Pressure Support Compensation and Demand Continuous Positive Airway Pressure: Discussion
This study confirms that endotracheal tubes and a ventilator demand CPAP system can markedly increase the inspiratory work of breathing. The additional work rate (Wi/L) due to these added resistances increases with increased flow demand and decreased internal diameter of the tube. Pressure support can be used to compensate for this added work when the level is adjusted for the endotracheal tubes diameter and mean inspiratory flow. The effect of the endotracheal tubes size on airway resistance and work of breathing is substantial. Bolder and associates2 have reported that every 1-mm decrease in the endotracheal tube s diameter results in an increase in work of 34 to 154 percent, depending on the respiratory rate and tidal volume. In the mechanical-lung phase of this study, our findings were similar, with each 1-mm change in the tube s diameter resulting in a 67 to 100 percent increase in Wi/L. The effect of demand CPAP systems on work varies among ventilators. Katz et al have shown that the Puritan-Bennett 7200 ventilator circuit results in 10 to 40 percent increase in additional inspiratory work, depending on inspiratory flow. Consistent with these studies, we found that the relative contributions to added work of an endotracheal tube and the ventilator circuit in this study were a function of tube size, with the proportion of work due to the endotracheal tube ranging from approximately 50 percent for a 9-mm tube to 70 percent for a 7-mm tube. natural breast enlargement cream
Inspiratory work per liter is a good measure of the mechanical properties of the lungs and airways in patients with respiratory failure. Several studies have related the level of inspiratory work to dependence on the ventilator. Peters et al have suggested that mechanical ventilation is necessary when Wi/L is greater than 0.18 kg-m/L and that ventilator independence is associated with Wi/L of less than 0.08 kg-m/L. We have found that patients requiring prolonged mechanical ventilation had a mean Wi/L of 0.090 kg-m/L when they were successfully weaned. During unassisted spontaneous inspiration, there is negative airway pressure generated to overcome the resistances of the demand valve, ventilator circuit, and endotracheal tube. This results in additional inspiratory work. In the present study of normal subjects, the Wi/L during breathing by mouthpiece (range, 0.013 to 0.040 kg-m) was comparable to published “normal” values. For the group of subjects, the mean increase in work due to both an endotracheal tube and demand CPAP together ranged from 0.013 to 0.072 kg-m/L, depending on tube size and Vt/Ti. This represented a 54 to 240 percent increase over baseline values. Since the Vt/Ti of patients being weaned from mechanical ventilation covers a range which was simulated in this study, increases in work of these magnitudes would be expected in clinical practice. Clearly, this degree of added work could affect the success or failure of a weaning trial. This may have particular impact on patients with either marginal respiratory muscle strength or those who already have a high level of respiratory work due to pulmonary parenchymal disease.