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12-Lead ECG Interpretation – Introduction and Fundamentals

Rationale & Scope of Learning

  • Primary goal: achieve competency in resting & exercise 12-lead ECG interpretation for use in clinical exercise physiology, cardiac rehabilitation, & emergency screening.

  • Contextual importance

    • Cardiovascular disease = leading cause of death in Australia ➜ earlier recognition saves lives.

    • Clinicians may be handed historical ECGs/reports by patients; you must independently verify safety before exercise prescription.

    • Sporting example: sudden-cardiac-arrest of Belgian footballer Anthony van Loo (07-06-2009) demonstrates life-saving value of real-time ECG awareness.

  • Assessment boundaries

    • Slides flagged with 🔸 (orange diamond) are NOT examinable in the mid-semester test.

    • Minor definitional discrepancies exist across textbooks/websites; default to definitions supplied in this unit.

Recommended Learning Resources (all free unless stated)

  • en.ecgpedia.org ("Wikipedia of ECGs").

  • medicine-on-line.com → succinct basics.

  • lifeinthefastlane.com ECG Library (LITFL) → pathology image bank & coupon LITFL for 50 % off lifetime student membership (USD 15 → 7.50).

  • ecgwaves.com “The ECG Book” – free chapters, optional paid upgrade.

  • University of Sydney Library → multiple hard-copy ECG texts.

  • LITFL list of “Top 20 Online ECG Courses”.

Basic Cardiac Anatomy Refresher

  • Chambers & inflow/outflow

    • RA (right atrium) ⇄ SVC (superior vena cava) / IVC (inferior vena cava), tricuspid (valve) → RV (right ventricle) ⇄ pulmonary semilunar valve → PA (pulmonary artery).

    • LA (left atrium) ⇄ four pulmonary veins, mitral valve → LV (left ventricle) ⇄ aortic semilunar valve → aorta (arch & descending).

  • Support structures: chordae tendineae, papillary muscles, interventricular septum (myocardium thickness ↑ in LV).

  • Coronary circulation not pictured but clinically relevant for ischaemic changes.

The Cardiac Cycle (Mechanical Events)

  1. Late diastole – all chambers relaxed, passive ventricular filling (80-90%).

  2. Atrial systole – atrial “kick” adds ≈ 20 % ventricular volume.

  3. Isovolumic ventricular contraction – AV valves close (first heart sound, S1), pressure ↑ but SL valves still shut.

  4. Ventricular ejection – ventricular pressure > arterial; SL valves open, stroke volume ejected.

  5. Isovolumic ventricular relaxation – SL valves snap shut (S2), pressure falls with no volume change.

  • Graphic relationships (Wiggers diagram)

    • LV pressure, aortic pressure, LA pressure, LV volume, heart sounds, ECG overlay.

Cardiac Output

  • Definition: CO = HR \times SV

  • Normal resting range: 4\text{–}8\,\text{L\,min}^{-1}.

  • Increases required by exercise or thermal stress achieved via ↑HR, ↑SV, or both.

Electrophysiology Fundamentals

  • Cardiac contraction requires membrane depolarisation.

  • Pacemaker cells (sinoatrial node - located RA; atrioventricular node - located between atria and ventricles):

    • Automaticity – spontaneous phase-4 depolarisation once threshold reached.

    • Unstable resting membrane potential (If “funny” current facilitates drift).

    • SA node

      • contracts 60-100 bpm

      • If fails to fire → AV node will fire

    • Atrioventricular node

      • contracts 40-60 bpm

      • if fails to fire → purkinje fibres will fire

    • purkinje fibres

      • contracts 20-40 bpm

    • extra beats: if AV node or purkinje fibres fire spontaneously, this can lead to extra beats which may manifest as premature ventricular contractions (PVCs) or other ectopic rhythms in the heart.

    • SA node not firing correctly: AV node and/or purkinje fibres will fire at a slower rate if they take over as the primary pacemaker, which can further impact heart rhythm and efficiency.

  • sinoatrial node → bachmann’s bundle; internodal tract (posterior - thorel’s; middle - wenckebach’s; anterior) → atrioventricular node → bundle of His → right bundle branch & left bundle branch (both middle) → purkinje fibres

Autonomic Control

  • Sympathetic (β₁ + circulating epinephrine)

    • Chronotropic ↑HR, dromotropic ↑ AV conduction, inotropic ↑ contractility, lusitropic ↑ relaxation rate.

  • Parasympathetic (vagal)

    • Negative chronotropy – dominant at rest, slows SA & AV nodes.

12-Lead ECG Acquisition

  • Hardware: 10 electrodes → 12 leads.

    • precordial/chest leads are placed in specific locations on the chest to capture the electrical activity of the heart from multiple angles, allowing for comprehensive analysis of cardiac rhythm and function.

      • V1: located in the fourth intercostal space at the right sternal border, it provides crucial information about the right ventricle and anterior wall of the heart.

      • V2: located in the fourth intercostal space at the left sternal border, it offers valuable insights into the anterior wall and the left ventricle's activity.

      • V3: located in the fifth intercostal space at the midclavicular line, it helps assess the anterior wall of the left ventricle and provides information about the septal region.

      • V4: located in the fifth intercostal space, at the left anterior axillary line, it plays a key role in evaluating the anterior wall of the left ventricle and contributes to understanding the heart's overall electrical activity.

      • V5: located in the fifth intercostal space at the left mid axillary line, it is crucial for analyzing the lateral wall of the left ventricle and can indicate potential ischemic changes in this region.

      • V6: located in the fifth intercostal space at the left anterior axillary line, it is essential for assessing the lateral wall of the left ventricle and provides valuable insights into cardiac health and function.

    • Limb Leads: the limb leads, which include I, II, III, aVR, aVL, and aVF, are positioned on the patient's arms and legs, providing crucial information about the heart's electrical activity from different angles, helping to identify arrhythmias and other cardiac issues.

      • LA: located on the left arm (or clavicle/torso during exercise), it forms part of the limb leads and is instrumental in monitoring the heart's electrical activity, particularly aiding in the detection of abnormalities in the heart's rhythm and axis.

      • RA: located on the right arm (or clavicle/torso during exercise), it also plays a critical role in the limb lead configuration, allowing for comprehensive analysis of the heart's electrical pathways and detecting any potential issues related to atrial activity.

      • LL: located on the left leg (or torso during exercise), it complements the limb lead system by providing additional insights into the heart's overall electrical function, particularly in assessing inferior wall abnormalities and detecting disturbances in conduction.

      • RL: located on the right leg (or torso during exercise), it finalizes the limb lead arrangement, enabling a complete view of the heart's electrical activity while helping to identify inferior wall discrepancies and aiding in the overall assessment of cardiac function.

Lead System & Viewing Planes

  • Chest/precordial electrodes (V1–V6)

    • Unipolar, positive exploring electrode; reference = Wilson central terminal (average of limb electrodes).

    • Horizontal (transverse) plane view.

  • Limb electrodes (RA, LA, LL)

    • Unipolar augmented leads aVR, aVL, aVF (reference = midpoint of other two limbs).

      • aVF: a unipolar lead that measures electrical activity from the left leg (LL - pos.) to the midpoint of the right arm (RA) and left arm (LA), providing insights into the inferior wall of the heart.

      • aVR: a unipolar lead that measures electrical activity from the right arm (RA - pos.) to the midpoint of the left arm (LA) and left leg (LL), mainly reflecting the activity from the right upper part of the heart.

      • aVL: a unipolar lead that assesses electrical impulses from the left arm (LA - pos.) to the midpoint of the right arm (RA) and left leg (LL), offering perspectives on the lateral wall of the left ventricle.

    • Bipolar leads I (LA-RA), II (LL-RA), III (LL-LA) forming Einthoven’s Triangle – frontal (vertical) plane.

      • lead I: A bipolar lead formed by the difference in electrical potential between the left arm (LA - pos.) and the right arm (RA - neg.) electrodes, providing insight into the heart's electrical activity from a specific angle - specifically, it allows us to assess the electrical forces directed from the right arm towards the left arm, highlighting any potential abnormalities in the superior-inferior axis of heart function.

      • lead II: This bipolar lead is created by measuring the electrical potential difference between the left leg (LL - pos.) and the right arm (RA - neg.) electrodes, effectively providing a view of the heart's electrical activity as it travels down towards the left leg. It is particularly useful for detecting issues in the inferior aspect of the heart.

      • lead III: This lead is established by the electrical potential difference between the left leg (LL - pos.) and the left arm (LA - neg.) electrodes, focusing on the heart's electrical journey from the left arm downward to the left leg, which assists in identifying abnormalities in the inferior and lateral aspects of cardiac performance.

Axial Reference

  • Standard hexaxial diagram (−150° to +210°). Quick quadrant review:

    • Lateral: 0° to −30°.

      • V5, V6, lead I, aVL

    • Inferior: +60° to +120°.

      • lead III, aVF, lead II

    • Anterior (right ventricle): −150° to −90°.

      • V1, V2, V3, V4

ECG Wave Components

  • P wave – atrial depolarisation.

  • PR interval – conduction time SA→AV→His–Purkinje; normal 0.12\text{–}0.20\,s.

  • QRS – ventricular depolarisation; duration < 0.12\,s

  • ST segment – iso-electric, early repolarisation; displacement indicates ischaemia/injury.

  • T wave – ventricular repolarisation (should be concordant with QRS axis).

  • QT interval – total ventricular activity; corrected (Bazett) QTc = \dfrac{QT}{\sqrt{RR}}, normal < 0.44\,s.

  • P wave to T wave = 1 cardiac cycle, which encompasses one complete depolarization and repolarization of the atria and ventricles, respectively.

Electrical Conduction Correlate

  • SA node fires (atrial depolarization) → P wave.

  • AV node delay → PR segment plateau.

  • His–bundle & bundle branches fire (ventricular depolarization; and atrial repolarization) → QRS.

  • ventricular depolarization complete → ST segment plateau.

  • Ventricular myocyte repolarisation (beginning at apex of heart) → T wave.

  • ventricular repolarization complete → TP segment plateau.

Positive vs Negative Deflections (Vector Concept)

  • Waveform polarity relates to direction of net electrical vector vs positive exploring electrode.

    • Vector toward electrode ➜ upward (positive) deflection.

    • Vector away ➜ downward (negative) deflection.

    • Vector perpendicular ➜ biphasic or isoelectric.

ECG Paper & Calibration

  • Standard settings: 25 mm s⁻¹ (x-axis), 10 mm mV⁻¹ (y-axis).

    • time: left to right (x-axis)

    • voltage: bottom to top (y-axis)

  • Small square = 1 mm = 0.04 s (x-axis) & 0.1 mV (y-axis).

  • Large square (5 mm) = 0.20 s & 0.5 mV.

  • Always verify the printed calibration bar before interpretation; alternate settings (50 mm s⁻¹, 5 mm mV⁻¹) change measurements.

  • rhythm lead: This represents the lead that shows the patient's heart rhythm best and is typically chosen as Lead II for its clear depiction of the P wave, QRS complex, and T wave. The rhythm lead is essential for diagnosing various arrhythmias and should be scrutinized closely during the interpretation process.

  • waveforms represent composite of electrical activity across the whole heart

    • the shape and timing of each component/waveform is important - if the shape of a QRS complexes changes, so has the pathway of conduction.

Systematic Approach Template (preview)

  1. Confirm patient, date, calibration.

  2. Rate → rhythm → axis.

  3. Intervals (PR, QRS, QT).

  4. Wave morphology (P, QRS, ST–T, U).

  5. Comparison with previous ECG if provided by patient.

Clinical & Ethical Considerations

  • Early detection of acute myocardial infarction, arrhythmias (AF, VT, VF), electrolyte disturbances, drug effects.

  • Recognising ECG patterns ensures exercise safety & reduces medicolegal risk.

  • Ethical duty: act on abnormal findings & refer appropriately; withholding or misinterpreting could be fatal.

Key Numerical & Equation Summary

  • CO = HR \times SV.

  • Normal CO: 4\text{–}8\,L\,min^{-1}.

  • PR: 0.12\text{–}0.20\,s.

  • QRS: <0.10\,s.

  • QTc (Bazett): <0.44\,s.

  • Paper speed: 25\,mm\,s^{-1} → 1\,mm = 0.04\,s.

  • Voltage calibration: 10\,mm\,mV^{-1} → 1\,mm = 0.1\,mV.

Take-Home Messages

  • Mastering 12-lead ECG interpretation integrates anatomy, physiology, physics & clinical reasoning.

  • Adhere to a consistent reading algorithm to minimise oversight.

  • Leverage free high-quality resources & practical examples; exposure to diverse pathologies accelerates pattern recognition.