Cardiac Pacemakers

Anatomy and Pacemaker Types

The heart acts as a pump, ensuring continuous blood flow. Pacemakers — specialized cells — are responsible for generating electrical impulses. Interestingly, they are located not only around the perimeter of the heart, but throughout the entire body.

There are two main types of pacemaker cells:

• Type One — transmit the electrical impulse evenly in a circle.
• Type Two — spread the signal in multiple directions (these are located in the sinoatrial node).

The primary pacemaker is the sinoatrial (SA) node. It is followed by the atrioventricular (AV) node. Next, the impulse travels through the bundle of His, specialized conduction pathways containing elements of nerve tissue. Notably, the right side conducts impulses faster than the left. On the ECG, this is reflected in wave morphology: waves are larger in the right chest leads and sharper in the left leads, as the signal arrives there later.

Normal Automatism and Its Disorders

Normally, the source of a regular, correct rhythm is the sinus node. On the ECG, this is confirmed by the presence of a normal polarity P wave preceding each QRS complex. However, automatic function can be impaired:

1. Sinus Tachycardia — the heart rate (HR) exceeds normal limits (greater than 90–100 bpm at rest in adults). An inappropriate form is distinguished separately: persistent pulse elevation without obvious causes, which begins gradually.
2. Sinus Bradycardia — a regular rhythm with an HR of less than 60 bpm. Without clinical symptoms, isolated bradycardia has no significance.
3. Sinus Arrhythmia — an irregular rhythm in which the difference between R-R intervals exceeds 0.16 seconds, while the pacemaker remains sinus.

Sick Sinus Syndrome (SSS)

If the automatism of the main node is persistently suppressed, SSS develops. The node loses its ability to maintain a rhythm adequate for the level of vital activity. The morphological basis is often dystrophy and fibrosis in the nodal zone, congenital or acquired heart defects, and the predominance of parasympathetic influences.

Key ECG signs of SSS:

• Tachy-brady syndrome (alternation of bradycardia and tachycardia).
• Episodes of sinus arrest (sinus arrest).
• Slow recovery of sinus rhythm after termination of tachycardia.
• Transient atrial flutter or fibrillation.

The pathophysiological outcome is chronotropic incompetence — the heart's inability to adequately increase its rate during exertion.

Heterotopic (ectopic) rhythms

When SA node activity decreases, lower centers take over as the pacemaker. Subsidiary ectopic rhythms emerge:

• Slow Atrial Rhythm — the focus is usually in the left atrium, with a rate of less than 70–80 bpm. It occurs in neuroses, heart defects, and cardiomyopathies.
• Atrioventricular (Junctional) Rhythm — the source is in the AV junction. The heart rate is 40–60 bpm. The P wave is absent on the ECG, superimposed on a narrow QRS, or becomes negative (retrograde) in leads II, III, and aVF.
• Idioventricular (Ventricular) Rhythm — a deep subsidiary rhythm with a rate of 20–40 bpm. Wide, deformed QRS complexes without a preceding P wave are visible on the ECG.

Migration of the Supraventricular Pacemaker is also possible, where the impulse source smoothly moves between the SA node, atria, and AV junction. In this case, the shape, amplitude, and polarity of the P wave change from cycle to cycle with variability in the P-R and R-R intervals.

Interaction of Pacemakers

The conduction system has a strict hierarchy, but special electrophysiological phenomena sometimes occur:

• Escape beats (escape beats) — a protective mechanism. If the main pacemaker is delayed, a subsidiary impulse arises from a lower automaticity center after a pause.
• Atrioventricular Dissociation with Interference — simultaneous independent activity of two pacemakers (e.g., sinus and AV nodal) without mutual suppression.
• Supraventricular Tachycardias (Re-entry) — associated with the re-entry mechanism of the excitation wave. Sinus nodal re-entry tachycardia begins suddenly and abruptly. A formal diagnostic criterion is the recording on the ECG of a "run" of three or more consecutive supraventricular premature beats.

Class and exam questions

What else to study on this topic

• Atrioventricular Blocks (First, Second, and Third Degree)
• Right and Left Bundle Branch Blocks
• Differential Diagnosis of Blocks and Ventricular Premature Beats
• Functions of the Cardiovascular System and Types of Blood Vessels
• Sinus Node Dysfunction and Sinoatrial Blocks

Other topics in the "Additional Topics" section

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