In his comprehensive review of the hyperventilation syndrome¹ Gardner points out the difficulties in terminology and definition that have dogged this complicated and confused area. As he states, it is physiologically inappropriate to use the term "hyperventilation" in the absence of demonstrated hypocapnia. The term "hyperventilation syndrome" has, however, gained wide currency both in research studies and in clinical practice, often without precise diagnostic criteria being specified or hypocapnia rigorously demonstrated. This situation may have arisen from the perception of many clinicians that there is a real but poorly defined clinical entity causing morbidity in real world practice resulting from breathing abnormalities. Abnormal breathing patterns may, indeed, result in hyperventilation and hypocapnia, but rapid, irregular and shallow breathing may not necessarily result in increased ventilation yet may still cause significant symptoms. Isocapnic hyperventilation studies have shown that many of these symptoms are independent of hypocapnia,² and other mechanisms have been . . . [Full text of this article]