Physiology Flashcards
What are the changes seen in lung function with obesity? What level of obesity do we need to start to get significant impact?
BMI > 40 is when we start to get worried.
Typically the changes are reduction in lung volumes, including VC, FEV1, TLC
There shouldn’t be a change in gas transfer, except Kco MAY BE INCREASED
what is the definition of clinically useful reversibility?
12% improvement AND 200mL improvement in FEV1, VC or FVC
ANY OF THESE PARAMETERS
how do we determine between a fixed airway obstruction and variable? (on RFTs)
variable would occur if there was no limitation during the other action.
that is, if it was extra-thoracic - this would cause an inspiratory limitation. If it was fixed, it would also limit expiration. If variable, it would only impact insp.
What should we think about if the flow rates during inspiration and expiration are similar?
this suggests a fixed obstruction. these are usually extra-thoracic
please explain how the DLco measures gas transfer.
a single breath is taken in. that breath contains about 0.3% carbon monoxide
the CO avidly binds any available Hb.
The CO uptake reflects the amount of “accessible Hb” in the pulmonary capillary bed.
what is the Kco used for?
the Kco is the rate constant for alveolar CO uptake.
This is the DLco with the alveolar volume (VA) excluded
what causes a low Kco?
This reflects a DECREASE in accessible Hb:
- diffuse alveolar-capillary damage such as emphysema and interstitial lung disease
- pulmonary vasc abnormality, such as pulm HTN, vasculitis
- intrapulmonary shunting - R to L (bypasses the lung)
- anaemia
what causes a high Kco?
This reflects an INCREASE in accessible Hb (in the areas of the lung that were ventilated):
- loss of respiratory units, e.g. pneumonectomy or consolidation (the amount of blood in the ventilated capillary beds has increased) (DOES NOT INCREASE DLCO)
- incomplete alveolar expansion of normal lung (chest wall) - (the amount of blood in the ventilated capillary beds has increased)
- asthma - increased pulmonary blood flow at the apices
- obesity - increase blood volume at the apices and pulmonary vascular congestion
- increased pulmonary blood flow - e.g. a left to right intra-cardiac shunt
- recent alveolar haemorrhage (not old bleed)
- polychthaemia
- exercise, because the capillary volume per Va increases
what changes do we see in lung volume reduction surgery compared to best medical care in advanced emphysema?
there is an increase in FEV1 by 30%
RV dec by 23%
6MWD inc by 30%
dyspnoea score improved
QOL not significantly improved
why is peak flow worse than FEV1 for measuring the degree of airflow obstruction in asthma?
basically because it is effort dependent
what does it mean if someone’s TLC is greatly increased (esp in setting of obstructive pattern)?
this suggests gas trapping, and points a lot towards emphysematous disease
what sort of thing moves the oxygen-dissociation curve to the right?
anything that increases activity.
that is, if you exercise, it moves it right. this is probably because EXERCISE is the RIGHT thing to do.
movement to the right will lead to easier dissociation of the Hb-O2.
Exercise leads to increased acid (pH down), inc DPG, inc temp, inc PaCO2
what moves the O2 curve to the left?
this is the opposite of exercise.
also, foetal Hbs are to the left. This is because they want to steal mum’s O2 preferentially.
what is the alveolar air equation?
For the purposes of this exam:
PAO2 = PiO2 - PaCO2/R
= 150 - 1.25*PaCO2
the amount of oxygen in the alveolus can be approximated as thus: the amount inspired (PiO2) minus the amount that was taken away from the alveolus (1.25 * PaCO2)
this is worked out by:
(760- 47 (water vapour)) * 21% = 150
the conversion of arterial CO2 to alveolar CO2 uses an R of 0.8
what is the A-a gradient?
this is a measurement of PaO2 - PAO2
the normal is about 5 - 15
there is normally a mild A-a gradient because of bronchial and thebesian veins
also in elderly people this can be up to 30! this is due to V/Q mismatch
