The cause of dyspnea is not certain, but it is thought that the sensation arises from the respiratory muscles. When hypoxemia and hypercapnia are present but dyspnea is absent, the inappropriate lack of dyspnea suggests that the chemical drives to breathe are depressed. When due to hypercapnia, these symptoms relate to cerebral pH better than to cerebral PaCO 2 . The somnolence may be related to the sleep breathing problems of these patients. Muscle twitching and convulsions may occur with severe hypoxemia.
Circulatory abnormalities are common in respiratory failure.
Other findings may include hypertension and warm skin with bounding pulses or hypotension. These are dependent both on the direct effects of hypoxia and acidosis and on the reflex responses to them, including release of catecholamines. Polycythemia or an increased red blood cell mass is a complication of longstanding hypoxemia due to increased levels of erythropoietin. Polycythemia often is not evident in patients with chronic hypoxemia due to lung disease. Pulmonary hypertension is regularly present in respiratory failure.