Hyperventilation is defined as breathing in excess of metabolic demands and, therefore, is purely a respiratory disturbance. It produces hypocapnia, respiratory alkalosis, cerebral vasoconstriction (which, in turn, reduces the CBF), a reduction in the availability of O 2 peripherally (through shifts of the O 2 dissociation curve), a reduction in the level of ionized serum Ca, and when sustained, significant hypophosphatemia. y Thus, neurological features are frequent, and patients with hyperventilation disorders are frequently referred to neurologists. Symptoms associated with hyperventilation vary widely and include lightheadedness, acral and perioral paresthesias (which may be painful), syncope and, less commonly, carpopedal spasms, muscle cramps, blurred vision, headache, dyspnea, and chest pain. y ,  Other reported symptoms include sweating of the hands, cold extremities, giddiness, ataxia, tremor, tinnitus, hallucinations, epigastric pain, a bloated feeling, vomiting, and unilateral somatic symptoms, which may result in the misdiagnosis of cerebral ischemia, seizure, or complicated migraine. y Once it is induced, hypocapnia can be maintained with only an occasional sigh, thereby making its visual recognition difficult. y These symptoms probably arise from a reduction in CBF and are associated with significant cerebral hypoxia. y For every 1 mm Hg drop in PaCO2 , the CBF drops by 2 percent.
Disorders associated with hyperventilation can be subdivided into psychogenic (e.g., anxiety, panic, sighing, air hunger, or factitious disorders), organic (usually multifactorial including respiratory disease, CNS disorders, pain, aspirin overdose), and physiological (progesterone ingestion, prolonged conversation, hypoxic stimulation of chemoreceptors at high altitude, pyrexia). y Hyperventilation is frequently reported in anxious individuals without any identifiable systemic disease. y However, because isolated anxiety (i.e., in the absence of hyperventilation) has no effect on CBF, it should not produce these symptoms. y Patients with organic disorders (e.g., asthma) can present with hyperventilation. Additionally, disorders producing anxiety (e.g., angina) can cause hyperventilation. Thus, the identification of hyperventilation should never be considered synonymous with anxiety, nor should these patients be taken less seriously. For that reason, whenever hyperventilation is recognized, organic etiologies should be excluded and treatment initiated.
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