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