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Positive Airway Pressure Therapy |   |
| The positive airway pressure adjuncts are similar to IPPB in that they use positive pressure to increase the transpulmonary pressure gradient and enhance lung expansion. However, unlike IPPB, positive airway pressure therapy does not require any complex machinery, and some techniques don't even use a pressurized gas source. |  |
Definitions and Physiologic Principles
The three positive airway pressure therapy modalities are: positive expiratory pressure (PEP), expiratory positive airway pressure (EPAP), and continuous positive airway pressure (CPAP).
With the PEP modality, patients exhale against a fixed-orifice flow resistor. With a flow resistor, expiratory pressures depend on the patient's expiratory flow. The higher the expiratory flow through an orifice, the greater the expiratory pressure. Patients are taught to actively but not forcefully exhale through the flow resistor in order to maintain a pressure between 10 to 20 cm H20.
PEP does not utilize pressurized gas sources, and like pursed-lip breathing, increases pressure within the airways. This back pressure helps keep the airways open throughout exhalation. PEP helps move secretions into the larger airways, and contributes to resolving atelectasis by preventing airway collapse during expiration.
With EPAP therapy, which is somewhat similar to PEP, a threshold resistor replaces the flow resistor. The pressure generated by a threshold resistor is independent of flow, and can be set to provide specific expiratory pressures independent of flow, usually between 10 to 20 cm H20. The physiologic effects of EPAP are not the same as PEP because the mechanical properties Of threshold resistors differ from flow resistors.
The inspiratory phase of EPAP is similar to spontaneous breathing. As expiration begins, however, airway and alveolar pressures quickly rise to the preset EPAP level. This increases the transpulmonary pressure gradient during exhalation, which prevents airway closure and elevates the FRC.
While PEP and EPAP create expiratory positive pressure only, CPAP maintains a positive airway pressure throughout both inspiration and expiration. CPAP elevates and maintains high alveolar and airway pressures throughout the full breathing cycle, increasing the transpulmonary pressure gradient throughout both inspiration and expiration.
The patient on CPAP breathes through a pressurized circuit against a threshold resistor, with pressures maintained between 5 to 20 cm H20. CPAP uses a source of pressurized gas in order to maintain system pressure throughout the breathing cycle. It is not known exactly how CPAP helps resolve atelectasis, but the following factors contribute to its beneficial effects:
- the recruitment of collapsed alveoli via an increase in FRC
- a decreased work of breathing due to increased compliance or abolition
of auto-PEEP
- an improved distribution of ventilation through collateral channels
(e.g., pores of Kohn)
- an increase in the efficiency of secretion removal