VT in Scar
When the ghost of a prior heart attack builds the perfect labyrinth for reentry.
A prior heart attack leaves behind a tombstone of dead tissue. But the real danger isn't the dead core—it's the border zone. The intricate maze of surviving muscle strands weaving through the dense collagen. This is the perfect trap for reentry.
When an artery occludes and myocardium dies, it doesn't leave a perfectly smooth border. The healing process is messy. Fibrosis invades the tissue, choking off normal muscle. What remains is a chaotic mix: dense islands of unexcitable scar, separated by narrow, tortuous channels of living myocytes struggling to survive.
This is the anatomical substrate for the most dangerous arrhythmia in the adult heart: Scar-related Ventricular Tachycardia (VT).
The Maze
To understand VT, you have to visualize the physical circuit. It has three essential components.
First, the central obstacle. This is a dense, unexcitable region of mature collagen. It acts as the physical barrier around which the electrical impulse will spin, much like the center of a roundabout.
Second, the isthmus. This is the critical vulnerability. The isthmus is a channel of surviving myocardium zigzagging between areas of dense scar. Here, the cellular architecture is heavily distorted. The gap junctions—the tiny pores that allow the action potential to jump rapidly from cell to cell (as we saw in Volume I)—are severely reduced and disorganized.
Because gap junctions are scarce, conduction through the isthmus is incredibly slow. The wavefront crawls cell by cell through the fibrotic maze. This agonizingly slow conduction is exactly what reentry needs: it gives the healthy tissue outside the scar enough time to recover its excitability before the wavefront emerges.
Third, the exit site. This is the point where the wavefront finally bursts out of the slow channel and spills into the healthy, wide-open ventricular myocardium. The moment it hits the exit site, the main ventricular mass depolarizes, creating the QRS complex on the surface ECG.
The Wide and Bizarre QRS
When you look at VT on a 12-lead ECG, the first thing that strikes you is the QRS duration. It is exceedingly wide, often greater than 140 or 160 milliseconds. Why?
In a normal heartbeat, the electrical impulse speeds down the bundle branches and Purkinje fibers, delivering the signal simultaneously to all corners of both ventricles. The entire ventricular mass depolarizes in a tightly synchronized snap, producing a narrow QRS.
But a premature beat that triggers VT originates in the scar. When the impulse bursts from the exit site, it is not on the fast-lane highway of the His-Purkinje system. It is in the middle of ordinary ventricular muscle. To depolarize the rest of the heart, the wavefront must spread from cell to cell, muscle fiber to muscle fiber, across the entire mass of the ventricles.
This cell-by-cell spread is inherently slow and inefficient. It takes a massive amount of time for the electrical wave to travel from an exit site in the left ventricle all the way across the septum to the right ventricle. The result on the ECG is a broad, slurred, bizarrely shaped QRS complex.
Two Hearts Beating as One
While the ventricles are locked in a vicious 180 bpm spin around the scar, what are the atria doing?
Most of the time, the sinus node doesn't care about the chaos below. The SA node continues firing at its normal rate of 75 bpm. The atria contract independently of the ventricles. The two chambers are completely divorced. This is called Atrioventricular (AV) Dissociation, and finding it on an ECG is the gold standard for diagnosing VT. You will see regular, wide QRS complexes marching at 180 bpm, with tiny P waves marching right through them at 75 bpm, totally disconnected.
Occasionally, a brilliant electrical collision occurs. A sinus P wave fires just at the right moment. The impulse travels down the AV node, through the His bundle, and down the bundle branches into the ventricles, right as a VT wavefront is bursting out of the scar.
The two wavefronts—one from above (narrow) and one from the scar (wide)—crash into each other in the ventricular myocardium. They fuse together. The resulting QRS complex on the ECG looks like a hybrid: slightly narrower than the VT beats, but wider than a normal beat. This is a Fusion beat. Seeing a fusion beat during a wide-complex tachycardia provides absolute, irrefutable proof that the rhythm is VT.
How do we find this invisible microscopic channel inside a massive scar to ablate it? We use a technique called Entrainment.
While the patient is in VT, we carefully slide an ablation catheter into the scar border zone. We start pacing from the tip of our catheter at a rate slightly faster than the VT. We "override" the tachycardia.
If our catheter tip is sitting perfectly inside the critical isthmus, the pacing wavefront will exit the scar through the exact same exit site as the clinical VT. The 12-lead ECG on our monitor will perfectly match the clinical VT morphology (a "concealed fusion" or exact match).
When we stop pacing, the circuit resumes. If the time it takes for the first native VT beat to return (the Post-Pacing Interval) exactly matches the VT cycle length, we know we are sitting directly in the active circuit. We have found the critical isthmus. We step on the pedal, deliver radiofrequency energy, and burn the channel. The VT terminates instantly.
Key Takeaways
- Mechanism: Reentry utilizing surviving strands of myocardium (the border zone) surrounded by dense, unexcitable scar.
- The Circuit: An isthmus of slow conduction (due to loss of gap junctions) leading to an exit site where the wavefront bursts into healthy tissue.
- ECG Hallmark: A regular, wide, and bizarre QRS complex because the wavefront spreads slowly, cell-by-cell, outside the His-Purkinje system.
- Diagnosis: AV dissociation (atria and ventricles beating independently) and fusion beats are the ultimate proof of VT.
- Cure: Identifying the critical isthmus through entrainment pacing and destroying the slow channel with ablation.