CARDIORESPIRATORY EFFECTS OF VOLUME-CONTROLLED AND PRESSURE-CONTROLLED VENTILATION AT VARIOUS I E RATIOS IN AN ACUTE LUNG INJURY MODEL/

Numerous approaches to the provision of mechanical ventilation during acute lung injury are currently available. Of these, pressure control inverse ratio ventilation has been considered superior to volume contr ol ventilation with PEEP with respect to improving gas exchange and mi nimizing cardiovascular compromise. However, no study systematically c ompares volume-controlled (VC) and pressure-controlled (PC) ventilatio n while maintaining mean airway pressure (MAP) constant at varying I/E ratios. We studied the effect of VC and PC with PEEP at normal (1:2) and inverse I/E ratios (2:1 and 4:1) on gas exchange, lung mechanics, and hemodynamics in a sheep lung injury model. Severe lung injury was induced in 12 sheep with bilateral lung ravages using normal saline; p relavage PO2 230 +/- 50 mm Hg, PEEP 5 cm H2O and postlavage, pretreatm ent PO2 70 +/- 20 mm Hg, PEEP 10 cm H2O both at Fl(O2) 0.50. MAP was k ept constant throughout the study at 25 +/- 2 cm H2O while ventilating all animals with a VT of 10 ml/kg and a rate of 20/min by randomized application of VC and PC with I/E ratios of 1:2, 2:1, and 4:1. Despite liberal fluid administration, all ventilatory modes depressed cardiac output compared with preinjury values. However, gas exchange and hemo dynamics did not differ among ventilation modes or I/E ratios. Of all the parameters investigated, the most striking changes occurred in pea k inspiratory pressure (PIP) and PEEP: PC (I/E 2:1 and 4:1) resulted i n significantly (p < 0.05) lower values for PIP and applied PEEP compa red with control data (VC I/E ratio 1:1), and VC (I/E 4:1) resulted in significantly (p < 0.05) lower values for applied PEEP than VC I/E ra tio 1:1. Auto PEEP levels during PC (I/E ratio 2:1 and 4:1) and VC (I/ E 2:1 and 4:1) were significantly higher (p < 0.05) than VC I/E ratio 1:1. When MAP remained constant, alterations in the ventilation mode ( PC or VC) or I/E ratio (1:2 to 4:1) produced no significant change in hemodynamic, oxygenation, or ventilation parameters in this sheep mode l.