Final answer:
A second-degree heart block is identified on an ECG by intermittent P waves not followed by QRS complexes, indicative of an impairment in the heart's electrical conduction system, specifically within the AV node for Mobitz type 1.
Step-by-step explanation:
A second-degree heart block, particularly Mobitz type 1, also known as Wenckebach, is a specific abnormality in the heart's electrical conduction system. This can be identified on an ECG when there is a progressive prolongation of the PR interval until one atrial impulse fails to conduct to the ventricles, resulting in a P wave not followed by a QRS complex. Subsequent P waves are then conducted with a shorter PR interval which then progressively lengthens again. When there is a complete (third-degree) heart block, the ECG will display no correlation whatsoever between the atrial activity (P waves) and the ventricular activity (QRS complexes). In these cases, the AV node can maintain a ventricular rate that is adequate to preserve consciousness, normally at a rate of 40-60 beats per minute.
The identification of such conditions on an ECG is crucial for understanding the heart's functioning and ensuring appropriate medical response. It is important to recognize that a second-degree Mobitz type 1 heart block reflects an impairment within the AV nodal area, while an infra-Hisian block would suggest a defect further down the conduction pathway, below the bundle of His.