Sinus tachycardia is a condition in which the rate of impulse discharge from the SA node is elevated. The standard heart rate is 60 to 100 per minute. A rate more than 100 per minute is considered as tachycardia. The regular heart rate differs in part with age and from individual to individual. Infants usually have a heart rate between 110 to 150 per minute and it gradually decreases as the age advances. In children that are older and adults the normal is considered to be between 65 to 85 beats per minute.
Causes: There are many causes for tachycardia. Physiological increase in heart rate is seen when the person exercises and in cases of fear, flight, fright, anger or stress. Other causes would be as follows: Fever – when the body temperature rises, there is a rise in pulse rate, Hyperthyroidism – which means an increase in the levels of thyroid hormones which control the metabolism, which increases the cardiac output and heart rate as well, anxiety and panic attacks, Anaemia – refers to a decrease in red blood cells or haemoglobin or decrease in the haemoglobin carrying capacity of the blood, Sepsis, Pheochromocytoma – is a tumour of the adrenal gland which causes excess release of epinephrine and nor epinephrine, both of which control the heart rate, blood pressure and metabolism, heart failure, hypoxia, chronic pulmonary disease, pulmonary embolism, hypoxia etc.
Symptoms: Sinus tachycardia is often asymptomatic. Some of the patients may have complaints of palpitations. In some cases if the heart rate is very high, due to rapid contraction of the ventricles and incomplete cardiac output, the output falls down to low levels. There is also increased oxygen demand of the myocardium and decrease in coronary blood flow, which may make the heart prone to ischemia and infarction.
Diagnosis of tachycardia: An ECG is useful for detecting the type of tachycardia. If the heart rate is higher than 140 per minute the P waves may be hard to differ from the earlier T waves and may be confused with either atrial flutter or even a paroxysmal supraventricular tachycardia.
There are a few kinds of sinus tachycardia such as inappropriate sinus tachycardia and postural orthostatic tachycardia syndrome. Inappropriate sinus tachycardia is also called chronic nonparoxysmal sinus tachycardia. These patients have increased heart rates in response to exercise. They seem to have no heart ailment or any other pathology to cause tachycardia. It may be due to abnormal autonomic control. Postural Orthostatic tachycardia syndrome refers to postural sinus tachycardia with or without orthostatic hypotension. It is usually found in women having no heart pathology. At rest however these patients seem to have normal heart rate.
Treatment: Patients having sinus tachycardia due to physiological reasons do not need any treatment. If there is any underlying cause however it must be treated. Following an acute myocardial infarction, one third of the patients may develop sinus tachycardia. Those patients in whom the tachycardia is sustained, a larger infarct should be suspected. Such patients may be treated with Beta blockers.
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