Ventilation and Alveolar Gases
~1 min read
Lesson 10 of 15
Notes
The composition of alveolar gas determines the partial pressures driving oxygen and carbon dioxide diffusion across the alveolar-capillary membrane. Two fundamental relationships govern alveolar gas composition: the alveolar gas equation and the inverse relationship between alveolar ventilation and alveolar PCO2.
The alveolar gas equation calculates the ideal alveolar PO2 (PAO2): PAO2 = PiO2 − (PACO2 / R), where PiO2 is the inspired PO2 (approximately 20 kPa after humidification at sea level), PACO2 is the alveolar PCO2 (assumed equal to arterial PCO2 ≈ 5.3 kPa), and R is the respiratory quotient (approximately 0.8 for a mixed diet). This equation is used to calculate the alveolar-arterial (A-a) oxygen gradient: A-a gradient = PAO2 − PaO2. A normal A-a gradient is less than 2 kPa in young adults (increasing with age). An elevated A-a gradient implies V/Q mismatch, diffusion impairment, or shunt; a normal gradient with hypoxaemia implies hypoventilation or a low FiO2.
Alveolar ventilation is inversely proportional to alveolar PCO2: if alveolar ventilation doubles, PACO2 halves. This relationship is clinically critical: PACO2 (and thus PaCO2 on arterial blood gas) is a direct measure of the adequacy of alveolar ventilation. Hypercapnia (elevated PaCO2) always indicates hypoventilation.
Radial traction is the mechanism by which surrounding lung parenchyma holds small airways open. The alveolar walls attached to the outer surface of bronchioles exert outward traction on the airway wall, opposing its tendency to collapse. In emphysema, destruction of alveolar walls eliminates this radial traction, causing premature airway collapse during expiration and contributing to dynamic airway compression and air trapping.
Pursed-lip breathing, used by patients with COPD, increases resistance to expiratory flow at the lips, raising pressure in the small airways and maintaining airway patency during expiration. This simple manoeuvre reduces dynamic airway compression, decreases air trapping, and improves alveolar ventilation.