What are R waves and S waves on an electrocardiogram (ECG)?

R Waves and S Waves on an Electrocardiogram (ECG)

R waves and S waves are specific deflections within the QRS complex that represent different phases of ventricular depolarization, with R waves appearing as upward (positive) deflections and S waves as downward (negative) deflections following an R wave.

Basic Definition and Characteristics

• R wave: The first positive (upward) deflection in the QRS complex
• S wave: The negative (downward) deflection that follows an R wave

These waves form part of the QRS complex, which represents ventricular depolarization on the ECG. The QRS complex typically consists of:

1. A Q wave (first negative deflection)
2. An R wave (first positive deflection)
3. An S wave (negative deflection following the R wave)

Physiological Basis

R and S waves reflect the electrical activity during ventricular depolarization:

• The electrical impulse spreads from the endocardium (inner heart layer) to the epicardium (outer heart layer) during ventricular depolarization 1
• This creates voltage gradients that are detected by electrodes on the body surface
• The amplitude and direction of these waves vary depending on the lead placement relative to the heart's electrical axis

Clinical Significance

Normal Patterns

• In normal hearts, R waves progressively increase in amplitude from V1 to V6 (R wave progression)
• S waves are typically deep in right precordial leads (V1-V2) and become progressively smaller toward the left precordial leads (V5-V6)
• The relative amplitudes of R and S waves in different leads help determine the heart's electrical axis

Abnormal Patterns

1. Ventricular Hypertrophy:

   • Left Ventricular Hypertrophy (LVH): Increased R wave amplitude in left-sided leads (I, aVL, V5-V6) and deeper S waves in right-sided leads (V1-V2)
   • Right Ventricular Hypertrophy (RVH): Tall R waves in right precordial leads (V1-V2) and deep S waves in left-sided leads (I, V5-V6) 1

2. Bundle Branch Blocks:

   • In right bundle branch block: RSR' pattern (a second R wave called R') in V1-V2
   • In left bundle branch block: Wide, notched R waves in leads I, aVL, V5-V6

3. Myocardial Infarction:

   • Loss of R wave amplitude or development of pathological Q waves in the affected region

Technical Considerations

• The fundamental frequency for the QRS complex at the body surface is approximately 10 Hz 1
• Most diagnostic information is contained below 100 Hz in adults
• Inadequate high-frequency response in ECG recording can result in underestimation of R wave amplitude and smoothing of notches 1
• Age-specific criteria must be applied when evaluating R and S wave amplitudes, particularly in children 2

Measurement and Interpretation

• R wave amplitude is measured from the baseline to the peak of the R wave
• S wave depth is measured from the baseline to the nadir (lowest point) of the S wave
• Combined measurements (like R+S amplitude) are often used in diagnostic criteria for ventricular hypertrophy

Common Pitfalls

• Electrode placement significantly affects R and S wave amplitudes
• Body habitus (obesity, thin chest wall) can alter R and S wave amplitudes
• Age-related changes must be considered when interpreting R and S wave amplitudes
• Technical factors like filter settings can artificially alter the appearance of these waves

Understanding R and S waves is fundamental to ECG interpretation and helps clinicians identify various cardiac conditions including ventricular hypertrophy, conduction abnormalities, and myocardial infarction.