ECG Interpretation: Characteristics of the Normal ECG

Overview

• ECG interpretation involves assessing the morphology (appearance) of waves and intervals on an ECG curve.
• Key components include P-wave, QRS complex, ST segment, T-wave, and U-wave.
• Understanding normal findings, variants, and pathological variants is crucial.

P-wave, PR Interval, and PR Segment

• P-wave: Represents atrial depolarization.
  • Small, positive, smooth wave.
  • Positive in lead II during sinus rhythm.
  • Biphasic in V1, possibly V2.
• PR Interval: From onset of P-wave to onset of QRS complex.
  • Normal duration: 0.12–0.22 seconds.
• PR Segment: Flat line between end of P-wave and onset of QRS complex.
  • Serves as baseline for measuring deflections.

QRS Complex

• Represents ventricular depolarization.
• Composed of Q, R, and S waves, though not all may appear.
• QRS Duration: Normal < 0.10 seconds, prolonged if > 0.12 seconds.
  • Prolonged QRS indicates slow ventricular depolarization.

J Point and ST Segment

• ST Segment: Corresponds to plateau phase of action potential.
  • Must be assessed for myocardial ischemia, which causes ST segment deviation.
• ST Segment Deviation:
  • Depression: Below the PR segment.
  • Elevation: Above the PR segment.
• J Point: Start of the ST segment.

T-wave

• Reflects rapid repolarization of contractile cells.
• Normal T-waves are slightly asymmetric and concordant with QRS.
• Changes in T-waves may indicate various conditions.

U-wave

• Occasionally seen positive wave after the T-wave.
• Amplitude usually one-fourth of T-wave.
• More prominent in slow heart rates.

QT Duration and Corrected QT (QTc) Interval

• QT Duration: Total duration of ventricular depolarization and repolarization.
  • Corrected for heart rate to give QTc.
  • Prolonged QTc increases risk of ventricular arrhythmias.

R-wave and its Progression

• R-wave Amplitude: High amplitudes may indicate ventricular hypertrophy.
  • Important references for measuring amplitudes.
• R-wave Progression: Gradual increase from V1 to V5, then diminishes.
  • Abnormal progression may indicate myocardial infarction or hypertrophy.

Q-wave

• Differentiates normal from pathological Q-waves (indicative of myocardial infarction).
• Criteria for pathological Q-waves: duration > 0.03 sec, amplitude > 25% of R-wave.

Causes of ST Segment Changes

• ST Depression: Indicates conditions like heart failure, ischemia.
• ST Elevation: Can suggest acute myocardial ischemia.
• Important to differentiate ischemic from non-ischemic causes of ST changes.

Axis of the Heart

• Electrical axis reflects the average direction of ventricular depolarization.
• Normal Axis: Between -30° and +90°.
• Deviations indicate conditions like hypertrophy or infarction.

Key Terms

• Primary ST-T Changes: Due to abnormal repolarization.
• Secondary ST-T Changes: Caused by abnormal depolarization affecting repolarization.

Important Figures

• Figure 1: Classical ECG curve.
• Figures 13 and 14: Illustrate ST segment elevation and depression.

Clinical Implications

• Accurate interpretation of ECG components is vital for diagnosing cardiac conditions.
• Understanding normal ECG is essential for recognizing pathological deviations.

These notes provide a structured overview of normal ECG characteristics, focusing on the key waves, segments, and intervals crucial for cardiac assessment.