Classic sleep apnea in the patient. Cessation of breathing causes nocturnal hypoxia and systemic hypertension/arrhythmic/sudden death can persist. PaO2 is normal during day and decreased during episodes and at night. Classically seen in Obese patients such as the patient in the question stem.

Lung volume will decrease in obesity hypoventilation syndrome. Even though this patient is obese, he has all the clinical features of sleep apnea

Sleep apnea

Repeated cessation of breathing > 10 seconds during sleep = disrupted sleep = daytime somnolence.

Diagnosis confirmed by sleep study.

Nocturnal hypoxia = systemic/pulmonary hypertension, arrhythmias (atrial fibrillation/flutter), sudden death.

Hypoxia = More EPO release = More erythropoiesis.

Daytime sleepiness, fatigue, frequent nighttime awakening, snoring, and obesity (BMI>30) are all symptoms of obstructive sleep apnea. Frequent cessations of breathing during sleep result in a decreased O2 saturation in his lungs, which causes hypoxic vasoconstriction in the pulmonary vessels, and over time this leads to irreversible pulmonary hypertension (loud S2), and the pulmonary HTN eventually causes right heart failure, which is what's causing his peripheral edema.