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ELECTROCARDIOGRAPHY Electrocardiography may reveal a disturbance of heart rate or rhythm. There may be evidence of cardiac ischemia, ventricular hypertrophy, or pericardial disease. The presence of heart failure is highly unlikely in patients with a normal electrocardiogram.18 CHEST RADIOGRAPHY Chest radiography may reveal chest wall abnormalities, lung parenchymal disease (e.g., COPD), neoplastic lesions, congestive heart failure, or pleural disease. COMPLETE BLOOD COUNT AND METABOLIC PANEL Profound anemia is an unusual but important cause of chronic dyspnea. Secondary erythrocytosis may be present in patients with advanced COPD. Changes in acid-base balance, reflected by the bicarbonate level in a metabolic panel, may provide a clue to dyspnea. Respiratory acidosis, occurring in patients with severe cases of COPD, interstitial lung disease, and neuromuscular disorders, leads to a metabolic compensation resulting in an elevated bicarbonate level. SPIROMETRY This test is useful in distinguishing obstructive lung disorders from restrictive lung disorders. It is highly effort dependent, which can be challenging in older or impaired patients, especially when measuring forced vital capacity (FVC). However, forced expiratory volume in six seconds (FEV6) has been shown to be an acceptable surrogate for FVC in the spirometric diagnosis of obstructive disease and possibly in restrictive lung disease.24 Most of the newer spirometers are equipped to measure FEV6. Although the FEV in one second (FEV1) and FVC are reduced proportionately in patients with restrictive lung disease, the FEV1 is reduced more than the FVC in those with obstructive lung disease. Therefore, a ratio of FEV1/FVC less than 0.7 or 0.8 is a common diagnostic criterion for COPD. PULSE OXIMETRY Desaturation at rest or after exercise is a sensitive indicator of gas exchange abnormalities. If abnormal, consideration should be given to obtaining arterial blood gas measurements. Selective Testing PULMONARY FUNCTION TESTING The primary limitation of spirometry is its inability to measure lung volumes, including the total amount of air in the lungs at full inspiration (total lung capacity [TLC]), the amount of air remaining in the lungs at the end of passive expiration (functional residual capacity [FRC]), or the amount of air remaining after maximal expiration (residual volume [RV]). The TLC is reduced in restrictive disorders and normal or increased in obstructive disorders as a result of air trapping. In restrictive disorders caused by lung parenchymal disease, all lung volumes are proportionately reduced. In contrast, with other restrictive diseases (e.g., neuromuscular disease or chest wall restriction) the RV and the RV/TLC ratio are increased. With the use of carbon monoxide, which is a highly diffusible gas, the gas-transfer function of the lung can be estimated by measuring the diffusing capacity of the lung for carbon monoxide (DLCO). This is reduced in patients with diseases affecting the lung parenchyma, vascular abnormalities, anemia, and conditions where there is a reduction of effective lung volume (e.g., after lung resection). DLCO may be elevated in conditions where there is an increased effective pulmonary blood volume, such as asthma, obesity, left-to-right cardiac shunts, and polycythemia. Performing a broncho-provocation challenge using methacholine can identify airway hyperreactivity. A 20 percent reduction in FEV1 is considered diagnostic of asthma. TESTS FOR PULMONARY VASCULAR DISEASE Pulmonary hypertension may be primary (rare) or secondary to a pulmonary, cardiac, or extrathoracic pathology. Two-dimensional echocardiography with Doppler flow studies is the most useful imaging modality to demonstrate elevated pulmonary artery pressures and the resultant tricuspid regurgitation.25 If the etiology of pulmonary hypertension remains unexplained after appropriate testing, chronic thromboembolism should be suspected. Ventilation-perfusion scanning or spiral computed tomography of the chest may be used to confirm this diagnosis. Right heart catheterization may be required to confirm or diagnose less common causes of pulmonary hypertension. TESTS FOR CARDIAC DISEASE The test of choice for diagnosing most cardiac causes of chronic dyspnea is echocardiography, especially if heart failure is suspected. However, as many as 40 percent of patients with clinical evidence of congestive heart failure have diastolic dysfunction with preserved left ventricular systolic function.26 In such cases, the diagnosis is suggested by the findings of left ventricular hypertrophy, dilated left atrium, and reversal of the normal pattern of flow velocity across the mitral valve. Other cardiac pathologies that can be demonstrated on echocardiography include valvular dysfunction, atrial tumors, and pericardial disease. Brain natriuretic peptide (BNP), also known as B-type natriuretic peptide, is a neurohormone synthesized by ventricular myocytes that is useful in the diagnosis of heart failure. It is released in response to pressure/volume overload resulting in increased wall tension. The magnitude of elevation is proportional to the severity of heart failure and the New York Heart Association functional classification.27 Using a threshold of 100 pg per mL, the test is 82 percent sensitive and 99 percent specific.27 Table 328 compares the specificity, sensitivity, and post-test probabilities of different cutoffs for an abnormal BNP test with echocardiographic diagnoses of left ventricular dysfunction (systolic or diastolic). This test is available as a point-of-care assay. Although an absolute standard for the diagnosis of congestive heart failure does not exist, the BNP test may be helpful, especially in patients who have coexisting cardiac and pulmonary disease and if there is uncertainty about the primary cause of dyspnea. |