Pressure Support Compensation and Demand Continuous Positive Airway Pressure: Subjects Work and Pressure Support

Pressure Support Compensation and Demand Continuous Positive Airway Pressure: Subjects Work and Pressure SupportSubjects Work and Pressure Support
To determine the usefulness of the relationship of pressure support and Vt/Ti derived from the mechanical model, trans-pulmonary inspiratory work was measured in four normal subjects. Each subject was trained to breathe at a Vt of500 ml at rates of 15,25, and 35 breaths per minute with I:E ratios of approximately 1:2. Throughout the study, subjects monitored volume using an oscilloscope (Tectronix 5113) which displayed the integrated flow signal. Respiratory rate and I:E ratio were held constant with the aid of a metronome. Each subject breathed through the same circuit as was used as in the first part of the study (Fig 1B). In addition, esophageal pressure (Pes) was measured using an esophageal balloon. A 10-cm latex balloon attached to a perforated polyethylene catheter was positioned transnasally into the esophagus and filled with the appropriate volume of air. Subjects were seated in an upright position and breathed through a mouthpiece into the circuit with an open glottis and occluded nares.

Esophageal pressure, airway flow, and volume were recorded simultaneously, and Vt/Ti was calculated as the mean of ten breaths. Inspiratory work done by the subject on the lung and airway was measured as the integral of the area enclosed by the plot of Pes and pulmonary volume during inspiration. The beginning of inspiration was defined as the point at which either inspiratory flow began (when Paw = 0) or when the sign of the Paw became negative prior to inspiratory flow (during resistive breathing). Inspiratory work per liter of ventilation (Wi/L) was calculated from the pressure-volume curves of ten representative breaths during each condition of measurement. At each respiratory rate, subjects breathed through a mouthpiece alone and through each of the three endotracheal tubes plus the ventilator CPAP circuit (CPAP = 0). For each tube size and level of mean inspiratory flow, pressure support was increased from zero in 2-cm H20 increments up to 6 cm H20 above the level predicted by the data from the mechanical lung.