Spirometry

OVERVIEW

• Capacity = any combination of 2 or more volumes

LUNG VOLUMES

FLOW VOLUME LOOP

PARAMETERS

• VC = 70mL/kg
  = IRV + TV + ERV
• IRV = 45mL/kg
• TV = 10mL/kg – volume breathed in & out during a normal breath (without extra effort)
• ERV = 15mL/kg – extra volume beyond normal expiration
• RV = 15mL\kg (not measured by spirometer)
• TLC = 85L\kg
• FRC = 30mL\kg
• FEV1 = forced expiratory volume in 1 second (normally 4L)
• FVC = forced vital capacity (a little lower than VC because of dynamic airway closure; normally ~5L)
• PEFR = peak flow rate over an expiration (normally 500L/min)
• FEV1/FVC (normal = 80%)
  – obstructive disease = FEV1 reduced more than FVC, low FEV1/FVC
  – restrictive disease = FEV1 & FVC reduced but FEV1/FVC normal or increased
  – after a relatively small amount of gas has been exhaled -> flow is limited by airway compression determined by (1) elastic recoil force of lung & (2) resistance of airways upstream of the collapse point.
• FEF50%
• FIF50%
• FEF/FIF50 = if > 1 -> inspiratory flow is affected more than expiratory -> extrathoracic site of obstruction

OBSTRUCTIVE DISEASE

• flow rate very low in relation to lung volume (c/o resistance to flow – scooped out appearance often seen following the point of maximum flow)
• total lung capacity is large, but expiration ends prematurely c/o early airway closure from increased smooth muscle tone of bronchi (asthma) or loss of radial traction from surrounding parenchyma (emphysema).
• equal pressure point is close to the alveolus and the transmural pressure gradient can become negative quickly -> collapse.
• encroachment of VC by an increased RV caused by hyperinflation (‘air trapping’)

RESTRICTIVE DISEASE

• total volume exhaled and flow rate reduced
• inspiration limited by reduced compliance of lung/chest wall or weakness of inspiratory muscles