Arterial blood samples were drawn from an arterial line to determine PaCO 2 and partial pressure of arterial oxygen (PaO 2 ). End-tidal carbon dioxide (ET-CO 2 ) concentration, and expiratory tidal volume (V Te ) were measured at the airway opening. ET-CO 2 was measured using a side-stream Microstream (Philips Medical Systems, Best, The Netherlands) and V Te was measured with a proximal flow sensor connected to the AVEA ventilator (Cardinal Health, Yorba Linda, CA, USA). The ventilator is designed to detect which gas is used and adjusts its pneumotachograph automatically in order to measure the correct V Te .