Bilevel PAP delivers an inspiratory pressure, called IPAP, and an expiratory pressure, called EPAP. Pressure support is the difference between them: PS = IPAP − EPAP. The values are usually expressed in centimetres of water pressure, written cmH₂O.
For example, a prescription written as 16/8 means IPAP 16 and EPAP 8, giving pressure support of 8 cmH₂O. This is an explanation of the notation, not a suggested setting.
What EPAP contributes
EPAP is the lower pressure during exhalation and the baseline from which inspiratory support rises. It helps maintain upper-airway patency. In some respiratory conditions it also affects lung volume and oxygenation.
EPAP is not wasted pressure. Lowering it for comfort can change how well obstruction is controlled. Conversely, a higher baseline is not automatically better and can affect comfort, leak and breathing mechanics.
What pressure support contributes
The increase from EPAP to IPAP assists inspiration. Depending on the person's respiratory mechanics and effort, it can reduce breathing work and help ventilation.
The same pressure-support difference does not deliver the same breath size to everyone. Leak, resistance, lung and chest-wall mechanics, respiratory drive and timing all influence the result. A displayed pressure is therefore not proof that carbon dioxide is being adequately cleared.
Why subtraction helps you read the prescription
Keeping IPAP fixed while raising EPAP reduces the pressure-support gap. Raising both by the same amount preserves the gap but changes the baseline. These changes are physiologically different even if one headline number looks familiar.
Some auto-adjusting prescriptions specify pressure limits and a support value or range instead of two fixed numbers. Read the mode and full prescription rather than assuming every menu uses the same arrangement.
The mode adds another layer
Spontaneous bilevel follows the person's breathing. A spontaneous-timed mode adds a backup timing function. Volume-assured modes adjust support within limits toward a prescribed ventilation target. Neither the pressure arithmetic nor a mode name establishes a device's life-support suitability.
If breathing feels out of step, leak increases or symptoms persist, bring the full prescription and download to the treating team. Prescribed pressures and timing controls should be reviewed together, not changed from the subtraction alone.
Related reading: Bilevel PAP backup rate: what the number means