A193 SLEEP, Volume 41, Abstract Supplement, 2018 Results: The mean age was 52.0 years±9.4 and all patients were male. Mean BMI was 28.8 kg/m2 ± 3.3. The mean pre-implantation AHI of 46.7/h ±12.2 was reduced to 14.5/h±8.9 at 6 months post-implanta- tion (p < 0.001). The mean MWT latency improved significantly from 25.0 ± 12.8 minutes at baseline to 36.8 ± 7.0 minutes after 6 months of treatment (p=0.004). A significant reduction in WASO (71.4min±32.4 vs 53.4min±13.5, p=0.06) and of N1% (9.7 ± 8.8 vs 3.7 ± 1.8, p=0.04) was observed. MWT latencies at 6 months were significantly corre- lated with the intensity of stimulation (r=-0.63, p=0.05). Intensity of stimulation was positively correlated with WASO (r=0.76, p=0.01). Conclusion: Significant changes in sleep architecture were observed six months post-treatment by HNS. To optimize daytime alertness and sleep quality, HNS should be calibrated not only on AHI but also on sleep structure and WASO. Because of the intrinsic properties of HNS, we strongly recommend titration of these therapeutic devices on both breathing events and sleep structure patterns. Support (If Any): NA. 0514 IMPACT OF ADAPTIVE SERVOVENTILATION THERAPY ON OUTPATIENT HEALTHCARE UTILIZATION Mansukhani MP, Kolla B, Naessens JM, Gay PC, Morgenthaler TI Mayo Clinic, Rochester, MN Introduction: Adaptive servoventilation (ASV) has been shown to provide significantly better control of central sleep apnea (CSA) and may be better tolerated than other treatment modalities. Prior studies have demonstrated a reduction in healthcare utilization with treatment of obstructive sleep apnea. In this study, we evaluated the impact of ASV on outpatient healthcare utilization in patients with CSA. Methods: All cases of CSA (n=1,237) from Olmsted County, MN were identified utilizing the Rochester Epidemiology Project database. Patients with CSA commenced on ASV were selected. The rate of Emergency Room (ER) visits, outpatient (OP) visits and outpatient prescription medi- cations per year in the 2 years before and after ASV were compared using the paired t-test. Multiple regression analyses were conducted accounting for adherence to ASV, CSA subtype and comorbidities. Manual review of charts was performed to obtain all relevant data. Those with a follow-up period of <1 month pre- and/or post-ASV were excluded. Results: A total of 309 patients (68.0 ± 14.6 years, 80.3% male) were included. Mean body mass index was 31.7 ± 6.5 kg/m 2 , mean apnea-hy- popnea index 41.6 ± 26.5/hour and mean central apnea index 14/hour. Treatment-emergent CSA (72.5%) was the most common subtype; 65% were fully adherent (≥4hrs/night on ≥70% nights) to treatment at 1 month and 16% partially adherent. Twenty-nine (9%) patients died during follow-up. There was no change in the rate of yearly ER visits [mean 1.19(SD=2.18) versus 1.26(SD=2.08), p=0.93], OP visits [mean 5.32(SD=47.25) versus 2.34(SD=8.72), p=0.43], or medications [mean 6.68(SD=12.0) versus 5.31(SD=5.86), p=0.70] in the 2 years before and after ASV. There was also no significant change in the rates of these outcomes pre- and post-ASV in sub-analyses accounting for adherence to ASV, CSA subtype or comorbidities (all p<0.05). Conclusion: Although reasonably well tolerated, the use of ASV in patients with CSA was not associated with a change in outpatient healthcare utilization, perhaps reflective of the overall ill health of this cohort. Support (If Any): ResMed Foundation. 0515 EXCESSIVE DAYTIME SOMNOLENCE IN PATIENTS WITH OBSTRUCTIVE SLEEP APNEA ADEQUATELY TREATED WITH POSITIVE AIRWAY PRESSURE Foster S 1 , Mysliwiec V 1 , Matsangas P 2 1 San Antonio Military Medical Center, San Antonio, TX, 2 Naval Postgraduate School, Monterey, CA Introduction: Patients with obstructive sleep apnea (OSA) commonly experience residual excessive daytime sleepiness (EDS) despite treatment with positive airway pressure (PAP). Yet, there are currently no guidelines on the evaluation of this common clinical problem. The purpose of this study was to determine the percentage of patients with OSA adequately treated with PAP who had objective residual sleepiness. Methods: We conducted a retrospective review of 31 adults (ages 18–64;80.6% male) with OSA on PAP therapy who underwent an evaluation for EDS. For inclusion criteria, patients had to have subjec- tive sleepiness by the Epworth Sleepiness Scale (ESS) and an attended in-lab polysomnogram with PAP followed by a multiple sleep latency test (MSLT) to determine objective sleepiness. Results: All patients were subjectively sleepy with an ESS score of >10 and met minimal PAP usage of 4 hours a night for at least 70% of nights with a residual AHI <10. The average PAP usage was 7 hours/ night. On MSLT testing, 10 (32.2%) patients had an average sleep onset latency (SOL) <8 minutes, 10 (32.2%) had a SOL between 8 and 11 minutes, and 11 (35.4%) had SOL > 11 minutes. Sixteen (51.6%) patients had a comorbid disorder (anxiety, depression, chronic pain, PTSD). Approximately 81% (13) of the patients with a comorbid dis- order had non-congruent subjective and objective sleepiness. Conclusion: Despite subjective symptoms, the majority of OSA patients were not objectively sleepy. Subjective and objective sleepi- ness corresponded in only 32.2% of our cohort. This questions the clinical practice of using wake promoting agents in this population without objective testing and/or evaluation for other etiologies of sub- jective symptoms. Support (If Any): 0516 EFFECTIVENESS OF POSITIVE AIRWAY PRESSURE TREATMENT IN VETERANS WITH MODERATE TO SEVERE OBSTRUCTIVE SLEEP APNEA Imayama I 1 , Doumit J 1, 1, Hussain J 1 , Tam M 2 , Prasad B 2 1 University of Illinois at Chicago, Chicago, IL, 2 Jesse Bronw VA Medical Center, Chicago, IL Introduction: Obstructive Sleep Apnea (OSA) is prevalent in Veterans leading to adverse cardiovascular outcomes. The effectiveness of Positive Airway Pressure (PAP) treatment is not well understood. We examined changes in functional outcomes, 24-hour blood pressure profile and sympathetic nervous system activity with 3-months PAP treatment in Veterans with OSA and hypertension. Methods: Veterans with hypertension and moderate-to-severe OSA (Respiratory Event Index [REI] ≥15/hour on type 3 Home Sleep Apnea Test) were enrolled in a trial that tested the effects of PAP therapy on blood pressure. Habitual sleep duration (average Total Sleep Time; TST) was measured by Actigraphy over 3–10 days. Epworth Sleepiness Scale (ESS), Functional Outcomes of Sleep Questionnaire (FOSQ), psychomotor vigilance test (PVT), 24-hour Ambulatory Blood Pressure (ABP), Central Aortic Blood Pressure (CABP) and 24-hour urine cathecolamine levels (dopamine, epinephrine, and nor- epinephrine) were assessed at baseline and 3 months of Autoadjusting PAP treatment. Paired t-Tests or Wilcoxon sign rank tests were used to compare 3-month changes in ESS, FOSQ, PVT, ABP, CABP and urine cathecolamine levels. STATA version 14 was used for analyses. Results: Veterans (n=145) with age mean (standard deviation [SD]) 52.8 [10.5], Body Mass Index 34.6 [5.8] kg/m 2 and REI 31.6 [18.6]/ B. 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