Introduction Despite the advances, weaning from mechanical ventilation is an important problem in daily operations of intensive care units (ICUs). The weaning period can represent more than 40% of the mechanical ventilation duration. As reported in different series, weaning failure varies widely (1-4). Several pathophysiological determinants of weaning failure have been identified, including inadequacy of pulmonary gas exchange, cardiovascular dysfunction, disturbance in respiratory muscle performance, and psychological factors (5). A stepwise reduction of the mechanical ventilatory support is a widespread weaning strategy to gradually transfer the work of breathing (WOB) from the ventilator to the respiratory muscles. It has been reported that, under certain pathologic conditions, spontaneous ventilation increases the respiratory oxygen ([O.sub.2]) demand from 1% to 3% up to 25% to 40% of the total [O.sub.2] consumption (6-9). In critically ill patients, and especially in those with chronic obstructive pulmonary disease (COPD), the oxygen cost of breathing is increased during the weaning process and may increase the proportion of total body [O.sub.2] delivery required by respiratory muscles to meet ventilation requirements (10, 11). This increased [O.sub.2] cost of breathing is met by an increase in blood flow to respiratory muscles, resulting in a blood flow diversion from other tissues, and may lead to hypoperfusion in some areas (12).