Automaticity and Escape Rhythms
Key facts about intrinsic pacemaker rates, phase 4 depolarization, and escape versus irritable beats.
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Questions Covered in This Set
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Automaticity
The ability of a cardiac cell to spontaneously depolarize to threshold and fire without outside stimulation.
Phase 4 depolarization (prepotential)
The slow upward drift of membrane potential in pacemaker cells, driven by the funny current (I_f), until threshold (~-40 mV) is reached.
What determines how fast a pacemaker fires?
The steepness of the phase 4 slope — steeper slope = faster firing rate. The SA node has the steepest slope.
Overdrive suppression
The fastest pacemaker fires first and depolarizes all other cells before they reach their own threshold, suppressing slower pacemakers.
Intrinsic rates of the pacemaker hierarchy
SA node 60–100 bpm; atrial foci 60–80; AV junction/His 40–60; ventricles 20–40 bpm (roughly -20 bpm per level down).
What does QRS width tell you?
Width tells altitude: impulses from above the ventricles use the Purkinje system and give a narrow QRS; ventricular impulses crawl muscle-to-muscle and give a wide QRS (>0.12 s).
Junctional escape rhythm on EKG
SA node fails; AV junction fires ~40–60 bpm (about 45). Regular narrow QRS with no upright P waves (absent, inverted, or after the QRS).
Ventricular (idioventricular) escape rhythm
Ventricular focus fires ~20–40 bpm (about 30) with wide, bizarre QRS and no related P wave — dangerous and unstable.
Agonal rhythm
A dying ventricular escape rhythm below 20 bpm with very wide complexes, slowing toward asystole.
Escape beat vs. irritable (ectopic) beat
Escape beats come LATE, after a pause, as a safety mechanism; irritable/ectopic beats (PVC, PAC) come EARLY from a hyper-excitable focus (ischemia, caffeine, hypokalemia, catecholamines, digoxin toxicity).
