Final answer:
A likely diagnosis for a patient with syncope, sinus pauses, long PR interval and long QRS on an ECG is a third-degree AV block. In this condition, atrial and ventricular activities do not correlate, leading to decreased cardiac output and the symptoms observed.
Step-by-step explanation:
If a patient experiences syncope accompanied by sinus pauses, a long PR interval, and a long QRS duration on an ECG, the likely diagnosis might be an atrioventricular (AV) block, possibly a third-degree or complete heart block. A third-degree block is characterized by no correlation between the atrial activity (the P wave) and ventricular activity (the QRS complex). With a complete heart block, impulses from the SA node do not pass to the ventricles at all. In second-degree blocks, some impulses do not result in ventricular contractions, hence not all P waves are followed by QRS complexes. These conditions can result in decreased cardiac output and symptoms such as fainting.
However, other problems such as ventricular tachycardia or bradycardia, myocardial infarction (MI), or other conditions might also cause similar ECG changes. A complete diagnosis should involve assessing the full clinical picture, including detailed history, physical examination, and considering other possible contributing factors such as electrolyte imbalances or drug effects.