Dyspnea, also called shortness of breath, is a patient's perceived difficulty to breathe. Sensations and intensity can vary and are subjective. It is a prevalent symptom impacting millions of people. It may be the primary manifestation of respiratory, cardiac, neuromuscular, psychogenic, or systemic illnesses, or a combination of these. Dyspnea on exertion is a similar sensation. However, this shortness of breath is present with exercise and improves with rest. Exercise is defined here as any physical exertion which increases metabolic oxygen demand above the body’s ability to compensate. Oxygen is vital to the human body as its used for oxidative phosphorylation, and it is the last acceptor of an electron in electron transport chain. The sensation of dyspnea mostly comes when our body is lacking the oxygen delivery.
Oxygen Delivery: Hb x 1.39 x SaO2 x Cardiac Output + 0.003 x Pao2
If a body has low Hb, hemoglobinopathies, some toxicities affecting Hb (like carbon monoxide toxicity), low cardiac output (congestive heart failure [CHF], myocardial infarction [MI], arrhythmia) a person will feel dyspneic.
Dyspnea on exertion is a symptom disease, rather than a disease itself. As such, its etiology can be designated as arising from two primary organ systems: the respiratory system and the cardiac system. Other systemic illnesses may by culprit as well as a combination of different etiologies.
Respiratory causes may include asthma, acute exacerbation or chronic congestive obstructive pulmonary disorder (COPD), pneumonia, pulmonary embolism, lung malignancy, pneumothorax, or aspiration.
Cardiovascular causes may include congestive heart failure, pulmonary edema, acute coronary syndrome, pericardial tamponade, valvular heart defect, pulmonary hypertension, cardiac arrhythmia, or intracardiac shunting.
Other systemic illnesses may include anemia, acute renal failure, metabolic acidosis, thyrotoxicosis, cirrhosis of the liver, anaphylaxis, sepsis, angioedema, and epiglottitis.
The epidemiology of dyspnea on exertion is highly variable depending on etiology.
Dyspnea on exertion is the sensation of running out of the air and of not being able to breathe fast or deeply enough during physical activity. It results from multiple signal interactions with receptors in the central nervous system (CNS), peripheral receptors chemoreceptors, and mechanoreceptors in the upper airway, lungs, and chest wall.
The respiratory center of the brain is comprised of 3 neuron groupings in the brain: the dorsal and ventral medullary groups and the pontine grouping. The pontine grouping further classifies into the pneumotaxic and apneustic centers. The dorsal medulla is responsible for inhalation. The ventral medulla is responsible for exhalation. The pontine groupings are responsible for modulating the intensity and frequency of the medullary signals where the pneumotaxic groups limit inhalation and the apneustic centers prolong and encourage inhalation. Each of these groups communicates with one another to unify the efforts as the pace making potential of respiration.
Mechanoreceptors located in the airways, trachea, lung and pulmonary vessels exist to provide sensory information to the respiratory center of the brain regarding the volume of the lung space. There are 2 primary types of thoracic sensors: slow adapting stretch spindles and rapid adapting irritant receptors. Slow-acting spindle sensors convey only volume information.
However, the rapid-acting receptors respond to both volume of the lungs and chemical irritation triggers such as harmful foreign agents that may be present. Both types of mechanoreceptors signal via cranial nerve X (the Vagus nerve) to the brain to increase the rate of breathing, the volume of breathing, or to stimulate errant coughing patterns of breathing secondary to irritants in the airway.
Peripheral chemoreceptors consist of the carotid and the aortic bodies. Both sites function to monitor the partial pressure of arterial oxygen in the blood. However, hypercapnia and acidosis increase the sensitivity of these sensors and play a partial role in the receptor’s function. The carotid bodies are located at the bifurcation of the common carotid arteries, and the aortic bodies are located within the aortic arch. Once stimulated by hypoxia, they send a signal via cranial nerve IX (the glossopharyngeal nerve) to the nucleus tractus solitarius in the brain, which in turn, stimulates excitatory neurons to increase ventilation. It has been estimated that the carotid bodies comprise 15% the total driving force of respiration.
Central chemoreceptors hold the majority of control over respiratory drive. They function through sensing pH changes within the CNS. Primary locations within the brain include the ventral surface of the medulla and the retrotrapezoid nucleus. pH changes within the brain and surrounding cerebrospinal fluid is derived primarily from increases or decreases in carbon dioxide levels. Carbon dioxide is a lipid-soluble molecule that freely diffuses across the blood-brain barrier. This characteristic proves to be useful in that rapid changes in pH within the cerebrospinal fluid are possible. Chemoreceptors responsive to pH change are located on the ventral surface of the medulla. As these areas become more acidic, sensory input is generated to stimulate hyperventilation, and carbon dioxide within the body is reduced through the increased ventilation. When pH rises to more alkalotic levels, hypoventilation occurs, and carbon dioxide levels decrease secondary to decreased ventilation.
Respiratory centers located within the medulla oblongata and pons of the brainstem are responsible for generating the baseline respiratory rhythm. However, the rate of respiration is modified by allowing for aggregated sensory input from the peripheral sensory system which monitors oxygenation, and the central sensory system which monitors pH and indirectly carbon dioxide levels along with several other portions of the cerebellar brain modulate to create a unified neural signal. The signal is then sent to the primary muscles of respiration, the diaphragm, external intercostals, and scalene muscles along with other minor muscles of respiration.
The history and physical exam should ascertain whether there are any chronic underlying cardiovascular or pulmonary illnesses. Key components of the history include onset, duration, aggravating factors, and alleviating factors. Presence of a cough may indicate the presence of asthma, chronic obstructive pulmonary disease (COPD), or pneumonia. A severely sore throat could indicate epiglottitis. Pleuritic quality chest pain may indicate pericarditis, pulmonary embolism, pneumothorax, or pneumonia. Orthopnea, nocturnal paroxysmal dyspnea, and edema suggest a possible diagnosis of congestive heart failure. Tobacco use is a common history finding that increases the likelihood of COPD, congestive heart failure, and pulmonary embolism. If indigestion or dysphagia is present, consider gastroesophageal reflux disease or gastric secretion aspiration in the lungs. A barking quality cough, especially in children may suggest croup. Presence of fever strongly suggests an infectious etiology.
The physical exam should begin with a rapid assessment of the ABCs (airway, breathing, and circulation). Once determined to be stable, a full physical exam can be done. To determine the severity of dyspnea, carefully observe respiratory effort, use of accessory muscles, mental status, and ability to speak. Distention of the neck veins may imply cor pulmonale caused by severe COPD, congestive heart failure, or cardiac tamponade. Thyromegaly may indicate hyperthyroidism or hypothyroidism. Percussion of the lung lobes for dullness can determine the presence or absence of consolidation and effusion. Hyperresonance on percussion is a worrisome finding that indicates possible pneumothorax or severe bullous emphysema. Lung auscultation may reveal absent breath sounds indicating the presence of region occupying mass such as pleural effusion or malignancy. The presence of wheezing is highly consistent with the diagnosis of an obstructive lung disease such as asthma or COPD. However, wheezing may be associated with pulmonary edema or pulmonary embolism. Pulmonary edema and pneumonia may present with rales on auscultation. Auscultation of the heart may reveal the presence of dysrhythmia, cardiac murmurs, or aberrant heart gallops. An S3 gallop indicates cardiac overfilling seen in left ventricular systolic dysfunction and congestive heart failure (CHF). An S4 gallop suggests left ventricular dysmotility and dysfunction. A loud P2 indicates possible pulmonary hypertension. Murmurs may indicate valvular dysfunction. Diminished heart sounds may indicate cardiac tamponade. Pericarditis may present with a rubbing cardiac sound on auscultation. On abdominal examination, hepatomegaly, ascites, positive hepatojugular reflux may suggest a diagnosis of CHF. Lower extremity edema is associated with CHF, and extreme swelling of the extremities suggests possible deep venous thrombosis that can lead to a pulmonary embolism. Digits clubbing is present in some forms of lung malignancy or severe chronic hypoxia. Cyanosis of the extremities indicates hypoxia.
Every evaluation should begin with a rapid assessment of the ABC status of the patient. Once these are determined to be stable and no life-threatening status present, a complete history and physical exam can be collected. Vital signs should be assessed for heart rate, respiratory rate, body temperature, body mass index (BMI), and oxygen saturation. Oxygen saturation may be normal at rest, so oxygen saturation with physical exertion should be obtained. In normal physiological conditions, the pulse oximetry improves as V/Q matching improves. Fever may indicate an infectious etiology. A chest x-ray is the first diagnostic test that should be utilized in evaluating dyspnea on exertion. If abnormal the disease process is likely cardiac or a primary pulmonary process. An echocardiogram is needed to evaluate cardiac function, pericardial space, and valvular function.
Additionally, an electrocardiogram should be obtained to evaluate for myocardial infarction or right-sided heart pattern strain. Elevated pro-brain natriuretic peptide (BNP) levels can further a congestive heart disease diagnosis. Exercise stress testing is also beneficial to determine cardiac function along with exercise oxygenation. If the chest x-ray is normal, then spirometry is needed to determine lung function. Abnormal spirometry can indicate either an obstructive pathology such as asthma, COPD, or physical airway obstruction or restrictive disease processes such as interstitial fibrosis. Spirometry can also indicate the presence of respiratory muscle weakness from muscular or neurological abnormalities. Normal spirometry indicates a need to evaluate for hypoxia as a source of dyspnea. The restrictive pathology can be confirmed with lung volumes, which will show reduced total lung capacity (TLC). In obstructive lung disease, the TLC is increased, and RV/TLC ratio is increased. Diffusion capacity is reduced in disease processes which affect the alveolar membrane area and or thickness. For example, it will be reduced with interstitial lung disease (ILD), emphysema, pulmonary embolism (PE), CHF, and obesity.
Arterial blood gas testing is used for this purpose as well as to calculate the A-a gradient and assess for an acidotic state. If hypoxic at PaO2 is low with a normal chest x-ray, then pulmonary embolism should be considered. The pH is mostly alkalotic in the setting of PE. This is to blow carbon dioxide to relatively increase the partial pressure of oxygen. In a pregnant female d-dimer with leg ultrasound and V/Q scan should be ordered first. Detection of a mismatch in 2 or more areas indicates pulmonary embolism. D-dimer testing has a low specificity and a high sensitivity. Spiral CT of the chest is an alternative to V/Q scanning. In acute settings, the CT chest with PE protocol is the gold standard. If the dyspnea on exertion is chronic, then chronic thromboembolic pulmonary hypertension (CTEPH) should be considered, and VQ scan is the test of choice and is considered the gold standard. The VQ scan in this setting has a “moth-eaten” appearance.
A normal scan necessitates cardiac catheterization to determine pulmonary hypertension, intracardiac shunting, or coronary artery disease. A normal cardiac catheterization diagnosis idiopathic dyspnea. If hypoxia is not present with a PaO2 greater than 70 mm Hg, correlation with oxygen saturation is needed. Abnormal oxygen saturation indicates possible carbon monoxide poisoning, methemoglobinemia, or an abnormal hemoglobin molecule.
Normal oxygen saturation requires a complete blood count (CBC) to evaluate hemoglobin content and hematocrit values. The white blood count also assesses for an immune response to possible infection. Hematocrit less than 35% is anemia.
Oxygen Delivery: Hb x 1.39 x SaO2 x Cardiac Output + 0.003 x Pao2
If one cannot determine the etiology of dyspnea, then we should order a cardiopulmonary exercise test (CPET). If the CPET does not show any cardiac or pulmonary etiology, then likely diagnosis for dyspnea on exertion is physical deconditioning.
All testing modalities should target toward clinical suspicion and the history and physical exam to avoid overtesting and minimize cost to the patient.
Treatment for dyspnea on exertion depends on its underlying etiology. The first intervention is to determine that there are no life-threatening etiologies present on an acute presentation by monitoring the ABC’s (airway, breathing, and circulation) of the patient. Once determined to be stable and that no immediate lifesaving interventions are necessary to further treatments can be assessed. If a patient is using smoking tobacco, this should be discontinued. Various inhaler therapies may be used in respiratory disease including short-acting or long-acting bronchodilators, inhaled antimuscarinics, and inhaled corticosteroids. Continuous supplemental oxygen therapy is used to ease discomfort associated with dyspnea on exertion if oxygen saturation is shown to decrease with exercise. Cardiac function should be optimized when a cardiac illness is identified. If a myocardial infarction is suspected based on ST changes on electrocardiogram or troponin marker evaluation, rapid percutaneous intervention should be performed by a cardiologist. Aspirin, statin, ACE inhibitor, beta-blocker, heparin, and nitro therapy should be initiated immediately if no contraindications. Occasionally, medications such as beta blockers and calcium-channel blockers can induce dyspnea on exertion by decreasing cardiac function, which can be picked up on a CPET. These should be decreased or discontinued when possible. In CHF, diuretic medications should be used to decrease vascular congestion from fluid overloading. If the dyspnea on exertion is due to obesity or deconditioning physical therapy and an exercise regimen should be pursued. If psychological problems are causing dyspnea on exertion, a selective serotonin receptor inhibitor can be trialed along with counseling sessions.
Acute dyspnea on exertion is most likely caused by acute myocardial ischemia, heart failure, cardiac tamponade, pulmonary embolism, pneumothorax, pulmonary infection in the form of bronchitis or pneumonia, or upper airway obstruction by aspiration or anaphylaxis.
Chronic dyspnea is most likely caused by asthma, chronic obstructive pulmonary disease, congestive heart failure, interstitial lung disease, myocardial dysfunction, obesity, or deconditioning.
The most common diagnosis underlying dyspnea on exertion is CHF.
In itself, dyspnea on exertion is harmless and a normal physiological finding; however, as it is a symptom and not an illness, it may indicate an underlying disease. The prognosis is highly variable depending on the exact etiology and patient demographics.