Two-to-One Sinoatrial Block

Symptomatic Bradycardia

“…1 Sudden halving of the heart rate could occur because of 2:1 sinoatrial block (SAB), and sudden doubling of the heart rate could be because of its resolution. 2 I agree wholeheartedly with Dr Littmann's advice to look for P waves, whether premature or not, whenever faced with what appears to be simple sinus bradycardia. However, sudden doubling or halving of a sinus rhythm in the absence of hidden P waves should raise the possibility of 2:1 SAB.…”

“…1 A common finding is that in type II block, acceleration of the sinus rate can trigger block whereas β-blocker or carotid massage, by decreasing the sinus rate, can paradoxically relieve the block and result in doubling of the heart rate. 1,2 However, there are distinct differences between type II sinoatrial block and type II atrioventricular (AV) block. First, type II sinoatrial block is rare, whereas type II second-degree AV block is more common.…”

“…Like the atrioventricular block case presented by Dr Littmann, mild acceleration of the sinus rate might bring on 2:1 or other degrees of SAB in a patient with that condition. By the same token, deceleration of the sinus rate (whether spontaneous or by applying carotid massage or administering β-blockers) may allow the sinoatrial junction to recover its excitability and paradoxically increase the sinus rate owing to resolution of SAB and resumption of 1:1 conduction …”

Carotid Sinus Massage Is the Message

“…It may slow down the sinus rate. The sinoatrial junction may then recover its excitability and allow 1:1 conduction, paradoxically increasing the heart rate and revealing the diagnosis . Additionally, CSM may help determine the site of type 1 second-degree AVB.…”

Symptomatic Bradycardia—In Reply

“…I agree that sudden halving of the heart rate can indicate type II second-degree block at more than 1 level within the sinoventricular conduction system. The 2 established sites are the sinoatrial junction and the His-Purkinje system . A common finding is that in type II block, acceleration of the sinus rate can trigger block whereas β-blocker or carotid massage, by decreasing the sinus rate, can paradoxically relieve the block and result in doubling of the heart rate .…”