A BiPAP prescription is often written as two numbers separated by a slash: 12/6, 16/8 or 20/10 cmH₂O. The first number is IPAP; the second is EPAP. Subtract the second from the first and you have pressure support.
That arithmetic is simple. What each pressure is doing — and why changing one is not equivalent to changing the other — is the important part.
The three terms in one table
| Term | Full name | When it is delivered | Main role |
|---|---|---|---|
| IPAP | Inspiratory positive airway pressure | During inhalation | Higher pressure that supports inspiration |
| EPAP | Expiratory positive airway pressure | During exhalation and between breaths | Baseline pressure that helps keep the airway/alveoli open |
| PS | Pressure support | Difference between IPAP and EPAP | Amount of inspiratory assistance above baseline |
All are measured in cmH₂O — centimetres of water pressure.
Pressure support is subtraction, not a third pressure
The relationship is:
Pressure support = IPAP − EPAP
Examples:
| Prescription | IPAP | EPAP | Pressure support |
|---|---|---|---|
| 12/6 | 12 | 6 | 6 |
| 16/8 | 16 | 8 | 8 |
| 20/10 | 20 | 10 | 10 |
Some auto-bilevel menus are written differently. Instead of displaying fixed IPAP/EPAP, they ask for minimum EPAP, maximum IPAP and a pressure-support value or range. The algorithm then moves the pressures within that envelope. On a fixed-PS VAuto-style prescription, IPAP remains EPAP plus PS as the baseline moves.
This is why “EPAP 6, PS 4” means IPAP 10 — it does not mean 6 + 4 added again to an existing IPAP.
What EPAP does
EPAP is the pressure present while you breathe out. It behaves like the airway-splinting baseline:
- helps prevent the throat from collapsing at end-expiration;
- supports oxygenation by maintaining end-expiratory lung volume in some respiratory conditions;
- provides the platform from which IPAP rises; and
- influences how hard exhalation feels.
In obstructive sleep apnoea treated with bilevel, residual obstructive apnoeas often indicate that the expiratory baseline may be insufficient — but leak, sleep position and event classification must be checked before assuming a pressure change is required.
EPAP is not “unused pressure.” If it is too low for the airway, obstruction can persist. If it is unnecessarily high, exhalation may become uncomfortable and problems such as leak or air swallowing can worsen.
What IPAP and the pressure-support gap do
IPAP is the higher pressure during inspiration. The change from EPAP to IPAP assists inspiratory flow. A larger PS gap can:
- reduce the muscular effort required to inhale;
- increase delivered tidal volume in some patients;
- support ventilation and carbon-dioxide removal; and
- make high baseline pressure more tolerable than one continuous CPAP pressure.
The same PS does not produce the same tidal volume in every person. Lung compliance, airway resistance, leaks, respiratory drive, sleep stage and patient–device synchrony all matter. That is why pressure support cannot be prescribed from weight or diagnosis alone.
CPAP, BiPAP-S and BiPAP-ST compared
CPAP holds one pressure throughout the breathing cycle. Comfort relief such as EPR briefly lowers expiratory pressure, but its limited drop is not equivalent to the full independently prescribed pressure support of a bilevel device. Read EPR, C-Flex and other relief settings.
BiPAP-S switches between IPAP and EPAP in response to the patient’s own breaths. If the patient does not initiate a breath, spontaneous mode waits.
BiPAP-ST adds a timed safety net. If breathing falls below the set rate, the machine delivers a timed breath using the prescribed inspiratory time and pressures. See BiPAP backup rate explained.
Pressure values therefore cannot be interpreted without the mode. A 16/8 S prescription and 16/8 ST prescription share pressures but not breath timing or safety behaviour.
Why two prescriptions with the same IPAP can feel different
Compare 16/12 and 16/8:
- Both reach IPAP 16.
- The first has PS 4.
- The second has PS 8.
The second provides a larger inspiratory assist but drops farther on expiration. It may feel easier to inhale, yet the lower EPAP may not control the same degree of upper-airway obstruction. Conversely, raising both pressures from 16/8 to 18/10 keeps PS at 8 while increasing the baseline airway-splinting pressure.
Trigger sensitivity, cycle sensitivity, rise time and Ti limits also change the feel without changing the headline IPAP/EPAP numbers. Those controls are explained in BiPAP trigger, cycle, rise time and Ti.
Reading common prescription formats
“BiPAP-S 14/8” Fixed IPAP 14, EPAP 8, PS 6; all breaths patient-triggered.
“BiPAP-ST 18/8, RR 12, Ti 1.0” IPAP 18, EPAP 8, PS 10, backup rate 12 breaths/min, with a timed inspiratory-time parameter.
“VAuto: min EPAP 6, max IPAP 20, PS 4” The algorithm may vary EPAP and IPAP while maintaining a pressure-support gap of 4, bounded by the minimum and maximum settings.
“VAPS/iVAPS/AVAPS” Pressure support may vary within clinician-set limits to pursue a target ventilation or tidal-volume goal. The displayed IPAP may therefore change as the algorithm responds. See TVAPS explained.
When the numbers need review
Bring the prescription and device download to the treating team if you experience:
- persistent obstructive events despite good use;
- a rising clear-airway or central-event index;
- significant bloating or air swallowing;
- inability to exhale comfortably;
- morning headache, persistent sleepiness or signs of hypoventilation;
- large leaks; or
- a feeling that the machine changes pressure before your breath is ready.
The download needs to be interpreted as a system: mode, pressures, leak, event types, respiratory rate, tidal volume/minute ventilation when available, oximetry and symptoms.
Takeaway
IPAP is the inspiratory pressure, EPAP is the expiratory baseline, and pressure support is the difference between them. EPAP mainly holds the airway open; PS supplies inspiratory assistance. A “16/8” prescription therefore carries three useful facts: IPAP 16, EPAP 8 and PS 8 cmH₂O.
Do not change a bilevel prescription from the arithmetic alone. The same numbers behave differently across modes, diseases and synchrony settings, and ST or volume-assured users may depend on them for ventilation.
Primary references: ResMed sleep-lab titration guide; AASM PAP treatment guideline; PAP technology review.