An electrocardiogram can be used to differentiate between cardiac and respiratory (COPD) causes of hypoxia.  Patients with COPD might have some electrocardiographic findings such as right heart strain and P pulmonale.
Chronic obstructive pulmonary disease electrocardiogram
Chronic bronchitis electrocardiogram
Chronic obstructive pulmonary disease echocardiography
The chest x-ray in cardiovascular disease
Carcinoid syndrome electrocardiogram
Coronary heart disease electrocardiogram
Sudden cardiac death electrocardiogram
Asthma electrocardiogram