Adherence to sleep apnea therapy and use of lipid-lowering drugs: a study of the healthy-user effect

Abstract

Background: Evidence that continuous positive airway pressure (CPAP) reduces cardiovascular morbidity comes largely from observational studies. This association may be confounded if CPAP adherents are healthier in ways not measured by investigators. We assessed whether patients adhering to lipid-lowering medications were more adherent to CPAP.

Methods: This was a retrospective cohort study undertaken at the Philadelphia Veterans Affairs (VA) Medical Center (2005-2006) of consecutive patients on lipid-lowering therapy newly initiating CPAP for obstructive sleep apnea. Adherence to medications dispensed via the VA closed-pharmacy system was measured as the proportion of days covered (>/=80% vs < 80%) in the year prior to CPAP initiation. CPAP adherence was defined as >/= 4 h/d of "mask-on" time, measured electronically daily during the first week of CPAP. We examined the association between medication adherence and CPAP adherence using multivariable logistic regression.

Results: Complete data were available for 117 of 142 (81.5%) subjects. After adjustment for age, race, medical comorbidity, and sleep apnea-related clinical factors, subjects with low medication adherence demonstrated a 40.1% (95% CI, 30.0-51.0) probability of using CPAP >/= 4 h/d compared with 55.2% (95% CI, 46.9-63.1) for subjects with adequate (>/=80%) medication adherence (adjusted for comparison, odds ratio (OR) = 1.8 [95% CI, 1.0-3.3], P = .04). Married patients were more adherent to medications and CPAP; inclusion of this factor reduced to nonsignificance the association of medication and CPAP adherence (OR = 1.6 [95% CI, 0.9-2.8], P = .12).

Conclusion: Patients consistently refilling lipid-lowering medications were more adherent to CPAP, suggesting that differences in medication adherence or other health-promoting behaviors should be investigated in future nonrandomized, observational studies linking CPAP adherence and cardiovascular outcomes.

Figure 1.

Philadelphia VA study cohort (2005-2006). CPAP = continuous positive airway pressure; VA = Veterans Affairs.

Figure 2.

Adjusted probability of initial CPAP use ≥4 h, by lipid-adherence subgroup (n = 117). Medication adherence is measured as the PDC, calculated as the number of d with medication available from the Veterans Affairs pharmacy divided by the total number of d exposed in the year before CPAP therapy. Estimates are adjusted for age, race, BMI, Epworth Sleepiness Scale, apnea-hypopnea index, in-laboratory vs unattended sleep study, Charlson Index, and CPAP use on first night vs subsequent nights. PDC = proportion of days covered. See Figure 1 for expansion of other abbreviation.