Tachypnea
chemoreceptors responding to increased levels of CO2 (decreased cerebrospinal fluid pH) and peripheral chemoreceptors (carotid and aortic bodies) responding to hypoxemia (PaO2 below 60 mm Hg), increases in PaCO2, and decreases in pH initiate increases in the respiratory rate.
Mechanoreceptors in the lungs and joints also influence ventilation. Lung inflation stimulates stretch receptors in the airways, which decreases the inspiratory effort, whereas mechanoreceptors in the joints are thought to be partly responsible for the increases in ventilation that occur during exercise. Pulmonary chemoreceptors that influence the respiratory cycle include irritant receptors in the airways, which are stimulated by dust and histamine, and J receptors, which apparently detect levels of interstitial fluid and are thought to be responsible for the respiratory pattern observed in animals with pulmonary edema.Tachypnea is classified as physiologic when it occurs in the absence of underlying disease and as pathologic when it is a manifestation of respiratory distress. Physiologic tachypnea is associated with pain, exertion, heat, fever, anxiety, and other stresses (Boxes 5.12 and 5.13). Factors predisposing to pathologic tachypnea are the same as those that cause respiratory distress: inadequate oxygenation of the blood (i.e., a need for additional oxygen); compensation for metabolic acidosis; excessive environmental heat; disorders that damage the central nervous system respiratory centers in the medulla (e.g., head trauma, inflammation, mass lesions) if they disrupt the control of breathing; disorders that cause dysfunction of motor nerves and/or weakness of respiratory muscles (e.g., botulism, hypocalcemia); and painful conditions involving the respiratory sensory nerves, muscles, pleurae, and ribs (e.g., chest trauma, pleural infection, neoplasia)66 (see the Respiratory Distress [Dyspnea] section later).
Approach to Diagnosis of Tachypnea
Because tachypnea is a manifestation of respiratory distress, the approach used for evaluating patients with tachypnea is the same as that used for evaluation of respiratory distress (see the Respiratory Distress [Dyspnea] section later).