Sinus bradycardia is a cardiac rhythm characterized by a rate below 50 beats per minute, originating in the sinus node. On the electrocardiogram (ECG), it shows regular RR intervals, with P waves preceding each normal-looking QRS complex and a 1:1 P:QRS relationship. The P waves are positive in leads I and aVF, confirming the sinus origin of the rhythm. It may be a normal finding in young individuals, athletes or during sleep, but it may also indicate conditions such as increased vagal tone, hypothyroidism or the effect of drugs such as beta-blockers.

Electrocardiographic Criteria

  • Sinus rhythm with HR < 50 bpm

ECG in Sinus Bradycardia

Electrocardiogram showing sinus bradycardia

Sinus bradycardia. ECG showing a HR of 44 bpm. Note the sinus P waves before each QRS complex.

References

  1. Samesima N, God EG, Kruse JCL, Leal MG, et al. Diretriz da Sociedade Brasileira de Cardiologia sobre a Análise e Emissão de Laudos Eletrocardiográficos – 2022. Arq Bras Cardiol. 2022;93(3 Suppl 2):2–19.