Final answer:
The Second degree AV block, Type I (Wenckebach) rhythm is usually not life threatening and transient, while Second Degree AV Block, Type II and Complete heart block (3rd Degree) are more serious and can be life-threatening.
Step-by-step explanation:
In this case, the rhythm that is usually not life threatening and transient is Second degree AV block, Type I (Wenckebach).
A second-degree AV block, Type I, also known as Wenckebach, is characterized by a progressive lengthening of the PR interval until a QRS complex is dropped. This rhythm is typically transient and self-limited, meaning it resolves on its own and does not pose a significant risk to the patient's life.
On the other hand, Second Degree AV Block, Type II and Complete heart block (3rd Degree) are more serious and can be life-threatening rhythms that often require medical intervention.