Differential Diagnosis in Primary Care
4th Edition

Dyspnea, Tachypnea, and Orthopnea
Dyspnea is the subjective feeling of rapid or difficult breathing. The patient will often say, “I can’t get my breath!” Tachypnea is the objective finding of a rapid respiratory rate, and may or may not be associated with the feeling of not being able to breathe properly. One is a symptom and the other is a sign, but the mechanisms for producing them are the same: inadequate oxygen for body needs or inability to excrete CO2. A few other mechanisms that produce hyperventilation and tachypnea will be discussed later on in this chapter. The best basic science for developing a list of the causes of dyspnea and tachypnea is pathophysiology. Difficulty breathing or rapid breathing will develop when there is decreased intake of oxygen, impaired absorption of oxygen, inadequate perfusion of the lungs with blood, inability of the body to transport enough oxygen to the tissues, increased demand of the tissues for oxygen, and inability of the body to excrete CO2 and other waste products of body metabolism. These are tabulated in Table 25.
Dyspnea and tachypnea
Disorders of oxygen intake
In this category are the conditions that may block the respiratory passages such as laryngitis, foreign bodies, an aortic aneurysm or mediastinal tumor pressing on the trachea or bronchi, bronchial asthma, acute infectious bronchitis, and pulmonary emphysema. Also considered in this category
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are conditions that interfere with the “respiratory pump” (thoracic cage, thoracic and diaphragmatic muscles, and respiratory centers in the brain) such as kyphoscoliosis, Pickwickian syndrome, myasthenia gravis, peritonitis, encephalitis, and brain tumors.
TABLE 25. Dyspnea, Tachypnea, and Orthopnea
  V I N D I C A T E
Vascular Inflammatory Neoplasm Degenerative Intoxication Congenital Allergic and Autoimmune Trauma Endocrine
Disorders of Oxygen Intake   Laryngitis
Bronchitis
Bronchogenic carcinoma Pulmonary emphysema Pneumoconiosis Kyphoscoliosis Bronchiectasis Bronchial asthma Foreign body Injury to ribs  
Disorders of Oxygen Absorption Pulmonary edema Pneumonia
Tuberculosis
Lung abscess
Alveolar carcinoma
Metastatic carcinoma
Pulmonary emphysema and fibrosis Lipoid pneumonia
Toxic pneumonitis
Shock lung
Atelectasis Periarteritis nodosa
Wegener granuloma
Sarcoidosis
Scleroderma
Pneumothorax  
Disorders of Perfusion Pulmonary embolism   Hemangioma Pulmonary fibrosis
Pulmonary emphysema
  Congenital heart disease      
Disorders of Transport Congestive heart failure Septicemia with shock   Aplastic anemia Methemoglobinemia
Shock from drugs and toxins
Sickle cell anemia
Congenital heart disease
Shock Hemorrhagic shock Waterhouse–Friderichsen syndrome
Disorders of Increased Oxygen Demands Polycythemia Fever Leukemia
Hodgkin lymphoma
Metastatic carcinoma
          Hyperthyroidism
Disorders of Excretion of Carbon Dioxide and Other Wastes of Body Metabolism   Septicemia with lactic acidosis Pulmonary emphysema   Uremia Lactic acidosis       Diabetic acidosis
Disorders of oxygen absorption
Lobar pneumonia, sarcoidosis, silicosis and various causes of pulmonary fibrosis, and pulmonary edema are considered here. Oxygen diffusion across the alveolocapillary membrane is affected in all of these. Alveolar proteinosis, shock lung, and the adult respiratory distress syndrome must also be considered here.
Disorders of perfusion of the pulmonary capillaries
Pulmonary emboli, hemangiomas of the lungs, and congenital heart increases such as tetralogy of Fallot belong in this category. In all of these conditions unoxygenated blood bypasses the alveoli. Also included in this category are diseases with a ventilation–perfusion defect. In other words, some alveoli are being ventilated but not perfused with blood, while at the same time some alveoli are being perfused but not ventilated. Pulmonary emphysema and the various conditions associated with pulmonary fibrosis (e.g., pneumoconiosis) cause dyspnea on this basis, as well as other physiologic reasons mentioned above.
Disorders of oxygen transport
The tissues will not get oxygen if there is not enough blood to transport it, as in anemia and hemorrhagic shock; if there is not enough blood pressure to perfuse the tissues, as in vasomotor and cardiogenic shock; or if the heart pump fails, as in CHF from many causes. In methemoglobinemia and sulfhemoglobinemia, there may be enough blood, but it is unable to carry the oxygen.
Increased tissue oxygen demand
During exercise and nervous stress, and in febrile states, leukemia and other malignancies, and hyperthyroidism there is an increase
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in tissue metabolism; consequently, tachypnea may develop to increase the supply.
Inadequate excretion of CO2 and other wastes of tissue metabolism
Inability to excrete CO2 may occur without anoxia in pulmonary emphysema and other chronic obstructive lung diseases and initiate dyspnea, especially on exertion. Other wastes of tissue metabolism may cause an acidosis and stimulate the respiratory centers in this fashion. Lactic acidosis, diabetic acidosis, and uremia may cause dyspnea on this basis.
From the above discussion, it should be evident that the clinician can develop an excellent list of the causes of dyspnea and tachypnea with an understanding of the pathophysiology involved. A few conditions cannot be recalled with this method: hyperventilation syndrome, ingestion of acids (e.g., methyl alcohol poisoning) and drugs that stimulate the respiratory centers (such as amphetamines), and atmospheric reduction in oxygen tension.
Approach to the Diagnosis
The history and physical examination will almost invariably disclose the cause of dyspnea. To confirm pulmonary disease one will order pulmonary function studies, a chest roentgenogram, and arterial blood gases. If routine pulmonary function studies are normal, more sophisticated studies such as the nitrogen washout test and perfusion and ventilatory scans may be necessary. To diagnose cardiac conditions, ordering an ECG and measuring venous pressure and circulation times may be necessary.
Any patient with dyspnea and normal physical findings deserves a circulation time to rule out early CHF. A hemogram will diagnose anemias but it will not
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diagnose methemoglobinemia. A determination of the erythrocytes methemoglobin, arterial oxygen saturation, and diaphorase I test must be done.
Other Useful Tests
  • CBC (anemia, polycythemia)
  • Sedimentation rate (pneumonia, subacute bacterial endocarditis [SBE])
  • Serial cardiac enzymes (acute myocardial infarction)
  • Sputum smear and culture (pneumonia)
  • Lung scan (pulmonary embolism)
  • Sputum for eosinophils (asthma)
  • Toxicology screen (drug abuse)
  • Echocardiogram (CHF, valvular heart disease)
  • Pulmonary angiogram (pulmonary embolism)
  • Trial of diuretics (CHF)
  • Forced vital capacity (FVC) with methacholine challenge (asthma)
  • B-type natriuretic peptide (BNP) assay (CHF)
  • Cardiac catheterization (CHF)