ECG killer!

Many medical practitioners, from students to seasoned doctors, grapple with ECG interpretation. This guide demystifies ECG readings, essential for OSCEs, written examinations, and clinical practice.

ECG interpretation is both complex and simple. Although ECG isn't always the primary diagnostic tool, it offers valuable insights into differential diagnoses. This guide emphasises foundational knowledge vital for every doctor.

Rate and Rhythm:

• Heart Rate:
  • Quick estimate: Count QRS complexes in a 10-second strip x 6.
  • Accurate measure: Count large squares between two R-waves and divide into 300.
• Tachycardia (>100 bpm):
  • Causes: Fever, anemia, hyperthyroidism, etc.
  • On ECG: Close QRS complexes, possibly reduced or absent T-waves.
• Bradycardia (<60 bpm):
  • Causes: Hypothyroidism, increased intracranial pressure, etc.
  • On ECG: Wide QRS complexes.
• Rhythm Analysis:
  • Regular or irregular? If irregular, is it consistently so or not?

P-Wave Analysis and Abnormalities:

• Basic P-Wave Analysis:
  • Presence: Are P-waves present?
  • Morphology: Upright in leads I, II, aVF and inverted in aVR?
  • Duration: < 0.12 sec.
  • Relationship with QRS: One P-wave should precede each QRS in normal sinus rhythm.
• Atrial Fibrillation (AF): Absent P-waves with an irregularly irregular ventricular rhythm.
• Atrial Flutter: Saw-tooth pattern with regular atrial contractions.

QRS Complex Analysis and Abnormalities:

• QRS Complex Basics:
  • Duration: < 0.12 sec. Prolonged indicates bundle branch block or ventricular rhythm.
  • Morphology: Pathological Q-waves for past myocardial infarction.
  • Axis: Check leads I and aVF for cardiac axis. Left or right axis deviation.
• Supraventricular Tachycardia (SVT): Fast, regular rhythm with narrow QRS. P-waves might be absent.
• Ventricular Tachycardia (VT): Series of wide and rapid QRS complexes.
• Ventricular Fibrillation (VF): Chaotic activity with no discernible waves.
• Wolff-Parkinson-White (WPW) Syndrome: Features a delta wave in the QRS complex due to an accessory pathway.

ST and T-Wave Analysis and Abnormalities:

• ST Segment:
  • Elevation or Depression: Indicative of myocardial ischaemia or infarction.
  • Morphology: Horizontal, upsloping, or downsloping.
• T-Wave:
  • Morphology: Check for peaked, inverted, or biphasic T-waves. Indicates electrolyte imbalances, myocardial ischaemia, etc.
• U Wave: A small wave following T-wave, typically in hypokalemia.

QT Interval and Abnormalities:

• QT Interval Basics: Measure from start of QRS to end of T-wave.
• Corrected QT (QTc): QT/√RR. Prolonged QTc may lead to ventricular arrhythmias.
• Long QT Syndrome: Prolonged QT interval possibly leading to a specific type of VT.
• Short QT Syndrome: Shortened QT can increase risk of ventricular arrhythmia.

Conduction Abnormalities:

• First Degree AV Block: Extended PR interval (>0.20 sec).
• Second Degree AV Block: Either progressive PR interval elongation till a QRS drop (Type 1/Wenckebach) or dropped beats without PR interval change (Type 2).
• Third Degree AV Block: Atria and ventricles beat independently.

Electrolyte Abnormalities:

• Hyperkalemia: Peaked T-waves, flat P-waves, prolonged PR, and wide QRS.
• Hypokalemia: Flattened T-waves, U-waves, and ST depression.
• Hypocalcemia: Prolonged QT.
• Hypercalcemia: Shortened QT.

Other Conditions and Assessments:

• Ischaemia/Infarction by Location and ECG Changes:

• Anterior Wall Infarction (usually due to occlusion of the left anterior descending artery - LAD):

  • ECG Leads: V1-V4 (sometimes V5).
  • Changes: ST elevation in these leads. Possible Q wave development.

• Inferior Wall Infarction (usually due to occlusion of the right coronary artery - RCA, or in some cases, the left circumflex artery - LCx):

  • ECG Leads: II, III, aVF.
  • Changes: ST elevation in these leads. Development of Q waves.

• Lateral Wall Infarction (usually due to occlusion of the left circumflex artery - LCx):

  • ECG Leads: I, aVL, V5, V6.
  • Changes: ST elevation in these leads. Possible Q wave formation.

• Posterior Wall Infarction (occlusion can be in the RCA or LCx):

  • ECG Leads: V7, V8, V9 (if used; otherwise, can be inferred from changes in V1-V3).
  • Changes: ST depression in V1-V3 (as it’s a reciprocal change), tall R waves, and upright T waves in the same leads. If V7-V9 are used, ST elevation would be seen there.

• Septal Infarction (usually due to occlusion of the septal branches of the LAD):

  • ECG Leads: V1-V2.
  • Changes: ST elevation in these leads. Development of Q waves.

• Right Ventricular Infarction (usually associated with an inferior MI and RCA occlusion):

  • ECG Leads: V3R, V4R (right-sided leads).
  • Changes: ST elevation in these right-sided leads. This is of clinical significance as patients with right ventricular involvement may be more sensitive to the preload reduction caused by medications like nitrates.
• General Signs of Infarction:
  • Pathological Q Waves: These are deep and typically last longer than 0.04 seconds (or 1 small square). They represent areas of transmural infarction.
  • T-wave Inversion: After an episode of ST elevation, the T-wave may invert during the healing process.
  • R-wave Reduction: Decreased R-wave amplitude over time can be a sign of infarction.

Brugada Syndrome:

• Definition: A genetic condition associated with an increased risk of sudden cardiac death due to ventricular arrhythmias.
• ECG Features:
  • Type 1: "Coved" ST-segment elevation ≥ 2mm followed by a negative T-wave in more than one of V1-V3.
  • Type 2: "Saddle-back" pattern with ST elevation ≥ 1mm.
  • Type 3: Either "coved" or "saddle-back" but with ST elevation < 1mm.
• Location: Usually seen in leads V1-V3.
• Other Info: It’s more prevalent in Asian populations. Fever can unmask or exacerbate the ECG findings.

Wolff-Parkinson-White (WPW) Syndrome:

• Definition: A condition in which there's an extra electrical pathway in the heart, leading to episodes of rapid heart rate (tachycardia).
• ECG Features:
  • Short PR interval: Less than 120 milliseconds (3 small squares).
  • Delta wave: A slurred upstroke in the QRS complex, leading to a widened QRS.
  • Secondary ST and T wave changes.
• Location: It can be seen in multiple leads, depending on the location of the accessory pathway.
• Other Info: Paroxysms of atrioventricular reentrant tachycardia (AVRT) can be seen in patients with WPW.

Long QT Syndrome (LQTS):

• Definition: A condition characterized by prolonged QT intervals which can lead to torsades de pointes, a type of ventricular tachycardia, and increase the risk of sudden death.
• ECG Features: Prolonged QT interval, which should be corrected for heart rate (QTc). A prolonged QTc is generally > 440 ms in males and > 460 ms in females.
• Location: It can be observed in multiple leads.

Short QT Syndrome:

• Definition: A genetic condition associated with a high risk of atrial and ventricular arrhythmias and sudden cardiac death.
• ECG Features: A shortened QT interval, usually < 340 milliseconds.
• Location: It can be observed in multiple leads.

Torsades de Pointes:

• Definition: A specific type of polymorphic ventricular tachycardia in the setting of a prolonged QT interval.
• ECG Features: Characteristic "twisting" pattern of the QRS complexes around the isoelectric line. It can self-terminate or degenerate into ventricular fibrillation.
• Location: It can be seen globally across the ECG.

Hypertrophic Cardiomyopathy (HCM):

• Definition: A condition where the heart muscle becomes thickened without an obvious cause.
• ECG Features: Left ventricular hypertrophy (LVH) pattern, deep narrow Q waves in the lateral leads (often > 40 ms in width), and giant negative T waves in the precordial leads.
• Location: It can manifest in various leads.

ECG Basics:

• Patient and ECG Information:
  • Patient Details: Confirm name, date of birth, and patient number.
  • Date and Time: Compare with prior tracings.
  • Paper Speed: 25mm/sec is standard (small square = 0.04 sec, large square = 0.20 sec).

Practice and Clinical Correlation:

• Clinical Correlation: Match ECG results with clinical scenario. Symptom and physical examination insights.
• Consistent Practice: Become familiar with ECGs across different settings for proficiency.