Tachycardia, like dyspnea, is usually a sign that the tissues are not getting enough oxygen to meet their demands. To recall a list of causes, pathophysiologyis applied. If tachycardia results from anoxia then the causes can be developed on the basis of the causes for anoxia, which may result from a decreased intake of oxygen, a decreased absorption of oxygen, and inadequate transport of oxygen to the tissues. Tachycardia also results when the tissues' demand for oxygen increases. Another cause is peripheral arteriovenous shunts. In addition, anything that stimulates the heart directly, such as drugs, electrolyte imbalances, or disturbances in the cardiac conduction system, will cause tachycardia. Let us review the conditions that may fall into each of these categories.
1. Decreased intake of oxygen. Anything that obstructs the airway and prevents oxygen from getting to the alveoli should be recalled in this category. Bronchial asthma, laryngotracheitis, chronic bronchitis, and emphysema are most important to recall. In addition, if the “respiratory” pump (thoracic cage, intercostal and diaphragmatic muscles, and respiratory centers in the brainstem) is affected by disease, especially acutely, there will be tachycardia. Poliomyelitis, myasthenia gravis, barbiturate intoxication, and intoxication by other central nervous system (CNS) depressants are examples of disorders in this category. Finally, the intake of oxygen may decrease if there is a low atmospheric oxygen tension. High altitude is an obvious cause, but hazardous working conditions must also be considered.
2. Decreased oxygen absorption. This may result from three mechanisms.
A. Alveolar–capillary block in sarcoidosis, pneumoconiosis, pulmonary fibrosis, congestive heart failure (CHF), alveolar proteinosis, and shock lung.
B. Diminished perfusion of the pulmonary capillaries in pulmonary emboli and pulmonary and cardiovascular arteriovenous shunts.
C. Disturbed ventilation/perfusion ratio in which alveoli are perfused but not well ventilated, in alveoli that are not well ventilated, or in alveoli that are ventilated but not well perfused. This is typical of pulmonary emphysema, atelectasis, and many chronic pulmonary diseases.
3. Inadequate oxygen transport. Severe anemia, shock, and CHF (regardless of the cause) fall into this category, as do methemoglobinemia and sulfhemoglobinemia.
4. Increased tissue oxygen demands. Fever, hyperthyroidism, leukemia, metastatic malignancies, polycythemia, and certain physical or emotional demands fall into this category.
5. Peripheral arteriovenous shunts. These shunts may occur in the popliteal fossa following a gunshot wound, in the sellar area following the rupture of a carotid aneurysm into the cavernous sinus, and in Paget disease.
6. Disorders that directly affect the heart. Stimulants of the heart such as caffeine, adrenalin (pheochromocytomas), thyroid hormone (hyperthyroidism), amphetamines, theophylline, and other drugs fall into this category. Nervous tension and neurocirculatory asthenia may be the cause. Electrolyte disturbances such as hypocalcemia and hypokalemia may precipitate ventricular tachycardia. Excessive amounts of digitalis may also provoke atrial or ventricular tachycardia.
Tachycardia of various types may occur from disturbances in the conducting system of the heart. Digitalis has already been mentioned, but the Wolff–Parkinson–White syndrome, focal myocardial anoxia from emboli or infarction, and distention of various chambers of the heart (atria in mitral stenosis, ventricles in essential hypertension and cor pulmonale) are also etiologies of this mechanism. Anticholinergic drugs such as atropine block the ability of the vagus to slow the heart and may cause or contribute to tachycardia. All of the above categories are outlined in Table 56 where a few diseases that are more specific are mentioned.
Approach to the Diagnosis
The association of other clinical signs and symptoms will often help to pinpoint the diagnosis. Tachycardia with tremor and an enlarged thyroid suggests hyperthyroidism. Tachycardia with respiratory wheezes suggests bronchial asthma. Tachycardia with a black stool suggests a bleeding peptic ulcer. If the blood pressure is low, the workup will proceed as that of shock (see page 260). In contrast, tachycardia with a normal blood pressure should prompt thyroid function studies, pulmonary function studies, arterial blood gases, and a venous pressure and circulation time. Electrolyte determinations, a drug screen, and 24-hour urine for catecholamine determinations may be indicated if there is hypertension as well.
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Tachycardia |
Other Useful Tests
1. Complete blood count (CBC) (anemia)
2. Sedimentation rate (infection)
3. Chemistry panel (liver disease, uremia)
4. Antinuclear antigen (ANA) (collagen)
5. Antistreptolysin O (ASO) titer (rheumatic fever)
6. Blood cultures (subacute bacterial endocarditis [SBE])
7. Febrile agglutinins (fever of unknown origin)
8. Serial electrocardiograms (ECGs) and cardiac enzymes (myocardial infarction)
9. Lung scan (pulmonary embolism)
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TABLE 56. Tachycardia |
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10. Holter monitoring (cardiac arrhythmia)
11. Echocardiography (CHF, valvular heart disease)
12. 5-hour glucose tolerance test (insulinoma)
13. Temperature chart (fever of unknown origin)
14. Sleeping pulse rate (anxiety neurosis)
15. Psychiatric consult
