This three-lead rhythm strip is from an 85 year-old-woman for whom we have no other information, unfortunately. It shows an obvious, regular bigeminal pattern. There are many rhythms that can cause bigeminy, with regular, paired beats. So, how do we analyze this rhythm?
First, we can see that all the QRS complexes are fairly narrow and look alike in each lead. So, we know this is a supraventricular rhythm. The rhythm “marches out”, with the same spacing between all the paired beats, and exactly one P-QRS missing after each pair.
Sino-atrial exit block occurs when the sinus node fires, but the impulse is unable to exit the sinus node. No P wave will occur, but the sinus node continues to fire in a regular rhythm. There are four types of SA block, just as there are four types of AV block. This ECG shows a Second-degree, Type II SA block.
First-degree SA block would not be decipherable on the ECG, as we do not see the SA node fire, and therefore cannot measure an interval between the SA node firing and the production of a P wave.
Second-degree SA block, Type I would have progressively prolonging exit times for the impulse leaving the SA node. This would produce progressively shortening R-to-R intervals, until the pause.
Second-degree SA block, Type II has regular R-to-R intervals until the pause, and the pause will equal two or more (usually two) R-to-R intervals.
Third-degree SA block produces NO P WAVES, and would have to be diagnosed on electrophysiology studies, as it would usually result in a junctional escape rhythm.
Differential diagnosis includes many causes of “atrial bigeminy”. When every other beat is a premature atrial contraction (APB or PAC), the pause will be caused by the PAC “re-setting” the sinus node, and will not be a multiple of the R-to-R interval on that strip. We can differentiate this rhythm from the AV blocks because in sinus block, the P wave is missing. In AV block, the P wave is present, but not always conducted.
To better illustrate what is happening in this ECG, we have included a laddergram. If you are not familiar with using laddergrams, please refer to the following links.
Clinically, we would want to investigate the possible cause of the SA block, and watch for worsening of the block, which could result in a dangerous bradycardia.