Pulmonary Function Tests Flashcards
Functions of the lungs?
- Gaseous exchange
- Acid base balance
- Oxygen in
- Carbon dioxide out
How do the lungs perform their functions?
air moves to the area of gas exchange
- this requires working respiratory muscles and a normal ribcage
What are the conducting airways?
- trachea
- lobar bronchi
- segmental bronchi
- subsegmental bronchi
- small bronchi, bronchioles and terminal bronchioles
What are pulmonary function tests?
objective methods to assess lung function
- primary function = gas exchange
Examples of PFTs?
- spirometry
- lung volumes by helium dilution or body plethysmography
- blood gases
- exercise tests
- diffusing capacity
- bronchial challenge testing
- pulse oximetry
Indications for PFTs?
- Diagnosis and monitoring of disease
- Disability/impairment evaluation
- Insurance
- Public health such as surveys and occupational health issues
- Derivation of reference equations
- Legal reasons
What is pulse oximetry?
Non-invasive procedure that estimates O2 sats of Hb in pulsating arterial blood
Describe a pulse oximeter?
Has a microprocessing unit that is connected to a sensor probe containing Light Emiiting diode (LED) and a photodetector
How does a pulse oximeter work?
- Light is transmitted at 660nm (red) and 940nm (infrared)
- The amount of light absorbed in the light above is compared to give O2 sats
Accuracy of a pulse oximeter?
Accuracy – at +/-2% for sats >70% but 3% between 50 to 70%
Factors that affect pulse oximetry?
- Temp, pH, PaCO2 and 2,3-DPG as in the oxygen dissociation curve
- Nail polish that absorbs light in similar wavelengths
- Other abnormal species of hemoglobin eg COHb or MetHb
- Penumbra (paene – almost, umbra - shade) effect refers to poor placement of the Pulse oximeter resulting in poor illumination of the photodetector by the LED
What is spirometry?
Measurement of volume of air that can be inhaled or exhaled
Note: cannot measure any volume that includes Residual Volume (RV)
– volume of air that remains in the lungs after maximal exhalation
What are the 4 lung volumes?
- inspiratory reserve volume
- tidal volume
- expiratory reserve volume
- residual volume
What are the 4 lung capacities?
- vital capacity
- inspiratory capacity
- functional residual capacity
- total lung capacity
What are the 2 ways to record results of a forced vital capacity maneuver?
- flow volume curve
- flow meter measures flow rate in L/s upon exhalation; flow plotted as function of volume - classic spirogram
- volume as a function of time
Measurements obtained from the FVC curve?
- FEV1
- FEF 25-75%
- FEV1/FVC
What is FEV1?
the volume exhaled during the first second of the FVC maneuver
What is FEF 25-75%?
the mean expiratory flow during the middle half of the FVC maneuver - reflects flow though the small (<2 mm in diameter) airways
What is FEV1/FVC?
the ratio of FEV1 to FVC X 100 (expressed as a percent)
Describe the FEV1/FVC ratios corresponding to the ages <40 to >60 years?
- <40 years old – at least 75% FVC should be exhaled
- 40 to 60 years old – 70% to 80% FVC to be exhaled
- > 60 years old – 65%
What are the FEV1/FVC ratios that coincide with mild, moderate and severe obstruction?
- 60 to 70% - mild obstruction
- 40 to 60% - moderate
- <40% - severe
Differentials for obstructive diseases?
- Bronchospasm
- Asthma
- Emphysema
Differentials of restrictive diseases?
- Scoliosis
- Lung resection
- Neuromuscular diseases
- Interstitial fibrosis
What is the spirometry pre and post bronchodilator?
- obtain flow-volume loop
- administer a bronchodilator
- obtain the flow-volume loop again a minimum of 15 minutes after administration of the bronchodilator
- calculate percent change in FEV1
- FEV1 = [(FEV1 Post - FEV1 Pre)/FEV1 Pre] X 100 - reversibility is with 12% or greater change
Advantages of flow volume loops?
- Shape – shows adequacy of the expiratory effort, early termination of effort drops abruptly, coughing and extra breaths are also picked
- Small airway obstruction – the expiratory effort is long, upwardly-concave indicating low flow rates of the FVC, consistent with small airways obstruction.
- Upper airway obstruction – flattens the peak. Both the expiratory and inspiratory phases are flattened (in fixed obstruction eg tracheal stenosis)
- Variable upper airway obstruction (eg bilateral vocal cord paralysis) affects the inspiratory limb – flattening it
Contraindications to spirometry?
- Acute disorders affecting test performance eg nausea, vomiting, vertigo
- Hemoptysis of unknown origin
- Pneumothorax
- Recent abdominal or thoracic surgery
- Recent myocardial infarction/ unstable angina
- Thoracic aneurysms
- Recent eye surgery (to prevent increased intra-ocular pressure)
Describe the diffusing capacity of the lungs?
- Uses Carbon monoxide inhaled
– a 10-second breath hold at full inspiration
– DLCO - CO has high affinity for Hb and therefore not affected by pulmonary blood flow but surface area and nature of air-blood interface around the alveolar-capillary membrane
NB. Affected by smoking, age, height
