B. Dyspnea

patient holding spirometer to mouth Spirometry is useful in assessing pulmonary mechanics which become abnormal in both obstructive and restrictive ventilatory disorders.

Introduction

Dyspnea, the sensation of breathlessness or inadequate breathing, is the most common complaint of patients with cardiopulmonary diseases. Evaluation of the complaint is complicated by the fact that in many circumstances shortness of breath is a normal consequence of exertion. Furthermore, perception of shortness of breath varies considerably among individuals at the same level of fitness and work and even in the same individual performing comparable work at different times. In disease states, perception of dyspnea can vary greatly among individuals. Consequently, assessment of the subjective sensation of dyspnea must balance the concepts of physiologic work and ventilatory demand with the individual’s perception of breathlessness. This chapter provides an overview of basic concepts on the mechanisms of dyspnea, lists those disease states in which the complaint is encountered, gives a diagnostic pathway for the evaluation of the complaint, and concludes with treatment options.

Basic Mechanisms

The physiologic system that regulates ventilation is extraordinarily complex. The headquarters for the spontaneous initiation of breathing and its control resides in the medulla and to a lesser extent the pons in the form of discrete aggregations of interconnected “respiratory” neurons. The medullary centers receive afferent neural input that originates in sensors that monitor the rate and depth of breathing and the levels of oxygen and carbon dioxide in the bloodstream; chief among these are receptors in the muscles and tendons that participate in breathing; chemosensitive cells in the carotid and aortic bodies; and receptors situated in the upper and lower airways and elsewhere in the lungs themselves. Afferent input also comes directly from chemosensitive cells close to the surface of the medulla that respond to changes in pH, which in turn is regulated largely by the level of PCO2, in the nearby cerebrospinal fluid. Efferent and presumably coordinated instructions to the muscles engaged in breathing come through two pathways in the spinal cord: the medullary control centers send messages through axons in the ventral portion, whereas the cortex communicates through axons in the more dorsal corticospinal tract.

Neural information about breathing that is received and integrated in the medulla and pons clearly is relayed to the cortex, where the sensation of dyspnea is perceived, but the anatomic pathways are poorly described. Increased afferent activity from one or more of the sensors that monitor the various acts of breathing (e.g., muscle contraction, airflow, and lung expansion) and their consequences (e.g., levels of PO2 and PCO2 in the bloodstream) are almost certainly involved. The exact mechanism underlying dyspnea is also largely unknown and probably varies with different medical conditions and may differ depending on which one or more receptors are involved.

The major physiologic components that are thought to contribute to dyspnea are shown in Table 2. The sense of effort is mediated primarily through cortical function and is basically a subjective assessment of afferent input or ventilatory need. Both peripheral mechanoreceptors and chemoreceptors influence the medullary center directly and can increase its efferent activity. Of the mechanoreceptors, muscle receptors in the intercostals seem to play a major role in enhanced afferent signals to the medullary and cortical centers. Airflow in the larger airways, particularly when there is airflow obstruction, results in enhanced afferent activity from lung and airway receptors. Dyspnea seems to occur most commonly when afferent input from peripheral receptors is enhanced or when cortical perception of respiratory work is excessive.

Disease States

Abnormalities of cardiopulmonary function are most commonly associated with dyspnea. All diseases of lung parenchyma and airways can cause dyspnea. These include COPD, asthma, fibrotic and infiltrative diseases, and pulmonary vascular disease. With the exception of asthma, most of these conditions first cause dyspnea with extreme exertion. As the disease progresses, dyspnea appears with less exertion, and finally is manifested at rest. Asthma constitutes the most important exception and is characterized by episodic onset of dyspnea not necessarily related to exertion.

Cardiovascular disease is a prominent cause of breathlessness. Many times the underlying problem is evident, such as in pulmonary edema or acute myocardial infarction. At other times the cause is less clear, as in atrial septal defect or early mitral stenosis. Particularly in patients with diabetes mellitus, myocardial ischemia from coronary artery disease can present as intermittent dyspnea without chest pain. Chronic heart failure is a troublesome cause of breathlessness, since the complaint will sometimes linger after apparently adequate treatment. In these cases the clinician must reevaluate the efficacy of treatment and look for other causes such as anemia or pulmonary embolism.

Neuromuscular disease is a well-known cause of dyspnea. Patients with Guillain-Barré disease, myasthenia gravis, amyotrophic lateral sclerosis, or late-occurring muscular dystrophies can present with this complaint. Severe weight loss from malnutrition, malignancy or chronic disease may also cause respiratory muscle weakness with associated dyspnea.

Anemia is a prominent cause of dyspnea when the hemoglobin concentration falls below 8-10 g/dl. As the hemoglobin declines further, dyspnea becomes more pronounced. This relationship is most prominent in acute anemia. Various compensatory mechanisms help to blunt the sensation of dyspnea in chronic anemia.

Renal disease leads to dyspnea from acidosis, anemia and volume overload. The complaint is much less common now than before owing to more effective dialysis and the availability of recombinant erythropoietin to increase red blood cell production.

Patients with chronic liver disease often complain of dyspnea but the mechanism is frequently obscure. One particular cause can be small arteriovenous shunts at the lung bases. This condition is classically associated with breathlessness and oxyhemoglobin desaturation on assuming the upright position as when arising from bed in the morning. This symptom is known as platypnea.

Endocrine abnormalities, particularly hyperthyroidism, can be associated with dyspnea. In this setting the sensation is probably related to the hypermetabolic state associated with thyroid over-activity. In the late stage dyspnea can be associated with high-output heart failure.

Early sepsis with bacteremia is associated with hyperventilation and sometimes with dyspnea. In some cases hyperventilation and dyspnea constitutes the presentation of sepsis. The cause is likely multifactorial and includes acidosis, tissue ischemia, and perhaps a direct effect on the brainstem respiratory center and carotid bodies by various mediators.

Clinical Evaluation

Evaluation of this complaint always begins with a careful history and physical examination. Careful attention should be paid to the duration and severity of dyspnea and to those activities that make it worse. Table 3 gives a severity scale for dyspnea developed by the American Thoracic Society. Activities and body positions that provoke dyspnea can often help to focus the diagnostic work-up. Key questions are listed in Table 4. Although the history alone rarely gives the diagnosis, these historical points are useful to point toward more specific testing. The physical examination should focus on the organ systems mentioned above, with meticulous attention to the respiratory and cardiovascular systems. Figure 1 shows a diagnostic pathway with the points where a particular diagnosis is frequently made.

Routine laboratory tests include spirometry, chest x-ray, ECG, and complete blood count. If a diagnosis has still not emerged, liver and kidney function tests may be helpful. If these tests are unrevealing, more specialized pulmonary function testing (lung volumes and single breath diffusing capacity) and echocardiography are useful. Consultation with a specialist is often helpful—particularly if the results of specialized tests are equivocal or therapy proves ineffective. When dyspnea is associated with exertion, a formal exercise test is sometimes necessary to differentiate myocardial ischemia from asthma, pulmonary vascular disease and physical deconditioning. When exercise testing is being considered, referral to a specialist is recommended.

Psychogenic dyspnea is a particularly interesting type of breathlessness because it is usually a diagnosis of exclusion. The malady occurs more commonly in women than men and tends to appear in the third or fourth decades of life. The condition should be considered when the physical examination, chest x-ray, ECG and spirometry are all normal. Patients with psychogenic dyspnea often exhibit extreme anxiety with concurrent symptoms of hyperventilation including visual complaints, dizziness, near-syncope and perioral and finger tingling and numbness. Arterial blood gases show a chronic respiratory alkalosis. Psychogenic dyspnea is better treated with counseling and biofeedback although the temptation to employ anxiolytics is great.

Sighing dyspnea appears in middle-aged individuals with mild heart or lung disease. The patient complains of inability to take a deep breath at rest and so periodically makes a conscious effort to sigh, which is invariably unsatisfactory since physiologic sighing is an involuntary action. This presentation is usually not associated with anxiety or symptoms of hyperventilation. Arterial blood gases are normal in most cases of sighing dyspnea. Sighing dyspnea usually responds to reassurance, nonspecific support, and treatment of the underlying condition.

It must be stressed again that in the great majority of cases, strong hints as to the cause of dyspnea emerge during the performance of a thorough medical history and physical examination. Use of the pathway outlined in Figure 1 allows a cost-effective approach to sequential diagnostic testing to help confirm the tentative diagnosis.

Treatment of dyspnea is best aimed at the underlying cause. When heart or lung disease can be improved, the sensation of dyspnea is often greatly ameliorated. Severe restrictive lung disease as manifested by pulmonary fibrosis or neuromuscular abnormality poses a particularly difficult problem. In these cases the complaint is often permanent and debilitating. The most effective treatment of dyspnea in cases of far-advanced pulmonary fibrosis is single lung transplantation.

Until recently the same was said of advanced emphysema. Now it appears that lung volume reduction surgery can significantly relieve dyspnea by reducing functional residual capacity, which reduces the work of breathing by improving the mechanical function of the lungs and diaphragm. Further studies are now in progress to evaluate the effect of this surgery on relieving dyspnea.

A number of studies have examined opiates and benzodiazepines in the treatment of intractable dyspnea. While anecdotal reports have indicated some short-term value, controlled clinical trials have failed to confirm long-term benefit; moreover, these studies have demonstrated deleterious events in a substantial number of patients.

When to Refer

Many patients with dyspnea can be evaluated and treated without referral to a specialist. However, unexplained dyspnea after routine evaluation usually warrants referral. Specifically, equivocal results after full pulmonary function testing or echocardiography or unsatisfactory response to preliminary treatment warrants referral. Referral is also warranted when cardiopulmonary exercise testing is being considered.

Medicolegal Considerations

Acute dyspnea can be associated with life-threatening diseases such as pulmonary embolism and myocardial infarction. Failure promptly and accurately to pursue these diagnoses in patients with unexplained dyspnea can lead to untimely deaths and subsequent lawsuits. Chronic dyspnea is often the only symptom of primary pulmonary hypertension or thromboembolic pulmonary hypertension. Failure to pursue these diagnoses, including failure to refer the patient for further evaluation, can lead to malpractice litigation as well.

Summary

Dyspnea is the most common symptom the Frontline physician encounters in managing the spectrum of cardiopulmonary diseases. The complaint is entirely subjective and highly variable, but a thoughtful, stepwise approach, beginning with a careful medical history and physical examination, leads to a satisfactory diagnosis in most patients. Dyspnea that remains unexplained after routine evaluation should be referred to a specialist. This chapter offers a sequential approach to dyspnea, beginning with simple inexpensive evaluations and proceeding to more sophisticated testing.

Reference

Manning HL, Schwartzstein RM. Pathophysiology of dyspnea. N Engl J Med 1995; 333:1547-53. A clear concise overview of dyspnea—its mechanisms, diagnoses and treatment.

Adams L, Guz A. Dyspnea on exertion. In Whipp BJ, Wasserman K (eds). Exercise: Pulmonary Physiology and Pathophysiology. New York, Marcel Dekker, 1991, 449-494. An old review but still one of the best available about the mechnisms of dyspnea during exercise.

Joffe D, Berend N. Assessment and management of dyspnoea. Respirology 2:33-43, 1997. A recent article with a nice review of the clinical problems of management.

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