Hemodynamic and Oxygen Transport Characteristics of Common Ventilatory Modes: Results
Results of this study show that patients on SIMV and PSV modes can achieve similar levels of Ve as when on ACV mode. However, Vt is significantly diminished and f is raised substantially during SIMV to reach similar Ve as achieved on the other two ventilatory modes. It is also noteworthy that in both SIMV and PSV modes, levels of ventilation and oxygenation equal to ACV mode are reached with significantly lower peak and mean inspiratory pressures. Therefore, as dictated by the design of our study, if adequate levels of pressure support necessary to reach similar Vt as with ACV are applied, Ve will be maintained with no change in f and with lower inspiratory pressures this mycanadianpharmacy.com. In our patient population, an average of 18.5 cm H2O pressure support (range: 15 to 35) was necessary to achieve this goal. The only case where relatively higher levels of pressure support became necessary was in patient No. 11 who suffered from pulmonary fibrosis (Table 1).
We used very modest rates of SIMV (f=75 percent of that on ACV) in this study. The increase in f was, therefore, moderate (mean of 20.3 breaths/min). In only one patient did the f rise above 30 breaths/min. Indexes of oxygen transport and tissue oxygenation were not affected with the SIMV mode using our study protocol. It remains to be studied whether lower rates of SIMV for longer periods of time will lead to more significant changes in these parameters.
The lower inspiratory pressures seen in SIMV and PSV modes (Table 2) could be of potential benefit to the patients; lower PIP may reduce the possibility of barotrauma, and lower MIP can minimize the effect of positive ITP on venous return to the right heart, maintaining right ventricular preload and output. Indeed, an estimate of the right atrial transmural pressure (right atrial pressure-mean inspiratory pressure) in the three groups revealed values of 3.3 ± 1.1 cm H20 in ACV, 6.7 ±1.2 cm H20 in SIMV, and 4.9 ±1.3 cm H2O in PSV modes (p=0.056 comparing ACV with SIMV). The relatively larger right atrial transmural pressure on the SIMV mode could have contributed to higher Cl on this mode which reached borderline significance when compared with the ACV mode (Table 3).