ECG and Myocardium

• This document contains lecture notes on ECG (Electrocardiogram) study for medical students

Myocardium

• Myocardium comprises cardiac muscle cells
• Myocardial cells are interwoven and may branch out, connected by intercalated disks that feature gap junctions
• Gap junctions facilitate cell-to-cell conduction of excitation allowing the myocardial cells to act as a functional syncytium
• Ventricular stimulation at any point leads to complete contraction of both chambers (all-or-none contraction)

Depolarization and Repolarization

• Action potential triggers contraction through abrupt reversal of membrane potential to a positive value
• Spread of excitation occurs through local electrical currents in the atria, ventricles, and the conducting system
• Active depolarized region features a positively charged membrane interior with a negatively charged resting zone ahead
• Positive charge flows through gap junctions, which depolarizes the next cell
• External positive charge flows in the opposite direction, reducing the charge outside of the resting membrane

Conduction Velocity

• Atria: 0.5 m/s
• AV node: 0.05 m/s
• Bundle of His, bundle branches: 1.0 m/s
• Purkinje fibres: 5.0 m/s
• Ventricles: 0.5 m/s

Depolarization of Atrial or Ventricular Muscle Cells

• This occurs through charge movement via gap junctions from previously depolarized cells
• Specifically, positive charge flows through these junctions to depolarize the next cell
• Resting membrane potential within cells has a negatively charged interior and positively charged exterior
• Atrial depolarization sees the interior of the cells as positively charged and the exterior as negatively charged
• Ventricular depolarization results in positive charge inside with the negative charge outside
• Ventricular repolarization leads to the interior of cells being negatively charged and positively charged outside

Principles of Electrocardiography (ECG)

• ECG involves recording potential changes on the skin surface, resulting from heart muscle depolarization and repolarization
• Cardiac excitation spreads and creates currents in the extracellular fluid
• Currents generate small potential differences across the body surface by around 1mV
• These can be recorded by a sensitive voltmeter connected to metal electrodes on the skin surface
• Readings go to a strip of moving paper or a computer screen to produce the ECG trace
• Paper speed is typically set at 25mm/s, with one large division every 0.2 seconds
• The magnitude of the skin potential difference depends on the mass of the myocardium
• The surface ECG measures activity of atrial and ventricular muscle, excluding the SA and AV nodes

ECG Vectors

• Electrical current from atrial and ventricular depolarization form a "vector"
• Vectors have magnitude and direction metrics
• At any given electrode, the polarity of any voltage change relies on direction metrics:
  • Depolarization towards the electrode shows a positive deflection
  • Depolarization away from the electrode shows a negative deflection
  • Repolarization away from the electrode: positive deflection
  • Repolarization towards the electrode: negative deflection
  • No electrical activity: baseline (isoelectric line)
  • Depolarization/repolarization at 90°: no deflection

Electrical Events Causing the ECG

• Standard limb lead II includes the detecting electrode (+) on the left leg directly below the heart
• SA nodal cells depolarize spontaneously, at the same time, right and left atria depolarize downward towards the AV node.
  • Depolarization goes towards the + and the electrode which gives a positive deflection (P wave)
• In the ventricular septum, the overall direction of depolarization is away, which gives a small negative deflection (q wave)
• In the ventricle, depolarization occurs from inside to outside where the detecting electrode gives a large positive deflection (r wave)
• In the last segment of the ventricle, depolarization happens from inside to outside at the same time away from the same electrode for a small negative deflection (s wave)
• Repolarization occurs from outside to inside near the apex with apex facing the top of the ventricles, and away from the electrode for a positive deflection (t wave)
• Atrial repolarization is not usually seen in a typical ECG reading

Standard Limb Leads

• The original electrocardiographic lead device was conceived by Einthoven
• Recording electrodes are placed on:
  • Left arm (LA)
  • Right arm (RA)
  • Left leg (LL)
• A fourth electrode on the right leg acts as an electrical earth
• Recordings are made between 2 electrodes
• A combination of 2 electrodes forms a "bipolar lead"
  • Lead I Records potential difference between the LA and RA, LA is at positive while RA is negative
  • Lead II Records potential difference between the LL and RA, LL is positive while RA is negative
  • Lead III Records potential difference between the LL and LA, LL is positive while LA is negative

Einthoven Triangle

• Einthoven triangle's frontal plane is two dimensional
• The average direction the dipole takes causing the QRS complex is called Mean QRS axis
  • Usually falls within 0° to 90° quadrant
  • Downward and towards the left
  • Coordinates to the anatomical pathway and wave of depolarization it takes

Augmented Limb Leads

• The same electrodes utilized for standard limb leads are used (RA, LA + LL)
• A singular electrode is positive while the other 2 electrodes are negative
• The three new leads this generates (unipolar): aVR, aVL, & aVF
• The term "aV" refers to the augmented voltage measured using these electrodes
• The hexaxial reference system involves unipolar (av) and bipolar (standard) limb leads recording electrical activity of the heart within the body's frontal plane
• Main deflection is positive for all leads excluding the aVR
• Leads are oriented every 30° increments
  • Lead I is oriented at 0°
  • Lead II is oriented at +60°
  • aVF is oriented at +90°
  • Lead III is oriented at +120°
  • aVR is oriented at -150°
  • aVL is oriented at -30°

Chest Leads

• Active electrodes make recordings in one of 6 positions on the chest
• Chest leads are called: V1, V2, V3, V4, V5 and V6
• Chest leads examine the heart within the horizontal plane
• Negative electrodes: LA, RA and LL connected together
• The right foot is earthed
• Chest leads produce large ECG deflections
• With V1 and V2, the area is in the right ventricle along with negative S as main QRS deflection
• With V3 and V4, the area is in the interventricular spectrum so this shows positive R/negative S pattern
• With V5 and V6, the area is in the left ventricle so this produces positive R QRS deflection

Positive and Negative Deflections in the QRS Complex

• The components of the QRS complex are arbitrarily labeled
  • If the first deflection in QRS is downwards, it's the Q wave
  • If the there's deflection upwards, it's the R wave whether or not it is preceded by a Q
  • Any deflection below the baseline after the R wave is called the S wave regardless if there has been a Q preceding

V1 versus V6

• One wave of depolarisation gives:
  • "positive" (R wave) and "negative" deflections (Q wave) in the QRS complex
  • There is a negative S wave in V1
  • There is positive R wave in V6
• Left ventricle depolarization is greater than in the right ventricle
• LV depolarization exists, and cancels out RV depolarization
• Both V1 and V6 are limited to a wave of depolarization going towards V6
• V1 shows a negative wave - S Wave
• V6 shows a positive wave - R Wave
• Ventricular depolarization as show on V1 demonstrates - RS
• Ventricular depolarization as show on V6 demonstrates - QR

12 Lead ECG

• Involves a total of 6 chest leads with 6 limb leads (standard and aV)

Calibration of the ECG Recording Device

• An impulse of 1mV generates a peak of 10mm, (2 large boxes, 10 small squares)
• The recorder runs at 25mm/s, (5 large boxes, 25 small squares)

Characteristics of the ECG

• ECG comprises of waves (P, Q, R, S, T), and segments (The relative distances between waves being the segments)
• The ST segment measures any distance between the end of the S wave along any beginning of the T wave

PR Interval

• On ECG, intervals capture both waves and segments
• The "PR interval" is measured from the initial point of atrial depolarization to the point just before ventricular depolarization
  • PR correlates with conduction time which moves through the AV node
  • Typically it is read at 0.12-0.20 s

Sinus Arrhythmia

• One form of irregular rhythm (arrhythmia) is both normal, named Sinus arrhythmia
• The physiological slowing of firing by the SA node occurs during both expiration and acceleration during inspiration
• A phasic rise through vagal activity occurs during expiration, initialed in areas where lung stretch receptors slow the heart rate
• Vagal activity typically declines as you breath during inspiration causing the heart rate to increase

AV-Nodal Block

• During normal sinus rhythm, all QRS complexes feature an preceding P wave read inside the PR 0.12-0.20 second interval.
• Conduction velocity here should it slow from AV node injury, the PR interval will grow longer (00.20s>) leading to first degree AV-nodal block
  • The AV node conducts on some of the supraventricular depolarizations which features P waves following the QRS complexes, and not following with other P waves
• With a second-degree AV block, shown in 3:2 ratios, with other ratios possible to read
• AV node injury has completely blocked the electric signal between the atria and the ventricles this is Third degree AV-nodal blocks
• the AV node will cease signals from reading and P waves will occur regularly although QRS may only be in the Low frequency (secondary pacemaker) state

Premature Ventricular Contractions (PVCs)

• The heartbeat normally originates within the atria (SA node)
• Heartbeats can stem directly from within the ventricle when faced with certain abnormal states like acute myocardial ischaemia.
• PVC is not preceded by a P wave due to the wave originating in an ectopic area of the ventricles.
  • The PVC shows as premature along with other QRS would appear as predicted but not accurately
  • Shape amplitude And length here come off as abnormal and the same wave will fail to connect with any typical conducting pathways to show ectopic beat
• The diastolic between the beat allows pause for increased ventricle filling although it will present before the next scheduled beat.

Re-entry

• Area "D" occurs when both currents cancel each other out

Injured Heart Tissue

• Electrical signals won't propagate with the normal heart tissue although they tend to conduct better under abnormal directions due to not meeting any new signals and then looping

Ventricular Fibrillation

• This requires rapid uncodified excitations that are caused due to cardiac outputs and result in some form of cardiac arrest within minutes of the event
  • Ventricular contraction occurs due to some from of lack of blood flow(Myocardial Ischaemia) due to a result of anaesthetic over does, or even electrocution.
  • Ventricular ectopics tend to only occur with a person has ischaemia.

Ventricle Defibrillation

• It involves use of the electrical shock delivered through pads or paddles into the persons chest over the heart.