Hemodynamic and Oxygen Transport Characteristics of Common Ventilatory Modes

We studied hemodynamic and oxygen transport parameters in 12 stable critically ill patients on assist control (ACV), synchronized intermittent mandatory (SIMV), and pressure support (PSV) ventilatory modes. Patients were optimally ventilated on ACV, were awake, and capable of spontaneous 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 rate 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 modes. Patients on SIMV and PSV maintained similar minute ventilation as when on ACV but with significantly lower peak and mean inspiratory pressures than on ACV. However, f increased and VT decreased significantly on SIMV. Hemodynamic and oxygen transport parameters were not significantly different among the three groups, although there was a tendency toward higher cardiac index, oxygen transport, and oxygen consumption 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 provide adequate ventilation with lower airway pressure and possibly less adverse effects on hemodynamic and tissue oxygenation parameters compared 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.