Atrial Tachycardia
A rogue cell stealing the show. When a tiny focus of atrial tissue outpaces the sinus node and hijacks the heart's rhythm.
Unlike flutter's massive anatomical racetrack or AVNRT's tightly wound nodal loop, atrial tachycardia is a sniper.
It is the story of a single, highly localized cluster of cells firing wildly. Somewhere in the vast muscular expanse of the right or left atrium, a microscopic focus decides it wants to be the pacemaker. It ignores the command of the sinus node, fires at a blazingly fast rate, and forces the rest of the heart to follow along.
Because it originates from a single tiny point, the impulse spreads outward from that spot like ripples in a pond, creating a distinct electrical signature on the surface ECG.
Two Ways to Spin
How does a sniper nest form? There are two distinct biophysical mechanisms that cause Atrial Tachycardia, and they behave very differently in the wild.
Mechanism 1: Enhanced Automaticity. A callback to Volume III. A cluster of atrial cells gets irritable. The inward leak of sodium during Phase 4 becomes unusually steep. These cells reach their threshold potential so quickly that they fire before the sinus node even has a chance.
Because automaticity is driven by a gradual change in cellular metabolism, this type of tachycardia acts like a volume dial. It "warms up" — starting at 110 bpm, creeping to 130, then 150. When it finishes, it "cools down," gradually slowing before returning control to the sinus node.
Mechanism 2: Micro-reentry. A tiny scar or structural defect in the atrial wall creates a microscopic spinning circuit. It is reentry, just like flutter, but happening on a millimeter scale.
Because reentry requires a highly specific premature beat to enter the circuit at the exact right moment, it acts like a light switch. It turns on instantly at full speed (e.g., suddenly jumping to 160 bpm) and stops just as abruptly.
The P-Wave is the Map
When the sinus node fires, electricity travels from the high right atrium downward and to the left. This normal path gives us our classic sinus P waves: upright in leads II, III, and aVF.
But when a rogue focus takes over, the electricity travels from a completely different starting point. The shape of the P wave tells you exactly where the sniper is hiding. It is a pure vector puzzle.
If the P wave is positive in V1, the electricity is moving toward the front of the chest. The origin must be in the back—specifically, the left atrium.
If the P wave is negative in lead aVL, the electricity is moving away from the high left shoulder. The origin must be sitting right at that high left spot, like the left atrial appendage.
If the P wave is negative in inferior leads (II, III, aVF), the electricity is moving from the bottom to the top. The origin must be low in the atrium, near the coronary sinus or the inferior vena cava.
The Timing: Long RP
In AVNRT, we learned that the atrium and ventricle fire almost simultaneously, causing a short RP tachycardia. Atrial Tachycardia plays by completely different rules.
In AT, the rogue focus fires in the atrium (creating a P wave). That impulse must then travel physically across the atrial tissue, enter the AV node, endure the normal AV node delay, and finally travel down the His-Purkinje system to fire the ventricles (creating a QRS).
Because the normal AV node delay is preserved, the P wave sits far away from the next QRS, but immediately precedes its own QRS.
The time from the R wave (ventricular beat) to the subsequent P wave (next atrial beat) is very long. This is the definition of a long RP tachycardia. The P wave is visually closer to the QRS that follows it than the QRS that precedes it.
To cure an atrial tachycardia, we have to find the sniper. We bring the patient to the EP lab and induce the tachycardia so the rogue focus is actively firing.
We then take a mapping catheter and systematically drag it around the interior walls of the atria. We are looking for the exact spot where the local electrical signal on our catheter happens earliest relative to the P wave on the surface ECG.
As we get closer to the sniper's nest, our local signal gets earlier and earlier, until it precedes the surface P wave by 30 to 50 milliseconds. When we find that spot—the center of the ripples—we deliver a targeted ablation. The sniper is neutralized, and the sinus node regains control.
Key Takeaways
- Mechanism: A single ectopic focus firing rapidly due to enhanced automaticity or micro-reentry.
- Onset: Automatic AT shows "warm-up" and "cool-down," while micro-reentrant AT starts and stops abruptly.
- ECG Hallmark: A regular tachycardia with abnormal P wave morphology different from the sinus P wave.
- Timing: Typically a long RP tachycardia, because conduction relies on the normal AV node pathway.
- Localization: The P wave axis and morphology map precisely to the anatomical origin of the rogue focus.