Ventilation/Perfusion relationships Flashcards
How is partial pressure for dry gas in air calculated?
- (PB- 47) x %gas
- example for O2: (760-47) x 0.21 = 150mmHg
- usually O2 is 160 mmHg
water exerts a saturated water vapour pressure of 47
How is arterial blood gas pressure measured?
- arterial blood sample + blood gas analyser
How is arterial PCO2 measured?
- measuring end-tidal volumes
What is Capnography?
- the continous recording of CO2 concentration in respiratory gas
How thick is the alveolocapillary membrane?
- 0.5µm
alveolar lining fluid, alveolar epithelium and capillary endothelium
What advantage does the thin alveolar-capillary membrane give?
- rapid and complete equilibration of O2 and CO2
What is Fick’s law of diffusion?
- gas exchange by simple diffusion
- proportional to pressure difference and surface area
- inversely proportional to distance
What happens to gas exchange in emphysema and lung fibrosis?
- REDUCED
- Reduced SA in emphysema
- Increased diffusion distance in lung fibrosis
What is diffusing capacity/transfer factor?
- extent to which a gas passes from air sacs of the lung into the blood
- DL = rate of transfer of gas from lung to blood/partial pressure difference
mmol/min/kPa
How is Diffusion capacity measured?
- small, non-lethal amounts of Carbon monoxide
- pCO in blood is zero, so partial pressure difference is alveolar pCO
- units = ml/min/kPa
What factors are dependent on DL?
- haemoglobin
- age
- sex
What is a normal DLCO?
- > 75%
Why is arterial PO2 slightly less than alveolar PO2?
Venous admixture:
- Anatomical shunt (bronchial and thesbian veins)
- V/Q mismatch
What is anatomical shunt?
- Blood bypasses the alveoli and enters systemic circulation without being oxygenated
Why is a greater than normal Arterial to alveolar PO2 gradient bad?
- suggests problems with gas exchange
Why happens if there is a rise in CO2 in the blood?
- can displace oxygen molecules
- hypoxemia
What is the RER?
- Respiratory exchange ratio
- CO2 production/O2 consumption
What is V/Q ratio?
- Ventilation/Perfusion matching
- normal is 0.8 for average of entire lung (ventilation is 80% of perfusion)
Is V/Q ratio varied in different parts of the lungs?
- Yes
- Near the apex = high VQ
- Near the base = low VQ
Why is V/Q higher at the apex?
- low ventilation
- even lower perfusion
- more air goes to base during inspiration
- TB more likely at apex since higher PO2
What does an increased V/Q mean?
- alveolar dead space increased
- wasted ventilation
What does a decreased VQ mean?
- shunting: deoxygenated venous blood bypasses alveoli, enters arteries
- hypoxaemia
What diseases could cause an increased VQ?
- Embolus, emphysema
- blockage in vessel
What diseases could cause a decreased VQ?
- COPD
- asthma
obstructive airway diseases
What is a “true shunt”?
- Blood flows through a region with zero ventilation
- abnormal right-left shunts in heart, atelectasis, consolidation
How does an increased VQ affect PO2 and PCO2?
- increased alveolar PO2
- decreased alveolar PCO2
- lung try to constrict airway + dilate vessel to combat
How does a decreased VQ affect PO2 and PCO2
- decreased alveolar PO2
- increased alveolar PCO2
- lung try to relax airway and constrict vessel
- HYPOXIC PULMONARY VASOCONSTRICTION
Why is Hypoxic vasoconstriction good in the lungs?
- Diverts blood flow away from unventilated areas, where blood flow will be wasted since gas exchange cannot occur, to ventilated areas
