BACKGROUND: Heliox is known to decrease PaCO2 in patients with increased airway resistance by increasing minute ventilation and reducing work of breathing(WOB). Besides these effect, heliox is expected to decrease functional anatomic dead space owing to improvement of peak expiratory flow rate(PEFR) and enhancement of gas distribution. We investigated whether heliox can decrease PaCO2 even at the same minute ventilation(VE) and WOB with N2-02 to speculate the effect of the heliox on the anatomic dead space. MATERIAL AND METHOD: The subjects were 8 mechanically ventilated patients with asthma or upper airway obstruction(M : F= 5 : 3, 68+/-10years) who were under neuromuscular paralysis. The study was consisted of three 15-minutes phases: basal N2-O2, heliox and washout Heliox was administered via the low pressure inlet of servo 900C, and respiratory parameters were measured by pulmonary monitor(CP-100 pulmonary monitor, Bicore, Irvine, CA, USA). To obtain the same tidal volume(Vt) in heliox phase, the Vt on monitor was adjusted by the factor of relative flow rate of heliox to N2-O2. Dead space was calculated by Bohr equation. RESULTS: 1) Vt, VE, peak inspiratory pressure(PIP) and peak inspiratory flow rate(PIFR) were not different between N2-O2 and heliox. 2) PEFR was higher on heliox(0.52+/-0.19L/sec) than N2-O2(0.44+/-0.13L/sec) (p=0.024). 3) PaCO2(mmHg) were decreased with heliox(56.1+/-14.1) compared to N2-O2(60.5+/-15.9) (p=0.027). 4) Dead space ventilation(%) were decreased with heliox(73+/-9 with N2-O2 and 71+/-10 with heliox) (p=0.026). CONCLUSION: Heliox decreased PaCO2 even at the same VE and WOB with N2-O2, and the effect was considered to be related with the reduction of anatomic dead space.