The wide complexes are ventricular premature beats (VPB's or PVC's). They occur EARLY in the cycle (before one would expect the next sinus beat), so they are not escape beats but are due to ventricular irritability (too much caffeine anyone?)
Also ischemia, meds (digoxin), hypercathecholamine states (incl anxiety, lack of sleep), electrolyte disturbances, and structural heart disease may be causes. PVC's are characterized by a) no preceding P wave, b) wide and occur earlier than expected, and c) (usually) have a 'fully compensatory pause'. Because they rarely penetrate the AV junction retrograde, they set up a refractory period in the node for the next sinus beat.
The sinus node keeps depolarizing, so if you march out the P waves (even if buried in the PVC), the sinus beat following the PVC occurs right on schedule. The net result is a long interval following the PVC (patients feel this pause and the strong beat that follows (Starling's law), but the sinus beat is right where it would be expected if the PVC hadn't intervened (“fully compensatory”). In contrast, PAC's reset the sinus node, so the interval following may vary.
NOTE: EKG's are for internal educational purposes of the University of Massachusetts Department of Family Medicine. Please do not forward without permission from Dr. Golding!
