Brainstem hypoperfusion in CFS.
Sir, Costa and his colleagues {QJ Med 1995; 88:767-73) are to be congratulated for providing more information about chronic fatigue syndrome. Hypocapnia is a powerful and readily available cerebral vasoconstrictor. 1 The 'cerebral vasoconstric-tion, and reduction in cerebral blood flow, are initiated when the arterial pCO 2 has fallen 2 mmHg below normal. When the pCO 2 has fallen by 25 mmHg, cerebral blood flow is decreased by about one third ... the maximum possible reduction of blood flow that can be achieved by respiratory alkalaemia is of the order of 40 per cent'. 2 Unless proved otherwise, it might be rational and prudent to attribute the findings of Costa's group to dysfunctional breathing, an essential element of effort syndrome. References 1. Lum LC. The syndrome of habitual chronic hyperventilation. In: Hill OW, ed. Modern trends in psychosomatic medicine. Sir, In 1948, Kety and Schmidt 1 were the first to report a generalized reduction of cerebral blood flow (CBF) associated with hyperventilation-induced alkalosis in young men. It is known that hypocapnia and resulting respiratory alkalosis may be either produced by voluntary hyperventilation or a secondary manifestation of a wide range of well defined pathological conditions. Dynamic stress (skeletal muscle exercise), fever, high altitude, hyperthyroidism (thyrotoxicosis), heart failure, pulmonary insufficiency, diabetes (aci-dotic state) and many pharmacological agents (sali-cylates, analeptics) may all produce hyperventilatory states as a secondary manifestation, sometimes with hypoxia, sometimes with hypercapnia, and frequently with hypocapnia. 2 The literature on metabolic and circulatory effects, particularly regional blood flow (including regional cerebral blood flow—rCBF) changes, is contradictory and confusing. As early as 1912, Krogh and Lindhard, 3 using the nitrous oxide method, found cardiac output to increase with thor-acic type breathing and reduce with abdominal type breathing. In general terms, it is not difficult to cause a severe alkalosis with dizziness, blurred vision, mental confusion, numbness of extremities, muscle spasm, and rarely convulsions with loss of consciousness due to forced hyperventilation. So much so that hyperventilation is sometimes used to provoke seizures in patients with epilepsy in order to identify the site of the epileptic focus. Although the magnitude of responses to raised pCO 2 differs in different regions of the brain, with increases in rCBF more marked in cerebral grey matter than in white matter, 4 the effects of reduced pCO 2 are not so clear. Whilst reduction of pCO 2 from 45 to 29 mmHg evokes the expected …