- Capacity = any combination of 2 or more volumes
FLOW VOLUME LOOP
- 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.
- FEF/FIF50 = if > 1 -> inspiratory flow is affected more than expiratory -> extrathoracic site of obstruction
- 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’)
- total volume exhaled and flow rate reduced
- inspiration limited by reduced compliance of lung/chest wall or weakness of inspiratory muscles