Session 3 - part II Flashcards
What is non invasive lung testing?
Lung function is inferred by measuring volumes, pressures, flows, composition (at mouth)
What limits vital capacity?
Maximum inspiration (effected by compliance of the lungs, force of muscles) Maximum expiration (effected by airway resistance)
Why might vital capacity be lower than expected?
The patient may not be able to breathe out or in maximally.
How can you investigate the reason for vital capacity being lower than expected?
Single breath spirometry (The patient fills their lungs with atmospheric air and breathes out as far and fast as possible)
What is a Vitalograph trace?
A plot of volume expired Vs time. Would be an initial rapid rise followed by a plateau
What is FEV1?
Volume expired in the first second
Define Forced vital capacity
Maximum volume that can be expired from full lungs which is usually 5L
What effects FEV1?
How quickly air slows down. So it is lower if the airways are narrowed. It is usually >70% FVC
What is FVC?
The percentage of the total volume that leaves in the first second
What can effect FVC?
Restrictive defects. The lungs are harder to fill due to them being stiff, muscle weakness or problems with the chest wall. This causes the lungs to begin being less full so FVC will be reduced. Air will still leave normally so FEV1 will still be >70% of FVC
What causes FEV1 to be reduced?
An obstructive defect. The airways are narrowed, but still easy to fill the lungs. However, resistance will increase in expiration so air will come out more slowly therefore FEV1 will be longer. FVC will be relatively normal
What are flow volume curves?
A plot of volume expired Vs flow rate. Derived from Vitalograph trace
When is flow rate maximal?
When the lungs are full as little air is expired. The airways are stretched so resistance is minimum
What is PEFR?
Peak Expiratory Flow Rate (Effected in asthma. Limited by larger airways)
When does flow rate fall?
As the lungs are compressed, more air is expired. The airways begin to narrow and resistance increases. The narrower the airways are to begin with, the more rapidly it falls
What is a good indicator of airway narrowing?
Flow volume curves
They can also show whether the narrowing airways is a large or small airway
How can you measure Residual volume?
With a helium dilution (Helium as it is not naturally occurring in the air and it is insoluble in blood)
How does a Helium dilution test work?
A known concentration of Helium is breathed in
Starting at the FRC and see how much the concentration is reduced by mixing with the residual air already in the lungs.
What is a nitrogen wash out used for?
Serial dead space and indirectly can measure ventilation perfusion matching
How is a nitrogen wash out performed?
The patient takes a breath of pure O2 (Mixes with the air already in the lungs)
Patient then breathes out via a meter measuring % nitrogen
O2 leaves, then O2 and air (including N2 from alveoli)
The volume expired at the transition is the serial dead space
Define conductance
How easily a gas passes across a barrier
How do you measure the conductance of O2?
Has a similar conductance to CO and it is not usually in the blood so use CO in the test,
Patient breathes in a known volume of CO, then can measure CO in blood so can infer how much has crossed the alveolar barrier
