One of the students wished for clarification between the clinical phenotypes of "pink puffer" versus "blue bloater" in COPD.
A "pink puffer" is a person where emphysema is the primary underlying pathology. As you recall, emphysema results from destruction of the airways distal to the terminal bronchiole--which also includes the gradual destruction of the pulmonary capillary bed and thus decreased inability to oxygenate the blood. So, not only is there less surface area for gas exchange, there is also less vascular bed for gas exchange--but less ventilation-perfusion mismatch than blue bloaters. The body then has to compensate by hyperventilation (the "puffer" part). Their arterial blood gases (ABGs) actually are relatively normal because of this compensatory hyperventilation. Eventually, because of the low cardiac output, people afflicted with this disease develop muscle wasting and weight loss. They actually have less hypoxemia (compared to blue bloaters) and appear to have a "pink" complexion and hence "pink puffer". Some of the pink appearance may also be due to the work (use of neck and chest muscles) these folks put into just drawing a breath.
A "blue bloater" is a person where the primary underlying lung pathology is chronic bronchitis. Just a reminder, chronic bronchitis is caused by excessive mucus production with airway obstruction resulting from hyperplasia of mucus-producing glands, goblet cell metaplasia, and chronic inflammation around bronchi. Unlike emphysema, the pulmonary capillary bed is undamaged. Instead, the body responds to the increased obstruction by decreasing ventilation and increasing cardiac output. There is a dreadful ventilation to perfusion mismatch leading to hypoxemia and polycythemia. In addition, they also have increased carbon dioxide retention (hypercapnia). Because of increasing obstruction, their residual lung volume gradually increases (the "bloating" part). They are hypoxemic/cyanotic because they actually have worse hypoxemia than pink puffers and this manifests as bluish lips and faces--the "blue" part.