Sinoatrial (SA) block is a disorder of impulse conduction from the sinus node to the atria. It can be divided into three degrees.

Electrocardiographic Criteria

First-Degree Sinoatrial Block

  • Delay between generation of the sinus impulse and its propagation to the atrium
  • Not visible on the ECG

Second-Degree Sinoatrial Block Type 1

  • The PP interval becomes progressively shorter until the block occurs (absent P wave)
  • The PP interval before the pause is shorter than the PP interval after the pause
  • The pause must be less than twice the preceding PP cycle

Second-Degree Sinoatrial Block Type 2

  • No progressive shortening of the PP interval before the block occurs (absent P wave)
  • The pause equals 2 previous PP cycles

Third-Degree Sinoatrial Block

  • Complete absence of P-wave formation
  • Seen as an escape rhythm

ECG in Sinoatrial Block

Electrocardiogram showing sinoatrial block

Sinoatrial block. This is a second-degree type 2 SA block, because there is no progressive shortening of the PP interval before the block occurs, and the pause equals 2 previous PP cycles.

References

  1. Scheffer MK, De Marchi MFN, de Alencar Neto JN, Felicioni SP. Eletrocardiograma de A a Z. São Paulo: Manole, 2024.
  2. 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.