RESPIRATORY RESPONSE TO CO2 DURING PRESSURE-SUPPORT VENTILATION IN CONSCIOUS NORMAL HUMANS

The respiratory response to CO2 during pressure-support ventilation (P SV) was studied in 16 conscious normal humans. The subjects breathed t hrough a mouthpiece connected to a ventilator in PSV mode, with pressu re set to tile highest comfortable level for each subject (10.1 +/- 0. 4 cm H2O, mean +/- SE). Compared with breathing spontaneously through the ventilator (CPAP mode with zero positive end-expiratory pressure), with PSV, tidal volume (VT) Increased significantly (1.16 +/- 0.1 ver sus 0.85 +/- 0.04 L), whereas breathing frequency (f) remained stable (16.0 +/- 0.9 versus 15.6 +/- 1.1 breaths/min). As a result, the subje cts hyperventilated decreasing significantly end-tidal PCO2 (PETCO2, 2 3.5 +/- 1.2 versus 35.5 +/- 1.1 mm Hg). Fraction of inspired CO2 (FICO 2) was then increased in steps, and changes in respiratory motor outpu t were quantitated from changes in f, VT, ventilation ((V) over dot I) , peak inspiratory flow ((V) over dot peak), and muscle pressure (Pmus ). Pmus was calculated by the equation af motion, based an respiratory system mechanics, which were measured previously by airway occlusion at end-inspiration. VT, (V) over dot I, and Pmus increased significant ly with increasing PETCO2, and the response was detectable even below eupneic levels; f remained relatively stable over a wide range of PETC O2 (23 to 45 mm NS) and increased significantly only when PETCO2 appro ached 50 mm Hg. These results indicate that in conscious normal humans during PSV, CO2 responsiveness extends well into hypocapnia and is ex pressed principally as an increase in intensity of respiratory motor o utput with little change in respiratory rate.