Hemodynamic and Oxygen Transport Characteristics of Common Ventilatory Modes: Recommendation
The V02 decreased only on PSV of 5 cm H20. Furthermore, mean inspiratory flow increased and the duty cycle decreased significantly with increasing level of PSV. Fassoulaki and Eforakopoulou compared pressure support levels of 0,7,14, and 21 cm H2O for 1 h each in patients with respiratory failure and during postoperative period. With increasing level of PSV, f declined but Vt and mean airway pressure increased progressively. No change in РаОг, РаСОг, V02, and Vcc>2 were observed. In another study, Tokioka and colleagues applied levels of PSV to patients with severe respiratory failure necessary to achieve similar peak airway pressures reached while on ACV mode. They then decreased the level of pressure support gradually to the lowest level allowing regular breathing pattern and an f less than 20 breaths/ min. The data obtained after 30 min on an unspecified level of PSV showed an increase in Vt but lower f and Ve on PSV compared with ACV. Peak airway pressure and the duty cycle were lower, and РаОг was higher on PSV. Canadian health&care mall in detail No differences in mean inspiratory flow, inspiratory work, and V02 were noted. Dries and associates compared PSV levels of 10,20, and 30 cm H2O with SIMV rate of 8 during the postoperative period in patients undergoing coronary artery bypass grafting. All patients were on 5 cm H2O of PEEP. Data obtained after 20 min on each mode showed no differences in hemodynamic parameters between SIMV and PSV of 30 cm H2O. As the level of pressure support decreased, heart rate, mean arterial pressure, PAOP, and central venous pressure increased. No change in SVR, PVR, and Cl were noted among various modes. While РаОг, РаСОг, and PVO2 increased on PSV, shunt fraction and V02 remained unchanged.
The ultimate goal of any ventilatory support mode is to provide adequate Vt with acceptable f and, thus, necessary Ve to maintain and preserve oxygen transport and tissue oxygenation. This would obviously require an optimum hemodynamic profile otherwise tissue oxygenation will be jeopardized. It appears to us, therefore, that applying predetermined and fixed levels of pressure support in patients with variable cardiorespiratory status and comparing the results with other ventilatory modes will not provide useful information. The level of pressure support in each patient should be adequate to maintain acceptable values of f, Vt, and Ve, as determined individually for each patient while on standard and conventional modes of ventilatory support. Once the optimum level of PSV has been determined, the respiratory, hemodynamic, and oxygen transport parameters could then be obtained to determine whether the given level of pressure support is an acceptable alternative mean of ventilatory support. Our data show that, indeed, in stable critically ill patients, using this strategy will provide adequate level of ventilation with lower airway pressures and with hemodynamic and oxygen transport profiles equal to those achieved on ACV. Although similar results are obtained with a lower rate of SIMV, due to significant increases in f and decreases in Vt, the long-term results may not be as favorable as with PSV. Studies using similar strategies for longer periods on SIMV and PSV, however, are needed to assess these issues satisfactorily.