Sinus Bradycardia with Respiratory Sinus Arrhythmia
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This ECG is from a healthy, 29 year old caucasian male with no medical history.
- Sinus bradycardia with varied ventricular rate
- Normal cardiac axis at approx 60 °
- Flattened T wave in aVL
- Voltage criteria (Sokolow-Lyon index) suggests Left Ventricular Hypertrophy
Sinus Bradycardia
Sinus bradycardia is normally considered clinically significant when then rate drops under 50 beats/min. However during sleep and in healthy, young individuals (especially athletes), heart rate may be as slow as 40 bpm without compromising cardiac output. Chou (2008: 329-330) states that a rate as slow as 35 bpm may be seen, but in healthy individuals the rate is usually > 40 bpm during waking hours. Increased vagal tone is often the cause of sinus slowing. In sinus bradycardia, the sinoatrial node fires slower than in normal sinus rhythm. The cell automaticity in the SA node is prolonged. When considering the action potential, this is equivalent with a longer phase 4 repolarization. This again manifests electrocardiographically as a prolonged TP segment.
Respiratory Sinus Arrhythmia
Variations in heart rate commonly occur in relation to the respiratory cycle. In respiratory sinus arrhythmia the heart rate increases with inspiration and decreases with expiration, due to changes in vagal tone during the respiratory cycle. Sinus arrhythmia occurs more frequently when the sinus rate is slow. This arrhythmia is considered benign and normal. Prevanlence is higher in infants and young individuals and tends to decrease with age.
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