What the note says

Why the T wave is dangerous

• The T wave is ventricular repolarization.
• Near its peak and downslope, ventricular cells are repolarizing at different rates. Some are still refractory and some have recovered.
• A shock or ectopic beat landing in this window excites only the recovered cells. The impulse travels in a broken, uneven pattern and sets up re-entry, which can degenerate into VF.
• Your note gives the window as 20 to 30 ms on the T wave. I couldn't confirm that exact figure in the library. Textbooks usually describe it as the area around the peak of the T wave and its upslope.

The clinical point: cardioversion vs defibrillation

• Synchronized cardioversion is used for organized rhythms with a pulse, such as AF, flutter, SVT, and VT with a pulse. The machine times the shock to the R wave, so it avoids the T wave.
• Defibrillation is unsynchronized. It is used for VF and pulseless VT, which have no organized QRS or T wave. Braunwald states that for VF "synchronization is not necessary because there is no vulnerable period of the T wave to avoid" - Braunwald's Heart Disease, Technique.
• Cecil notes that a nonsynchronized shock coincident with the vulnerable period of the T wave may precipitate VF. The risk therefore applies to an unsynchronized shock given to a patient who still has an organized rhythm - Goldman-Cecil Medicine, Transthoracic Cardioversion and Defibrillation.

Quick recall

Enhanced automaticity

Normal pacemaker behavior

• The SA node has the fastest intrinsic rate, so it is the dominant pacemaker. The AV node and Purkinje system are subsidiary pacemakers that fire more slowly and are normally suppressed by overdrive suppression.
• Phase 4 depolarization in the SA node comes from two linked processes:
  • The "membrane clock" is the funny current (I_f, through HCN4 channels) plus other voltage-dependent currents.
  • The "Ca clock" is spontaneous sarcoplasmic reticulum Ca release in late diastole.
• Fuster and Hurst's The Heart (p. 1075) shows isoproterenol triggering spontaneous SR Ca release during phase 4 in the canine SA node, which speeds the heart rate. The same text states that the Ca clock also contributes significantly to AV node automaticity.

Ways a focus can fire faster

1. Phase 4 slope increases. This is the commonest mechanism. Sympathetic stimulation, catecholamines, hypokalemia, ischemia, and stretch all steepen the slope.
2. Threshold becomes less negative... or more negative. A threshold that moves closer to resting potential means less depolarization is needed to fire.
3. Maximum diastolic potential becomes less negative. The cell starts closer to threshold.

Two types

Triggers

• Catecholamines, sympathetic surge, and sympathomimetic drugs
• Hypokalemia, hypoxia, acidosis, and ischemia or reperfusion
• Myocardial stretch, heart failure, and structural remodeling. In heart failure, HCN4 expression and Ca handling are abnormal, which alters automaticity (Fuster and Hurst's The Heart, Automaticity as a Mechanism of Cardiac Arrhythmias).
• Digitalis (also causes triggered activity) and hyperthyroidism

Clinical pictures

• Sinus tachycardia
• Focal atrial tachycardia
• Accelerated junctional rhythm
• Accelerated idioventricular rhythm, such as after reperfusion in STEMI
• Premature ventricular contractions, which Harrison's attributes to enhanced automaticity, triggered activity, or localized reentry
• Ventricular tachycardia in STEMI, which Fuster lists as often triggered by ischemia, electrolyte abnormalities, reentry, and enhanced automaticity

How to recognize it clinically

• Warm-up and cool-down. The rate gradually accelerates at onset and slows at termination.
• Not started or stopped by programmed pacing. Triggered activity and reentry are more reactive to pacing.
• Cardioversion is unreliable. Braunwald notes it is not established whether cardioversion can terminate tachycardias caused by enhanced automaticity or triggered activity (Braunwald's Heart Disease, Mechanisms). The tachycardia usually recurs soon after the shock, and catecholamine release after the shock can perpetuate it.

Treatment principles

• Correct the cause first: potassium, hypoxia, ischemia, and adrenergic drive.
• Target phase 4. Harrison's lists calcium channel blockers, beta-blockers (indirectly, by reducing adrenergic input), and ivabradine for automaticity-based arrhythmias (Harrison's, Underpinnings of the Treatment of Arrhythmias).
• Class I and III drugs can also suppress automaticity, such as amiodarone and some sodium channel blockers.
• Catheter ablation removes a discrete focus in cases such as focal atrial tachycardia.

Compared with the other mechanisms

Caveats

• The "threshold" point above has a typo. The correct statement is that a less negative maximum diastolic potential or a more negative threshold brings the cell closer to firing. The first half of that item was worded confusingly. A threshold closer to resting potential means a more negative threshold, which makes firing easier.
• No matching image came back from the image library, so I included none.
• A PubMed check of recent systematic reviews returned nothing on the mechanism that changes this answer.

Why hypokalemia causes arrhythmia

1. Reduced K⁺ currents and prolonged repolarization (long QT and torsades)

• Hypokalemia speeds up internalization and degradation of the cardiac HERG channel. HERG carries I_Kr, the main K⁺ efflux current in phases 2 and 3.
• Less I_Kr means slower phase 3 repolarization, a longer action potential, and a longer QT interval.
• The text states that this "provides an elegant explanation" for the link to long QT syndrome and torsades de pointes.
• The effect is worse with hypomagnesemia, QT-prolonging drugs, or underlying channelopathy mutations.
• Low extracellular K⁺ also reduces the conductance of other K⁺ channels, including I_K1. Both effects lower the "repolarization reserve."

2. Paradoxical depolarization of Purkinje and other cardiac cells

• By the Nernst equation, low extracellular K⁺ should hyperpolarize a cell, and it does in skeletal muscle. That causes weakness.
• In human cardiac cells, especially Purkinje fibers, hypokalemia instead causes paradoxical depolarization.
• The cause is K2P1 (TWIK-1) channels. In low K⁺ they start to conduct Na⁺ into the cell.
• A resting potential closer to threshold raises automaticity and favors ectopic firing. It also makes conduction heterogeneous, which supports reentry.
• This ties to your previous question: hypokalemia steepens phase 4 and promotes enhanced automaticity.

3. Na⁺/K⁺-ATPase inhibition, which leads to Ca²⁺ overload and afterdepolarizations

• Hypokalemia downregulates cardiac Na⁺/K⁺-ATPase activity. Intracellular Na⁺ rises.
• The Na⁺/Ca²⁺ exchanger then removes less Ca²⁺, so intracellular Ca²⁺ rises.
• Higher Ca²⁺ activates CaMKII, which turns on late Na⁺ and Ca²⁺ currents and reduces repolarization reserve.
• This favors early afterdepolarizations and arrhythmias such as torsades and polymorphic VT.
• Goodman and Gilman note the same Na⁺ and Ca²⁺ accumulation increases the risk of afterdepolarizations and drug-induced arrhythmia (Adverse Effects and Precautions).
• Delayed afterdepolarizations (DADs) from Ca²⁺ overload are also promoted. This is why hypokalemia is dangerous in digoxin toxicity. K⁺ competes with digoxin at the Na⁺/K⁺-ATPase binding site, so low K⁺ intensifies digoxin's effect (Goldman-Cecil, Digitalis Overdose).

4. Heterogeneous repolarization

• Regional differences in repolarization create a dispersion that supports reentry. Prolonged repolarization in some zones, together with the triggered beats above, makes sustained VT or VF more likely.

ECG findings

• Flat or broad T waves
• ST depression
• Prominent U waves (the source text lists the first three. U waves are classic, but this chapter doesn't mention them)
• Prolonged QT (QT-U) interval
• The changes are most marked when serum K⁺ is below about 2.7 mmol/L (Harrison's and Brenner).

Arrhythmias seen

• PACs and PVCs
• Atrial fibrillation, flutter, and atrial tachycardia
• Torsades de pointes and polymorphic VT
• VF

Mechanism summary

Clinical points

• Correct K⁺ and Mg²⁺ together. Hypomagnesemia makes the effect worse and blunts K⁺ repletion.
• Be most careful with digoxin, QT-prolonging drugs, loop and thiazide diuretics, and heart disease.
• Keep K⁺ at 4.0 mmol/L or above in patients with ventricular arrhythmia, per common practice (I didn't verify a specific guideline threshold here).

Correction to my previous answer

Recent literature check

Side-by-side

Where your reasoning is right

• Both produce premature or tachycardic ectopic beats from a focus, not from a reentrant circuit.
• Both are triggered by sympathetic stimulation, ischemia, electrolyte disorders, and drugs. Hypokalemia, as in your last question, promotes both.
• Clinically they are hard to tell apart. Textbooks often group them (Harrison's says PVCs can be due to "enhanced automaticity, triggered automaticity, or localized reentry"). Neither responds reliably to cardioversion.
• They share an abnormal Ca²⁺ and Na⁺ handling background, in particular Ca²⁺ overload.

Where it needs refining

• A DAD or EAD is not the cell firing on its own. It is an oscillation that follows a normal action potential. Only if it reaches threshold does it trigger a new beat.
• So enhanced automaticity is a pacemaker problem, while triggered activity is a repolarization or Ca²⁺ handling problem.
• EADs occur in phase 2 or 3, during a prolonged action potential (long QT, bradycardia, hypokalemia, class Ia and III drugs). DADs occur in phase 4, after repolarization is complete, driven by spontaneous SR Ca²⁺ release (digoxin, catecholamines, CPVT, heart failure).

One link worth knowing

Exam shortcut

• Automaticity: spontaneous, "no trigger needed," warm-up.
• Triggered activity: "afterdepolarizations," needs a prior action potential. EAD with long QT and torsades. DAD with digoxin toxicity and Ca²⁺ overload.

Intrinsic automaticity

• Fuster and Hurst's The Heart (cited earlier) lists the AV node and Purkinje system as "also capable of generating automatic activity." The Ca clock contributes significantly to AV node automaticity, as it does in the SA node.
• Histology (Ross) says that if the SA node fails, the area with the next fastest intrinsic rate takes over. The AV node then drives the heart at about 50 beats/min.
• Typical intrinsic rates are:

Why it doesn't fire normally: overdrive suppression

• Costanzo Physiology (Latent Pacemakers): when the SA node drives the heart, the latent pacemakers are suppressed. This is overdrive suppression.
• The SA node fires fastest, and its impulses spread to the other tissues before they reach threshold, resetting them each time.
• Each time the AV node is driven faster than its own rate, its Na⁺/K⁺-ATPase pumps harder. This produces a hyperpolarizing outward current that suppresses its phase 4 depolarization.
• The result is that the AV node is activated by the atrial impulse, not by its own phase 4. It acts as a conducting tissue rather than a pacemaker.

What happens when the SA node stops

• The latent pacemaker emerges after a delay, because suppression takes time to wear off. Guyton and Hall describe the same delay in the ventricles in incomplete AV block: ventricular excitability is first suppressed "because the ventricles have been driven by the atria at a rate greater than their natural rate."
• After SA arrest, a junctional escape rhythm appears with a narrow QRS at about 40 to 60/min. In complete heart block, the ventricles use their own slower intrinsic rate.
• Escape rhythms are protective. They are physiologic automaticity doing its job, not a pathologic rhythm.

Link to your earlier question

• An accelerated junctional rhythm (above about 60/min) from digoxin toxicity, ischemia, or catecholamines is a case of enhanced automaticity. The junction is firing faster than usual and can compete with the SA node.
• A fast junctional rhythm that competes with sinus rhythm shows that the AV node's own rate has risen above the SA node's rate, so suppression no longer holds.

Summary