It is the characteristic rhythm of the healthy human heart and its rate is usually regular, but it will vary depending on the autonomic inputs in the sinus node.
There may be an irregularity in sinus frequency and when this occurs, it is called ” sinus arrhythmia “. A sinus rhythm faster than the normal range is called sinus tachycardia, while a slower rhythm is called sinus bradycardia.
The sinoatrial node, due to its small mass, does not have a visible manifestation in the electrocardiogram (ECG). The behavior of the sinoatrial node, therefore, must be inferred from the atrial response.
The right upper atrium is depolarized first, followed by simultaneous depolarization of the rest of the right and some of the left atrium, and finally by depolarization of the left atrial appendage.
Normal Sinus Heart Rate
The normal heart rate has been considered between 60 and 100 beats per minute, although there is some disagreement with respect to the normal frequency in adults.
The range (defined by percentiles 1 and 99) is between 43 and 102 beats per minute in men and between 47 and 103 beats per minute in women.
There is also a significant variability in age in young children. The normal heart rate is 110 to 150 beats per minute in babies, with a gradual decrease during the first six years of life.
A normal sinus rhythm refers to both a normal heart rate and a rhythm.
The shape of the electrocardiogram (ECG) tracing will show certain key attributes that will be considered normal, as explained below. With normal sinus rhythms, the electrical impulse of the heart beat originates in the sinoatrial node.
The P waves are vertical and appear before each QRS and have the same shape. The intervals between P waves are regular, although some variations with breathing may occur. Sinus rhythms are classified as:
Normal Sinus Rhythm
This is the normal rhythm of the heart. The electrical impulse originates within the sinoatrial node and travels through the atria. After a short delay, the impulse travels through the branches of the bundle.
Normal heart rate in children
Expressed in beats per minute (bpm):
- Newborns : 110 – 150 bpm.
- 2 years : 85 – 125 lpm.
- 4 years : 75 – 115 lpm.
- 6 years : 60 – 100 lpm.
Normal heart rate in adults
Normal sinus rhythm in a healthy adult of 18 years:
- Regular rhythm at 84 beats per minute.
- The morphology and the axis of the normal P wave.
- Narrow QRS complexes.
- Each P wave is followed by a QRS complex.
- The PR interval is constant.
Factors that can alter Normal Sinus Rhythm
- Stimulation (for example, pain)
- Lower myocardial infarction.
- Disease of the Sinus node.
- Anorexia nervosa.
- Electrolyte abnormalities; hyperkalemia, hypermagnesemia.
- Hernia of the brainstem (the Cushing reflex).
- Beta blockers.
- Blockers of calcium channels (Verapamil and Diltiazem).
- Central agonists alpha-2 (Clonidine and Dexmedetomidine).
- GABA-ergic agents (Barbiturates, Benzodiazepines, Baclofen).
- Poisoning by organophosphates.
The sinus pause includes stop and blockade of sinus output. Sinus arrest is caused by the failure of the sinus node to create an impulse. An interruption in regularity can be observed, with the blockade of sinus output, the node generates an impulse but is blocked before being transmitted through the cardiac atria .
Sinus arrhythmia seems normal, except for slight irregularities. A frequent cause of sinus arrhythmia may be rhythm variations caused by breathing.
The PP range varies widely from 1.04 seconds (heart rate ~ 57 bpm) to 0.60 seconds (heart rate ~ 100 bpm); a variability of more than 400ms.
Sinus bradycardia is a rhythm with a frequency of 40-60 beats per minute.
Sinus tachycardia is a normal rhythm but with a heart rate of more than 100 bpm. It is a normal response to exercise, excitement and some diseases.