College Answer

Although  usually  coexistent  these  problems  (emphysema/chronic  bronchitis)  have  theoretically different effects. Chronic bronchitis leads to increased airway resistance from mucosal oedema, secretions, bronchospasm, loss of elastic tissue supporting small airways leading to dynamic airway compression. Emphysema leads to loss of alveolar spaces and capillary bed. The end result is airflow limitation, prolonged expiration, hyperinflation (reducing diaphragm efficiency and increasing work of breathing), pulmonary hypertension, V/Q mismatch and tendency to degrees of hypoxia and hypercarbia. Chronic hypercarbia may lead to reliance on hypoxic drive and chronic hypoxia to cor pulmonale and polycythaemia. Skeletal muscle dysfunction may be prominent due to malnutrition, steroids, electrolyte abnormalities and reduced muscle blood flow.

These effect management by necessitating avoidance of gas trapping during ventilation (long exp time, bronchodilators etc), ensuring enteral nutrition and sputum clearance with physiotherapy, aiming to maintain the patient’s usual PCO2 and PO2 with a normal pH.

Discussion

A systematic approach is called for:

Emphysema

• increased lung compliance
• decreased gas exchange surface
• increased dead space

Chronic bronchitis

• increased airway reactivity
• decreased airway diameter

COPD in general

• hyperinflation, decreased diaphragm excursion
• increased intrinsic PEEP

Consequences of chronic respiratory failure

• chronic hypercapnea
• decreased respiratory drive (reliance on hypoxic drive)
• chronically increased work of breathing, thus cachexia and malnutrition