Sinoatrial Block Type II, 2:1 Block
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Patient: Woman, 84 y/o with no known cardiac history. Lives in a nursing home and has reportedly been struggling with fatigue, dizzyness and general weakening the last two weeks. No syncopal episodes. She has recently been treated for a pneumonia. When presented in the ER, she is hemodynamically stable, but hypotensive with an initial BP of 80/40. Her SAT is 90 % with oxygen administered at two litres/min. She has a slow, palpable and irregular pulse in radialis, bilaterally. When palpated, the pulse is counted to around 35 bpm. Skin is normal and dry, but shows signs of dehydration. No cyanosis or diaphoresis. She is awake and conscious with a GCS of 15.
At first, a 12 lead ECG is obtained (above). This shows:
- Regular, narrow-complexed bradycardia of supraventricular origin
- Sinus bradycardia at 48 bpm
- Normal cardiac axis, at approx. 30°
- Minimal ST-depression laterally
- Prolonged QT interval, probably rate related. QTc is 470 ms
ECG comments: This ECG shows sinus bradycardia, but as the next strips will show, this is in fact a 2:1 SA Block. Persistent 2:1 SA Exit Block cannot be distinguished from marked sinus bradycardia, since the RR intervals are regular. The rhythm is so slow here, that this ECG only shows three cycles. A slow pulse at around 35 was palpated when she arrived, and considering her history of fatigue and dizzyness, a type of heart block should be suspected. In such cases, a longer rhythm strip must be obtained.
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These strips reveal the true problem: Long and frequent sinus pauses, resulting in a mean ventricular rate of approximately 35 bpm. This patient was hypotensive, which could be due to low cardiac output as a result of fewer ventricular contractions. Although, she was also clinically dehydrated. There are P waves preceding every QRS complex, with a normal and regular PR interval. The QRS complexes are sinus paced and does not represent any ectopy or escape. By using a caliper, a mathematical relationship between the normal cycles and the pauses is quickly established. Each long cycle is a multiple of the normal cycles. In fact, each pause is twice as long as the normal cycles. Every now and then, a P wave “falls out” without disturbing the underlying rhythm. Sinoatrial block may be due to failing SA Node automaticity or blocked conduction out of the node. These two mechanisms cannot be distinguished from a surface ECG.
Since the long cycles are twice as long as the normal cycles, we can establish the diagnosis of SA Block Type II. Same as with Second Degree AV Block, Type II, the giveaway here is the dropped complexes. Here, the P waves are dropped, not the QRS complexes as in an AV Block. In these strips, the block is intermittent, which is also why we can see it. If this 2:1 block was persistent, the rhythm would present as a marked sinus bradycardia and could not be seen on the ECG.
Leave a Comment January 24, 2009