Hyperventilation

To the Editor.—  In reference to the letters "Hyperventilation and Sphygmomanometry" by Dr. Harry F. Klinefelter (226:81, 1973) and Dr. J. Belogorsky (227:77, 1974), our fellow readers may be interested to recall that a fall in arterial carbon dioxide (CO₂) tension from hyperventilation lowers the arterial blood pressure in normal subjects as well as in those with hypertension (Circulation 9:89, 1954; 10:362, 1954). This fall in pressure results from an overall net decrease in peripheral vascular resistance. Vasodilation is pronounced in muscle beds; concomitantly, the pulse rate and cardiac output increase, skin and brain flow decrease, the hands become cold and cyanotic, and consciousness is altered—the patient is having a "spell."Since this hypotensive response is greatly augmented in patients with autonomic nervous system dysfunction and is not prevented by induced autonomic blockade, it may be due to local effects of hypocapnia on blood vessels rather than being mediated