Provided is an ECG image of sinus bradycardia. Identify the wave pattern and describe its characteristics.

Choice A rationale Sinus bradycardia is defined by a heart rate of less than 60 beats per minute originating from the sinoatrial node. On an ECG, this rhythm must demonstrate all the hallmarks of a normal sinus rhythm, including a P wave preceding every QRS complex, a constant PR interval between 0.12 and 0.20 seconds, and a regular R to R interval. The only abnormality is the slow rate. It can be physiological in athletes or a result of vagal stimulation. Choice B rationale An absence of P waves combined with a slow heart rate of 40 beats per minute typically indicates a junctional escape rhythm. In this scenario, the sinoatrial node fails to fire, and the atrioventricular node takes over as the primary pacemaker. Because the atria are not depolarized from the top down, P waves are either absent, inverted, or hidden within the QRS complex. This is distinct from sinus bradycardia, where the sinoatrial node is still functioning as the pacemaker. Choice C rationale A prolonged PR interval greater than 0.20 seconds is the defining characteristic of a first-degree atrioventricular block. While this condition can occur alongside sinus bradycardia, the prolongation of the PR interval indicates a delay in conduction through the atrioventricular node rather than a slow discharge rate from the sinoatrial node itself. Simple sinus bradycardia maintains a normal PR interval. Lab values for cardiac output may be monitored if the rate becomes too slow to maintain tissue perfusion. Choice D rationale A regular rhythm with a heart rate between 60 and 100 beats per minute is the definition of normal sinus rhythm. In this range, the sinoatrial node is firing at an appropriate physiological frequency for a resting adult. All ECG components, including P waves, QRS complexes, and T waves, are present and occur at regular intervals. Sinus bradycardia specifically requires the rate to drop below the 60 beats per minute threshold while maintaining the other characteristics of sinus rhythm.