Normal rhythm and sinus tachycardia
To identify abnormalities, a clear understanding of normal criteria is essential. In a normal sinus rhythm, the heart rate (HR) is 60–90 bpm. A key marker is the P wave: it must be positive in leads II and III, negative in aVR, and its morphology must remain constant within a single lead throughout the entire tracing. Atrioventricular conduction (P-Q interval) falls within 0.12–0.20 sec.
Sinus tachycardia is a regular rhythm with an increased heart rate ranging from 90 to 180 beats per minute.
The causes of its onset are diverse:
- Physiological: physical exertion (the rhythm increases gradually), stress, and emotions due to sympathetic nervous system activation.
- Pathological extra-cardiac: fever, anemia, thyrotoxicosis, vegetative-vascular dystonia.
- Cardiac: heart failure, myocarditis.
- Pharmacological: reaction to atropine, adrenaline, or aminophylline administration.
On the ECG, each QRS complex is preceded by a P wave. However, at a very high heart rate, the T wave may merge with the P wave of the next cardiac cycle, which requires differentiation from atrial or AV nodal reentrant tachycardia.
Sinus bradycardia
This condition is characterized by a regular sinus rhythm, but with an HR of less than 60 beats per minute. Often, patients are asymptomatic; however, with a marked drop in heart rate, central nervous system ischemia may develop, manifesting as dizziness and syncope.
Main causes of bradycardia:
- Physiological norm — frequently found in trained individuals due to elevated vagal tone (n. vagus) and reduced sympathetic influence.
- Pathology — e.g., in hypothyroidism (decreased thyroid function).
- Iatrogenic factors — use of $\beta$-blockers or antiarrhythmic drugs (amiodarone).
The diagnosis is confirmed exclusively via ECG: HR below 60, a normal P wave precedes each QRS, the P to QRS ratio is strictly 1:1, and the P-Q interval is normal or slightly prolonged.
Irregular sinus arrhythmia
Sinus arrhythmia is defined as an irregular rhythm in which periods of slower and faster heart rates smoothly alternate with each other. The pathogenesis is based on fluctuations in the automaticity of the sinoatrial node under the influence of vagal tone.
A specific and most common case is respiratory arrhythmia, typical for children, adolescents, and athletes. The mechanism is simple: the heart rate increases during inspiration and decreases during expiration. This is a variant of the physiological norm. When vagal influence is eliminated (during physical exertion or after administration of adrenomimetics), this arrhythmia disappears completely.
ECG presentation: irregular rhythm, the difference between R-R intervals exceeds 0.16 seconds, but the pacemaker remains sinus (P wave is present before each QRS).
Sick sinus syndrome (SSS)
SSS is a persistent depression of automaticity function in the sinus node. The anatomical substrate is usually dystrophy or fibrosis of the node secondary to acquired diseases or congenital heart defects.
Key manifestations of SSS:
- «Brady-tachy» syndrome (alternation of persistent bradycardia periods with tachycardia paroxysms).
- Activation of ectopic rhythms (from the AV junction, atria) or the appearance of atrial fibrillation/flutter due to failure of the primary pacemaker.
- Chronotropic incompetence — the inability of the heart to adequately increase HR in response to physical exertion.
Clinically, SSS manifests with symptoms of cerebral ischemia due to a drop in cardiac output (syncope). In symptomatic cases, pacemaker (PM) implantation is indicated.
Briefly: answers to topical questions
Which drug classes can cause iatrogenic sinus bradycardia?
Drug classes that can cause iatrogenic sinus bradycardia include:
- Antiarrhythmic drugs — for example, amiodarone.
- Beta-blockers.
What vagal maneuvers exist for terminating or diagnosing tachycardias?
To terminate and differentially diagnose tachycardias, reflex maneuvers that increase vagal tone (n. vagus) are used.
The following types of vagal maneuvers are distinguished:
- Carotid sinus massage — stimulation of the carotid sinus region (strictly unilateral).
- Valsalva maneuver — breath-holding with forceful straining.
- Aschner reflex — pressure applied to the eyeballs.
- Breath-holding — performed at the maximum deep inspiration.
- Gag reflex provocation — stimulation of the tongue root.
- Dive reflex — immersion of the face in cold water.
- Mechanical pressure on the solar plexus — pressure applied with a fist.
To remember respiratory arrhythmia: INspiration — INcreased rate (HR rises), EXpiration — EXtinction (HR falls).
Class and exam questions
How to distinguish sinus tachycardia from paroxysmal tachycardia if P and T waves merge?
Vagal maneuvers (Valsalva maneuver, carotid sinus massage) are used. They transiently increase vagal tone, slow the rhythm, and allow visualization of hidden P waves.
Does sinus arrhythmia always require treatment?
No. If the arrhythmia is linked to respiratory phases and disappears with exertion, it is a functional norm. Only arrhythmia independent of respiration is considered pathological.
In which leads should the P-Q interval be assessed if the Q wave is not prominent?
In such cases, evaluation should be performed in lead II or the right chest leads.
How does ECG recording speed affect rhythm analysis?
Standard paper speed is usually 25 mm/s. If the tracing is recorded at a speed of 50 mm/s, this must be taken into account when calculating heart rate by counting grid boxes, as their number between complexes visually doubles.
What else to study on this topic
- Procedure and contraindications for vagal maneuvers
- Differential diagnosis of escape rhythms in SSS
- Mechanisms of cardiac chronotropic incompetence development
- Pathophysiology of autonomic nervous system influence on the SA node
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