The paper by Malmberg et al in the current issue of Thorax¹ deals with the difficult subject of the hyperventilation syndrome and finds that these patients have a disproportionately high ventilatory response to change of body position from supine to standing. The authors suggest that this can be used as a diagnostic criterion for hyperventilation syndrome. Hyperventilation is a confused and poorly documented subject and the publication of this paper provides an opportunity to review some of the particularly controversial aspects of this subject.

The first issue concerns the basis for the labelling of these patients as "hyperventilation syndrome". Some of the controversies about the use of this term have recently been reviewed by Folgering² and by Gardner.³ The physiological definition of hyperventilation is alveolar ventilation that is inappropriately high for the metabolic production of carbon dioxide, leading to reduction of arterial PCO₂ (PaCO₂) below the normal . . . [Full text of this article]