How to Read the Numbers on a BiPAP Machine

A BiPAP machine displays several numbers that describe how much air pressure it delivers, how well your breathing responds, and whether the mask seal is holding. The two most prominent figures are IPAP and EPAP, the high and low pressures the machine cycles between as you breathe in and out. But beyond those, most BiPAP screens and data reports show leak rates, tidal volumes, breathing rates, event counts, and usage hours. Each number tells you something specific about how your therapy is going, and understanding what they mean can help you spot problems before your next clinic visit.

IPAP and EPAP

The defining feature of a BiPAP (bilevel positive airway pressure) machine is that it delivers two different pressure levels. IPAP stands for inspiratory positive airway pressure, the higher pressure the machine pushes when you breathe in. EPAP stands for expiratory positive airway pressure, the lower pressure it drops to while you breathe out. Both are measured in centimeters of water pressure, abbreviated cm H₂O, and you will see them displayed as two numbers, something like 14/8 or 18/6.

The original BiPAP design works by superimposing a pressure boost on top of a baseline continuous pressure, so EPAP is the baseline and IPAP is the boosted level during inhalation.¹ A standard CPAP machine, by contrast, delivers one fixed pressure throughout the breathing cycle. The reason BiPAP uses two pressures is that many people find it uncomfortable or difficult to exhale against a high constant pressure. Having the machine drop to a lower EPAP during exhalation makes breathing feel more natural, and the gap between the two pressures does actual therapeutic work.

Pressure Support and Why the Gap Matters

The difference between your IPAP and EPAP is called pressure support. If your machine is set to an IPAP of 16 and an EPAP of 8, your pressure support is 8 cm H₂O. This number is clinically significant because it represents how much extra push the machine gives your lungs during each breath. A larger gap means the machine is doing more work on your behalf, helping your respiratory muscles and improving the clearance of carbon dioxide from your blood.²

For people using BiPAP primarily for obstructive sleep apnea, the EPAP is usually the number doing the heavy lifting in keeping the airway open, much like a standard CPAP would. The IPAP then adds a boost to ease breathing. For people with conditions like COPD or obesity-hypoventilation syndrome, the pressure support gap becomes the more critical therapeutic element. Research on high-intensity noninvasive ventilation in COPD patients has found that it is specifically the high-pressure component that drives improvements in blood gases, more so than other settings like backup breathing rate.³ So when you look at your pressure numbers, understand that it is not just the absolute values that matter, it is the spread between them.

Breathing Mode

Most BiPAP machines display which mode they are operating in, and this affects how the machine decides when to switch between IPAP and EPAP. There are three main modes:⁴

S (Spontaneous): The machine detects when you start breathing in and delivers IPAP, then detects when you start breathing out and drops to EPAP. You control the timing entirely.
ST (Spontaneous-Timed): Same as spontaneous, but with a safety net. If the machine does not detect a breath within a set time window, it automatically delivers a breath at the IPAP level for a set inspiratory time. The backup rate, displayed as breaths per minute (BPM), is the minimum number of breaths the machine will ensure you take.
T (Timed): The machine controls everything. Both the respiratory rate and the inspiratory time are fixed, and the machine cycles between IPAP and EPAP on its own schedule regardless of your breathing effort.

Most people using BiPAP at home for sleep-related conditions are on ST mode, which lets you breathe naturally but steps in during apneas or periods when your breathing drive weakens during deep sleep. If you see a backup rate number on your machine (say, 12 BPM), that means the machine will deliver at least 12 machine-initiated breaths per minute if you are not triggering breaths on your own. A high number of machine-triggered breaths in your data report can indicate that your body is not initiating enough breaths during sleep, which is information worth flagging for your provider.

Tidal Volume and Minute Ventilation

Many modern BiPAP machines display estimated tidal volume and minute ventilation. Tidal volume is the amount of air moved in a single breath, measured in milliliters (mL). Minute ventilation is the total volume of air moved in one minute, usually in liters per minute (L/min). It is essentially tidal volume multiplied by breathing rate.

These numbers tell you whether you are actually getting meaningful air exchange. A tidal volume that is consistently low might mean the pressure support is not high enough, the mask is leaking, or you are taking very shallow breaths. Research in patients with obesity-hypoventilation syndrome found that when volume targeting was activated on a BiPAP, both mean tidal volume and total ventilation increased, and nighttime carbon dioxide levels improved.⁵ Tidal volume numbers in the range of roughly 400 to 600 mL are typical for adults, but what counts as adequate for you depends on your body size and your underlying condition. If your machine’s data report consistently shows tidal volumes that look low, bring it up with your sleep or pulmonary provider rather than adjusting settings yourself.

Leak Rate

Leak rate is one of the most important numbers on your BiPAP screen, and also one of the trickiest to interpret. It is measured in liters per minute (L/min) and represents how much air is escaping from the mask system. Every BiPAP mask has built-in exhaust vents that create intentional leak, which is necessary to flush out the carbon dioxide you exhale. The number you see on your screen or data report may or may not include that intentional component, and this varies by manufacturer.

A bench study testing multiple ventilator devices found that manufacturers do not all measure or report leaks in the same way. Some devices subtract the expected intentional leak and show you only unintentional leak (the actual mask seal problem), while others report the total of both intentional and unintentional leak combined. At least one device estimated leaks only during the lower expiratory pressure phase, making its numbers misleading compared to other machines.⁶ This means you cannot simply compare a leak number from one brand’s machine to another brand’s threshold without understanding what each is actually counting.

As a general rule, a leak reading that stays within the range your machine labels as acceptable (often shown with a green indicator or a “normal” range on the data screen) means your mask seal is fine. A persistently high or rising leak reading, especially one flagged in yellow or red, usually points to a problem: the mask has shifted, the cushion is worn, or your mouth is opening during sleep if you use a nasal mask. Significant unintentional leak reduces the machine’s ability to deliver accurate pressures and can undermine the entire therapy.

AHI and Respiratory Event Counts

Your BiPAP machine tracks respiratory events during the night and calculates a residual AHI (apnea-hypopnea index), which counts the average number of apneas and hypopneas per hour of use. If you started BiPAP therapy because of obstructive sleep apnea, this number tells you how well the machine is controlling your airway events. The goal is usually to get the residual AHI below 5, and ideally much lower.

Some machines break this number down further, distinguishing between obstructive events (where your airway physically collapses), central events (where your brain temporarily stops sending the breathing signal), and hypopneas (partial blockages that reduce airflow). Seeing which type dominates matters. A case report described a patient whose BiPAP showed a persistent residual AHI of about 13, with most events being obstructive despite optimized compliance and multiple interventions, ultimately requiring a combination therapy approach.⁷ A residual AHI that stays stubbornly elevated despite good mask seal and adequate usage hours is a sign that your current settings or mask may need to be changed, not that the machine is failing.

Keep in mind that the machine’s built-in event detection algorithms are estimates. Ventilator software helps clinicians gauge ventilation, tidal volume, leaks, and triggering, but independent validation of these signals is still limited.⁸ The machine is not performing a sleep study every night. It is making its best guess based on airflow patterns. Treat these numbers as useful trends rather than lab-quality measurements.

Usage Hours and Compliance

Every BiPAP machine logs how many hours you use it each night and over time. This number matters both for your health and, in the United States, for insurance purposes. Many insurance plans and Medicare require that you use your PAP device for at least four hours per night to maintain coverage and keep the equipment. A technical expert panel from major sleep and respiratory medicine organizations identified this four-hour threshold as one of the most significant coverage barriers, because it does not reflect actual clinical need and can result in patients losing access to their machine.⁹

In research settings, adherence is typically defined as an average of four or more hours of daily use, while nonadherence is anything below that threshold.¹⁰ Your machine will display nightly usage and often a rolling average, like the number of hours over the past 30 or 90 days. If your insurance company or equipment provider downloads your data, the compliance percentage they report is usually the proportion of nights where you hit that four-hour minimum. Falling short even occasionally can affect your standing, so it is worth keeping an eye on this number and understanding how the average is calculated. If you take the mask off at 3 a.m. most nights, the data will show it.

From a health standpoint, more hours are generally better. Four hours is an administrative threshold, not a biological one. If you sleep seven or eight hours, wearing the machine for only four means you are untreated for the other half of the night.

Comfort Settings You Might See

Depending on your machine, you may notice settings or displayed values labeled rise time, pressure relief, or max/min inspiratory time. These affect how the therapy feels rather than the core pressure targets, but they still show up in your data and are worth understanding.

Rise time controls how quickly the machine ramps from EPAP up to IPAP when you inhale. A short rise time means the pressure jumps up fast, which delivers the full boost sooner but can feel like a gust of wind. A longer rise time eases into the higher pressure more gradually, which some people find more comfortable. Adjusting rise time, pressure relief, and maximum and minimum IPAP durations can improve both comfort and how well the machine stays synchronized with your natural breathing pattern.⁴

Pressure relief (sometimes called EPR, Flex, or similar brand-specific names) slightly lowers the pressure during early exhalation to make breathing out feel easier. If this is activated on your machine, you might see a setting from 1 to 3 indicating the degree of relief. It does not change your prescribed EPAP in a meaningful therapeutic way; it is a comfort feature that reduces the sensation of exhaling against resistance.

Max and min inspiratory time set boundaries on how long the machine stays at IPAP during a single breath. If your machine holds IPAP for too long or too briefly relative to your natural breathing rhythm, you end up fighting the machine, which increases discomfort and the work your muscles have to do. These settings are generally adjusted during a titration study or by a clinician reviewing your data, not something you would typically change on your own.

Advanced Modes and Automatic Pressure Adjustment

Some BiPAP machines offer modes where the pressure is not fixed but adjusts automatically. The most common is AVAPS (average volume-assured pressure support), which sets a target tidal volume and then automatically raises or lowers the IPAP to hit that target. If your breathing becomes shallower, the machine increases the pressure; if you are breathing deeply, it backs off. This is a hybrid approach that combines pressure and volume targeting.¹¹

If your machine runs in an AVAPS or similar auto-titrating mode, the IPAP number you see on the screen may fluctuate breath to breath rather than sitting at a fixed value. Your data report will typically show a range or an average IPAP for the night rather than one set number. You might also see the target tidal volume displayed, which is the goal the machine is trying to achieve. When reviewing your data, pay attention to whether the machine is spending most of its time near the maximum IPAP allowed. If it is constantly hitting the ceiling pressure and your tidal volume is still below target, that suggests the upper limit may need to be raised, or there is another issue like significant leak undermining delivery.

Waveforms and What They Show

Some machines and most clinical data-download software can display waveforms: graphical tracings of pressure, flow, and volume over time. These squiggly lines are mainly a clinical tool, but if you have access to them through software like OSCAR (an open-source CPAP/BiPAP data viewer), they can give you a visual window into what happens while you sleep.

A pressure waveform shows the machine cycling between EPAP and IPAP. If the tracing looks smooth and rectangular, with clean transitions, the machine is delivering stable pressures. Irregularities in the tracing can suggest problems. A flow waveform shows air moving in and out. Asynchrony between you and the machine, where the machine’s pressure delivery is out of step with your breathing effort, shows up as specific distortions in these tracings. Anything that disrupts the coordination between your breathing and the machine’s cycling results in asynchrony, which increases discomfort and makes your respiratory muscles work harder than they should.¹²

For most home users, waveform analysis is not something you need to do nightly. But if you are experiencing unexplained discomfort, frequent awakenings, or a feeling that the machine is “fighting” you, waveform data can help a clinician pinpoint the problem, whether it is trigger sensitivity, inspiratory time mismatch, or cycling issues. If you use data-viewing software, look for patterns: breath-to-breath consistency is good, wild swings or irregular shapes suggest something is off.

Putting the Numbers Together

No single number on a BiPAP machine tells the full story. The pressures are the prescription. The leak tells you whether that prescription is actually being delivered. The tidal volume tells you how much air you are moving. The AHI tells you whether your airway events are controlled. The usage hours tell you (and your insurer) whether you are getting enough therapy time. And the comfort settings shape the feel of the experience.

When reviewing a data report, look for patterns across nights rather than fixating on a single night’s numbers. One night with a high leak probably means your mask shifted. A week of high leaks means the cushion needs replacing or the fit needs adjusting. A slowly climbing residual AHI over months could reflect weight gain, positional changes, or a need for pressure adjustment. Studies in COPD patients using high-intensity BiPAP have shown that sustained use over years can keep blood gases improved and reduce hospitalization rates, but only when settings are kept appropriately calibrated.¹³ The numbers are a feedback loop: the machine tells you how therapy is going, and you and your clinician use that information to keep it working.

When the Numbers Do Not Match How You Feel

One frustrating scenario is when your BiPAP data looks perfect on paper, with low leak, low AHI, good tidal volumes, and solid usage hours, but you still feel lousy in the morning. This disconnect is more common than people expect. The machine’s event detection algorithms are approximations, not polysomnography-grade measurements. A machine might undercount respiratory effort-related arousals (brief awakenings that fragment sleep without meeting the formal definition of an apnea or hypopnea). It might not capture positional issues or sleep-stage effects that disrupt your rest in ways that do not show up in the airflow signal.

The reverse also happens: someone whose data shows a residual AHI of 3 or 4 and moderate leak might feel great because the remaining events are brief and the leak is not at a level that actually compromises pressure delivery. The numbers are tools, not verdicts. They work best when you combine them with your subjective experience: how rested you feel, whether you wake with headaches or a dry mouth, whether you recall the mask coming off. If the numbers and your symptoms do not align, that is worth a conversation with your provider, and bringing a printout of your data makes that conversation far more productive than trying to describe how things “feel” in the abstract.