Flashcards in Lung Function Tests And Oxygen In The Blood Deck (40)
Describe how spirometry works
Subject breathes from a closed chamber over water
Volume changes with ventilation
Coupled apparatus with a pen draws a trace on a rotating drum
Define inspiratory reserve volume
The extra volume that can be breathed in above the tidal volume
Define expiratory reserve volume
The extra volume that can be breathed out after tidal volume
What is vital capacity?
Maximum expiration to maximum inspiration
Approx 5L in a typical adult
What is the functional residual capacity?
The volume of air left in the lungs at resting expiratory level
What is total lung capacity?
Volume of air in the lungs at the end of maximal inspiration
Approx 5.8L in a normal adult
Name some factors influencing the vital capacity
Force of inspiratory muscles
Describe single breathe spirometry
Breathe out as fast and fully as possible
Typically do 3 tests and take an average
Requires the subject to understand the test
What is a vitalograph trace?
Plotting volume expired against time
When is flow maximal?
When volume in the lungs is maximal due to the pressure gradient
What is FEV1?
The volume expired in the first second
Affected by how quickly air flow slows down if airways are narrowed
What is the normal ratio for FEV1:FVC?
Describe an obstructive disease
Airways are narrowed
Lungs are still easy to fill but resistance increases with expiration
Air will come out more slowly
(FEV1 decreased, FVC relatively normal)
If the narrowing of airways is reversible what does this suggest?
Describe a restrictive disease
Lungs are difficult to fill
Cannot achieve maximal volume of lungs (FVC lowered)
Air will come out at same rate so FEV1 normal
Ratio >70% generally
How can we measure PEFR?
Peak flow metre
Cheap and easy to use
Blow into tube as hard and fast as possible
Arrow shoots up scale to an estimate PEFR
Describe the flow volume loop of an obstructive disease
Reaches (almost) the PEFR
Then flow rate falls rapidly
Describe a restrictive flow volume loop
Shape of flow curve generally the same as normal
Doesn't reach as high a volume
Describe the helium dilution test
Measuring residual volume
Helium isn't metabolised therefore pt breathes a known volume of gas containing a known conc of helium
Helium conc diluted into the larger volume in the lungs
Measure the conc when we breathe out to work out what volume of air the helium expanded into
How do we measure serial/anatomical dead space?
Nitrogen wash out
Subject takes one normal breath of pure oxygen
Breathes out via meter reading the percentage of nitrogen
The volume expired half way between just oxygen and normal concs of nitrogen is the volume of serial dead space
What do we use the CO transfer factor for?
Measuring the diffusion capacity of the lung
Not done routinely
Patient inhales to total lung capacity - mixture containing CO
Hold breath for 10 sec
Rate of diffusion of CO estimated
Is oxygen soluble in water?
What is the equation for working out the concentration of oxygen dissolved?
Solubility of oxygen x pO2
What does the dissociation curve of myoglobin look like?
(Saturation = y, partial pressure = x)
Describe the T state of Hb
Low affinity for oxygen
Difficult for O2 to bind to Hb
When the pO2 is low, Hb is in which state?
What does the dissociation curve of Hb look like?
Above what approx kPa is Hb saturated?
What is the normal Hb concentration?