Final answer:
A sinus arrest duration of 3 seconds or longer on an EKG tracing is considered a medical emergency, requiring immediate medical response due to the heart's critical role in pumping blood.
Step-by-step explanation:
An EKG technician should consider sinus arrest to be a medical emergency when the arrest reaches a duration of 3 seconds or longer. Cardiac arrest indicates that the heart has stopped pumping blood effectively, which is a critical condition that requires immediate medical intervention. The heart's activities, including the cardiac cycle and phases of contraction, namely atrial systole and ventricular systole, are crucial for maintaining blood circulation throughout the body. Electrical events represented by the ECG, such as ventricular depolarization (QRS complex) and atrial depolarization (P wave), dictate the heart's mechanical contractions, ensuring adequate blood flow. Extended periods without appropriate electrical activity can be life-threatening and warrant urgent attention.