Orthostatic increase of respiratory gas exchange in hyperventilation syndrome

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 w as to assess these orthostatic changes by non-invasive pulmonary gas exchan ge measurements and to evaluate whether these responses discriminate patien ts with HVS from healthy subjects. Methods - Respiratory gases were collected with a face mask and pulmonary g as exchange was measured after 10 minutes at rest and after eight minutes s tanding 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 th e standing position. Results - At rest the variables of respiratory gas exchange did not differ significantly between the groups. As a response to standing, minute ventila tion 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 en d tidal CO2 fraction (FETCO2) and in ventilatory equivalents for oxygen (VE /VO2) and for CO2 (VE/VCO2) during the orthostatic test were arise signific antly 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 VE/VO2 (mean difference 18.4; 95% CI 7.7 to 29.0) and VE/VCO2 (mea n difference 11.7; 95% CI 4.8 to 18.6) were significantly higher in HVS pat ients 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% a nd 77%, respectively, and by using the cut off level of 37 for VE/VO2 they were 93% and 100%, respectively. In the HVS patients arterial PCO2 and FETC O2 were closely correlated during the orthostatic test (r = 0.93, p<0.0001) . Conclusions - As a response to change in body position from supine to stand ing, patients with HVS have, an accentuated increase in ventilation which d istinguishes them from healthy subjects. These findings suggest that non-in vasive measurements of pulmonary gas exchange during orthostatic tests are useful in the clinical evaluation of patients with hyperventilation disorde rs.