Medicine Marrow Pg No 377-386 (ECG)

ECG Axis Determination

• Method: Use leads I, II, III, aVL, and aVF to count the height of the QRS complex
• Equiphasic Leads: Leads where the R wave height equals the S wave height
• Axis Location: Identify the lead showing the biggest QRS complex deflection
• Perpendicular Axis: The axis perpendicular to identified lead will show significant deflections in the leads

QT Interval

• Definition: Represents time for ventricle to recover and prepare for the next heart beat
• Measurement: Time between start of QRS complex and end of T wave
• Features: Includes QRS, ST segment and T wave
• Normal Duration:
  • Females: ≤ 0.44 seconds
  • Males: ≤ 0.42 seconds
• Prolonged: > 0.48 seconds

ECG Axis Diagram

• Contains different axes angles and leads, including normal, northwest, rightward and leftward axis

ECG Approach

• QRS Complex Width: Narrow/normal or wide
• Rhythm: Examine for different rhythm types: Idioventricular, accelerated idioventricular, ventricular arrhythmias, Bundle Branch Block (BBB), Atrial rhythm, Junctional rhythm, etc.
• PR Intervals: Normal, short, prolonged
• P Wave: Normal, Abnormal, Absent
• Presence of Other Waves: P, QRS, T
• AV Block: Degree (1st, 2nd, or 3rd-degree)
• Ventricular Tachycardia: Wider QRS complexes

Leads

• Bipolar Leads:
  • Lead I: Between Right Arm (RA) and Left Arm (LA)
  • Lead II: Between Right Arm (RA) and Left Leg (LL)
  • Lead III: Between Left Arm (LA) and Left Leg (LL)
• Augmented Unipolar Leads:
  • AVR: Perpendicular to Lead III
  • AVL: Perpendicular to Lead II
  • AVF: Perpendicular to Lead I

Criteria for Myocardial Infarction (MI)

• V5, V6: Concordant ST elevation > 1 mm, Discordant ST elevation > 5 mm, Concordant ST depression > 1 mm
• VI, V4: Abnormally discordant ST segment ≥ 5 mm, Discordant ST segment ≥ 1 mm, Concordant ST segment ≥ 1 mm

ECG Tracings

• LBBB No MI: Shows Left Bundle Branch Block (LBBB) but no MI
• LBBB + Acute MI: Shows LBBB and acute MI

Acute MI With LBBB

• Concordant Elevation > 1 mm: Acute MI with LBBB and a consistent ST segment elevation over 1 mm
• Discordant Elevation > 5 mm: ST segment elevation deviated in direction opposite to QRS deflection

ECG Findings

• Sokolow Lyon Index:
  • V5 or V6 R wave + VI or V4 S wave → >35mm LVH
  • aVL > 11mm → LVH
  • 2° ST/T changes (positive) due to strain/pressure overload (e.g., aortic stenosis/chronic HTN)
  • Left bundle branch block (LBBB)
• Right Ventricular Hypertrophy (RVH):
  • Axis: Between leads III (120°) and aVF (90°)
  • Rightward axis
  • Causes:
    • RVH
    • VI: R/S ratio > 1
    • 2° ST changes
    • Mitral stenosis (ms) + RVH
• P-pulmonale:
  • In leads II, III, aVF
  • Most common cause: COPD
  • Hyperinflated lung
  • Downward diaphragm push
  • Axis shifts to 90°
• COPD Signs:
  • Prominent complexes in aVF and lead II
  • Absent complexes in lead I → Lead I sign
  • QRS complex: Narrow
  • VI to V6: Poor R-wave progression
  • P pulmonale
  • Lead I sign (+)

Vectors

• Vector 1:
  • Interventricular (IV) septum
  • Left Bundle Branch (LBB) shorter than Right Bundle Branch (RBB): Left side of IV septum activated first
  • Direction: Right side, anteriorly and inferiorly
  • Right sided leads: Positive deflection
  • Left sided leads: Negative deflection
  • Most predominant, right lead
• Vector 2:
  • Free wall depolarization
  • Direction: Right to Left + Endocardium → Epicardium
  • Left posteriorly and superiorly
  • Right sided: Negative deflection in lead V1
  • Left sided: Positive deflection in leads V5 and V6 (Big R)
• Vector 3:
  • Last part (Posterobasal portion) to be depolarized
  • Direction: Superiorly and Posteriorly
  • No deflection

Wide QRS: Causes

1. Ventricular activation via alternate pathway:

   • 0.16 s

   • Myocardial cell-to-cell conduction

   • Ventricular generated rhythm:

     • Idioventricular: 15-40 bpm
     • Accelerated idioventricular: 40-100 bpm
     • Ventricular tachycardia: >100 bpm

RIGHT BBB (RBBB)

• RVH with RBBB:
  • Axis: B/W lead 1 & aVL → Leftward axis
  • R/S ratio > 1: RVH
  • RVH + Leftward axis: Biventricular hypertrophy

Blocks

• RBBB only: Unifascicular block
• RBBB + Lt anterior/posterior fascicular block (LAFB/LPFB): Bifascicular block
• LBBB: Bifascicular block (By itself)
• Bifascicular block + ↑ PR interval/mobitz block: Trifascicular block

LEFT BBB (LBBB)

• Axis: B/W lead I & AVL → Leftward axis
• V5, V6, AVL: LBBB morphology

Sgarbossa Criteria

• Used to diagnose MI along with LBBB

ECG Findings

• VI or va:
  • Concordant change: ST depression
  • Discordant change: ST elevation
• V5 or V6:
  • Concordant ST elevation

Chronology of Reading ECG

• Start from lead II/rhythm strip:
• QRS Complex:
  • Big R:
  • Small Q:
  • Small R: (leads to Deep S)
• Regularity of QRS:
• P Wave:
• Rate:
• Axis:
• QT Interval:

QRS Complex

• Features: Maximum deflection. Synchronous ventricular depolarization
• Duration:
  • Normal/narrow QRS: 2-2.5 small boxes (100ms/0.1s)
  • Wide QRS: > 0.12 s
  • Very wide QRS: > 0.16 s
• 1 small box = 40 ms

Patient Parameters

• Time: 00:40:48
• Rhythm: Regular
• Rate: 150/min (1500/no.of small boxes/2 QRS complexes)
• Heart Rate: 60-100 bpm (Sinus) / 40-60 bpm (Atrial/Junctional) / 15-40 bpm (Ventricular)

Normal Pathway With Block

• 0.12 - 0.165
• Right Bundle Branch Block (RBBB) / Left Bundle Branch Block (LBBB)

Regularity of QRS Complexes

• Regular, fixed intervals

P Wave Analysis

• Appearance: First wave. Produced by atrial depolarization from the SA node
  • Right atrial depolarization: Ascending limb
  • Left atrial depolarization: Descending limb
  • Shape: Smooth and rounded
  • Height and Width: Equal to 2.5mm
  • Leads II, III or aVF: Upright (Normal), (may be inverted=P-pulmonale)
  • Lead V₁: Inverted/Biphasic
• Normal: Every QRS is preceded by a P wave

PR Segment

• Definition: Impulse from P wave start to QRS complex start
• Parts:
  • Atria
  • AV node
  • Intra-AV nodal (Normal AV nodal delay)
  • Upper Part of Bundle of His
• Electrophysiology: Electrically inert; No deflection

PR Interval Analysis

• Duration: 0.12 -0.2 seconds (3-5 small boxes)
• Prolonged PR intervals (> 0.25): Defect/block (on regular pathway)
• Shortened PR intervals: Aberrant pathway between atria and ventricle (Bundle of Kent)

Atrial & Ventricular Enlargement

Atrial Enlargement

• Right atrial enlargement:
  • Abnormal P waves: Pathology of atrial origin
  • Pointed P (P-pulmonale): Right atrial enlargement
  • Tall & peaked: COPD
  • Rate > 100 bpm: Atrial tachycardia
• Left atrial enlargement:
  • Bifid P wave: P mitrale

Ventricular Hypertrophy

• Left ventricular hypertrophy (LVH):
  • Axis: aVL(30°) - lead I (0°) → Leftward axis
  • Causes:
    1. LVH (Left ventricular hypertrophy)
       • ↑ muscle mass on left
       • ↓ vector pushed to left
       • Left side: ↑+ve deflection
       • Right side: |-ve deflection
  • Note: Corrected QT interval: QT=QTRRQT = \frac{QT}{\sqrt{RR}}QT=RR​QT​

Route of Depolarization

• Sinus Node → Atria → AV Node → Bundle of His
• Bundle of His → Right Bundle Branch → Right Ventricle
• Bundle of His → Left Bundle Branch → Left Anterior Fascicle → Left Ventricle
• Bundle of His → Left Bundle Branch → Left Posterior Fascicle → Left Ventricle
• Purkinje Fibres → Right Ventricle
• Purkinje Fibres → Left Ventricle

Depolarization Vectors

1. Septal depolarization: Vector 1
2. Free wall depolarization: Vector 2
3. Posterobasal LV depolarization: Vector 3

• QRS complex is the sum of these three vectors (Vector 1 + Vector 2 + Vector 3)

Flow of Current

• Downward deflection: Current flows away from the positive electrode
• Upward deflection: Current flows towards the positive electrode

ECG Instrumentation Diagram

• Galvanometer: Measures electrical signals of the heart
• Electrodes: Placed on right and left arm
• Lead: Connects the galvanometer
• Potential Difference: Measured between right and left electrodes
• ECG Recording: Electrical signal recorded as ECG

This quiz focuses on understanding the ECG axis determination and the QT interval. It covers key concepts such as equiphasic leads, axis location, and the normal duration of the QT interval for different genders. Test your knowledge on these important aspects of electrocardiography.