Sinus Rhythms

Sinus Rhythms may manifest in the following forms:-

  1. Normal Sinus Rhythm

  2. Sinus arrhythmia

  3. Sinus tachycardia

  4. Sinus bradycardia

Normal Sinus Rhythm

Normal sinus rhythm is reflected by the inscription of normal P waves at a rate which ranges between 60 and 100 per minute in the adult. A normal P wave is positive in leads I and aVL, and in inferior leads, lead aVR is negative and V1 is positive or biphasic with an initial positive component. Normal sinus rhythm is usually associated with normal intraventricular conduction, and is thus reflected by the sequential inscription of P-QRS-T complexes. There is normally an associated slight to moderate sinus arrhythmia.

Sinus Arrhythmia

Sinus Arrhythmia is characterized by alternating periods of slow and rapid rates; it is due to an irregular fluctuating discharge of the SA node. The condition is most commonly associated with the phases of respiration – respiratory sinus arrhythmia. The periods of faster rate occur towards the end of inspiration and the periods of slower rate towards the end of expiration. The mechanism is mediated by reflex stimulation of the vagus nerve from receptors in lungs.

Diagnosis:- The PP interval varies alternately with normal P wave morphology and the relationship between P wave and QRS complexes is normal and QRS width is normal.

Significance:- Respiratory sinus arrhythmia is a normal physiological phenomenon and is most marked in young persons. It may cause considerable irregularity of the pulse in childhood.

Sinus Tachycardia

Sinus tachycardia occurs when SA Node discharges at a rate faster than 100 per minute in the adult. The normal resting rate in infants averages 120-130 beats per minute, slowing gradually to reach the adult rate at puberty.

Diagnosis:- The P-QRS-T complexes are recorded in rapid succession. It varies with emotion, respiration and exercise. Vagotonic procedures, for example carotid sinus massage, result in slight and gradual slowing.

Significance:- Sinus tachycardia is the normal physiological response to exercise and emotion. A sinus tachycardia is the normal physiologic response to exercise and emotion. It occurs in anxiety, thyrotoxicosis, toxaemia, cardiac failure and acute carditis. It is a normal accompaniment of fever. The sinus rate will increase by 8 beats per minute for every one degree rise of temperature.

Sinus Bradycardia

Sinus Bradycardia occurs when the SA node discharges at a rate slower than 60 per minute.

Diagnosis:- Sinus bradycardia in the absence of a complicating conduction disturbance, is characterized by normal P-QRS-T complexes, which are recorded in slow succession. It is commonly associated with respiratory sinus arrhythmia.

Significance:- Sinus bradycardia occurs as a normal phenomenon in athletes. Slowing of the sinus rate at times to bradycardiac levels is the physiologic response to sleep. Sinus bradycardia is accentuated by Digitalis and vagotonic procedures, such as carotid sinus compression. The rate quickens gradually with exercise, emotion and amyl nitrate.

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