number by 6. Each started ECG paper reads at 25mm/s therefore 1 ECG represents 10 seconds of activity.2

Thaler 2015

 

B. Intervals:

Identify the length of the PR and QT Intervals as well as the width of the QRS complexes

Normal Interval Lengths5:

  1. PR = 0.12 – 0.20 sec
  2. QT = varies with overall heart rate
  3. QRS = 0.05 – 0.10 sec

 

 

 

 

 

 

 

C. Rhythm5:

  1. P waves present and normal?
  2. QRS complexes wide or narrow? General pattern – regular, regularly irregular or irregularly irregular?
    1. Wide = >0.12 sec
    2. Narrow = <0.12 sec
  3. Relationship between P waves and QRS complexes
  4. Overall rhythm regular or irregular?

 

D. Axis

  1. The ECG electrodes record the average direction of flow of electrical current within the heart.
  2. Lead I is the zero reference point, any axis lying below is deemed positive while those lying above are deemed negative.
  3. When the wave of depolarization begins, any lead that views this wave as moving towards it will record this as a positive deflection on the ECG paper.
  4. Assessment of P Wave Axis:
    • Atrial depolarization begins at the sinus node in the right atrium and follows a right to left and inferior direction. This depolarization of the right to left atria should demonstrate a positive deflection in leads aVL, I, II and aVF.
  5. Assessment of QRS Complex Axis:
    • As the wave of depolarization moves through the interventricular septum the current moves in a left to right direction. This wave may not be visible on the ECG but when apparent appears as a negative deflection in leads I, aVL (V5 and V6). As a result of the increased size of the left ventricle in comparison to the right, the remainder of the QRS complex vector of flow is directed leftward and is demonstrated as the positively deflected R wave in most left lateral and inferior leads. The aVR lead will record a deep negative deflection based on the direction of flow being away from this lead.

 

 

Common Arrhythmias1

1. Sinus Tachycardia

  • HR >100 bpm
  • Can be normal or pathologic, strenuous exercise can cause HR above 100.

 

2. Sinus Bradycardia

  • HR <60 bpm
  • Can be normal or pathologic, many well-conditioned athletes maintain a resting HR below 60.

 

3. Paroxysmal Supraventricular Tachycardia

  • HR 150-250 bpm
  • Narrow complex QRS
  • Very common, sudden onset, sudden termination.
  • Clinical Symptoms: palpitations, shortness of breath, dizziness. Possibly induced by alcohol, caffeine or extreme excitement.

 

4. Atrial Flutter

  • P waves 250-350 bpm
  • Atrial depolarization occurs so rapidly that discrete P waves are indiscernible.
  • Leads II and III demonstrate a prominent saw-tooth
  • AV node cannot handle the number of atrial impulses therefore there is an unequal number of P waves to QRS complexes – some electrical impulses from the sinus node bump into a refractory node and go no further, this is called AV Block. 2:1 block is most common while 3:1 and 4:1 are also frequently observed.
  • Clinical Symptoms: shortness of breath, angina type discomfort.

 

 

5. Atrial Fibrillation

  • AV Node may receive >500 impulses per minute
  • More common than atrial flutter, most commonly sustained arrhythmia.
  • No true P waves are discernible, AV node allows occasional impulses to pass through to the ventricles, creating an irregularly irregular ventricular rate often in the range of 120-180 bpm.
  • Clinical Symptoms: some patients experience no symptoms, others experience shortness of breath, chest pain, palpitations and dizziness.

 

6. Premature Ventricular Contractions

  • Most common ventricular arrhythmia.
  • Retrograde P wave or no P wave prior to the QRS.
  • Wide QRS of at least 0.12 seconds in majority of the leads often followed by a compensatory pause before the subsequent beat.
  • Often occur randomly and rarely require treatment unless an isolated PVC is noted in the setting of acute MI as it may trigger ventricular tachycardia or ventricular fibrillation.
  • When to worry:
    • Frequent PVCs
    • Consecutive runs, 3+ in a row
    • Multiform – demonstrating variation in the site of origin
    • Occurring on the T wave – “R-on-T” phenomenon
    • PVC in the setting of an acute MI

 

 

7. Ventricular Tachycardia

  • Rate 120-200 bpm
  • Wide complex QRS
  • A run of 3+ consecutive PVCs.
  • Prolonged ventricular tachycardia is an emergency requiring immediate treatment to prevent cardiac arrest.
  • May be uniform or polymorphic, uniform being more closely associated with healed infarctions and polymorphic waveforms more commonly associated with acute coronary events.

 

8. Ventricular Fibrillation

  • Spasmodic tracings or coarse ventricular fibrillation or fine ventricular fibrillation without any true QRS complexes.
  • Heart generates no cardiac output, CPR and defibrillation are required immediately.
  • Most common arrhythmia in adults who experience sudden death.
  • Common predisposing factors:
    • Myocardial ischemia/infarction
    • Heart failure
    • Electrolyte disturbances
    • Hypoxemia or hypercapnia
    • Hypotension or shock
    • Overdoses of stimulants especially when used in combination with others

 

 

Summary

 

 

 

 

Suggested Resources

Teaching Medicine – Rhythm Strip Interpretation Practice

ECG Guide Mobile Smartphone App

  • Available through itunes app store

The Only EKG Book You’ll Ever Need

  • PDF available online through Dalhousie Library

 

References

  1. Thaler, M. S. (2015). The Only EKG Book You’ll Ever Need (9th ed.). Lippincott, Williams & Wilkins.
  2. Andrade, J. (2013). ECG Guide [Mobile application software]. Retrieved from http://itunes.apple.com
  3. Dubin, D. (2000). Rapid interpretation of EKG’s: An interactive course (6th ed.). Tampa, Fla.: Cover Pub.
  4. McKinley, M. P., OLoughlin, V. D., Harris, R. T., & Pennefather-O’Brien, E. E. (2015). Human anatomy (4th ed.). New York, NY: McGraw-Hill Education.
  5. Khan, M. (2008). Rapid ECG interpretation (3rd ed., Contemporary cardiology (Totowa, N.J). Totowa, N.J.: Human Press.
  6. Thomas, V. (n.d.). Premature Ventricular Contractions Treatment Cape Town. Retrieved from https://cardiorhythm.co.za/premature-ventricular-contractions/
  7. https://inside.fammed.wisc.edu/medstudent/pcc/ecg/axis.html
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