Answer to Question 1
Rhythm Ventricular regular; atrial regular (Ps plot through on time).
Rate Atrial rate is twice the ventricular rate.
P waves Normal in size and shape; every other P wave is not followed by a QRS complex (i.e., more Ps than QRSs).
PR interval Constant.
QRS duration May be narrow or wide; complexes are absent after every other P wave.
Answer to Question 2
The ventricles may assume responsibility for pacing the heart if the sinoatrial (SA) node fails to discharge, an impulse from the SA node is generated but blocked as it exits the SA node, the rate of discharge of the SA node is slower than that of the ventricles, or an irritable site in either ventricle produces an early beat or rapid rhythm.