Sinus tachycardia is a sinus rhythm of the heart, with an increased rate of electrical discharge from the sinoatrial node, resulting in a tachycardia, a heart rate that is higher than the upper limit of normal (90–100 beats per minute for adult humans).
The normal resting heart rate is 60–90 bpm in an average adult. Normal heart rates vary with age and level of fitness, from infants having faster heart rates (110-150 bpm) and the elderly having slower heart rates. Sinus tachycardia is a normal response to physical exercise or other stress, when the heart rate increases to meet the body's higher demand for energy and oxygen, but sinus tachycardia can also be caused by a health problem.
thumb|An elite athlete's heart recorded during a maximum effort workout maintaining over 180 bpm for 10 minutes.
Signs and symptoms
Tachycardia is often asymptomatic. It is often a resulting symptom of a primary disease state and can be an indication of the severity of a disease. If the heart rate is too high, cardiac output may fall due to the markedly reduced ventricular filling time. Rapid rates, though they may be compensating for ischemia elsewhere, increase myocardial oxygen demand and reduce coronary blood flow, thus precipitating an ischemic heart or valvular disease.
- Fever
- Malignant hyperthermia
- Hypovolemia with hypotension and shock
- Anemia
Diagnosis
thumb|A 12 lead ECG showing sinus tachycardia
Sinus tachycardia is usually apparent on an ECG, but if the heart rate is above 140 bpm the P wave may be difficult to distinguish from the previous T wave and one may confuse it with a paroxysmal supraventricular tachycardia or atrial flutter with a 2:1 block. Ways to distinguish the three are:
- Vagal maneuvers (such as carotid sinus massage or Valsalva's maneuver) to slow the rate and identification of P waves
- administer AV blockers (e.g., adenosine, verapamil) to identify atrial flutter with 2:1 block
Heart sounds should also be listened to.
- Rhythm: Regular.
Postural orthostatic tachycardia syndrome
Usually, in women with no heart problems, this syndrome is characterized by normal resting heart rate but exaggerated postural sinus tachycardia with or without orthostatic hypotension.
Metabolic myopathy
Upon exertion, sinus tachycardia can be seen in some inborn errors of metabolism that result in metabolic myopathies, such as McArdle Disease (GSD-V) Metabolic myopathies interfere with the muscle's ability to create energy. This energy shortage in muscle cells causes an inappropriate rapid heart rate response to exercise. The heart tries to compensate for the energy shortage by increasing heart rate to maximize delivery of oxygen and other blood borne fuels to the muscle cells. Notable in McArdle Disease (GSD-V) is the phenomenon of second wind where after approximately 6–10 minutes of aerobic exercise, such as walking without an incline, the heart rate drops as blood borne fuels, predominantly from free fatty acids, produce energy via oxidative phosphorylation.
Rare diseases, such as McArdle disease, are often misdiagnosed.