Dysrhythmias III

Cardiovascular System


An 18-year-old man is seen at a university clinic in South Florida for a pre-participation sports physical. He was on the cross-country track team in high school. He has been in good health, reports no symptoms, and denies recent travel out of the area. The physical examination is within normal limits. A routine electrocardiogram (ECG) is obtained and shows a heart rate of 58 and a PR interval of 300 milliseconds. The ECG is otherwise within normal limits.


Which of the following is the most likely cause of the prolonged P-R interval?


Background

This asymptomatic athlete has a prolonged PR interval, indicating a delay in conduction from the atria to the ventricles. Given the unlikelihood of addtional pathology based on the history and physical examination, the most likely explanation is that he has first degree atrioventricular (AV) block. Although many points along the conduction pathway may be involved in first degree AV block, it most commonly involves the atrioventricular node. This condition is present in about 35 percent of athletes and occurs due to increased vagal tone.


(A)

Left ventricular hypertrophy

Left ventricular hypertrophy may occur physiologically in well-conditioned athletes. However, it would not cause a prolonged PR interval. In addition, left ventricular hypertrophy would cause a QRS voltage increase. This is not the most likely explanation.

(B)

Hyperkalemia

Hyperkalemia may cause a prolonged PR interval, and electrolyte imbalances may result from vigorous training, especially in warm weather. However, other ECG abnormalities such as a widened QRS would be expected. This is not the most likely explanation.

(C)

Tricuspid valve abnormality

Ebstein anomaly of the tricuspid valve may cause a prolonged PR interval and may be asymptomatic. However, the prevalence is 1 in 20,000 live births and would be an uncommon cause of the finding. In addition, clinical findings such as widely split S1 and S2 and a tricsupid regurgitation murmur are common. ECG would show tall P waves and right bundle branch block. This is not the most likely explanation.

(D)

Subacute infective endocarditis

Subacute infective endocarditis may cause first degree AV block. However, clinical findings such as fever and one or more cardiac murmurs would likely be present. This is not the most likely explanation.

(E)

Increased vagal tone

Increased vagal tone is frequently found in athletes and can cause a conduction delay at the atrioventricular node, leading to first degree AV block. This is the most likely explanation.

References

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