Herala Merja, Stålenheim Gunnemar, and Bomm Gunnar. 2010. “Effects of Positive Expiratory Pressure (PEP), Continuous Positive Airway Pressure (CPAP) and Hyperventilation in COPD Patients With Chronic Hypercapnea”. Upsala Journal of Medical Sciences 100 (3). https://doi.org/10.3109/03009739509178908.