Narrow QRS, progressive increase in PR interval until a ventricular beat is dropped, then sequence is repeated (See Fig. 122-1B and Fig. 122-1D). Seen with drug intoxication (digitalis, beta blockers), increased vagal tone, inferior MI. Usually transient, no therapy required; if symptomatic, use atropine (0.6 mg IV, repeated × 3-4) or temporary pacemaker.