Mednotes

L11 CVS Physiology 5 ECG

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tagscvs, lecture, physiology
aliasesL11 CVS Physiology 5 ECG

9 min read

Overview

This lecture builds the electrocardiogram (ECG) up from first principles: the cardiac excitation-conduction system and its conduction velocities, two contrasting cardiac action potential shapes, how the spread of depolarisation/repolarisation across the heart changes the pattern of charge on the cell surface, the vector rules that translate that spread into an ECG deflection, the standard ECG waveform (waves, segments, intervals) and how each wave is generated, and the limb, augmented limb and chest lead systems that together make up the 12-lead ECG, illustrated with real printed tracings.

Cardiac conduction system

  • Coordinated cardiac contraction depends on the excitation-conduction system, made up of:
    (a) the SA node (sinoatrial node)
    (b) the AV node (atrioventricular node)
    (c) the bundle of His
    (d) the left and right bundle branches
    (e) the Purkinje fibres
  • To reach the ventricles, the action potential must pass through the atrioventricular ring.
  • Conduction velocities:
    • Atria: 0.5 m/s
    • AV node: 0.05 m/s (slowest)
    • Bundle of His / bundle branches: 1.0 m/s
    • Purkinje fibres: 5.0 m/s (fastest)
    • Ventricles: 0.5 m/s

The conduction-system diagram includes two blue arrows pointing toward the SA/AV node region that are not labelled or explained on the slide.

Cardiac action potentials

Two contrasting action potential / membrane-permeability profiles are shown; the slides do not state which cell type each belongs to.

  • Profile 1 (resting around -55 mV): potential rises from a less negative resting level, with Na+ and Ca2+ entry near threshold, peaks near 0 mV, then falls as K+ exits. Tracked permeabilities: fast Na+ (P_Na+(F)), T-type Ca2+ (P_Ca2+(T)), L-type Ca2+ (P_Ca2+(L)), K+ (P_K+).
  • Profile 2 (resting around -90 mV): a steep, sharp upstroke driven by Na+ entry, a plateau near +10 mV maintained by L-type Ca2+ entry, and repolarisation driven by K+ exit.

These two action potential slides carry no title or caption; the transcript notes they appear to contrast two different cardiac cell types, but this is not stated on the slide.

Cellular basis of the ECG: spread of charge

At rest, cardiac cells are negatively charged inside and positively charged outside.

  • Atrial depolarisation: the atria become positive inside / negative outside as depolarisation spreads across them (the ventricles remain positive outside at this point).
  • Ventricular depolarisation: the ventricles become positive inside / negative outside as depolarisation spreads across them (the atria are already depolarised).
  • Ventricular repolarisation: the ventricles return to negative inside / positive outside as repolarisation spreads across them.

ECG vectors and polarity

The polarity recorded at a positive electrode depends on the direction the wave of potential change is moving relative to it:

  • Depolarisation moving towards the positive electrode -> positive potential (upward deflection).
  • Depolarisation moving away from the positive electrode -> negative potential (downward deflection).
  • Repolarisation moving away from the positive electrode -> positive potential (upward deflection).
  • No depolarisation or repolarisation occurring -> no potential difference (isoelectric, flat line).

Standard ECG waveform: waves, segments and intervals

The ECG is made up of:

  • Waves: P, Q, R, S, T.
  • Segments: the distances between waves.
    • ST segment: from the end of the S wave to the beginning of the T wave.
  • Intervals: comprise both waves and segments.
    • PR (PQ) interval: time for excitation to spread through the atria, AV node and bundle of His; normally 0.12-0.20 s.
    • QS interval: time for excitation to spread through the ventricles; normally 0.12 s or less.

How each ECG deflection is generated (lead II, positive electrode on the left leg below the heart)

  1. SA nodal cells depolarise spontaneously; the right and left atria depolarise, spreading down towards the AV node - towards the + electrode -> positive deflection = P wave.
  2. In the ventricular septum, the overall direction of depolarisation is away from the + electrode -> small negative deflection = Q wave.
  3. In the ventricle, depolarisation spreads from inside to outside, towards the detecting electrode -> large positive deflection = R wave.
  4. In the last portion of the ventricle, depolarisation continues from inside to outside but is directed away from the detecting electrode -> small negative deflection = S wave.
  5. Ventricular repolarisation is directed away from the detecting electrode -> positive deflection = T wave.

Lead systems

Einthoven triangle and standard limb leads

  • Einthoven’s original lead system places the heart at the centre of a triangle (the Einthoven triangle) formed by the right shoulder, left shoulder and pubic area, oriented in the frontal plane of the body.
  • Recording electrodes sit on the left arm (LA), right arm (RA) and left leg (LL); a fourth electrode on the right leg (RL) acts as an electrical earth.
  • Standard limb leads are bipolar (each records the potential difference between two electrodes):
    • Lead I: LA positive, RA negative.
    • Lead II: LL positive, RA negative.
    • Lead III: LL positive, LA negative.
  • Lead II’s positive electrode (left leg) lies directly below the heart.

Augmented limb leads

  • Use the same three electrodes (RA, LA, LL): one is made positive while the other two are made negative, generating three unipolar leads: aVR, aVL, aVF (“aV” = augmented voltage).
  • Standard and augmented limb leads together record electrical activity in the frontal plane of the body.
  • The main QRS deflection is positive in all limb leads except aVR, which is predominantly negative.

Chest (precordial) leads

  • Active electrodes sit in one of 6 positions on the chest wall: V1-V6.
  • The negative electrodes (LA, RA, LL) are connected together; the right foot is earthed.
  • Chest leads give large ECG deflections and examine the heart in the horizontal plane.
  • Main QRS deflection by position:
    • V1 and V2 (right ventricle): negative S predominates.
    • V3 and V4 (interventricular septum): mixed positive R / negative S.
    • V5 and V6 (left ventricle): positive R predominates.

The 12-lead ECG

3 standard limb leads (I, II, III) + 3 augmented limb leads (aVR, aVL, aVF) + 6 chest leads (V1-V6) = the 12-lead ECG.

Historical note and real tracings

  • The ECG was first demonstrated to the Royal Society in 1909 using Waller’s pet bulldog, Jimmie, whose front and hind paws stood in pots of normal saline connected to a galvanometer.
  • The lecture includes several real printed 12-lead and rhythm-strip ECG tracings (e.g. a 12-lead recording with limb leads I/aVR/V1/V4, II/aVL/V2/V5, III/aVF/V3/V6 plus a rhythm strip, recorded at 25 mm/sec) used for teaching purposes.

"), transcribed only as far as could be read with reasonable confidence.

Self-test

  1. List the five components of the excitation-conduction system, in the order the action potential passes through them.
  2. State the conduction velocity for each of the atria, AV node, bundle of His/bundle branches, Purkinje fibres and ventricles.
  3. Describe the two contrasting cardiac action potential profiles shown, including the approximate resting potential and the main ions responsible for the upstroke, plateau and repolarisation of each.
  4. Describe how the distribution of charge across the cell membrane (inside vs outside) changes during atrial depolarisation, ventricular depolarisation and ventricular repolarisation.
  5. State the rule linking the direction of depolarisation or repolarisation relative to a positive electrode to the direction of the ECG deflection, including the isoelectric case.
  6. Define the PR (PQ) interval and the QS interval, and give their normal durations.
  7. Define the ST segment.
  8. Explain the origin of each wave of the ECG (P, Q, R, S, T) in lead II, in terms of the region of the heart involved and the direction of depolarisation/repolarisation relative to the detecting electrode.
  9. Describe what forms the Einthoven triangle and how it is oriented relative to the body.
  10. Distinguish standard limb leads (I, II, III) from augmented limb leads (aVR, aVL, aVF) in terms of how each is derived from the RA, LA and LL electrodes.
  11. Which limb lead normally shows a negative main QRS deflection?
  12. Distinguish the standard/augmented limb leads from the chest leads in terms of the plane of the heart each examines.
  13. For chest leads V1/V2, V3/V4 and V5/V6, state the cardiac region each overlies and the main QRS deflection typically recorded.
  14. List the 12 leads that make up a standard 12-lead ECG, grouped into their three sets.
  15. Describe the historical demonstration of the ECG referenced in the lecture.

Answers

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