Orthostatic increase of respiratory gas exchange in hyperventilation syndrome

Abstract

Background - Hyperventilation syndrome (HVS) is a common disorder which is difficult to diagnose because of somatic symptoms and its episodic nature. In previous studies respiratory alkalosis in arterial blood was often found during orthostatic tests in patients with HVS. The purpose of this study was to assess these orthostatic changes by non-invasive pulmonary gas exchange measurements and to evaluate whether these responses discriminate patients with HVS from healthy subjects. Methods - Respiratory gases were collected with a face mask and pulmonary gas exchange was measured after 10 minutes at rest and after eight minutes standing upright in 16 patients with HVS and 13 healthy control subjects. In patients with HVS arterial blood samples were also drawn at rest and in the standing position. Results - At rest the variables of respiratory gas exchange did not differ significantly between the groups. As a response to standing, minute ventilation increased in both study groups but significantly more in the patients with HVS (mean difference 5.4 l/min (95% CI 1.1 to 9.6)). The changes in end tidal CO, fraction (FETCO2) and in ventilatory equivalents for oxygen (V̇E/V̇O2) and for CO2 (V̇E/V̇CO2) during the orthostatic test were also significantly larger in patients with HVS than in healthy controls. During standing FETCO2 was significantly lower (mean difference -1.1 kPa; 95% CI -1.5 to -0.6) and V̇E/V̇O2, (mean difference 18.4; 95% CI 7.7 to 29.0) and V̇E/V̇CO2, (mean difference 11.7; 95% CI 4.8 to 18.6) were significantly higher in HVS patients than in healthy controls. By using the cut off level of 4% for FETCO2 the sensitivity and specificity of the test to discriminate HVS were 87% and 77%, respectively, and by using the cut off level of 37 for V̇E/V̇O2 they were 93% and 100%, respectively. In the HVS patients arterial PCO2 and FETCO2 were closely correlated during the orthostatic test (r = 0.93, p < 0.0001). Conclusions - As a response to change in body position fi om supine to standing, patients with HVS have an accentuated increase in ventilation which distinguishes them from healthy subjects. These findings suggest that non-invasive measurements of pulmonary gas exchange during orthostatic tests are useful in the clinical evaluation of patients with hyperventilation disorders.