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Electrocardiogram (ECG): A Comprehensive Essay

Allied Health / Paramedic Level | 10 Marks

Introduction

The electrocardiogram (ECG) is a non-invasive diagnostic tool that records the electrical activity of the heart over time, displaying it as a waveform on graph paper or a monitor. It remains one of the most valuable and widely used investigations in emergency and primary care settings. For paramedics and allied health professionals, proficiency in ECG acquisition and systematic interpretation is a core clinical skill that directly influences patient outcomes. A structured approach to ECG analysis - covering rate, rhythm, axis, intervals, and waveform morphology - allows the clinician to identify life-threatening conditions rapidly.

Physiological Basis

The heart generates electrical impulses through a specialised conduction system. The sinoatrial (SA) node, situated at the junction of the right atrial appendage and the superior vena cava, acts as the primary pacemaker. Its spontaneous phase 4 depolarisation initiates each beat at a normal resting rate of 60-100 beats per minute (bpm). The impulse spreads across both atria, reaches the atrioventricular (AV) node, traverses the Bundle of His, divides into the left and right bundle branches, and finally propagates through the Purkinje fibre network to depolarise ventricular myocardium in a co-ordinated fashion.

The cardiac action potential underpins these events. Phase 0 (rapid depolarisation) is driven by a fast inrush of Na⁺ ions; phase 2 (plateau) reflects Ca²⁺ influx; and phase 3 (repolarisation) involves K⁺ efflux back to the resting phase 4 potential of approximately -80 to -90 mV in working myocardium. These ionic events are ultimately captured on the surface ECG as distinct waveforms. (Braunwald's Heart Disease, Foundations of Cardiac Electrophysiology)

ECG Paper and Technical Setup

A standard ECG records at a paper speed of 25 mm/second. On the time (x) axis, each small square = 0.04 seconds and each large square = 0.2 seconds; five small squares make one large box. On the amplitude (y) axis, each small square = 0.1 mV. A standard 12-lead ECG runs for 10 seconds, so 300 large boxes represent one minute - this fact underpins the most common heart rate calculation method.

A standard 12-lead ECG uses:

Limb leads (I, II, III, aVR, aVL, aVF) - view the heart in the frontal plane
Precordial leads (V1-V6) - view the heart in the horizontal plane

Proper electrode placement is essential; incorrect placement can simulate pathological changes, produce axis deviation, and lead to misdiagnosis.

Systematic Approach to ECG Interpretation

A systematic approach is universally recommended and includes the following components:

1. Rate

Two methods are standard:

Rate-by-squares method: Count the number of large boxes between two consecutive R waves and divide 300 by that number (e.g., 5 large boxes = 300/5 = 60 bpm). This works well for regular rhythms.
QRS-count method: Count all QRS complexes on the 10-second strip and multiply by 6. This is preferred for irregular rhythms such as atrial fibrillation.

Normal: 60-100 bpm. Bradycardia: <60 bpm. Tachycardia: >100 bpm. (Kaplan & Sadock's Comprehensive Textbook of Psychiatry, Electrophysiology and ECG Interpretation)

2. Rhythm

Assess regularity by measuring R-R intervals. If they are equal, the rhythm is regular; if not, determine whether the irregularity is patterned (regularly irregular, e.g., Wenckebach block) or random (irregularly irregular, e.g., atrial fibrillation). A normal sinus rhythm requires a P wave before every QRS complex, a consistent PR interval, and a normal QRS morphology.

3. Axis

The electrical axis represents the net vector of ventricular depolarisation across the frontal plane. Normal axis lies between -30° and +90°. Axis is assessed primarily using leads I and aVF:

Both positive = normal axis
Lead I positive, aVF negative = left axis deviation (LAD) - seen in left ventricular hypertrophy from sustained hypertension, or left anterior fascicular block
Lead I negative, aVF positive = right axis deviation (RAD) - associated with right ventricular hypertrophy, pulmonary hypertension, or pulmonary embolism
Both negative = extreme axis ("northwest territory") - often indicates ventricular tachycardia or severe pathology

4. Intervals and Segments

Intervals measure the duration of electrical events:

Interval / Wave	Normal Duration	Significance
P wave	0.06-0.12 s (<3 small squares)	Atrial depolarisation
PR interval	0.12-0.20 s (3-5 large squares)	AV conduction time
QRS complex	<0.12 s (<3 small squares)	Ventricular depolarisation
QT interval	<0.44 s (corrected)	Ventricular repolarisation

A prolonged PR interval indicates first-degree heart block or AV nodal delay. A widened QRS (>0.12 s) suggests a bundle branch block or an ectopic ventricular origin. A prolonged corrected QT interval (QTc >440 ms in men, >460 ms in women) carries risk of polymorphic ventricular tachycardia (Torsades de Pointes).

5. Waveform Morphology

Each waveform carries specific diagnostic significance:

P wave - Atrial depolarisation; should be upright in leads I and II. Peaked P waves (P pulmonale) suggest right atrial enlargement; broad, bifid P waves (P mitrale) indicate left atrial enlargement.

QRS complex - Ventricular depolarisation. Normally begins with a small downward Q wave, followed by a tall upward R wave, and a downward S wave. Pathological Q waves (>1 mm wide and >2 mm deep, or >25% of the R wave height) indicate previous myocardial infarction.

ST segment - Connects the end of the QRS (J point) to the start of the T wave, normally isoelectric. ST elevation is the hallmark of a STEMI (ST-elevation myocardial infarction) and reflects transmural ischaemia. Other causes of ST elevation include pericarditis (saddle-shaped, diffuse), myocarditis, stress (Takotsubo) cardiomyopathy, and early repolarisation. ST depression indicates subendocardial ischaemia or reciprocal changes.

T wave - Ventricular repolarisation; normally upright in leads I, II, and V3-V6. T-wave inversion may indicate ischaemia, ventricular hypertrophy, or bundle branch block. Hyperacute (tall, peaked) T waves are an early sign of myocardial infarction.

U wave - A small deflection following the T wave, best seen in V2-V3. Prominent U waves occur with hypokalaemia or bradycardia. (Kaplan & Sadock's Comprehensive Textbook of Psychiatry; Braunwald's Heart Disease)

Clinically Important ECG Patterns

Myocardial Infarction (MI)

STEMI evolves through hyperacute T waves, ST elevation, development of Q waves, and eventually T-wave inversion as the injury area repolarises. Lead localisation identifies the culprit artery:

Inferior MI (II, III, aVF) - right coronary artery
Anterior MI (V1-V4) - left anterior descending artery
Lateral MI (I, aVL, V5-V6) - circumflex artery

Heart Blocks

1st degree: prolonged PR >0.20 s, all impulses conduct
2nd degree Mobitz I (Wenckebach): progressive PR lengthening until a QRS is dropped
2nd degree Mobitz II: fixed PR with intermittent dropped QRS - higher risk of progression
3rd degree (complete): no association between P waves and QRS; escape rhythm takes over

Atrial Fibrillation

Irregularly irregular rhythm, absent P waves replaced by fibrillatory baseline, variable ventricular rate. A common and clinically significant arrhythmia associated with stroke risk.

Bundle Branch Blocks

Left bundle branch block (LBBB): broad QRS with "M" pattern in V5/V6 (William); Right bundle branch block (RBBB): broad QRS with "M" pattern in V1 (Marrow). New LBBB in the context of chest pain should be treated as a STEMI-equivalent.

Role of the Paramedic / Allied Health Professional

In the prehospital setting, early 12-lead ECG acquisition and transmission to the receiving hospital has been shown to significantly reduce door-to-balloon time in STEMI. Paramedics must:

Ensure correct electrode placement to avoid artefact and lead misplacement errors
Apply a systematic interpretation approach on every ECG
Correlate ECG findings with the clinical picture - the ECG is never read in isolation
Recognise STEMI, life-threatening arrhythmias (VT, VF, complete heart block), and critical conduction disease requiring urgent intervention

Correct interpretation also requires ruling out mimics (e.g., Brugada syndrome, electrolyte disturbances such as hyperkalaemia, and medication effects such as QTc prolongation from antipsychotics or tricyclic antidepressants).

Conclusion

The ECG is a rapid, non-invasive, and highly informative investigation that forms the cornerstone of cardiac assessment in emergency and allied health practice. A systematic approach covering rate, rhythm, axis, intervals, and waveform morphology ensures that no critical finding is missed. Understanding the underlying electrophysiology - from the SA node action potential to ventricular repolarisation - provides the framework for interpreting both normal and pathological tracings. For the paramedic, early and accurate ECG interpretation is directly linked to time-critical interventions and improved patient survival.

Key References:

Kaplan & Sadock's Comprehensive Textbook of Psychiatry - Electrophysiology and ECG Interpretation
Braunwald's Heart Disease (10e) - Foundations of Cardiac Electrophysiology; Cardiac Conduction System
Robbins, Cotran & Kumar - Pathologic Basis of Disease (Conduction System)
Ganong's Review of Medical Physiology - Cardiac Conduction System

Word count: ~1,100 words | Suitable for a 10-mark allied health / paramedic exam

Tips for exam writing:

Always open with a definition and clinical relevance
Use a structured subheading format - markers award marks per section
Include a normal values table (it shows factual recall quickly)
Discuss at least 2-3 pathological ECG patterns with lead localisation
End with clinical application (why it matters in your specific role)

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