Difficulty or shortness of breath associated with marked awareness of the effort of respiration is called dyspnea. In the left-sided heart failure and in hypoventilatory states, the patient becomes more dypsneic in the recumbent posture and considerable relief is obtained by sitting up. This is referred to as orthopnea. Attacks of severe breathlessness occurring during sleep at night may awaken the patient and assumption of the erect posture gives relief. This is termed paroxysmal nocturnal dyspnea. This is also characteristic of left-sided heart failure. Respiratory disorders that lead to dyspnea may fall into different groups.
• Central causes for dyspnea affect the respiratory center, e.g encephalitis or cerebrovascular accidents.
• Significant airways obstruction is a common cause for dyspnea. Obstruction to the airway may be mechanical as due to a foreign body or functional as due to spasm. Larger airways may be obstructed by aspirated foreign bodies. Diphtheritic membrane, tumors, blood or secretions. Obstruction to the larynx produces inspiratory stridor and in-drawing of the chest wall. Dyspnea is felt both during inspiration and expiration. Obstruction to the smaller airways occurs in asthma, emphysema, chronic bronchitis, and extensive bronchiectasis. In these conditions the difficulty is felt more for expiration and the characteristic expiratory wheeze may be heard.
• Disorders that impair the process of gas exchange eg, massive pulmonary collapse, pulmonary embolism, respiratory distress syndrome, fibrosing alveolitis, pulmonary fibrosis and extensive parenchymal diseases such as tuberculosis, cystic disease and malignancy.
• Disease that prevent expansion of the lung, eg, pneumothorax, pleural effusion, kyphoscoliosis, injury to the chest wall, paralysis of respiratory muscles etc.
• Dyspnea is commonly the first symptoms when the inspired air does not supply adequate amounts of oxygen to the individual. This happens when the oxygen tension is low as in high altitudes or in gas poisoning.
• Hysterical hyperventilation presents as dyspnea. In this, the subject voluntarily hyperventilates. Other traits of the hysterical personality may be evident. Excessive removal of carbon-dioxide due to overventilation leads to repiratory alkalosis and tetany.
• In diseases like pneumonia and pleurisy, painful restriction of respiratory movements leads to hypoventilation and dyspnea.
Bluish discoloration of the skin and mucous membranes due to the presence of excess of reduced hemoglobin in peripheral blood is called cyanosis. In extensive diseases of the lungs, central cyanosis occurs due to defective oxygenation of arterial blood or the development of functional arteriovenous shunts. In chronic bronchitis and emphysema, the main defects are those of ventilation and perfusion. In fibrosing alveolitis, the defect is mainly one of diffusion. Differentiation between respiratory and cardiac causes of cyanosis can be made on clinical grounds in many cases. In respiratory diseases inhalation of oxygen helps in clearing the cyanosis, whereas this is not so in cardiac lesions with right to left shunts.