Pressure Support Compensation and Demand Continuous Positive Airway Pressure: Results
The net added inspiratory work (Waw) increased progressively with decreasing size of the endotracheal tube and with increasing Vt/Ti (Fig 3). Each 1-mm decrease in the tubes diameter resulted in a 67 to 100 percent increase in work. Adding the ventilator circuit increased the work further. The proportion of added work due to the endotracheal tube alone, when compared to the added work for the tube and ventilator together, was a function of the tubes size. On average, a 9-mm tube contributed 50 percent of the total added work, with 8-mm and 7-mm tubes contributing 60 and 70 percent, respectively.
The net additional Waw decreased progressively with increasing pressure support (Fig 4). The optimal level of pressure support (resulting in Waw = 0) was plotted as a function of Vt/Ti for each tube size for each of the three ventilators tested. A linear regression was fitted through these points for each tube size (Fig 5). There was a high level of correlation between optimal pressure support and Vt/Ti. In each condition of tracheal tube size and Vt/Ti, there was never more than a 2-cm H20 variation in optimal pressure support among the three ventilators tested. Reading here
Subjects Work and Pressure Support
Figure 6 shows the mean Wi/L for the four subjects as a function of the endotracheal tubes size and Vt/Ti. During breathing by mouthpiece, the mean Wi/L increased from 0.024 kg-m (range, 0.013 to 0.031 kg-m) at a mean Vt/Ti of 0.38 L/sec to 0.030 kg-m (range, 0.022 to 0.040 kg-m) at a mean Vt/Ti of 0.68 L/sec. The endotracheal tube and ventilator circuit increased work significantly compared with mouthpiece breathing, and this increase ranged from 54 percent (0.013 kg-m/L) with a 9-mm tube and mean Vt/Ti of 0.38 L/s to 240 percent (0.072 kg-m/L) with a 7-mm tube and mean Vt/Ti of 0.68 L/s.
At each respiratory rate, each endotracheal tube, together with the ventilator circuit, increased the inspiratory work significantly when compared with mouthpiece breathing. This added work increased progressively with increased inspiratory flow demand and decreasing tube size, as predicted by observations with the mechanical lung.
For each condition of tube size and respiratory rate, increasing pressure support decreased the inspiratory work. The optimal pressure support was that level at which Wi/L was closest to that while breathing through a mouthpiece (Fig 7). The optimal level of pressure support was plotted as a function of Vt/Ti for each subject breathing through each endotracheal tube and was remarkably similar to that pressure support predicted by the relationship derived using the mechanical breathing model as shown in Figure 8.
The peak change in Pes also was evaluated as an indicator of the patients effort. With pressure support equalling zero, this also increased with decreasing endotracheal tube diameter and with increased Vt/Ti. At those levels of pressure support which optimally compensated for added measured inspiratory work, peak change in Pes was similar to that while breathing through a mouthpiece alone (Fig 9).
Figure 3. Net additional work (Waw) to mechanical respiratory system model breathing through 7-mm, 8-mm, and 9-mm endotracheal tubes (ETT) alone (open bars) and through endotracheal tubes and ventilator circuit (CPAP = 0; pressure support = 0) together (hatched bars) at five respiratory rates.
Figure 4. Net additional work by mechanical respiratory system (positive Waw) due to endotracheal tube (ETT) and ventilator circuit with increasing pressure support for 9-mm, 8-mm, and 7-mm endotracheal tubes at respiratory rate of 20/min and Vt of 0.5 L.
Figure 5. Relationship between mean inspiratory flow and “optimal” level of pressure support (PS) necessary to compensate for additional inspiratory work due to ventilator circuit and 7-mm (A), 8-mm (B), and 9-mm (C) endotracheal tubes.
Figure 6. Inspiratory work per liter in four subjects (mean + SE) while breathing on open mouthpiece and each endotracheal tube with ventilator circuit at each of three respiratory rates. Asterisks indicate p<0.005 by paired Mest, compared with mouthpiece breathing.
Figure 7. Plot of change in Pes and pulmonary volume for subject 3 during spontaneous breathing (rate = 25/min; Vt = 0.5 L). Data points represent measurements made at 0.1-s intervals during subjects breath. Solid lines and broken lines trace periods of inspiration and expiration, respectively. Hatched area signifies inspiratory work. Plots are shown for representative respiratory cycles while breathing through mouthpiece (A), through 7-mm endotracheal tube and ventilator circuit without pressure support (B), and through 7-mm endotracheal tube and ventilator circuit with level of pressure support (6 cm H20) sufficient to result in Wi/L equivalent to that during mouthpiece breathing (C). Inspiration is denoted by positive changes in Pes.
Figure 8. Comparison of data from subjects to data from mechanical model. Data points represent plot of optimal pressure support vs Vt/Ti for four subjects breathing through 7-mm (top), 8-mm (center), and 9-mm (bottom) endotracheal tube and ventilator circuit Line is relationship derived from mechanical model, as shown in Figure 5.
Figure 9. Mean peak changes in Pes in four subjects while breathing through mouthpiece (open bars) and while breathing through each endotracheal tube and CPAP circuit without pressure support (hatched bars) and with optimal pressure support (<shaded bars). Single asterisks indicate p<0.05 compared with mouthpiece (paired f-test). Double asterisks indicate not significant compared with mouthpiece.