F6 + F7 - diffusion and V/Q Flashcards
How long does it take a blood cell to pass through the pulmonary capillaries?
0.75 seconds
Carbon dioxide takes how long to complete diffusion - equalisation of partial pressures?
0.1 seconds
How logn does oxygen take to equalise its partial pressures/complete diffusion at the respiratory membrane
0.3-0.4 seconds
Relate Fick’s diffusion laws to the diffusion capacity of the lungs analysed?
Small quantity of carbon monoxide in inhaled gas and the amount taken up analysed by difference between inspired and expired gas. It is highly soluble so the uptake is not flow limited
Utilising Fick’s law of diffusion the amount fo gas transferred across a sheet fo tissue is proportional to area, diffusion constant and difference in partial pressure, and inversely proportional to thickness. It is re-written as area nd thickness cannot be measured as
What factors are incorporated in the ‘diffusion capacity of the lung’ as measured using carbon monoxide
area, thickness, diffusing proprties of the membrane and gas –>
Describe the measurement technique for diffusion capacity
Define dead space
Dead space is the fraction of tidal volume which does not participate in gas exchange.
What components are there of dead space
physiological dead space and apparatus dead space
What is physiological dead space composed of
Anatomical deead space
Alveolar dead space
What is anatomical dead space
the volume of gas in the conducting airways from the lips to the innermost terminal bronchioles i.e. the volume of gas (150mls) exchaled before the CO2 concentration rises to its alveolar plateau
What is alveolar dead space
the fraction of tidal volume which passes beyond the anatomical dead space to mix with alveolar gas without participating in gas exchange i.e. where ventilation exceeds perfusion to a lung segment; otherwise known as the difference between
What is artifiical dead space
is the volume of gas not involved in respiration in an artificial breathing circuit. It can reduce or increase the dead space
What is the Bohr equation
Difference between exhaled CO2 and alveolar CO2 - hard to measure
◦ V(D)/V (T) = [F(A)CO2 - F(E)CO2]/F(A)CO2 ◦ VTx F(e)CO2 = (VT - VD) x FaCO2 (alveolar) ◦ F is for fraction; can use partial pressures to yield the same result ◦ Problem is regional alveolar CO2 varies throughout the lung significantly due to different V/Q ratios
What is the equation for the Bohr equation
◦ V(D)/V (T) = [F(A)CO2 - F(E)CO2]/F(A)CO2
◦ VTx F(e)CO2 = (VT - VD) x FaCO2 (alveolar)
◦ F is for fraction; can use partial pressures to yield the same result
◦ Problem is regional alveolar CO2 varies throughout the lung significantly due to different V/Q ratios
What is the problem with PACO2 in the Bohr equation
Difficult to measure
And regional alveolar CO2 varies throughout the lung significantly
What modificationo to the Bohr equation is instead used?
Enghoff modification
PaCO2 instead of PACO2
What is the basis for PaCO2 being used instead of PACO2 in the Enghoff modification of the Bohr equation
easier to measure and represented an average of the CO2 across all the alveolar units assuming good gas exchange and no shunting)
What is the Enghoff equation
Vd/VT = PaCO2 - P-mixed expired CO2/PaCO2
What is the assumptions of the Enghoff equation
Assumes good gas exchange
Assumes no shunting
What flaws are there in the Enghoff modification of the Bohr equation
Right to left shunt will appear as dead sapce
V/Q heterogeneity
Diffusion impairment and bronchospasm makes finding plateau for expired CO2 difficult
What can be used to calculate anatomical dead space
Fowlers method
What is Fowlers method
Attach the patient to a pneumotachograph to measure flow over time with a sensitive nitrogen sensor
What is the pneumotachograph
measures flow over time when attaching a patient to it
What does the patient breathe in Fowlers method of anatomical dead space calculation?
Single breath 100% FiO2
Same tidal volume as usual and do not pause between breaths
What happens as a patient inhales 100% oxygen for 1 breath
O2 replaces nitrgoen in the anatomical dead space
How is a Fowlers method test done
◦ Single breath of 100% oxygen, same tidal volume as usual and gap between breaths cannot be too long
◦ Oxygen replaces nitrogen in anatomical dead space
◦ Exhaled breath has its volume and nitrogen concentration measured (flow over time graph –> i.e. volume)
◦ Graph of nitrogen concnetration over volume cna be used to caclulate the anatomical dead space
◦ Phase 1 –> pure Oyxgen from dead space
◦ Phase 2 –> exhaled nitrogen rises rapidly and represents gas from fast time constant alveoli mixing with gas from more distal airways, half of this is counted as dead space
◦ Phase 1 and half of phase 2 in the single breath nitrogen washout test =dead space
◦ Phase 3 = plateau –> alveolus entirely
◦ Phase 4 - another increase in nitrogen concentration representing closing capacity as small airways in more compliant regions close and only poorly compliant alveoli exhale nitrogen rich gas without it being diluted
Describe the phases of the single breath nitrogen washout test
◦ Single breath of 100% oxygen, same tidal volume as usual and gap between breaths cannot be too long
◦ Oxygen replaces nitrogen in anatomical dead space
◦ Exhaled breath has its volume and nitrogen concentration measured (flow over time graph –> i.e. volume)
◦ Graph of nitrogen concnetration over volume cna be used to caclulate the anatomical dead space
◦ Phase 1 –> pure Oyxgen from dead space
◦ Phase 2 –> exhaled nitrogen rises rapidly and represents gas from fast time constant alveoli mixing with gas from more distal airways, half of this is counted as dead space
◦ Phase 1 and half of phase 2 in the single breath nitrogen washout test =dead space
◦ Phase 3 = plateau –> alveolus entirely
◦ Phase 4 - another increase in nitrogen concentration representing closing capacity as small airways in more compliant regions close and only poorly compliant alveoli exhale nitrogen rich gas without it being diluted
What is the impact of increased dead space
Reduction in TV
- Decreased CO2 clearance for a given minute volume
- Decreased oxygenation due to increased alveolar CO2
- Increased work of breathing due to decreased efficacy
- Proportional increase in required minute ventilation to the change in ratio of dead space to alveolar ventilation
if you had a large lobar PE and this halved your alveolar ventilation what do you have to do to reach normal gas exchange
Double minute volume
If you add dead space through apparatus how do you account for this in target TV
add the volume of the increased dead space to the TV
How do you measure alveolar dead space
You don’t but if you measure physiological dead space (Bohr method) then subtract anatomical dead space with Fowlers method you get alveolar dead sapce
Draw a diagram to represent Fowlers method
Alveolar ventilation =
= Tidal ventilation - dead space ventilation
= VCO2/PCO2 x K
VCO2 =
VCO2 = VA x %CO2/100
%CO2/100 = FCO2
Therefore
VCO2 = FCO2 x VA
As PCO2 = FCO2 x k
VA = VCO2/PCO2 x K
Alveolar ventilation equation
VA = VCO2/PCO2 x K
Why is there V/Q mismatch in the lung?
Gravity and its effect on blood flow through low presurre pulmonary circulation
Variation in pulmonary perfusion due to
- Lung volume
- pulmonary vascular architecture
- Hypoxic pulmonary vasoconstriction (improves V/Q mismatch)
Vertical gradient of pleural pressure - transpulmonary pressures affecting alveoli are different throughout the lung, as changes to the shape of the horacic cavity are unequal. Base expands more than apex.
Compliance differences
- more compliant areas are better ventilated for a given pressure, apical lung is more inflated and distended at baseline, leaving the base more compliant and therefore better ventilated
Ventilation based pathology
Perfusion based pathology
V/Q ratio in the normal upright lung
- Apex
- V/Q 1
- Bases
Apex V/Q >1 usually around 3’
At the 3rd rib equal to the midzones is the V/Q ratio of 1
Lung bases have aV/Q ratio of 0.6 usually when upright
What does V/Q of infinity mean
Dead space ventilation
What does a V/Q of 0 mean
No ventilation, shunt
Average V/Q ratio of the lung is?
0.8
What is the V/Q ratio when supine?
Close to 1 throughout the lung
The lower the V/Q ratio what happens?
Close to true shunt and mixed venous blood
What is the relationship between PaO2 and V/Q? How does this compare to PACO2
◦ The relationship between PaO2 and V/Q is steeper and more sigmoid than the relationship between PaCO2 and V/Q.
Draw the relationship between PaO2 and VQ
◦ The relationship between PaO2 and V/Q is steeper and more sigmoid than the relationship between PaCO2 and V/Q.
Draw the relationship between PaCO2 and VQ
◦ The relationship between PaO2 and V/Q is steeper and more sigmoid than the relationship between PaCO2 and V/Q.
Draw the relationship between blood oxygen content and VQ
◦ The relationship between PaO2 and V/Q is steeper and more sigmoid than the relationship between PaCO2 and V/Q.
What effect does low V/Q have on oxygen? How can it be reversed? Under what conditions can it not be reversed?
◦ The relationship between PaO2 and V/Q is steeper and more sigmoid than the relationship between PaCO2 and V/Q.
◦ Low V/Q values (V/Q ratios between 0 and 1) result in hypoxia ◦ The hypoxia due to low V/Q ratio is reversible with increased FiO2 ◦ "True" shunt where V/Q = 0 does not improve with increased FiO2
Why does Low VQ effect oxygen so markedly
‣ Areas with reduced ventilation have marked reduction in oxygen delivery in proportion to blood flow and have an effluent oxygen saturation markedly reduced comparted to V/Q 1
‣ Areas with higher V/Q ratios elsewhere in the lung will have reduced blood flow - and due to fixed amoutn of oxygen blood is able to carry due to the maximum oxygen carrying capacity of haemoglobin (plateau of oxyhaemoglobin dissociation curve) the oxygen content of the combined blood flows is not markedly raised by these areas
What is normal minute vnetilation
4L/min
Draw a diagraph presenting blood flow vs ventilation as a function of areas of the lung
What is the normal V/Q ratio for the lungs? Relate this to normal physiology
0.8
Ventilation - 4L/min on average
Cardiac output 5L/min on average
Why is there regional varition in perfusion 4
Gravity and its effect on blood flow through low presurre pulmonary circulation
- Pulmonary vascular architecture
‣ Lung volume (atelectasis increases pulmonary vascular resistance)
‣ Hypoxic pulmonary vasoconstriction
Why is there regional variation in ventilation 4
Gravity - weight of lung producing a vertical gradient i pleural pressure
Posture - changes the direction of the pleural gradient
Anatomical expansion potential - bases have more room to expand than the apices
Lung compliance - improved in the bases compared to the apices
Pattern of breathing
Explain the pleural pressure gradient
What is V/Q scatter
- The distribution of lung units along a spectrum of V/Q ratios is referred to as “V/Q scatter”
- In a normal young person, this “scatter” spans a V/Q range between 0.6 and 3.0
Draw a V/Q scatter plot for the effect of age on lungs
This reflects rising closing capacity and basal shunt
2 methods of measurnig V/Q mismatch
Functional techniques - MIGET, 3 compartment model
Imaging techniques - radionucleitide imaging, SPECT V/Q sacns, PET scans and MRI using IV gadolinium and 3He or 129Xe
What is the MIGET model used to measure? How does it do this?
V/Q mismatch
Multiple inert gas elimination technique using 6 dissolved gasses infused IV
- AV difference concentration and known blood:gas partition coeffcient is used to determine the distribution of V/Q
What is the 3 compartment model and what is it used for?
Assumes 3 gas exchange units
- Dead space
- True shunt
- V/Q 1
Requires PaO2, PaCO2 and estimation fo alveolar O2 and CO2 partial pressures
Magnitude of shunt as a proportion of cardiac output and magnitude of dead space
How do imaging technqiues for V/Q mismatch work
◦ SPECT V/Q scans- regional distribution of blood flow and ventilation can be calculated - poor V/Q matching is visually inspected. Regional distribution of these radionuclides is measured by a camera detecting gamma rays, perfusion measured using IV Tc labelled albumin; 133 Xenon used for ventilation
◦ PET scans - same as above but positron emitter isotopes (13N2) instead of gamma ray emitters
◦ MRI using IV gadolinium and 3He or 129Xe
What is the difference between V/Q scatter and true shunt
Distinguishing true shunt from V/Q scatter - V/Q scatter is where areas have reduced ventilation and therefore contribute a reduced oxygen supply to the blood supply, but for it to be true shunt the V/Q ratio needs to be zero
* V/Q scatter hypoxia will improve with supplemental O2 –> as even with reduced ventilation you can deliver a normal oxygen supply to the alveolus just with reduced volume and hgiher FiO2
What area of V/Q does the largest change in gas exchange occur
betwen 0.1 and 1
Improving the ventilation of a severely underventilted region from V/Q 0.01 to 0.1 doesn’t accomplish much
What effect do high V/Q areas have in the lung
- Due to alveolar content having high oxygen and low CO2 the increased equilibration can lead to dramatic changes in effluent blood
- Effluent blood closely resembles alveolar gas - what little blood flows is maximally ventilated and therefore increasing ventilation further has no effect on increasing gas exchange - and increasing FiO2 does not add a great deal as oxygenation is already maximal
- Their total contribution to gas exchange is minimal because blood flow as a proportion is minimal
What impact does emphysema have on V/Q
Large excess ventilation with poor perfusion - large dead space
What impact does chronic bronchitis have on V/Q
Large amount of blood flow to poorly ventilated regions –> hypoxia as effluent resembles mixed venous blood
What effect does asthma have on V/Q
25% of blood flow to poorly ventilated lung units with V/Q of 0.1 - similar to chronic bronchitis patients this leads to hypoxia. The low shunt value is probably due to collateral pendelluft and therefore get enough oxygen
In the shunt equation what is the numberator and what does this mean
(CcO2 - CaO2) is the difference in oxygen content between “perfect” endcapillary blood and systemic arterial blood; this drop in oxygen content is due to the venous admixture.
In the shunt equation what is the denominator and what does that mean?
(CcO2 - CvO2) is the difference in oxygen content between mixed venous and “perfect” endcapillary blood
Shunt vs PaO2 graph
Draw a diagram relating PaCO2 to ventilation and what FiO2 is required to normalise PaO2
Draw a diagram showing the effect of shunt on PaO2 at different FIO2
