Dyspnea is defined as abnormal or uncomfortable breathing in the context of what is normal for a person according to his or her level of fitness and exertional threshold for breathlessness.^1–4 Dyspnea is a common symptom and can be caused by many different conditions. It often has multiple etiologies. Although other causes may contribute, the cardiac and pulmonary organ systems are most frequently involved in the etiology of dyspnea.⁵

Pathophysiology

The physiology of normal respiration and gas exchange is complex, and that of dyspnea is even more so. Ventilation is related to the metabolic demands of oxygen consumption and carbon dioxide elimination necessary to meet a given level of activity.

The carotid and aortic bodies and central chemoreceptors respond to the partial pressure of oxygen (PO₂), partial pressure of carbon dioxide (PCO₂) and pH of the blood and cerebrospinal fluid.² When stimulated, these receptors cause changes in the rate of ventilation. The rate and pattern of breathing are also influenced by signals from neural receptors in the lung parenchyma, large and small airways, respiratory muscles and chest wall.

For example, in a patient with pulmonary edema, the accumulated fluid activates neural fibers in the alveolar interstitium and reflexively causes dyspnea.² Inhaled substances that are irritating can activate receptors in the airway epithelium and produce rapid, shallow breathing, coughing and bronchospasm. The central nervous system, in response to anxiety, can also increase the respiratory rate.³ In a patient who experiences hyperventilation, subsequent correction of the decreased PCO₂ alone may not alleviate the sensation of breathlessness. This reflects the interaction between chemical and neural influences on breathing.^2,3

Etiology

The broad differential diagnosis of dyspnea contains four general categories: cardiac, pulmonary, mixed cardiac or pulmonary, and noncardiac or nonpulmonary (Table 1).

Cardiac causes of dyspnea include right, left or biventricular congestive heart failure with resultant systolic dysfunction, coronary artery disease, recent or remote myocardial infarction, cardiomyopathy, valvular dysfunction, left ventricular hypertrophy with resultant diastolic dysfunction, asymmetric septal hypertrophy, pericarditis and arrhythmias.

Pulmonary causes include obstructive and restrictive processes. The most common obstructive causes are chronic obstructive pulmonary disease (COPD) and asthma. Restrictive lung problems include extrapulmonary causes such as obesity, spine or chest wall deformities, and intrinsic pulmonary pathology such as interstitial fibrosis, pneumoconiosis, granulomatous disease or collagen vascular disease.

Mixed cardiac and pulmonary disorders are also common sources of dyspnea^6,7 and include COPD with pulmonary hypertension and cor pulmonale, deconditioning, pulmonary emboli and trauma.

Noncardiac or nonpulmonary disease must be considered in patients with minimal risk factors for pulmonary disease and no clinical evidence of cardiac or pulmonary disease. These disorders include metabolic conditions such as anemia, diabetic ketoacidosis and other, less common causes of metabolic acidosis, pain in the chest wall or elsewhere in the body, and neuromuscular disorders such as multiple sclerosis and muscular dystrophy. Obstructive rhinolaryngeal problems include nasal obstruction due to polyps or septal deviation, enlarged tonsils and supraglottic or subglottic airway stricture.

Dyspnea can also occur as a somatic manifestation of psychiatric disorders, such as an anxiety disorder, with resultant hyperventilation.

History

As with all undifferentiated symptoms, a carefully taken history is important because it yields clues, if not the actual diagnosis, in many cases (Table 2).

Pertinent queries can provide valuable information and diagnostic clues to the cause of dyspnea. Factors such as the duration of the dyspnea, precipitating circumstances such as exertion, daytime or nighttime occurrence, the presence of chest pain or palpitations, the number of pillows the patient uses during sleep, how well the patient sleeps, concomitant coughing, exercise tolerance, and the ability to keep up with peers can all help narrow the differential diagnosis.^8,9

Other factors to be considered include past and current use of tobacco, exercise tolerance, environmental allergies, occupational history and the presence of asthma, coronary artery disease, congestive heart failure or valvular heart problems. A family history of asthma, lung problems (e.g., chronic bronchitis, bronchiectasis, serious pulmonary infections), allergies or hay fever must also be considered.⁹

When evaluating a patient with a possible psychiatric component of dyspnea, it is helpful to know if the feelings of dyspnea and anxiety are concurrent, if associated paresthesias of the mouth and fingers exist, and if the anxiety precedes or follows dyspnea.

Physical Examination

A complete physical examination, like a carefully taken history, is likely to lead the clinician toward the proper diagnosis and minimize unnecessary laboratory testing (Table 2).

Oropharyngeal or nasopharyngeal pathology may be found by identifying a grossly obstructive abnormality of the nasal passages or pharynx. Palpation of the neck may reveal masses, such as in thyromegaly, which can contribute to airway obstruction. Neck bruits are indicative of macrovascular disease and suggest concomitant disease of the coronary arteries, especially if the patient has a history of diabetes, hypertension or smoking.

Examination of the thorax may reveal an increased anteroposterior diameter, an elevated respiratory rate, spine deformities such as kyphosis or scoliosis, evidence of trauma and the use of accessory muscles for breathing. Kyphosis and scoliosis can cause pulmonary restriction. Auscultation of the lungs provides information regarding the character and symmetry of breath sounds such as rales, rhonchi, dullness or wheezing. Rales or wheezing can indicate congestive heart failure, and expiratory wheezing alone may indicate obstructive lung disease.

Cardiovascular examination may reveal murmurs, extra heart sounds, an abnormal location of the point of maximum impulse or an abnormality of the heart rate or rhythm. A systolic murmur can indicate aortic stenosis or mitral insufficiency; a third heart sound can indicate congestive heart failure and an irregular rhythm can indicate atrial fibrillation. Peripheral perfusion of the extremities should be evaluated by assessing pulses, capillary refill time, edema and hair growth pattern.

Psychiatric examination can reveal anxiety accompanied by tremulousness, sweating or hyperventilation.^2,4,8

Diagnostic Examination

Many diagnostic modalities used to evaluate dyspnea can be performed in the family physician's office.¹⁰ The basic evaluation is directed by the probable causes suggested in the history and physical examination. The most common organic causes of dyspnea are cardiac and pulmonary disorders.⁶

The most useful methods of evaluating dyspnea are the electrocardiogram and chest radiographs. These initial modalities are inexpensive, safe and easily accomplished. They can help confirm or exclude many common diagnoses.

The electrocardiogram can show abnormalities of the heart rate and rhythm, or evidence of ischemia, injury or infarction. Voltage abnormality suggests left or right ventricular hypertrophy if the voltage is excessive, or pericardial effusion or obstructive lung disease with increased chest diameter if the voltage is diminished.

A chest radiograph can identify skeletal abnormalities, such as scoliosis, osteoporosis or fractures, or parenchymal abnormalities, such as hyperinflation, mass lesions, infiltrates, atelectasis, pleural effusion or pneumothorax. An increased cardiac silhouette can be caused by increased pericardial size or increased chamber size.

A finger-stick hemoglobin determination or a complete blood count can quantify the severity of suspected anemia. Thyroid abnormalities rarely present with dyspnea and can be assessed by measurement of the serum thyroid-stimulating hormone level.^4,8

The history, physical examination and preliminary diagnostic modalities such as chest radiography and electrocardiography usually reveal the underlying cause or causes of dyspnea, but in selected cases further diagnostic evaluation may be needed. Useful second-line tests include spirometry, pulse oximetry and exercise treadmill testing. These tests can clarify the diagnosis if initial modalities indicate an abnormality or are inconclusive.

Spirometry

Spirometry depends on patient effort; if the patient is unable to give a maximal effort, the test has limited value. To perform the test, most patients require specific demonstration of the appropriate technique and coaching during the test in order to produce a maximal effort. The patient exhales fully, then takes a maximum inhalation and blows out as hard and as fast as possible, continuing the exhalation as long as possible to ensure that maximal volumes are measured. The test may be repeated until the results are consistent. Spirometry is extremely safe and has virtually no risk of serious complications.^4,9 The most common errors in technique are failure to exhale as fast as possible and failure to continue exhalation as long as possible.

Spirometry can help differentiate obstructive lung disease from restrictive lung disease (Table 3). COPD (chronic bronchitis or emphysema) and asthma are the most common causes of an obstructive spirometry pattern. A restrictive pattern can be caused by extrapulmonary factors, such as obesity; by skeletal abnormalities, such as kyphosis or scoliosis; by compressing pleural effusion, and by neuromuscular disorders, such as multiple sclerosis or muscular dystrophy. A number of systemic diseases, such as rheumatoid arthritis, systemic lupus erythematosus and sarcoidosis, can cause interstitial lung disease, which leads to a restrictive pattern on spirometry. Other causes of interstitial disease include farmer's lung and other pneumoconioses, infiltrating malignancy, fibrosis due to side effects of some medications (e.g., some chemotherapeutic agents, amiodarone [Cordarone]) and idiopathic interstitial fibrosis, which constitutes the largest single category of interstitial lung disease.⁹

Pulse Oximetry

Pulse oximetry uses an infrared light source to determine the hemoglobin oxygen saturation. However, the percentage of oxygen saturation does not always correspond to the partial pressure of arterial oxygen (PaO₂). The hemoglobin desaturation curve can be shifted to the left or right depending on the pH, temperature (e.g., oximeter used on a cool extremity) or arterial carbon monoxide or carbon dioxide level. Thus, a borderline-normal oxygen saturation percentage may actually reflect an abnormally low PaO₂ in some cases.¹⁰ Pulse oximetry is, however, valuable as a rapid, widely available and noninvasive means of assessment and is accurate in most clinical situations.

Arterial Blood Gases

Arterial blood gas measurement can provide information about altered pH, hypercapnia, hypocapnia or hypoxemia. This measurement is more commonly used for the evaluation of acute dyspnea but it can also be used in the evaluation of patients who have gradually become dyspneic or who are chronically dyspneic. Arterial blood gas measurement can be normal, however, in patients with clinically significant pulmonary disease. An abnormality of arterial blood gas parameters may sometimes be seen only during exercise, with a rapid return to normal during rest. Normal arterial blood gas measurements do not exclude cardiac or pulmonary disease as a cause of dyspnea.²

Complete Pulmonary Function Testing

Complete pulmonary function testing can be obtained if screening office spirometry is inconclusive. Measurement of all types of lung volume, such as total lung capacity and residual volume, can show combinations of obstructive and restrictive disease (Table 3). The diffusing capacity of the lung for carbon monoxide (DLCO) is often included in complete pulmonary function testing. The DLCO is used to indirectly measure the gas exchange of oxygen and carbon dioxide across the alveolar surface. Reduced diffusing capacity can occur in a variety of alveolar or interstitial abnormalities, such as edema, inflammation, infection, infiltration and malignancy. Reduced oxygen diffusion can markedly contribute to dyspnea; however, it usually occurs with some spirometric abnormality.^2,4,10

Exercise Treadmill Testing

Exercise treadmill testing can target ischemia as a cause of dyspnea.¹¹ This test can be performed when symptoms are atypical for exertional angina or when silent ischemia is suspected as a cause of dyspnea on exertion. A patient's ability to perform a treadmill test can be limited by poor aerobic conditioning, by lower extremity pathology such as arthritis, claudication or edema, or by coincidental pulmonary disease. Exercise treadmill testing is relatively safe and has few risks: only one in 10,000 patients dies of malignant arrhythmia or acute myocardial infarction, and only two in 10,000 have serious but nonfatal arrhythmia or another complication.¹¹

The normal physiologic response to exercise testing is an increase in blood pressure and heart rate. To achieve maximal effort, the heart rate should reach at least 85 percent of the target heart rate for the patient's age. Underlying heart disease may be signified by ST-segment changes, by arrhythmias or by inappropriate blood pressure changes during exercise. There are limitations to the sensitivity and specificity of treadmill testing, however, and interpretation of the results may vary. Negative results on treadmill exercise testing in a patient who has dyspnea but no chest pain or other cardiac risk factors suggest that dyspnea is caused by something other than coronary artery disease. When the results are equivocal or difficult to interpret, further diagnostic testing or consultation should be considered.^7,8

Echocardiography

Echocardiography can detect a valvular abnormality and may be diagnostically helpful in patients with questionable murmurs in the context of dyspnea. Chamber size, hypertrophy and left ventricular ejection fraction can also be assessed. A multigated cardiac acquisition (MUGA) scan or radionucleotide ventriculography can also be used to quantify the ejection fraction.

Cardiopulmonary Exercise Testing

Cardiopulmonary exercise testing quantifies cardiac function, pulmonary gas exchange, ventilation and physical fitness. Cardiopulmonary exercise testing may be used in selected cases when the diagnosis is still unclear after the inital examination. It can be particularly useful in cases where obesity, anxiety, deconditioning, exercise-induced asthma or other problems preclude standard exercise treadmill testing.

The test is usually performed on a treadmill or bicycle ergometer and requires that the patient breathe into a mouthpiece during exercise. The patient performs progressively more difficult exercise to the point of exhaustion. During exercise, oxygenation is measured by using either a pulse oximeter or an arterial line, and interpretation of the complete test requires analysis of oxygen consumption, carbon dioxide production, anaerobic threshold, heart rate and rhythm, blood pressure, minute ventilation, continuous monitoring of gas exchange, severity of perceived exertion, dyspnea, chest pain and leg discomfort. Cardio-pulmonary exercise testing can help define whether an abnormality lies in the pulmonary, cardiac or skeletal muscle systems.^2,4

Final Comment

In most patients, the cause or causes of dyspnea can be determined in a straightforward fashion by using the history and physical examination to identify common cardiac or pulmonary etiologies. In selected cases, specific diagnostic testing or consultation may be needed to confirm the diagnosis or to provide assistance with therapeutic management.