ECG Learning Center - An introduction to clinical electrocardiography (2024)

Topics for study:

1. Heart Rate
2. PR Interval
3. QRS Duration
4. QT Interval
5. QRS Axis

Heart Rate

In normal sinus rhythm, a resting heart rate of below 60 bpm is called bradycardia and a rate of above 90 bpm is called tachycardia.

PR Interval

(measured from beginning of P to beginning of QRS in the frontal plane)

• Normal: 0.12 - 0.20s
• Short PR: < 0.12s
  • Preexcitation syndromes:
    • WPW (Wolff-Parkinson-White) Syndrome: An accessory pathway (called the "Kent" bundle) connects the right atrium to the right ventricle (see diagram below) or the left atrium to the left ventricle, and this permits early activation of the ventricles (delta wave) and a short PR interval.

    • LGL (Lown-Ganong-Levine): An AV nodal bypass track into the His bundle exists, and this permits early activation of the ventricles without a delta-wave because the ventricular activation sequence is normal.
  • AV Junctional Rhythms with retrograde atrial activation (inverted P waves in II, III, aVF): Retrograde P waves may occur before the QRS complex (usually with a short PR interval), in the QRS complex (i.e., hidden from view), or after the QRS complex (i.e., in the ST segment).
  • Ectopic atrial rhythms originating near the AV node (the PR interval is short because atrial activation originates close to the AV node; the P wave morphology is different from the sinus P)
  • Normal variant
• Prolonged PR: > 0.20s
  • First degree AV block (PR interval usually constant)
    • Intra-atrial conduction delay (uncommon)
    • Slowed conduction in AV node (most common site)
    • Slowed conduction in His bundle (rare)
    • Slowed conduction in bundle branch (when contralateral bundle is blocked)
  • Second degree AV block (PR interval may be normal or prolonged; some P waves do not conduct)
    • Type I (Wenckebach): Increasing PR until nonconducted P wave occurs
    • Type II (Mobitz): Fixed PR intervals plus nonconducted P waves
  • AV dissociation: Some PR's may appear prolonged, but the P waves and QRS complexes are dissociated (i.e., not married, but strangers passing in the night).

QRS Duration

(duration of QRS complex in frontal plane):

• Normal: 0.06 - 0.10s
• Prolonged QRS Duration (> 0.10s):
  • QRS duration 0.10 - 0.12s
    • Incomplete right or left bundle branch block
    • Nonspecific intraventricular conduction delay (IVCD)
    • Some cases of left anterior or posterior fascicular block
  • QRS duration ≥ 0.12s
    • Complete RBBB or LBBB
    • Nonspecific IVCD
    • Ectopic rhythms originating in the ventricles (e.g., ventricular tachycardia, pacemaker rhythm)

QT Interval

(measured from beginning of QRS to end of T wave in the frontal plane)

• Normal: heart rate dependent (corrected QT = QT_c = measured QT, sq-root RR in seconds; upper limit for QT_c = 0.44 sec)
• Long QT Syndrome - "LQTS" (based on upper limits for heart rate; QT_c ≥ 0.47 sec for males and ≥ 0.48 sec in females is diagnostic for hereditary LQTS in absence of other causes of increased QT)
  • This abnormality may have important clinical implications since it usually indicates a state of increased vulnerability to malignant ventricular arrhythmias, syncope, and sudden death. The prototype arrhythmia of the Long QT Interval Syndromes (LQTS) is Torsade-de-pointes, a polymorphic ventricular tachycardia characterized by varying QRS morphology and amplitude around the isoelectric baseline. Causes of LQTS include the following:
    • Drugs (many antiarrhythmics, tricyclics, phenothiazines, and others)
    • Electrolyte abnormalities (↓K⁺, ↓Ca⁺⁺, ↓Mg⁺⁺)
    • CNS disease (especially subarrachnoid hemorrhage, stroke, trauma)
    • Hereditary LQTS (e.g., Romano-Ward Syndrome)
    • Coronary Heart Disease (some post-MI patients)

Frontal Plane QRS Axis

• Tutorial in Measuring QRS Axis
• Normal: -30 degrees to +90 degrees
• Abnormalities in the QRS Axis:
  • Left Axis Deviation (LAD): ≥ -30° (i.e., lead II is mostly 'negative')
    • Left Anterior Fascicular Block (LAFB): rS complex in leads II, III, aVF, small q in leads I and/or aVL, and axis -45° to -90°
    • Some cases of inferior MI with Qr complex in lead II (making lead II 'negative')
    • Inferior MI + LAFB in same patient (QS or qrS complex in lead II)
    • Some cases of LVH
    • Some cases of LBBB
    • Ostium primum ASD and other endocardial cushion defects
    • Some cases of WPW syndrome (large negative delta wave in lead II)
  • Right Axis Deviation (RAD): ≥ +90° (i.e., lead I is mostly 'negative')
    • Left Posterior Fascicular Block (LPFB): rS complex in lead I, qR in leads II, III, aVF (however, must first exclude, on clinical basis, causes of right heart overload; these will also give same ECG picture of LPFB)
    • Many causes of right heart overload and pulmonary hypertension
    • High lateral wall MI with Qr or QS complex in leads I and aVL
    • Some cases of RBBB
    • Some cases of WPW syndrome
    • Children, teenagers, and some young adults
  • Bizarre QRS axis: +150° to -90° (i.e., lead I and lead II are both negative)
    • Consider limb lead error (usually right and left arm reversal)
    • Dextrocardia
    • Some cases of complex congenital heart disease (e.g., transposition)
    • Some cases of ventricular tachycardia

Test your knowledge on lessons 3 and 4!