A preterm infant (GA 28+3, BW 1.020g) presented with hemodynamic instability following successful cardio version from VT within the first few hours after birth. Despite high amounts of administered catecholamines, the infant had bradycardia of 80 bpm.
When assessing the child, monitoring showed the following:
Clearly there is bradycardia and arterial hypotension. however, when taking a closer look to see P waves you find the following:
There are P waves in front of every QRS complex, as with sinus rhythm. However, there are also P waves inscribed within the initial portion of the T wave. Thus, bradycardia results from functional 2:1 block as every second P waves falls within the refractory period of the ventricles and can not conduct.
To solve this situation one needs to shorten QT. This was achieved by intravenous Ca++ substitution and, as obscure as it may sound, administration of propranolol. In other words, use beta blockers to treat "bradycardia".
By shortening the QT interval the second P wave moved to the terminal portion of the T wave, finally restoring sinus rhythm with 1:1 conduction at 160 bpm. Hemodynamics suddenly improved and tapering of catecholamines was possible.
Further reading: Kurath-Koller S, Noessler N, Pichler G, Höller N. Functional 2:1 AV block in a preterm. Arch Dis Child Fetal Neonatal Ed. 2021 May 5:fetalneonatal-2021-321741. doi: 10.1136/archdischild-2021-321741. Epub ahead of print. PMID: 33952627.