Respiratory Function Test
aka Pulmonary Function Test
Restrictive
plethysmography: measures volume change (body box), and airway resistance
Peak flow (bed side) (variable correlation with FEV1)
Spirometry
Single breath occlusion
Compliance best measured FVC (i.e. End expiration) where the chest/lungs are in equilibrium, i.e. lung compliance=chest compliance
(i.e. inspiration is active, expiration is passive recoil generally (not in asthma))
Measures of gas exchange
Neuromuscular disease: expiratory weakness = increased RV, with insp weakness = reduced VC best measure
Restrictive
- reduces TLC and VC
- reduced peak flow
- reduced FEV1 correlate with severity (forced expiratory volume in 1s post max inspiration)
- reduced maximal mid expiratory flow rate (average flow during middle 50%) more sensitive for mild obstruction
- FIF 50% : FEF 50% used to differentiate between intrathoracic and extrathoracic
- trapping increases RV and thus FRC
- look at FEV1 if normal, not mixed despite low volumes (can have low volumes in obstruction)
- the 2 largest (F)VC need to be within 5% of each other (or <100ml if (F)VC <1L)
- expiration needs to last 3s or show plateau (<25ml/s for 1s)
plethysmography: measures volume change (body box), and airway resistance
Peak flow (bed side) (variable correlation with FEV1)
Spirometry
- Volumes: VC and subdivisions (TV, IRV, ERV, IC (TV+IRV)) can't do FRC/TLC as can't measure RV
- Flow: FEV1, maximal midexpiratory flow rate ( ?V75-25%)
- FEV1/FVC gives indication of obstruction (more useful than FEV1 as compensates for restrictive impact)
- flow rate under 75% VC are less effort more airway dependent
- impair flow rates at high volumes suggest large airway obstruction
- Repeat after bronchodilator
- Bronchoprovocation challenges: methacholine, histamine, and cold or dry air.
Single breath occlusion
Compliance best measured FVC (i.e. End expiration) where the chest/lungs are in equilibrium, i.e. lung compliance=chest compliance
(i.e. inspiration is active, expiration is passive recoil generally (not in asthma))
Measures of gas exchange
- Diffusing capacity for carbon monoxide (DLCO) measures effective alveolar capillary surface area (Rheum/Immunol/Radiation)
- perfusion-ventilation xenon scan: shows regional gas exchange
- Arterial blood gas
Neuromuscular disease: expiratory weakness = increased RV, with insp weakness = reduced VC best measure