CPVT: Catecholaminergic Polymorphic VT

The most common form, CPVT1, involves mutations in the gene encoding the ryanodine receptor (RyR2) on the sarcoplasmic reticulum membrane. RyR2 is the calcium release channel that opens during excitation-contraction coupling: when a small amount of calcium enters the cell through L-type calcium channels during the plateau, RyR2 opens and floods the cytoplasm with stored calcium, driving contraction.

Under catecholamine stimulation, cyclic AMP (cAMP) rises inside the cell and activates protein kinase A (PKA). PKA phosphorylates RyR2, increasing its open probability so the cell can contract more forcefully during exercise. In a healthy heart, this system is tightly regulated. The channel opens on demand and stays shut during diastole.

In CPVT1, the mutant RyR2 channel loses its ability to stay closed under adrenergic phosphorylation. At rest, when catecholamine levels are low, the channel behaves well enough. But during exercise or emotional stress, the PKA-driven phosphorylation makes the mutant channel leaky. It releases calcium spontaneously during diastole, when the cell should be relaxing and reloading.

The less common form, CPVT2, is autosomal recessive and involves mutations in calsequestrin-2 (CASQ2). Calsequestrin-2 is the major calcium-buffering protein inside the SR lumen. It binds calcium ions and keeps the free calcium concentration within the SR at a manageable level.

When CASQ2 is defective, the SR cannot properly buffer its calcium stores. Under adrenergic stimulation, as more and more calcium is pumped into the SR, the free calcium concentration inside the lumen rises to abnormal levels. This overwhelms the remaining buffering capacity and destabilizes the RyR2 channel from the luminal side.

The end result is the same as CPVT1: diastolic calcium leak under stress. The pathway is different (a buffering defect rather than a gating defect), but the downstream arrhythmia mechanism is identical.

When calcium leaks out of the SR during diastole, the cell has a problem. Free cytoplasmic calcium must be cleared quickly, or the cell will remain in a state of partial contraction. The sodium-calcium exchanger (NCX) steps in: it extrudes one calcium ion in exchange for three sodium ions entering the cell.

This exchange is electrogenic. Three positive charges come in (3 Na⁺), two positive charges go out (one Ca²⁺ carrying two charges). The net effect is a transient inward current (I_ti) that depolarizes the membrane during diastole. This voltage blip is a delayed afterdepolarization (DAD).

If the DAD is small, the cell's resting potassium conductance damps it out, and nothing happens. If the calcium leak is large enough, the DAD reaches threshold and triggers a full action potential: a premature beat. Under sustained adrenergic drive, the leaks fire repetitively. Each beat generates more calcium cycling, more leak, more DADs. The result is continuous triggered activity producing runs of ventricular tachycardia.

Bidirectional VT is the hallmark rhythm of CPVT. On the surface ECG, the QRS morphology alternates beat-to-beat between two distinct patterns: typically an RBBB configuration with the frontal axis swinging between left and right on alternating complexes.

The alternation likely reflects alternating activation from the two fascicles of the left bundle branch. DAD-triggered beats arise from Purkinje fibers at different sites in the ventricular conduction system. One beat exits via the left anterior fascicle, producing a right-axis QRS. The next exits via the left posterior fascicle, producing a left-axis QRS. The two sources alternate as different Purkinje regions reach threshold at different times.

Bidirectional VT is nearly pathognomonic for CPVT. The only other condition that produces it reliably is digitalis toxicity, which causes a similar calcium-overload state through inhibition of the sodium-potassium ATPase. In a young patient without digitalis exposure, bidirectional VT during exercise is CPVT until proven otherwise.

The exercise stress test is the key diagnostic tool. The resting ECG is normal. The echocardiogram is normal. Standard screening reveals nothing abnormal. The arrhythmia shows itself only when catecholamines rise.

The classic progression during graded exercise follows a reproducible sequence. At low workload, the rhythm is normal sinus. As the heart rate climbs past 100 to 110 bpm, isolated premature ventricular complexes (PVCs) begin to appear. With further exertion, the PVCs become more frequent and organize into couplets. As the patient approaches peak exercise, the couplets give way to runs of bidirectional VT. At maximal effort, the rhythm may degenerate into polymorphic VT.

This predictable, dose-dependent progression mirrors the physiology. Higher catecholamine levels produce more calcium leak, more DADs, and more triggered beats. The arrhythmia burden scales directly with adrenergic tone.

Beta-blockers are the first-line therapy. Nadolol is generally preferred because of its long half-life (providing consistent 24-hour coverage) and because it has no partial agonist activity. A beta-blocker with intrinsic sympathomimetic activity (such as pindolol) could paradoxically worsen the calcium leak. The goal is to blunt the catecholamine drive that triggers the arrhythmia.

Flecainide is added as second-line therapy when beta-blockers alone do not fully suppress arrhythmias during exercise testing. In addition to its well-known sodium channel blockade, flecainide directly inhibits the RyR2 channel, reducing the diastolic calcium leak at its source. This dual mechanism (suppressing the trigger and reducing the substrate) makes it uniquely effective in CPVT.

Left cardiac sympathetic denervation (LCSD) is reserved for patients who remain symptomatic or inducible despite maximized medical therapy. The procedure removes the lower half of the left stellate ganglion and the T2 through T4 sympathetic ganglia, cutting the sympathetic nerve supply to the left ventricle. It does not eliminate catecholamine sensitivity entirely but reduces the adrenergic load enough to prevent arrhythmia in most refractory cases.

An ICD is considered as a backup in patients at high risk. However, the ICD carries a specific hazard in CPVT. A shock itself is a massive adrenergic stimulus. The pain and stress of a discharge can trigger a surge of catecholamines that worsens the calcium leak and provokes more VT or VF. This can create a vicious cycle of shock followed by arrhythmia followed by shock. For this reason, medical therapy and LCSD are optimized aggressively before resorting to device implantation.

Exercise restriction is a lifelong requirement. Competitive sports and vigorous exertion are prohibited. Patients are counseled to avoid sudden bursts of physical effort and to recognize warning symptoms such as palpitations or presyncope during activity.