Prehospital Academy Lesson 16.5
When the primary and secondary conduction pathways of the heart fail, the cardiac system relies on increasingly primitive and slow pacemakers to maintain life. These "escape" rhythms, originating in the AV junction and the ventricles, represent the heart's last stand against total electrical failure. This guide explores the morphology of junctional and ventricular rhythms, the mechanics of retrograde conduction, and the terminal states of fibrillation and asystole.
Junctional Rhythms: The Middle Backup
When the SA node fails to fire or the impulse is blocked at the atria, the AV junction takes over as the primary pacemaker with an intrinsic rate of 40–60 bpm. The defining characteristic of a junctional rhythm is its relationship with the P-wave, which is determined by the location of the electrical foci within the junction:
High Junctional: The atria depolarize before the ventricles, resulting in an inverted P-wave with a short PR interval (<0.12s).
Mid-Junctional: The atria and ventricles depolarize simultaneously. The larger QRS complex "buries" the P-wave, making it invisible on the tracing.
Low Junctional: The ventricles depolarize first, followed by the atria. This produces an inverted P-wave that appears after the QRS complex.
Despite these P-wave anomalies, a junctional rhythm remains a narrow-complex rhythm, as the electricity still utilizes the normal ventricular conduction system (Bundle of His and Purkinje fibers).
Ventricular Rhythms: The Bottom of the Barrel
Rhythms originating in the ventricles are the heart’s final failsafe. Because they do not use the rapid-conduction bundle branches, the electricity must travel cell-to-cell through the myocardium, resulting in a wide and aberrant QRS complex (>0.12s).
Idioventricular Rhythm (IVR): The "dying heart" rhythm, with a rate of 20–40 bpm.
Accelerated Idioventricular Rhythm (AIVR): An IVR at a rate of 40–100 bpm. This is often seen as a "reperfusion rhythm" following the successful opening of a blocked coronary artery.
Ventricular Tachycardia (V-Tach): A regular, wide-complex rhythm >100 bpm. It can be stable (with a pulse) or unstable (pulseless), representing a critical emergency.
Torsades de Pointes: A specialized form of polymorphic V-Tach characterized by the "twisting of points" around the isoelectric line, often associated with a prolonged QT interval.
The Lethal State: Fibrillation and Pulselessness
Ventricular Fibrillation (V-Fib) is a state of total electrical and mechanical chaos. The ventricles do not contract; they merely quiver. On the monitor, this appears as a chaotic, wavy baseline with no discernible waves or complexes. Without immediate defibrillation, V-Fib is 100% fatal.
Pulseless Electrical Activity (PEA): It is critical to remember that the ECG measures electrical activity only. PEA occurs when the monitor shows an organized rhythm (such as Sinus Bradycardia or Idioventricular), but the mechanical pump has failed or the patient has no circulating volume. Always treat the patient, not the monitor.
Ectopic Beats: PACs, PJCs, and PVCs
Irritable foci can fire early, creating premature complexes. These are identified by their morphology:
PAC: Premature, narrow QRS with a preceding (often abnormally shaped) P-wave.
PJC: Premature, narrow QRS with no P-wave or an inverted P-wave.
PVC: Premature, wide and aberrant QRS with no preceding P-wave. Frequent PVCs can indicate myocardial irritability or electrolyte imbalances.
Critical Thinking Questions
In a mid-junctional rhythm, why is the P-wave "buried" rather than simply absent? Explain the timing of the depolarization of the chambers in this scenario.
A patient is in an Idioventricular Rhythm at 30 bpm. Why is it clinically dangerous to administer a drug like Lidocaine (which suppresses ventricular foci) to this patient?
How does the cellular mechanism of "cell-to-cell" conduction explain why ventricular rhythms produce a wider QRS complex than supraventricular rhythms?
You encounter a patient in V-Tach at 180 bpm. They are alert and oriented with a strong pulse. How does the presence of "AV Dissociation" (P-waves marching through the QRS) help confirm this is V-Tach rather than SVT with a bundle branch block?
Relate the concept of "Reperfusion" to Accelerated Idioventricular Rhythm (AIVR). Why might a paramedic see this rhythm after successfully administering nitroglycerin or during transport to a cath lab?