HEMODYNAMIC AND OXYGEN-TRANSPORT CHARACTERISTICS OF COMMON VENTILATORY MODES

We studied hemodynamic and oxygen transport parameters in 12 stable cr itically ill patients on assist control (ACV), synchronized intermitte nt mandatory (SIMV), and pressure support (PSV) ventilatory modes. Pat ients were optimally ventilated on ACV, were awake, and capable of spo ntaneous breathing. After baseline measurements on ACV, patients were placed on SIMV and PSV for 30 min each and measurements were repeated at the end of each period. The SIMV rate (f) was 75 percent of the rat e on ACV. The PSV was set at a level enough to maintain a tidal volume (VT) equal to that on ACV. The FIO2 was kept constant on all three mo des. Patients on SIMV and PSV maintained similar minute ventilation as when on ACV but with significantly lower peak and mean inspiratory pr essures than on ACV. However, f increased and VT decreased significant ly on SIMV. Hemodynamic and oxygen transport parameters were not signi ficantly different among the three groups, although there was a tenden cy toward higher cardiac index, oxygen transport, and oxygen consumpti on on SIMV and PSV. We conclude that in stable critically ill patients , SIMV and PSV used according to our study protocol for 30 min can pro vide adequate ventilation with lower airway pressure and possibly less adverse effects on hemodynamic and tissue oxygenation parameters comp ared with ACV. Because of a significant decrease in VT and an increase in f seen with SIMV, PSV may be a more desirable mode for ventilatory support.