Ventilation and perfusion Flashcards
Other than gas exchange, what roles does the respiratory system have?
1) Reservoir for blood and oxygen
2) Metabolism of some circulating compounds - bradykinin, histamine, RAAS system
3) Filter blood - break down clots
4) Immune defence
What does the upper respiratory tract consist of?
Mouth, nose, sinuses and pharynx
What are the functions of the upper respiratory tract?
Conducts air to the larynx, the entrance to the lower respiratory tract.
1) Warms and humidifies air
2) Protects lower airways
What happens in conducting airways?
NO gas exchange (anatomical dead space) - bulk flow
What happens in respiratory airways?
Gas exchange - diffusion
Describe the stages in ventilation?
1) Expansion of cage –> negative pressure within the pleural cavity.
2) This causes lungs to expand.
3) Increase in volume reduces the pressure in respiratory airways.
4) Gas flows down pressure gradient into lungs.
5) Relaxation of muscles leads to reversal of process.
What is the difference between lung volumes and lung capacities?
Lung volumes change with breathing pattern, whereas lung capacities do not
Define Total Lung Capacity; what is it a sum of?
Total volume of air present in lungs at end of maximal inspiration.
=IRV+TV+ERV+RV
Define inspiratory capacity; what is it a sum of?
The biggest breath that can be taken from resting expiratory level (lung volume at the end of quiet expiration)
=IRV+TV
Define vital capacity; what is it a sum of?
Measured from maximum inspiration to maximum expiration (biggest breath that can be taken) - Key in monitoring neuromuscular disease as gives easily measurable indicator of degree of disease and warning about pending respiratory failure.
=IRV+TV+ERV
Define functional residual capacity; what is it a sum of?
The volume of air remaining in the lungs at the resting expiratory level - key in anaesthesia and intensive care.
=ERV+RV
What is (total) minute ventilation?
The amount of air moved into and out of the lungs per minute.
MV = TV x RR
Typical minute ventilation is 6-8l/min
What is serial dead space?
The volume of the conducting airways (anatomical dead space)
What is distributive dead space?
The parts of the lung that are not airways but cannot support gas exchange eg. damaged alveoli or alveoli with poor perfusion (physiological dead space)
How do you calculate the Alveolar Ventilation Rate (AVR)?
AVR = (Tidal volume - dead space volume) x RR
How will a change in AVR affect the diffusion rate?
Diffusion rate α (A. D .(∆P))/T
Increasing AVR will remove CO2 from the alveoli, thus lowering alveolar pCO2 and increasing ∆P, thus increasing CO2 diffusion from the blood (and removal from the body)
What happens if there is a physiological mismatch (V/Q mismatch) in healthy people?
Nothing, this is insignificant in normal conditions. V and Q vary in different areas of lung.
What happens to the V/Q ratio if there’s no ventilation?
V/Q –> 0
ie. shunt
What happens to the V/Q ratio if there’s no perfusion?
V/Q –> infinity
ie. deadspace
How does perfusion vary across the lung?
In an upright patient, blood flow is greater at the bases due to the effect of gravity causing higher blood flow to more dependant areas of the lung.
Zone 1 - Perfusion is absent.
Zone 2 - Perfusion is sporadic.
Zone 3 - Perfusion is constant.
How does ventilation vary across the lung?
Basal lung relatively compressed in its resting state – more potential for expansion than lung at the apex
Therefore more air moved in and out i.e. increased ventilation at bases
(Due to gravity)
How does the V/Q ratio vary across the lung?
As a result the V/Q ratio decreases moving down the lung
The base is relatively over-perfused (low V/Q ratio) and thus will have lower oxygen and higher carbon dioxide gas tensions in both alveoli and arterial blood
Apical alveolar PO2 is relatively high due to the lower blood flow (high V/Q ratio)
How can the V/Q ratio at the apex of the lung change with exercise?
Apical alveolar PO2 is relatively high due to the lower blood flow (Q), as oxygen uptake is perfusion dependent
Distensible apical blood vessels that are usually collapsed open during exercise to accommodate the increased cardiac output and this augmented perfusion increases the oxygen uptake at the top of the lung
Give a clinical example of a reduction in ventilation
Reduction in ventilation (decreasing V/Q ratio or shunt) of alveoli occurs in pneumonia; blood perfusing these areas is less oxygenated
Give a clinical example of a reduction in perfusion
Reduction in perfusion occurs in pulmonary embolus resulting in increase V/Q ratio or deadspace
