ECG

To determine rate, count the number of QRS complexes in 6 seconds (30 large squares) and multiply by 10.

• Sinus rhythm
  • every P wave followed by a QRS
• QRS rate = 60-100/min
  • rate < 60 = bradycardia
  • rate > 100 = tachycardia
  • variation > 10% = sinus arrhythmia
• Tachycardia
  • narrow QRS:
    • sinus tachycardia
    • supraventricular tachycardia (atrial flutter, atrial fibrillation)
    • junctional tachycardia
  • broad QRS:
    • ventricular tachycardia
    • supraventricular tachycardia with bundle branch block

• Represents atrial depolarisation
• Axis 0-75 degrees
• +ve in all leads aVR; biphasic in V1, may by -ve in aVL if the QRS is also predominantly -ve
• In lead II:
  • Height < 2.5 mm
  • Width < 120 ms (3 small squares)

• Measure from beginning of P wave to beginning of Q wave
• Width 120–200 ms (3-5 small squares)
  • > 200 ⇒ AV block
  • < 120 ⇒ WPW syndrome; Lown-Ganong-Levine syndrome
• Relationship of P to QRS:
  • 1:1 relationship = normal
  • not 1:1
    • AV block (2^nd or 3^rd degree)
    • ventricular arrhythmia

• Represents AV conduction
• Isoelectric

• Normal is -30^o to 90^o
• Positive in both I and II (refer to Table 2 below)
• Look at:
  • lead I: predominantly +ve (-ve = right axis deviation)
  • lead II: predominantly +ve (-ve = left axis deviation)

• Represents ventricular depolarisation
• Normal duration < 120 ms (3 small squares)
• Precordial lead transition point V3/V4

• End of S wave to end of T wave
• Normal, elevated or depressed
• ST-segment-elevation myocardial infarction (STEMI)
  • anterior: elevation V1-V6 (LAD artery)
  • septal: elevation V1-V4 (LAD artery)
  • lateral: elevation I, aVL, V5, V6; depression II, III, aVF (LAD, CX arteries)
  • inferior: elevation II, III, aVF (RCA, CX arteries)
  • posterior: elevation V7, V8, V9; depression V1-V3, dominant R wave V1-V3 (CX artery)
  • right ventricle: elevation V1, V4R; depression I, aVL (RCA)
• Pericarditis
  • elevation all leads except aVR
• Pulmonary embolism
  • elevation V1, aVR

• Measured from beginning of Q wave to end of T wave
• Corrected QT (QTc) Bazett method
  • QT corrected for heart rate (QTc) = QT ÷ √ RR interval /1000 
  • Normal values
    • Female: 355 – 482 msec
    • Male: 346 – 472 msec

• Normally upright except in aVR and V1
• T wave inversion in III can be a normal variant