Some patients will exhibit significant arterial oxygen desaturation with PAP treatment despite apparently adequate airflow, especially during REM sleep. Persistent hypoxemia on PAP in these conditions may be due to hypoventilation or ven-tilation-perfusion mismatch (often due to chronic lung disease). Under such circumstances, PAP can be increased to eliminate unrecognized high upper airway resistance, or CPAP can be changed to bilevel PAP to augment ventilation (higher tidal volume). If these measures are not effective at increasing the oxygen saturation or if higher pressure is not tolerated, supplemental oxygen can be added to PAP.
It is important to recognize that the effective oxygen concentration administered to a patient using PAP will depend both on the supplemental oxygen flow and the machine flow. Increases in machine flow associated with higher pressures or mask/mouth leak can dilute a given flow of supplemental oxygen. As an example, consider a patient with chronic lung disease and OSA who requires supplemental oxygen at 2 L/min to maintain an arterial oxygen saturation of 94% while awake. With CPAP treatment of 12 cm H2O, the required supplemental oxygen flow will likely be higher (3 to 4 L/min) depending on the total flow delivered. This assumes that the required fractional concentration of oxygen is the same or higher than the concentration required during wakefulness without CPAP.
Split-Night (Partial) PAP titration
Split-night polysomnography was introduced to obtain a diagnosis and determine an effective PAP on a single night. Underestimation of the severity of sleep apnea (no or minimal REM sleep in the first half of the night) and inadequate time for PAP titration are potential disadvantages of this approach. In a retrospective review of 412 consecutive patients with an apnea index > 20/h, Iber and coworkers found that 78% of the PAP titrations were adequate (the lower of AHI 40/h had similar optimal pressures on a split-night and subsequent full-night CPAP titration. In patients with milder disease a higher average pressure was documented during a full-night PAP titration. The following have been suggested as criteria for a split-night study: a minimum of 2 h of diagnostic monitoring, AHI > 40/h or 20 to 40/h with severe desaturation or other indications for an immediate titration, and 3 h remaining for the PAP titration. Medicare guidelines in some locales require at least 2 h of sleep during the diagnostic portion. The effects of split-night vs full-night PAP titration on adherence have not been studied by a prospective randomized trial. Using a case-controlled design, McArdle et al found no decrease in adherence in patients treated with a split-night study.
Alternative Methods of PAP Titration
The use of APAP devices for autotitration to select a fixed CPAP treatment pressure has been studied in both the attended and unattended settings. The information stored in the device can be transferred to a computer for analysis. Information about time at pressure (adherence), pressure vs time information, leak, and residual AHI information is available to determine the quality of the titration. It is common practice to choose the 95th or 90th percentile pressures as the CPAP prescription pressure. On the basis of a systematic review of APAP evidence published in 2003, the standards of practice committee of the American Academy of Sleep Medicine did not endorse routine use of unattended APAP to select a CPAP treatment pressure. A subsequent large multicenter study found that unattended APAP titration resulted in similar outcomes as attended CPAP titration in a carefully selected population with an AHI > 30/h. Of note, 23% of the potential study group were excluded. The quality of APAP titrations was assessed in detail (duration of use, residual AHI values). Unacceptable titrations were repeated, or patients were referred for an attended titration if necessary. Thus, it appears that in carefully selected populations with proper education and careful review of titration results that unattended APAP titration can be successful in many patients. APAP titration is a useful option when access to polysomnography is not possible or delayed, when the results of an attended titration are inconclusive, or when there is a need to check the efficacy of a given prescription pressure (recent weight gain).
A number of other alternatives to attended PAP titration have been proposed. These include initiating treatment using a pressure prediction equa-tion, with further pressure adjustments made by medical personnel based on bed partner observa-tions or by the patient based on comfort and perceived efficacy (self-titration).