How PAP Therapy Actually Works: CPAP, BiPAP, and Pressure Support Explained

Let's start with anatomy. When you breathe, air travels from your nose or mouth, through the back of your throat (the pharynx), and down into your lungs via the trachea (windpipe).

Here's the critical thing: your trachea is a rigid tube, reinforced with C-shaped cartilage rings. It holds its shape no matter what. But the pharynx above it? That's a flexible, collapsible tube made of soft tissue and muscle. When you're awake, your throat muscles actively hold this tube open. During sleep, especially during REM sleep, that muscle tone drops significantly. The tube becomes floppy, like a wet paper towel roll.

For people with obstructive sleep apnoea or upper airway resistance syndrome, this floppy section of the airway narrows or collapses during sleep, partially or completely blocking airflow. That's the entire problem PAP therapy is designed to solve.

Cross-section of the pharynx. The orange ring represents surrounding soft tissue. Left: muscles hold the airway open while awake. Centre: during sleep, gravity and relaxed muscles collapse the flexible tube. Right: PAP pressure inflates the airway from inside, like a balloon.

Here's something most people don't realise: breathing in is what makes your airway collapse, not breathing out. When you inhale, your diaphragm (the big muscle below your lungs) contracts and pulls downward. This creates negative pressure inside your chest, like pulling back a syringe plunger. That suction is what draws air in through your nose and down into your lungs.

The problem? That same suction also pulls on the walls of your pharynx. Remember, the pharynx is a flexible tube. Negative pressure inside a flexible tube makes it want to collapse inward, just like sucking too hard on a paper straw.

It gets worse. As the airway narrows, air has to flow faster through the remaining opening (think of putting your thumb over a garden hose). Faster airflow through a narrow passage creates even lower pressure on the walls, a phenomenon called the Bernoulli effect. This creates a vicious cycle: narrowing leads to faster flow, which leads to lower pressure on the walls, which leads to more narrowing. Left unchecked, this cascade ends in full airway collapse, an apnoea.

This is why snoring tends to get louder before an apnoea event. The vibrating tissue is narrowing, airflow is speeding up, and the physics of the situation are pulling toward total collapse.

EPAP stands for Expiratory Positive Airway Pressure. It is the baseline pressure that your machine maintains at all times, including during exhalation. On a CPAP machine, EPAP and CPAP are effectively the same thing (there is only one pressure). On a BiPAP machine, EPAP is the lower of the two pressure settings.

EPAP is the foundation of PAP therapy. Its job is deceptively simple: keep positive pressure inside the airway at all times, so the airway never has the chance to collapse. Think of it as a pneumatic splint, like inflating a tent from inside. The fabric might be floppy on its own, but with positive pressure inside, it holds its shape even against wind and gravity.

Red arrows show forces trying to collapse the airway (gravity, tissue weight, inspiratory suction). Blue arrows show EPAP pushing outward from inside. As long as the outward force exceeds the inward forces, the airway stays open.

The crucial point: EPAP doesn't just help during exhalation. It provides continuous positive pressure that counteracts all the forces trying to collapse your airway. Without it, every single breath you take during sleep is a battle between your inspiratory muscles (pulling inward) and the structural integrity of your pharynx (trying to stay open). EPAP changes the equation by adding outward pressure from inside the tube.

CPAP stands for Continuous Positive Airway Pressure. The word "continuous" tells you everything: one pressure level, maintained continuously during both inhalation and exhalation. If your CPAP is set to 12 cmH2O, you get 12 cmH2O whether you're breathing in or breathing out.

For most people with obstructive sleep apnoea, this is all that's needed. The continuous pressure splints the airway open, preventing collapse. You breathe normally against a pressurised airway. Your inspiratory muscles still do the work of pulling air in, but now they're pulling air through an open tube instead of fighting a collapsing one.

The tradeoff? You're also exhaling against that same pressure. Many people describe it as "breathing out against a wall." This is uncomfortable for some, which is why most CPAP machines offer a comfort feature:

CPAP works well for many people, but not everyone. Some patients need something more, and the reasons usually fall into a few categories:

• Persistent flow limitation: Even with CPAP pressure high enough to prevent apnoeas, the airway can still narrow partially. This flow limitation means you're getting air through, but not as much as your body needs with each breath. Increasing the CPAP pressure doesn't always fix this because higher pressure during exhalation makes it harder to breathe out.
• High work of breathing: Some patients have to work very hard to pull air in against the continuous pressure and through a partially narrowed airway. Their respiratory muscles fatigue, and they hypoventilate, moving less air with each breath than they should.
• Neuromuscular or chest wall issues: Conditions like obesity hypoventilation, COPD, or neuromuscular diseases can reduce the strength of the breathing muscles themselves. These patients need active assistance with ventilation, not just a splint.
• Comfort intolerance: Some people simply cannot tolerate exhaling against high pressure. They need a lower expiratory pressure (EPAP) but still need high inspiratory pressure to keep the airway open and move enough air.

For all of these situations, the answer is the same: use two different pressures. A lower pressure during exhalation (to keep the airway splinted and make exhaling comfortable) and a higher pressure during inhalation (to push extra air through and support ventilation). That's BiPAP.

BiPAP (Bilevel Positive Airway Pressure) delivers two distinct pressure levels that alternate with your breathing:

The machine continuously monitors your airflow. When it detects the start of inspiration (you begin breathing in), it swings up to IPAP. When it detects expiration (you start breathing out), it drops back to EPAP. This happens automatically with every breath, typically 12-20 times per minute.

Top: CPAP delivers a constant 12 cmH2O regardless of breath phase. Bottom: BiPAP oscillates between EPAP (8 cmH2O during exhalation) and IPAP (16 cmH2O during inhalation). The difference between them is Pressure Support (PS = 8). The purple line shows the actual pressure delivered to the patient over time.

Notice the key difference: with CPAP, you breathe in and out against the same pressure. With BiPAP, the machine works with your breathing rhythm. Lower pressure when you breathe out (easier exhalation, but the airway stays splinted), higher pressure when you breathe in (actively assisting your inhalation).

Pressure Support (PS) is the difference between IPAP and EPAP. If your IPAP is 16 and your EPAP is 8, your PS is 8 cmH2O. This number is arguably the most important setting on a BiPAP machine, because it determines how much the machine actively helps you breathe.

Remember our earlier distinction: EPAP is a splint, it holds the airway open. PS is a ventilatory assist. It actively pushes extra air volume into your lungs during each breath. Here's what that means in practice:

Left: breathing on CPAP, your respiratory muscles do all the work. Right: with pressure support, the machine pushes extra air during inhalation (green area), increasing tidal volume. The dashed blue line shows the CPAP-equivalent breath for comparison. More volume per breath means better ventilation.

Here's what happens during a single breath cycle on a BiPAP machine, step by step:

Now that you understand the mechanics, here's a quick reference for the settings you'll see on your machine or in your data:

Understanding how PAP therapy works isn't just background knowledge. It directly helps you interpret your data and have productive conversations with your sleep physician:

• High flow limitation despite good EPAP? Your airway is splinted, but air isn't getting through efficiently. You may benefit from BiPAP with pressure support to push through the restriction. This is especially relevant for UARS patients who have persistent flow limitation without frank apnoeas.
• Flow limitation worse during REM? Muscle tone is at its lowest during REM sleep. The airway is floppiest, and the splinting pressure needed is highest. Many auto-titrating machines raise pressure during REM, and seeing this pattern in your data helps explain why.
• Better numbers in the second half of the night? Auto-titrating machines learn throughout the night. If your flow limitation improves after hour 2-3, the machine may have found the right pressure. Your H1/H2 split data can show this.
• High EPR causing flow limitation? EPR reduces expiratory pressure for comfort, but too much EPR can let the airway narrow between breaths. If you see significant flow limitation with EPR 3, try reducing to EPR 1 or 2 (with your doctor's guidance).
• Understanding NED scores? Negative Effort Dependence directly reflects the physics we discussed. A high NED score means breathing harder is producing less airflow (the paper straw collapsing). This is a direct signal that EPAP may be too low or that PS could help overcome the restriction.

The better you understand the physics, the more actionable your data becomes. You're not just seeing numbers anymore. You're seeing the story of how air moves (or doesn't move) through your airway, and why.