ECG changes in borderline states

Disorders of sinus automaticity

The basis of normal hemodynamics is a regular rhythm generated in the sinus node. On the ECG, this is confirmed by the presence of an unchanged P wave of normal polarity preceding each QRS complex. Borderline shifts include:

• Sinus tachycardia: heart rate exceeds 90–100 beats per minute at rest. Of particular interest is non-physiological (inappropriate) sinus tachycardia. It occurs at rest or with minimal exertion without obvious causes and has a persistent character. Unlike sinoatrial re-entry tachycardia, it begins gradually rather than suddenly.
• Sinus bradycardia: slowing of the rhythm to less than 60 beats per minute. If detected in isolation (e.g., during monitoring) and not accompanied by clinical symptoms, it has no significant pathological meaning.
• Sinus arrhythmia: the pacemaker remains sinus, but R-R intervals become irregular. An important ECG criterion is that differences in their duration exceed 0.16 seconds.

Sick Sinus Syndrome (SSS)

This syndrome develops with persistent depression of automaticity, when the node loses the ability to provide a rhythm adequate to the metabolic demands. Morphologically, this is often associated with fibrosis of the nodal zone or autonomic imbalance with parasympathetic predominance.

The most informative ECG signs of SSS include:

1. "Tachy-brady" syndrome: constant or periodic sinus bradycardia alternating with bouts of tachycardia.
2. Transient atrial flutter or fibrillation.
3. Episodes of sinus arrest (sinus arrest).
4. Delayed restoration of the baseline rhythm after termination of tachycardia.

SSS leads to chronotropic incompetence (the inability to accelerate the rhythm upon exertion) and falls under the broader concept of sinus node dysfunction (SND). SND also includes sinoatrial blocks of various degrees (ranging from prolonged conduction time to complete impulse block).

Heterotopic (ectopic) rhythms

When the automaticity of the SA node decreases or ceases, subsidiary escape centers are activated. The ECG picture depends on their localization:

• Slow atrial rhythm: the ectopic focus is located in the atria (often the left), generating rare impulses (less than 70–80 per minute).
• Atrioventricular (junctional) rhythm: the source of impulses is in the AV junction. The QRS complex remains narrow and unchanged. The P wave may be absent, buried in QRS, or retrograde (negative in leads II, III, aVF). Heart rate is typically 40–60 per minute.
• Idioventricular rhythm: the ventricles act as the source. Characterized by wide and deformed QRS complexes without preceding P waves. The rate is very low (20–40 per minute).
• Wandering supraventricular pacemaker: gradual shifting of the pacemaker site between the SA node, atria, and AV junction. Manifests as changes in P-wave shape, amplitude, and polarity from cycle to cycle with variable P-R and R-R intervals.
• Escape beats (escape beats): single subsidiary impulses from lower centers occurring after a pause when the main rhythm is delayed.
• Interference dissociation: simultaneous independent activity of two pacemakers without mutual suppression.

Impact of systemic metabolic shifts on ECG

Borderline electrocardiographic changes may occur against the background of endocrine pathology, such as hypothyroidism syndrome. Reduced basal metabolism and thyroid hormone deficiency lead to a drop in myocardial sensitivity to catecholamines.

Key ECG findings in hypothyroidism:

• Pronounced bradycardia due to insufficient cardiostimulatory action of $T_3$ and $T_4$ hormones.
• Low QRS voltage, which often indicates the presence of hydropericardium (fluid accumulation in the pericardial sac due to mucinous edema).
• QT interval prolongation caused by electrolyte shifts, metabolic changes, and myocardial dystrophy ("myxedema heart").

Class and exam questions

What else to study on this topic

• Pathogenesis of mucinous edema (myxedema) and its effect on the cardiovascular system in hypothyroidism.
• Methods and efficacy criteria for electroconvulsive therapy (ECT) in endogenous depressions.
• Use of extracorporeal detoxification methods (plasmapheresis, hemoperfusion) in acute psychoses.
• Light therapy and sleep deprivation: indications and limitations in psychiatry.
• Laboratory markers and hormonal profile in primary and secondary hypothyroidism.

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