BJSM thetic nervous systems. In healthy adults, sinus rhythm < 60 beats/min is considered as ‘sinus bradycardia’ (figure 2). In well-trained athletes, resting sinus bradycardia is a common finding due to increased vagal tone. In endurance athletes, aerobic training also may induce intrinsic adaptations in the sinus node with decreased automaticity resulting in a high prevalence of sinus bradycardia.16 17 In the absence of symptoms such as fatigue, dizziness or syncope, a heart rate ≥30 beats/min should be considered normal in a well-trained athlete. Sinus bradycardia disappears with an increase in heart rate during physical activity. Figure 4 ECG of a 28-year-old asymptomatic Caucasian handball player demonstrating a junctional escape rhythm. Note the constant RR interval between beats. This figure is only reproduced in colour in the online version. Sinus arrhythmia The heart rate usually increases slightly during inspiration and decreases slightly during expiration (figure 3). This response called sinus arrhythmia can be quite exaggerated in children and in well-trained athletes resulting in an irregular heart rhythm which originates from the sinus node. It has been estimated that up to 55% of well-trained athletes have sinus arrhythmia.6 11 Figure 5 ECG shows an ectopic atrial rhythm. The atrial rate is 63 beats/min and the P wave morphology is negative in leads II, III and aVF (arrows), also known as a low atrial rhythm. This figure is only reproduced in colour in the online version. sinus node dysfunction (sick sinus syndrome). Differentiating features that suggest sinus node dysfunction include lack of rhythmic changes in the heart rate, abrupt sustained rate increases and decreases and an inappropriate rate response to exercise (both a slowed acceleration and an inappropriately rapid deceleration), as well as any association with clinical symptoms such as exercise intolerance, presyncope and syncope. While the heart rhythm is quite irregular in sinus arrhythmia, the P wave axis remains normal in the frontal plane. Accelerating the heart rate with physical activity normalises the heart rhythm. Sinus arrhythmia is considered as a normal finding in an athlete. Junctional escape rhythm A junctional or nodal rhythm occurs when the QRS rate is faster than the resting P wave or sinus rate which is slowed in athletes due to increased vagal tone (figure 4). The QRS rate for junctional rhythms is typically less than 100 beats/ min, and the QRS complex usually narrow unless the baseline QRS has a bundle branch block. Sinus rhythm resumes with increased heart rates during exercise. Figure 6 ECG shows first-degree AV block (PR interval >200 ms). The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. This figure is only reproduced in colour in the online version. Ectopic atrial rhythm In an ectopic atrial rhythm, P waves are present but are a different morphology compared to the sinus P wave. Ectopic P waves are most easily seen when the P waves are negative in the inferior leads (II, III and aVF; figure 5). The atrial rate is typically less than 100 beats/min. There also 28 Sport & Geneeskunde | mei 2013 | nummer 2 This should not be confused with Pagina 27
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