The term Biphasic Positive Airway Pressure (BiPAP) also is fairly self-explanatory. “Bi” means two, and you already know that “PAP” refers to positive airway pressure. Therefore, BiPAP is the application of two separate pressures to the patient via the circuit-and-mask interface. Just as the name suggests, it is “biphasic,” or unfolding in two phases: inspiration and expiration. In BiPAP, the pressure starts at a baseline pressure (similar to CPAP) that remains throughout inspiration and expiration. Where BiPAP differs from CPAP is when the patient starts to initiate a breath: BiPAP changes to a higher pressure where CPAP stays constant.
The most significant difference between CPAP and BiPAP centers around the supportive inspiratory pressure that BiPAP can deliver. By helping augment the patient’s natural tidal volume, BiPAP allows the patient to exhale more carbon dioxide. Similar to invasive mechanical ventilation, BiPAP can aid in correcting high CO2 from respiratory failure. Since BiPAP contributes to the clearance of CO2, it is a method of ventilation. BiPAP can also be referred to as non-invasive ventilation or NIV.
BiPAP or NIV senses that the patient is starting their inspiration and bumps up the pressure to a higher set pressure. At the end of inspiration, as the patient exhales, the pressure drops back down to the baseline lower pressure. Similar to CPAP, the patient breathes against the lower pressure, benefitting from the “splinting” effect and contributing to lung recruitment. The lower pressure is referred to as the Expiratory Positive Airway Pressure (EPAP). The inspiratory phase with the higher pressure helps facilitate the delivery of a larger breath than the patient would have been able to take on their own—it helps “push” the air into the patient’s lungs.
NIV is similar to pressure support (PSV) during invasive ventilation. The higher pressure is referred to as Inspiratory Positive Airway Pressure (IPAP). The higher the pressure delivered, the larger augmentation that is seen in the patient’s breath. In a way, BiPAP/NIV can be seen as the non-invasive version of PSV. Many of the same rules apply. A patient must have an intact drive to breathe and must be actively involved in triggering a breath so NIV can kick up to the higher pressure. PSV helps augment the patient’s normal tidal volume, but allows the patient to control how long they breathe in and the ending of inspiration. NIV is the same way.
Let’s compare PSV and NIV, with the following table:
|Needs an intact drive to breath
|Patient triggered assisted breaths
|Constant pressure at the end of the breath delivered
|Pressure delivered when breath initiated
|Patient controls the length of inspiration and breath cycling