A COMPARISON OF THE EFFECTS OF ASSIST-CONTROL, SIMV, AND SIMV WITH PRESSURE SUPPORT ON VENTILATION, OXYGEN-CONSUMPTION, AND VENTILATORY EQUIVALENT

Objective: To quantify the ventilatory efficiency of different modes o f mechanical ventilation used to achieve full ventilatory support in n ormal subjects. Modes compared were assist-control, synchronized inter mittent mandatory ventilation (SIMV), and SIMV with 10 cm H2O (0.98 kP a) of pressure support. Design: Prospective, randomized blocks repeate d measures design. Subjects served as their own controls. Setting: A u niversity affiliated pulmonary laboratory. Subjects: Ten healthy volun teers, aged 31-54 years. Outcome Measures: Minute volume, respiratory rate, average tidal volume, oxygen consumption, and ventilatory equiva lent. Intervention: Baseline spontaneous ventilation data collection w as followed by mechanical ventilation by mouthpiece in each of three m odes in a random sequence. All modes used a machine set rate of 12 bre aths per minute, Vt of 10 cc/kg of ideal body weight, inspiratory time of 1 second, square wave flow pattern and a sensitivity of -1 cm H2O (-0.09806 kPa) to achieve full ventilatory support. Data were collecte d continuously for 5 minutes and the mean values were reported. Ventil atory equivalent for oxygen is a measure of the efficiency of the vent ilatory pump at various work loads and was calculated by dividing Ve ( BTPS) by the Vo2 (STPD). Results: There were significant differences b y mode of mechanical ventilation in average tidal volume (p = 0.02), m inute volume (p = 0.02), oxygen consumption (p = 0.04), and ventilator y equivalent (p = 0.01) using ANOVA. There was no significant differen ce (p = 0.66) by mode of ventilation in respiratory rate. Pairwise fol low-up comparisons for these variables found that SIMV with pressure s upport produced a significantly greater average tidal volume, minute v olume, oxygen consumption, and ventilatory equivalent than SIMV alone. SIMV with pressure support also produced a significantly greater minu te volume and ventilatory equivalent than assist-control. There were n o significant differences between assist-control and SIMV. All three m odes produced a lower ventilatory equivalent and higher oxygen consump tion than spontaneous breathing. Conclusions: SIMV with pressure suppo rt significantly increased minute volume and ventilatory equivalent wh en compared with assist-control or SIMV alone, and thus was the most e fficient mode of full ventilatory support for our subjects. We found n o difference in ventilatory efficiency between assist-control and SIMV . All three mechanical modes were less efficient for our subjects than spontaneous breathing. The inspiratory time of 1 second used in this study, although common in clinical practice, may be inadequate for som e patients.