Final answer:
The SA node may stop initiating impulses due to damage, electrolyte imbalances, high parasympathetic activity, medication effects, or myocardial infarction. In a third-degree heart block, impulses from the SA node don't reach the ventricles. A defibrillator can reset the SA node to restore normal heart rhythm.
Step-by-step explanation:
The sinoatrial (SA) node would stop initiating impulses if it were to become damaged or defective, if there were an electrolyte imbalance affecting the heart's electrical activity, or during high levels of parasympathetic nervous system activation which releases acetylcholine and slows the heart rate. Additionally, the SA node can fail to initiate impulses due to medication effects, myocardial infarction affecting the right atrium where the SA node resides, or other pathological conditions.
In situations such as a third-degree heart block, the impulses from the SA node may fail to be conducted to the ventricles. An external defibrillator can be used to momentarily stop the heart and allow the SA node to reestablish a normal sinus rhythm. This process is critical, as the SA node maintains the heart's rhythm and pacing, thus ensuring the efficient circulation of blood throughout the body.