1. Volume-Assured Pressure Support Ventilation (VAPSV)
Marcelo Britto Passos Amato, M.D., Carmen Silvia Valente Barbas, M.D., Jorge Bonassa, Paulo Hilario Nascimento Saldiva, M.D., Walter Araujo Zin, M.D., Carlos Roberto Ribeiro de Carvalho, M.D. 
CHEST 
Volume 102, Issue 4, Pages (October 1992)
DOI: /chest
Copyright © 1992 The American College of Chest Physicians Terms and Conditions

2. FIGURE 1
Tracings recorded during volume-assured pressure support ventilation (VAPSV), at the moment when the patient was receiving an intravenous dose of midazolam. Note that the patient's effort (reflected by the negative inflections on Ptr at the beginning of inspiration) decreased progressively during the recording period. The result was a progressive reduction in the demand flow arising from the PS source (reflected by the progressive decrease in the peak flow) at the same time that the controlled “square wave” flow (CF) became progressively evident (large arrow). Thus, on the first cycle of the recording, the preset CF was hidden by the high demand flow delivered, and a flow and pressure curve resembling the conventional PSV was delineated (an exponentially decreasing flow). However, as shown right after—and differently from PSV—a fixed CF was imperceptibly working in parallel with the demand flow until the moment when the prearranged VT was completed (the small arrows on this first cycle are indicating the point when the preset VT was completed and the CF was cut off; after this point, a short inspiratory pause followed—0.15 s—during which the PS source remained solely activated). In the following cycles, as the demand flow fell down to the preset CF, but the accumulated volume (ΔV) had not yet exceeded the VT value prearranged on the ventilator, the fixed CF was maintained activated and appeared as a squarely shaped protuberance on the flow curve, prolonging the inspiratory time until the completion of this volume chosen. Note that as long as the CF became “visible” on the flow tracings of VAPSV, a small increment in the peak tracheal pressure followed. This small increment is normally responsible for a peak tracheal pressure close to that achieved during VAV (as long as the same CF rate is used) but never exceeding it. For further considerations about this phenomenon, see Appendix A.
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Copyright © 1992 The American College of Chest Physicians Terms and Conditions

3. FIGURE 2
Individual recording of V (L/min), Ptr (cm H2O), and Pes (cm H2O) of patient 3, presenting a severe airway obstruction. Ptr in this figure represents the “actual” tracheal pressure, obtained by subtracting the pressure losses of endotracheal tube (see “Methods”). In the figure, the tracings representing VAPSV were recorded immediately after the period of the study protocol. As the large VT achieved during VAPSV was excessive, we readjusted the PS level to a lower than that preset during the study protocol. This procedure was taken to illustrate the possibility of decreasing peak Ptr by using VAPSV, without changing the preset VT. After the institution of VAPSV, the intrinsic PEEP (PEEPi—represented in the figure as the gradient of baseline Pes before and after VAPSV) decreased from 23.5 cm H2O to 8 cm H2O, and the PTPb (the shaded areas in the figure are representing the lung component of the PTPb) decreased from 21.2 cm H2O.s to 9.1 cm H2O.s. Despite a higher peak flow achieved during VAPSV (110 L/min during VAPSV vs 75 L/min during VAV), a significant reduction of peak Ptr could be observed (from 43 cm H2O to 35 cm H2O). The better synchrony between the patient and the ventilator during VAPSV can also be inferred through the absence of negative inflections on Ptr traces during inspiration, a phenomenon frequently observed during VAV (arrow). In this figure, the Ptr tracing during VAPSV resembles the plateau of pressure normally achieved during conventional PSV, indicating that the demand flow was being responsible for a great part of the inspired flow. This situation is in marked contrast with that shown in Figure 1.
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Copyright © 1992 The American College of Chest Physicians Terms and Conditions

4. FIGURE 3
Representative tracings of Pes and volume during VAV and VAPSV and the corresponding Campbell's diagrams. The hatched areas represent the time interval during which the work diagram was plotted (VT vs Pes). The area A is representing the work performed to inflate the lung, whereas the area B is the work performed to distend the chest wall. By adding up both areas, we can obtain the total inspiratory work of breathing performed during this cycle (WOB). The relaxation curve of the chest wall (the dashed inclined line) was obtained from the mean of 15 esophageal pressure curves during relaxed expiration and fitted by linear approximation. The dotted line (splitting the work area into A and B) is representing the Pes value collected at the beginning of the inspiratory effort (the elastic recoil pressure of chest wall). The use of VAPSV resulted in a decreased WOB, an increased VT, and a lower Pes at end-expiration, which is representing a lower degree of hyperinflation or intrinsic PEEP (PEEPi).
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Copyright © 1992 The American College of Chest Physicians Terms and Conditions

5. FIGURE 4
The reduction in WOB/L achieved with the use of VAPSV (the WOB/L performed during VAPSV subtracted from the WOB/L performed during VAV—or also the “relief” provided by VAPSV) plotted against the WOB/L performed during the VAV period. Each point (triangles) represents the mean performance calculated from 15 representative breaths in each condition for each patient. The dashed line represents the regression line of these points. In the figure, the highest individual values of WOB/L during VAV were associated with the highest individual values of pulmonary impedance and ventilation requirement. The benefits of VAPSV were especially marked in this situation.
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Copyright © 1992 The American College of Chest Physicians Terms and Conditions