Sleep-disordered breathing with positive airway pressure devices

PAP equipment involves three basic parts:

• A device with a motor,
• a mask that covers either or both the mouth and nose,
• And a tube that connects the device to the mask.

PAP has improved greatly. Major landmarks in the evolution of PAP are the first commercially available units by Respironics in 1985 and the self-sealing “bubble mask” in 1990.

Current noninvasive positive pressure ventilation units are much more complex and may include an air filter, sensors (motor speed, gas volumetric flow rate, pressure, and snore transducer), microprocessor-based controller, data storage, multilingual displays, and humidifier with heated tubing.

• CPAP maintains a continuous PAP throughout inspiration and expiration. Auto adjusting (Auto) CPAP can gradually increase or decrease the pressure based on respiratory events, but similarly maintains the same pressure throughout the respiratory cycle. This can be helpful for patients who may need a higher pressure in rapid eye movement (REM) or supine position, but cannot tolerate the higher pressure through the entire night. Auto CPAP can also be used diagnostically to determine a fixed pressure setting.
• BiPAP provides a higher pressure during inspiration and lower pressure during expiration. This may improve tolerance and help with ventilation. A backup rate can be added to give a breath with weak or absent respiratory effort. Auto BiPAP may adjust either the expiratory positive airway pressure (EPAP) and inspiratory positive airway pressure (IPAP) with a fixed pressure support (PS) or may adjust them independently.
• SV is a bilevel system that continuously changes the inspiratory PS on a breath-by-breath basis in order to achieve a target ventilation or flow for a more constant breathing pattern, especially in the treatment of periodic breathing or Cheyne–Stokes respiration (CSR). Auto forms of SV also increase EPAP in response to obstruction.
• Volume-assured pressure support (VAPS) is a variable bilevel PAP that allows the target volume or ventilation to be programmed, which allows more control of ventilation. This is useful for patients with combined periodic breathing and hypoventilation or patients with REM-related hypoventilation related to conditions like chronic obstructive pulmonary disease (COPD), neuromuscular disorders, or obesity, who may need different PS levels at different times.

• Motor/flow generator and transducers

In order to provide a constant desired pressure at the patient’s airway, adjustments in flow must be made to account for the loss of pressure between the flow generator and the patient’s airway, as well as for breathing fluctuations and leak.

Transducers monitor the motor speed, airflow, and the pressure at a fixed point downstream from the flow generator. Because the sensor is within the device and not in the mask, the device must calculate the predicted pressure at the mask based on the flow measurements at a different point in the system. The pressure at two points varies with the flow.

In order to maintain a stable mask pressure, the microprocessor must adjust the turbine speed in response to deviations in pressure that occur from leak or normal swings in air pressure from breathing. The flow signal is sent through low and high pass filters to separate the respiratory flow signal from artifacts. Many devices adjust automatically to altitude.

• Leak compensation

Unlike invasive ventilation, leak is an important factor that must be compensated for. Leak affects aspects of performance including pressure delivered, cycle and trigger thresholds, and respiratory event determination. Leak compensation works by constantly monitoring flow and looking for deviations from the expected respiratory flow and will compensate the motor speed for the leak. Because there are normal variations in the patient’s breathing cycle the expected leak is usually averaged over several breaths. If leak is high, auto devices may compensate by lowering the pressure, which may seal the mask and reduce leak.

The leak can be determined from the flow rate at the end of exhalation. Normal leak includes that from exhalation ports on the mask, which varies by mask type and pressure level, and unintentional leak from the mouth or around the mask.

Determination of the inspiratory and expiratory cycles is essential not only to provide bilevel PAP but also for expiratory pressure relief and determination of inspiratory flow limitation in auto algorithms.

The start of inspiration is marked by a switch from negative to positive flow (relative to baseline). The point at which the flow signal switches from positive to negative flow is the start of expiration.

• CPAP

Most CPAP devices allow for pressure settings between 4 and 20 (all pressures in cm⋅H₂O). An EPAP of 4 is the lowest pressure needed to provide enough flow to clear the dead space from the device, tubing, and airway to prevent rebreathing of exhaled air. The goal of CPAP is to increase upper airway pressure enough to provide a pneumatic splint to open the airway, which may collapse during inspiration. Typically, the pressure is set to prevent hypopnea, apnea, snoring, flow limitation, and arousals. By providing positive end expiratory pressure, CPAP may recruit alveoli and improve ventilation.

• Auto CPAP

The aim of Auto CPAP is to adjust the pressure in response to respiratory events without adjustment to artifacts caused by leak or other factors. These variations are important to understand for appropriate clinical care as they affect the patient’s tolerance of the devices and the clinical efficacy. If a patient enters REM sleep or changes position, the degree of obstruction may suddenly increase and by the time the device is able to adjust to the needed pressure the patient may have had desaturations or arousals. This is why most studies reporting the equivalence of Auto CPAP to in-lab titration recommend changing the EPAP minimum to the pressure the device is at or below 90–95% of the time. In our experience, many patients left on Auto CPAP 4–20 are undertreated and may present with awakenings a couple hours into sleep, residual symptoms, or difficulty tolerating PAP. Some patients are sensitive to the pressure changes, so if patients are not doing well with Auto CPAP, fixed CPAP should be tried.

Most new Auto CPAP systems use snore detection in combination with flow detection. The flow is sampled many times per second, scaled with a low pass filter to remove artifact, and then a mean flow can be determined for any time length. Peak flow can be a poor measure of breath volume, which can lead to over- or underestimation of an apnea or hypopnea.

Apneas and hypopneas are typically defined as a reduction in ventilation below a percentage of recent breathing for at least 10 seconds, with varying methods used by different devices.

Since responding to central apneas can lead to over titration, testing for airway patency allows for differentiation of central from obstructive apneas. 

The first method looks for cardiogenic pulse artifact in the flow, which is only present if the airway is open. In the second method, the device provides single pressure pulse or small oscillation in the flow (e.g. 1 cm, 4–5 Hz or forced oscillation technique), which is only reflected back to the flow sensors if the airway is closed.

Respironics uses pressure pulses and also defines an obstructive apnea if there is a larger than expected breath after apnea termination. A mixed apnea can be determined if the airway is open for only part of the flow.

ResMed from >9 onward uses force oscillation technique to define central apneas and defines central apneas if leak is >30 L/min.

DeVilbiss Auto adjust 2 uses a modulating micro-oscillation to determine airway patency during apneas.

In order to evaluate flow limitations, Respironics determines roundness, flatness, skewness, and WPF to rate the most recent four breaths as better, worse, or the same compared to baseline. Roundness is determined by the similarity of the WPF between 5% and 95% values to a sine wave. Flatness is determined by the absolute value of the variance between 20% and 80% of inspiratory flow from the average of all the values in the same period, divided by 80% volume point. Skewness is determined by dividing the average of the highest 5% of flows in the mid third of the breath by the average of the highest 5% of flows in the first third of the breath.

ResMed also determines flow limitation. S8 AutoSet defines flow limitation using flatness of an inspiratory breath. The flatness index is calculated by the RMS deviation from unit scaled flow calculated over the middle 50% of a normalized inspiratory breath. From the S9 onward, flow limitation is calculated using a combination of flatness index, breath shape index, ventilation change, and breath duty cycle. Ventilation change is the ratio of the current breath ventilation to recent 3-minute ventilation. Breath duty cycle is the ratio of current breath time of inspiration to total breath time of recent 5 minutes. If a breath is severely flow limited, the flow limitation index will be closer to one and when the breath is normal or round, the flow limitation index will be zero.

ResMed AutoSet evaluates flow every breath looking for apneas, snore, and flow limitation, but responds to flow limitation on a 3-breath average, has faster decreases in the absence of flow limitation and has a higher rate of pressure change to all responses (apnea, snore, and flow limitation) than AutoSet for Her.

In comparison, ResMed AutoSet for Her evaluates the flow for every breath looking for apneas, snore, and flow limitation, and delivers a proportional increase in pressure depending on the degree of deviation of the event from normal, modulated by the current pressure setting and leak rates. If pressure is >10, then the response to flow limitation reduces and a louder snore is required to produce a response.

Respironics REMstar Auto uses layers of control including ramp, leak, snore, apnea/hypopnea, variable breathing, and flow limitation. If there is no snore, apnea/hypopnea, variable breathing, or flow limitation breathing for 3–5 minutes, it will enter a testing period in which it will first decrease the pressure until either P minimum or the flow characteristics (peak, flatness, roundness, and skew) worsen, which is the P critical, then quickly increase by 1.5 and holds for 10 minutes unless further events or flow limitation occurs.

If there is snore, apnea/hypopnea, variable breathing or flow limitation, the pressure increases by 0.5/min until there is no further improvement or worsening, then decreases by 1.5, which is set as the P optimal pressure.

REM star also uses several mechanisms to avoid over titration, which include nonresponsive apnea/hypopnea logic, variable breathing, and leak control.

Most devices start at the set EPAP minimum (EPAPmin) each night. Respironics REMstar Pro and REMstar Auto have CPAP check mode, which checks the 90% pressure every 30 hours, then decides whether to leave the EPAP unchanged, or changes the EPAP up or down by one but not more than three from set EPAP.

DeVilbiss’s IntelliPAP Auto Adjust allows setting of amplitude and duration cut-points for apneas and hypopneas to change sensitivity. Auto Adjust does not detect flow limitation and defines central apneas as <5% of airflow for 10 seconds. It continuously scores events, but only decides once per minute whether to adjust pressures. Auto Adjust 2 similarly adjusts pressures once per minute, but also responds to flow limitation based on inspiratory flatness. It uses modulating micro-oscillation during apneas to test for patency and has an algorithm to define periodic breathing, looking for cyclic breathing as short as a 20-second cycle. Auto Adjust 2 holds or lowers pressures in response to central apneas and periodic breathing.

• BiPAP

BiPAP provides a higher pressure during inhalation and lower pressure during exhalation. Pressures generally range from EPAP minimum of four to IPAP maximum of 25–30. Most devices use a flow trigger to determine when to change to IPAP. The trigger is set above zero flow to sense a significant patient effort. Different methods including flow, shape, and volume are used to cycle to EPAP in efforts to minimize dyssynchrony. The flow cycle algorithm changes to EPAP when the flow drops below a percentage (e.g. 25%) of the peak flow so the patient will not encounter resistance to exhalation. Shape cycling algorithm uses shape of flow, and volume cycle algorithm uses exhaled volume to cycle to EPAP. There can be significant variation between devices in terms of how quickly pressure levels are met and whether a device has a delay or premature cycle, especially in the setting of leaks. If there is a mismatch between the patient’s respiratory cycle and the device control cycle there can be patient discomfort.

In older BiPAP devices, the motor was stopped at the transition point from higher to lower pressures and the motor was accelerated when the device transitioned from lower to higher pressure, which affected synchrony and tolerability. One method to improve comfort is to allow the blower motor to spin freely on transition between inspiration and expiration. Newer devices allow for a smoother transition in pressure changes, and waveforms can be square, exponential, ramp, or sinusoidal. ResMed has a sharkfin-shaped “Easy-Breathe” waveform. The shape of the waveform may be affected by the compliance and resistance of the patient’s respiratory system and the breathing effort, as well as mechanical constraints of blower momentum and propagation delays. In general, BiPAP provides a square wave of PS, but manual or automatic adjustments can give more of a smooth pressure change, which may help with comfort. Most BiPAP devices allow for adjustment of the rise time (angle of the pressure change) from 100 ms to 600 ms.

Inspiration time typically ranges from 0.3 seconds to 2 seconds, often with a default of 1.5 seconds. The higher the baseline respiratory rate, the shorter the inspiratory time (Ti) recommended. Short Ti settings can be helpful for COPD patients for whom pressure does not quickly equilibrate throughout the lung, so the patient may need to actively exhale to cycle the end of expiration. Not only can this cause dyssynchrony and discomfort, it may also lead to air trapping and reduced tidal volume if expiration time is too short.

ST mode is most often used for primary central apnea or central apneas due to respiratory depression. ST mode may also be used for neuromuscular disease patients, whose respiratory efforts fall during REM sleep, which may make them unable to trigger inspiration. Timed mode is often used for patients with severe neuromuscular weakness or spinal cord injury, which is unable to trigger inspiration.

BiPAP may worsen central apneas due to CSR by increasing breath size of spontaneous breaths and forcing a triggered breath during the apneic portion, which is when the partial pressure of carbon dioxide (PCO₂) level is already at its lowest. By further decreasing the PCO₂, respiratory drive is reduced further and the duration of the apnea will often lengthen, although the oxygenation may improve with the deeper or forced breath.

Sometimes the improved oxygenation and PS will help to eventually stabilize the patient’s breathing, but in our experience patients with CSR often find BiPAP intolerable or still have a suboptimal clinical response including fluctuations in the respirations and electroencephalogram.

• AutoBiPAP

Like AutoCPAP, not all AutoBiPAP devices work in the same way. Some devices only allow a fixed PS, others set a PS maximum (PSmax), and others allow for both PS minimum (PSmin) and PSmax.

Thus AutoBiPAP may not provide adequate ventilator support if PSmin cannot be set. AutoBiPAP devices generally do not have an ST option, so are not recommended for central apneas.

ResMed’s VPAP AutoBiPAP and DeVilbiss AutoBiPAP have a fixed set PS. Respironics Series 50 AutoBiPAP fixes PSmin at two and allows setting PSmax, while Series 60 AutoBiPAP allows setting both PSmin and PSmax. Within the limits of PSmin and PSmax, Respironics AutoBiPAP changes EPAP in response to apneas (two apneas or one apnea and one hypopnea) and snoring, and IPAP in response to hypopneas (two hypopneas) and flow limitation, with algorithms similar to REMstar AutoCPAP.

Because higher PAP pressures and high PS can induce periodic breathing and CSR, devices have been developed to try to even out the breathing over several breaths.

ResMed’s standard ASV uses a set fixed EPAP, samples flow 50 times per second, and alters IPAP throughout inspiration to achieve target minute ventilation. The PS range can be 0–20, but default is usually a PSmin of three and PSmax of 15. The device calculates a target minute ventilation based on the recent average weighted minute ventilation, weighted toward the last 3 minutes. ASV uses fuzzy logic to determine the part of the respiratory cycle and whether the current ventilation is below or above the desired target and then adjusts the PS throughout the cycle to achieve that target, thus avoiding abrupt pressure changes. The change in pressure is calculated by multiplying a gain of 0.3 cm H₂O/L/min/s by the difference between the target minute ventilation and the actual minute ventilation. An automatic backup rate starts with the current spontaneous rate based on moving average calculated over several breaths and gradually adapts during an apnea to 15 bpm.

ASVAuto also adjusts the EPAP level similar to the AutoSet algorithm in response to obstructive apneas, flow limitation, and snoring.

The hope is that by stabilizing breathing, the periodic breathing pattern will subside. If the device is cycling the pressures frequently to maintain the target ventilation, it indicates that the underlying periodic pattern is still present and often the patients will either not tolerate the device or there will be a suboptimal clinical response.

Respironics BiPAP AutoSV Advanced is set with EPAP minimum and maximum, PSmin and PSmax, max pressure, and auto or fixed rate. Ti, rise time, and Bi-Flex can also be set for comfort. The level of PS is targeted based on instantaneous average inspiratory flow, which is the sum of the inspiratory flows during a time divided by the number of samples during a time in order to adjust for spurious values. The target peak flow is generally set at 90%–95% of the mean peak flow of the last 4 minutes.

The respiratory cycle of AutoSV Advanced is determined by using the average length of the breathing cycle over recent breaths and calculating the expected midpoint of inspiration.

Compared to multiple small adjustments in the PS throughout the breath cycle with ResMed’s ASV, the inspiratory PS is determined based on the prior breath’s PS plus a gain based on a moving average of the pressure needed in the prior 30 breaths multiplied by the difference between the target peak inspiratory flow and the current breath’s peak inspiratory flow. An intra breath PS is given if the actual flow is less than target flow in the 100 ms prior to the expected halfway point of the inspiration of the current breath. If actual flow is larger than target flow, the PS will be decreased for the following breath.

The older BiPAP AutoSV did not automatically titrate the expiratory pressure, and the algorithm for the automatic backup rate was not proportional to the baseline breathing rate, but it would give a breath if no spontaneous breath occurred within 8 seconds of end of expected breath length or within 4 seconds, if there was recent triggered breath.

• Humidification,
• heated tubing,
• ramp,
• automatic start and stop functions,
• alarms,
• And expiratory pressure relief is device functions that are designed to improve patient comfort and compliance.

Humidifiers include a water chamber with a heating plate through which the airflow is blown. Temperature sensors or humidification sensors allow for regulation of temperature or humidification level.

Increasing humidification in the air helps reduce nasal irritation and congestion that can result from the airflow on the nasal passages. If the humidified air cools in the tubing, water may condense in the tube or mask, commonly referred to as “rainout”.

For travel, standard PAP devices can be used with external battery packs or with converters to allow them to be powered by a car, boat, or other vehicles. 

Data from the device can be retrieved on the interface with a data card, cable, and wireless or by cellular and Bluetooth to an online platform. Data cards can either be used to download locally or to an online website, which allows for sharing. Summary data from the last month is often viewable on the device.

Data collected varies among devices and can include pressure settings, leak, average, 90%–95% pressures and maximum pressures, PS, tidal volumes, minute ventilation, and apnea hypopnea index. Some devices report more detailed data about respiratory events, which may include central, mixed, obstructive and undetermined apneas, hypopneas, flow limitation, snoring, expiratory puffs, and percentage of time in periodic breathing, tidal volume, and minute ventilation. Reports can show summary data over days and months as well as detailed data with the timing of events over the course of one night. Often data can be searched for a 30 day compliance period having over 4 hours of use on 70% of the nights. 

There are a wide range of PAP devices and a wide range of different algorithms used to provide PAP to treat sleep-disordered breathing. For this reason, devices in the same category may differ greatly in their clinical efficacy and comfort. Understanding how PAP devices function can help the clinician select the best PAP device, appropriately titrate, troubleshoot, and optimize settings for a particular patient. Many comfort features have improved the function and performance of devices, which allows many patients who have been unable to tolerate PAP in the past to become compliant. Many devices track compliance and provide important clinical data to help care for the patients.