Resp Session 3 Flashcards
How does tracheal air differ from dry air?
Saturated with water vapour meaning 6% is water vapour hence oxygen and nitrogen are slightly diluted
What does Fick’s law of diffusion state?
Flux of molecules across a barrier is proportional to the permeability of the molecules x transfer SA available x concentration gradient
Why does anything affecting diffusion only change oxygen transport?
This is limiting due to its much lower solubility than carbon dioxide and there has been a loss of the compensatory larger pressure gradient for oxygen
What factors affect the rate of diffusion of a gas?
Pressure difference Cross sectional area of fluid Molecular weight of the gas Solubility of the gas in solution Diffusion distance Temperature
What does Graham’s law of diffusion state?
Lighter gas effusion more rapidly
Considering Graham’s law why does oxygen diffuse slower than carbon dioxide?
Although oxygen is lighter in liquids the solubility of the gas also affects diffusion and carbon dioxide is much more soluble so it’s concentration gradient can be much smaller
Why is the partial pressure of alveolar oxygen lower than external environment?
Exchange and dilution
What is exhaled air diluted by?
Dead space air
How is carbon dioxide transported in the blood?
5% dissolved in plasma
5% as carboxyhaemoglobin on proteins
90% as HCO3- in plasma due to reaction with water in RBCs to form carbonic acid
What is the normal pO2 of alveolar air?
13.3 kPa
What is the normal pCO2 of alveolar air?
5.3 kPa
Why are the partial pressures of gases in the alveolar air and alveolar capillary blood equal?
They are at equilibrium due to gas exchange
On healthy lungs is surface area a limiting factor for gas exchange?
No
How does the distance between blood and alveolar air change in inhalation?
Decreases as lung extends
What forms the diffusion barrier in the lungs?
Epithelial cell of alveolus Tissue fluid Endothelial cell of capillary Plasma RBC membrane
Why is diffusion impaired by oedema in the lungs?
Increases tissue fluid gas has to move through therefore increases diffusion distance
When will pressure differences between gases on either side of the diffusion barrier change and affect diffusion?
In some pathologies
How does fibrotic lung disease affect diffusion in the lungs?
Exchange surface thickens –> increased diffusion distance so pO2 in capillary blood is decreased but pCO2 is normal due to faster diffusion rate
How does emphysema affect diffusion in the lungs?
Decreases surface area of alveoli so pO2 in capillary is reduced but pCO2 ok
Why does blood spend 3x as long as it needs to for diffusion of O2 to be effective in the capillary?
Built in redundancy allows for increased bloodflow in exercise to ensure gas exchange is not limiting
How many breaths are needed to totally exchange alveolar air?
7-8
Why is only 50% of alveolar air removed at normal ventilation rate?
Remaining 50% guards against sudden changes in gas levels so if respiration is temporarily interrupted blood gas levels and pH are ok
Why does rate of alveolar ventilation have to increase in exercise?
So alveolar pO2 can be maintained
How is alveolar ventilation rate calculated?
Pulmonary ventilation rate - dead space ventilation rate
= (TV x RR)-(DSV x RR)
What is lung perfusion in the average man?
Cardiac output from R ventricle = 5 l per min
What is the AVR for the average man?
~5 l per min
What is the overall V/Q ratio in the lungs in health?
Ideally 1
What happens if V/Q >1 at the alveolar level?
pO2 increases and pCO2 decreases as in PE
What is serial dead space?
Volume of airways
What is distributive dead space?
Volume of lungs not participating in gas exchange due to tissue damage/death or poorly perfused alveoli
What is physiological dead space?
Serial dead space + distributive dead space
When will the value of physiological dead space change?
In pathology
How does AVR change with rapid, shallow breathing?
Decreases
How does AVR change with slow, deep breathing?
Increases
Why do we not breathe as slowly and deeply as possible as this gives a more effective AVR?
More energy intensive so need to compromise energy use and ventilation
What does the amount of water vapour depend on in dry air?
Relative humidity and temperature
What do lung function tests assess?
Mechanical condition of lungs
Resistance of airways
Alveolar membrane
What are the advantages of simple lung function tests which can be carried out in primary care?
Non-invasive
Cheap
Technically simple
What is the problem with using simple lung function tests in primary care?
They are very user dependent - pt must fully understand clinician’s explanation and use sufficient effort
What is residual volume?
Volume remaining after maximal exhalation which cannot be measured by spirometry
Does residual lung volume contribute to total lung capacity?
Yes
What is the difference between volumes and capacities in the pattern of breathing cycle?
Volumes can change over cycle but capacities, which are the combination of two or more volumes, are fixed
What is vital capacity?
Biggest breath that can be taken from max inspiration to max expiration, typically 5l
= IRV+TV+ERV
What is inspiratory capacity?
Biggest breath than can be taken from resting expiratory level, typically 3l
What is functional residual capacity?
Volume of air at resting expiratory level
= ERV+RV
What is the clinical relevance of FRC?
A large number of factors can affect it
Provides considerable O2 reserve which is important when assessing hypoxia risk in anaesthetics
What is total lung capacity?
Volume of gas in the lungs at the end of maximal inspiration, typically 5.8l
What does TLC depend on?
Size of person
What can be predicted by plotting a pt’s height and weight on a gender specific nomogram?
Forced vital capacity (FVC)
Forced expired volume (FEV1)
How is single breath spirometry conducted?
Pt fills lungs then breathes out for as long and as fully as possible. Carried out 3x
What are important factors to consider when using single breath spirometry?
Pt competency and clarity of procedural explanation
How can single breath spirometry be used to assess the likelihood of a pt having asthma?
Perform before and after treatment with bronchodilator to see if obstructive pattern is reversible
What is a normal FEV1/FVC ratio?
> 70%
What does an FEV1/FVC
Obstructive lung disease
What does an FeV1/FVC»70% indicate?
Restrictive lung disease
How does an obstructive pattern appear on single breath spirometry?
Lungs fill easily but resistance to air flow –> normal FVC but reduced FEV1 (vol expired vs time curve becomes straighter)
How does a restrictive pattern appear on single breath spirometry?
Lungs difficult to fill –> decreased FVC but FEV1 normal
Vol expired vs time curve same shape but values lower
When is maximum flow rate seen on a flow volume curve?
When lungs are full so airways are at their most stretched
Why is using a simple PEFR device insensitive for testing airway narrowing?
Most affected by trachea and bronchial resistance not small airways so if PEFR is found to be reduced further tests are needed
What are the advantages of flow volume curves compared to single breath spirometry?
More sensitive
Can detect changes earlier in the course of disease
Discriminates where in the respiratory tract the problem lies
How does an obstructive pattern appear on a flow volume curve?
PEFR normal but curve scooped out
Why is the flow volume curve scooped out in obstructive lung disease?
Exacerbation of already narrowed airways during expiration reduces flow rate
How does a restrictive pattern appear on a flow volume curve?
PEFR reduced
Curve generally same shape but narrowed
How can helium dilution be used to measure residual volume?
Helium not metabolised so pt breathes in a known volume of gas containing a known [He] starting from FVC and then [He] can be monitored as it is diluted in the larger volume
How is nitrogen washout used to measure serial dead space?
Take one breath of pure O2 then breathe out via % nitrogen meter - initially pure O2 but them mixed with alveolar air until a plateau is reached where gas exhaled = alveolar gas. Find mid point of curve up to plateau and area under is the dead space
When is diffusion conductance using carbon monoxide used?
If spirometry suggests reduced VC, RV +/- TLC
How is a diffusion conductance test carried out to assess all gas exchange barriers in the lungs?
Pt inhales gas mix with CO up to TLC, holds breath for 10s so CO moves into RBCs and then rate of CO diffusion can be estimated to measure CO conductance
