Sinus tachycardia
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.
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 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.
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. Sinus tachycardia accompanying a myocardial infarction may be indicative of cardiogenic shock.Cause
Sinus tachycardia is usually a response to physiological stress, such as exercise, or an increased sympathetic tone with increased catecholamine release, such as stress, fright, flight, and anger. Other causes include:- Pain
- Fever
- Anxiety
- Dehydration
- Excess cortisol
- Malignant hyperthermia
- Hypovolemia with hypotension and shock
- Anemia
- Hyperthyroidism
- Mercury poisoning
- Kawasaki disease
- Pheochromocytoma
- Sepsis
- Pulmonary embolism
- Acute coronary ischemia and myocardial infarction
- Chronic obstructive pulmonary disease
- Hypoxia
- Intake of stimulants such as caffeine, theophylline, nicotine, cocaine, or amphetamines
- Hyperdynamic circulation
- Electric shock
- Drug withdrawal
- Porphyria
- Acute inflammatory demyelinating polyradiculoneuropathy
- Postural orthostatic tachycardia syndrome
- Mitral valve prolapse
- Metabolic myopathies
Diagnosis
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 to slow the rate and identification of P waves
- administer AV blockers to identify atrial flutter with 2:1 block
ECG characteristics
Rate: Greater than or equal to 100.Rhythm: Regular.P waves: Upright, in leads I, II and aVL, and negative in lead aVR;- each P wave is followed by a QRS and T waves
Inappropriate sinus tachycardia
In inappropriate sinus tachycardia, patients have an elevated resting heart rate and/or exaggerated heart rate in response to exercise. These patients have no apparent heart disease or other causes of sinus tachycardia. IST is thought to be due to abnormal autonomic control. IST is a diagnosis of exclusion.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 and Hereditary myopathy with lactic acidosis. 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.In one such category of metabolic myopathies, muscle glycogenoses, individuals are unable to create energy from muscle glycogen, and depending on the muscle GSD, may not be able to utilize blood glucose within the muscle cell either. As skeletal muscle relies predominantly on glycogenolysis for the first few minutes as it transitions from rest to activity, as well as throughout high-intensity aerobic activity and all anaerobic activity, individuals with glycogenoses experience during exercise: sinus tachycardia, tachypnea, muscle fatigue and pain, during the aforementioned activities and time frames. Notable in McArdle Disease 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. The inappropriate rapid heart rate response to exercise may be misdiagnosed as inappropriate sinus tachycardia.