Respiratory Physiology - Ventilation and Work of Breathing Flashcards
Draw and explain the work of breathing graph for a tidal volume breath (presssure-volume loop)
Consider the lung as a single unit
IMAGE
Clarify if exponentially is correct
X axis in cmH2O (of negative intrathoracic pressure), Y axis in L (Volume)
In IPPV, the graph is similar but the X axis is positive
Point A on the X axis at (-0.5kPa) represents the negative pressure generated by the thoracic wall, ‘suspending’ the lungs in the intrapleural space prior to inspiration.
Point C, at around 1-.5kPa on the X axis, represents full inspiration.
In a perfect system without any resistance, inspiration would follow a straight line from A-C, and expiration from C-A.
In reality, energy is lost in work of breathing (in frictional and viscous loss), meaning that more pressure is required to generate the required flow.
The graph is drawn anticlockwise, and the area within ABCA being the work required to overcome dynamic airflow resistance.
The stored potential energy (static elastic resistance) is reflected in the area of ACDA.
During expiration, the area within CB’AC is the work done to combat dynamic airway resistance.
Since this is within ACDA, no active effort should be required, as this can simply be released from the elastic potential energy of the lung tissue.
If the CB’AC curve crosses the the AD line because of increased obstruction, then active work is required to exhale.
ACDA-CB’AC is the energy dispersed as heat (wasted)
Draw and explain the pressure-volume loop for a vital capacity breath
Consider the lung as a single unit
IMAGE - Graph
X axis in cmH2O (of negative intrathoracic pressure), Y axis in L (Volume)
In IPPV, the graph is similar but the X axis is positive
Inspiratory limb:
From full expiration, there is a simoid curve, reflecting low compliance to begin with at residual volume (more pressure change is needed to generate a change in volume).
At FRC the inspiratory limb is at its steepest - the point of maximal compliance, flattening out as lungs distend and compliance decreases.
Expiratory limb:
The shape remains sigmoid, but differs to the inspiratory limb due to elastic recoil of lung tissue.
At higher volumes, the compliance remains low, with a big pressure change before significant decrease in volume.
FRC represents the point of maximal compliance, and the steepest part of the sigmoid curve.
Zone differences:
Different areas of the lung sit at different points on the curve - the upper lobs begin higher on the curve, as they are more distended than the lower zones.
This is why upper areas of the lung are relatively less compliant
During IPPV, everything shifts down the curve, so the upper alveoli become more compliant
A tidal breath has been super-imposed on the graph for reference.
Explain the factors behind work of breathing
Effort is required to lift and expand the thoracic cage, to combat the elastic nature of lung tissues, and overcome resistance to air flow
Gentle expiration uses this stored potential and elastic energy, and is considered passive in healthy lungs
Forced expiration requires additional force to overcome obstructions to exhalation
How does the body detect a rise in CO2?
CO2 dissolves readily in plasma to form carbonic acid, which dissociates into H+ and HCO3-
It can cross the blood-brain barrier easily, dissolving into the CSF
Peripheral chemoreceptors respond to PaCO2 and pH via carotid and aortic bodies (Glossopharyngeal and vagus nerves, respectively)
Central chemoreceptors respond to pH changes in the CSF
How does a rise in PaCO2 affect MV?
IMAGE (CO2 graph)
The relationship is represented by a sigmoid curve on a graph.
At low PaCO2 (below 5kPa), minimal change in MV
From 5-10kPa, there is a relatively linera increase in minute ventilation (Doubling PaCO2 increases MV by 4x)
The effects of narcosis are seen above 10kPA
The graph is shifted rightward in chronic CO2 retainers (lower sensitivity to CO2)
Opioids reduce CO2 sensitivity, causing right-shift and vertical compression.
How does MV affect PaCO2?
IMAGE - Graph
As alveolar minute ventilation doubles, PACO2 halves.
This is a rectangular hyperbolic graph
How does PaO2 affect MV?
IMAGE - Graph
Above a PaO2 of 10kPA, there is minimal effect on MV
Below 8kPa, MV increases dramatically as peripheral chemoreceptors trigger the respiratory centre (Hypoxic drive)
In concurrent hypercapnoea, the graph is shifted upwards and rightwards (accounting for the added influence of central chemoreceptors)
How does MV affect PaO2?
IMAGE - Graph
??Why isn’t subscript working
Hyperbolic graph passing through a MV of 6L/min and PaO2 of 13.3kPA, without reaching either axis
Hyperventilation has minimal improvement on PaO2, but even a small amount of hypoventilation can cause rapid desaturation
Clinically this is relevant when extubating patients (especially obese patients, or those with higher O2 demand), when desynchronisation can cause a few minutes of low MV
Increasing the FiO2 shifts the graph upwards, and is the only effective way to increase PaO2
What is resistance in context of lung physiology?
Clinical significance and expansion of this might be nice
Change in pressure (in cmH2O) for a given change in flow of air through the airways
Measured in cmH2O/L/sec
Total resistance = chest wall resistance + lung resistance
Explain the term ‘time constant’ in context of lung physiology
A product of compliance and resistance, that describes how rapidly a given unit fo lung will either empty or fill up
Also defined as the time taken for the lung unit to inflate to 64% of its total volume
τ (seconds) = compliance x resistance
High compliance or resistance increases τ, meaning the lungs are slower to fill and empty (Asthma)
Less compliance or resistance lowers τ, meaning the lungs are quicker to fill and empty (ARDS)
Physiologically, this means that different areas of the lung can fill at different times and rates
