Device Characteristics

Many brands and models of APAP devices are currently approved for treatment. The devices differ in the respiratory variables that are monitored and in the algorithms used to adjust the delivered pressure. The devices typically monitor one or more of the following: airflow (or motor speed), airflow profile (flattening), snoring (airway vibration), or airway impedance (forced oscillation technique). The algorithms used to adjust pressure are proprietary but determine if the delivered pressure should be increased or decreased. Depending on the type of respiratory event that is detected the delivered pressure is increased by a certain amount. Typically, pressure changes occur slowly over several minutes to prevent pressure-induced arousals. If no respiratory events are detected within a certain time window the delivered pressure is slowly decreased. Thus, the lowest effective pressure is delivered. In some of the devices machine adjustment is available for various mask types and for the type of humidifier that is being used.

Studies comparing different APAP devices provide evidence that devices from different manufacturers will not deliver the same pressure for a given clinical circumstance (7,8). These studies used a mechanical lung/upper airway with set patterns of apnea or hypopnea to challenge different APAP devices. In Figure 2 the

TABLE 1 Potential Uses of Auto-Positive Airway Pressure

Auto-titrating mode Attended auto-titration in CPAP naïve patient (technologist extender) Unattended auto-titration in CPAP naïve patient Check prescription pressure after weight gain/loss Salvage a failed manual CPAP titration Auto-adjusting mode Initial chronic treatment of OSA (no titration needed) Chronic treatment in patients not tolerating CPAP Chronic treatment in patients with difficult mask/mouth leak

Abbreviations: CPAP, continuous positive airway pressure; OSA, obstructive sleep apnea.

responses of several devices differs markedly in response to apnea. Some of the devices increased the delivered pressure in response to apnea while others did not (7). Kessler et al. (9) compared the 95th percentile pressure of one APAP device based on airflow to another based on the forced oscillation technique using a randomized crossover study design. There was poor agreement between the optimal pressure identified by the two devices. The 95th percentile pressure determined by the APAP machine based on flow was on average 3 cm H2O higher than that of the other device. Senn et al. (10) compared an APAP device responding to apnea, hypopnea, and snoring with a second device responding to the previous variables as well as airflow limitation (airflow profile flattening). The adherence and clinical outcomes were similar although the median applied pressure was slightly higher with the device that responded to airflow limitation (10). Thus, differences in the devices do not always translate into differences in outcomes.

The problems of mask/mouth leak and central apnea have provided a challenge for the designers of APAP algorithms. However, these two problems are familiar to technologists manually titrating CPAP. Mask/mouth leaks tend to raise the baseline flow delivered by blower units and diminish the variations in flow during inspiration and expiration. The resulting airflow signal may be interpreted as an apnea or hypopnea and prompt an increase in pressure that may further increase leak. Teschler and Berthon-Jones (3) reported on their clinical experience in 1000 patients using the AutoSet T™ (ResMed, North Ryde, Australia) APAP device and estimated that leak exceeds 0.4 L/s (considered high leak) on average for 10% of an attended APAP night and 15% on an unattended APAP night. To handle the leak problem many APAP units have algorithms that limit pressure increases when leak exceeds certain values or when increases in blower speed no longer result in increases

Body SLJP'ne -

Position Lateral ---

FIGURE 1 This schematic tracing of delivered pressure over an entire night illustrates that the patient slept at a lower pressure for most of the night than a single fixed pressure that would be effective in all body positions. The APAP device increased pressure when the patient was supine. Abbreviation: APAP, auto-positive airway pressure.

FIGURE 2 Response of the auto-positive airway pressure (APAP) devices (D1-D5) when subjected to a disturbed flow (V) breathing pattern consisting of repetitive apneas. P and V are the actual pressure and flow, respectively, measured at the entrance of the APAP device (V' > 0: inspiration). Source: From Ref. 7.

FIGURE 2 Response of the auto-positive airway pressure (APAP) devices (D1-D5) when subjected to a disturbed flow (V) breathing pattern consisting of repetitive apneas. P and V are the actual pressure and flow, respectively, measured at the entrance of the APAP device (V' > 0: inspiration). Source: From Ref. 7.

in mask pressure. Other units have leak alarms that can prompt the patient to adjust the mask. Mouth leaks can be approached by using a chin strap or full-face mask.

Central apnea during APAP treatment/titration is another difficult problem in some patients (1). Central apneas of the Cheyne-Stokes type are common in patients in congestive heart failure. Other patients with OSA may have central apneas during CPAP titration (treatment-emergent central apneas). Algorithms often include limits on upward titration of pressure for apnea to avoid the delivery of high pressure for central apneas. For example, pressure is not increased above 10 cm H2O unless apnea is associated with snoring or airflow profile flattening. Of note, many published studies of APAP excluded patients with congestive heart failure or frequent central apneas on the preceding diagnostic sleep study.

Recently, expiratory pressure relief (C-Flex™, Respironics, Inc., Murrysville, Pennsylvania, U.S.) is now available for one brand of APAP devices. This mode allows a reduction in pressure during early expiration with a return to the current set pressure at end expiration. This feature could improve patient tolerance to pressure. However, only one published study has demonstrated an advantage for CPAP with C-Flex compared to traditional CPAP treatment (11). Another recent development is the availability of APAP machines providing automatic adjustment of bilevel PAP (BPAP) (Fig. 3). These devices vary inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP) according to a proprietary algorithm. The physician sets the minimum EPAP, maximum IPAP, and maximum IPAP-EPAP difference. The option of using inspiratory and expiratory pressure relief (Bi-Flex®, Respironics, Inc., Murrysville, Pennsylvania, U.S.) is currently available for one device brand. Inspiratory pressure relief allows a drop in pressure at end inhalation (IPAP) while expiratory pressure relief allows a pressure drop at the start of exhalation (EPAP). The superiority of APAP devices with either C-Flex or the ability to deliver bilevel positive pressure (with or without Bi-Flex) remains to be demonstrated. These new features do increase treatment alternatives.

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