Other lung function tests (chapter 3) Flashcards
What can you measure with a simple spirometer - what requires additional measurements
- measurement of vital capacity, tidal volume, expiratory reserve volume can be done just with spirometer
- residual volume, functional residual capacity and total lung capacity require additional measurements
Measurement of functional residual capacity (FRC) -plethysmograph
- measure with plethysmograph
1) mouthpiece obstructed while patient makes a rapid inspiratory effort
2) as patient expands the gas volume in the lungs the air in the plethysmograph is compressed slightly and its pressure rises
3) Use boyles law to obtain lung volumes
Measurement of FRC - helium dilution technique
-spirometer of known volume and helium concentration is connected to the patient in a closed circuit
-from the degree of dilution of helium the unknown lung volume can be calculated
(and can then find the residual volume by subtracting FRC from expiratory reserve volume)
In what diseases are the FRC and RV typically increased
In diseases in which there is an increased airway resistance (emphysema, chronic brnchitis, asthma)
Why is RV elevated in emphysema
Because airway closure occurs at an abnormally high lung volume
In what disease are the FRC and RV typically reduced
-in diseases that reduce lung compliance -diffuse interstitial fibrosis (lung is stiff and therefore recoils to a smaller resting volume)
Comparison FRC measured by plethysmogrpahic vs. gas dilution
- plethysmograph measures ALL the gas in the lung
- the dilution technique sees only those regions of the lung that communicate with the mouth (regions behind closed airways result in a higher value for plethysmographic than dilution procedure)?
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Measurement of lung elastity
1) Small balloon at end of catheter passed down through the nose/mouth
2) Difference between mouth and esophageal pressures recorded as patient exhales in steps of 1 L from TLC to RV
3) Graph of % predicted TLC (accounting for differences in body size and therefore reducing variability of results) vs. pressure –> slope = compliance
Pressure volume curve where elastic recoil is reduced
-pressure volume curved displaced to the left and has a steeper slope
Some conditions that reduce lung elastic recoil
- emphysema
- some patients with asthma
Some conditions that increase lung elastic recoil
- interstitial fibrosis
- interstitial edema
Measurement of airway resistance
-measured as the pressure difference between the alveoli and the mouth divided by the flow rate
Measurement of airway resistance with a plethysmograph
- In airtight box, Pants through a flow meter
- alveolar pressure deduced from the pressure changes in the plethysmograph because when alveolar gas is compressed the plethysmograph gas volume increases slightly –> causing a fall in pressure
What reduces airway resistance (generally)
- reduced by an increase in lung volume
- because expanding parenchyma exerts traction on the airway walls
Conditions that increase airway reistance
- Chronic bronchitis
- Asthma
- Emphysema
- Inhaled irritants (cigarette smoke)
Conditions that decrease airway resistance
- Increased lung volume
How chronic bronchitis increases airway resistance
-lumen of a typical airway contains excessive secretions
+ wal thickened by mucous gland hyperplasia and edema
How emphysema increases airway resistance
- many of the airways loose the radical traction of the tissues surrounding them
- -> driving pressure for flow rate during expiration when exercise= static recoil pressure of the lung –> this is reduced in these conditions
How bronchial asthma increases airway resistance
-contraction of the bronchial smooth muscle with resultant bronchoconstriction, mucous plugs occluding many airways and eema of their walls
How tracheal obstruction increases airway resistance
- caused by compression from the outside (ex an enlarged thyroid) or by intrinsic narrowing caused by scarring or a tumor
- obstruction is usually apparent during inspiration
