Pressure Support Compensation and Demand Continuous Positive Airway Pressure
Then an intubated patient breathes spontaneously through an intermittent mandatory ventilation (IMV) or demand continuous positive airway pressure (CPAP) ventilator circuit, both the resistance of the endotracheal tube and the ventilator circuit can contribute to the inspiratory work of breathing. The level of this additional work increases as the diameter of the endotracheal tube decreases and minute ventilation increases. Increased inspiratory work can contribute to respiratory muscle fatigue and interfere with successful weaning from mechanical ventilation.
Pressure support is a mechanical ventilator option which applies a predetermined amount of positive pressure to a patients airway during the inspiratory phase of any spontaneous breath while in the CPAP or IMV mode. It has been proposed that pressure support may be used to compensate selectively for the increased inspiratory work due to breathing through an endotracheal tube and ventilator circuit; however, guidelines for use of pressure support for this purpose are not available. The intention of this study was to determine whether pressure support can compensate for the added inspiratory work due to breathing through endotracheal tubes and a ventilator demand-valve gas delivery system and to establish guidelines for its use.
Materials and Methods
Mechanical Model of Lung and Computation of Inspiratory Work
A reciprocating pump (Harvard Apparatus Co) was used as a mechanical model to simulate a patients spontaneous breathing effort (Fig 1A). The “mouth” of this model was connected to an endotracheal tube alone or to endotracheal tube and a ventilator demand CPAP circuit. Air flow (V) was measured proximal to the endotracheal tube using a screen pneumotachygraph (Electronics for Medicine, adult-size) which was calibrated to flows of 0 to 1.5 Us. Volume (V) was integrated from the airway flow signal using a respiratory integrator (Hewlett-Packard 8815A). Airway pressure (Paw) was measured between the pneumotachygraph and endotracheal tube with a pressure transducer (Validyne DP45).
Inspiratory work was computed as the integral of the product of Paw and V during inspiration. Inspiratory work measured when Paw was negative represented work done by the simulated respiratory system (Wrs). Inspiratory work measured during times of positive Paw represented work done on the respiratory system by the Ventilator (Wv). Net airway work (Waw) was defined as Wrs minus Wv. Airway pressure, flow, volume, and work were recorded on a four-channel time-based strip-chart recorder (Hewlett-Packard 7754A). Inspiratory work per liter of ventilation (Wi/L) and mean inspiratory flow (Vt/Ti) were calculated as the mean of ten cycled breaths during each condition of measurement.
Figure 1. Apparatus for measurement. A. Mechanical system used to simulate spontaneous breathing effort; Paw and V were recorded. B. Trained subjects breathed through same system while airway flow and Pes were recorded. All measurements were displayed on multichannel strip chart.
Tags: airway, endotracheal tubes, pressure