eLFH - Respiratory Physiology Part 2 Flashcards
Compliance definition
Volume change per unit change in pressure
Compliance equation
C = V / P
Compliance = Volume / Pressure
Units of compliance
ml/cmH2O
or
L/kPa
Measurement of compliance
Measured on pressure-volume (PV) graph
Gradient of line represents degree of compliance
Steeper gradient = greater compliance and easier for lungs to expand
Values for lung compliance
1.5 - 2 L/kPa
Values for chest wall compliance
1.5 - 2 L/kPa
Values for total thoracic compliance
0.75 - 1 L/kPa
How to add compliances together
Static compliance definition
Lung compliance when gas flow has ceased
Dynamic compliance definition
Lung compliance during the respiratory cycle while gas flow is ongoing
Which is higher, static compliance or dynamic compliance
Static compliance usually higher
There is time for pressure and volume to equilibrate
Why doesn’t Pressure-Volume (PV) curve start at zero
Lungs are never completely collapsed so always some volume present
Why are PV curves different for inspiration and expiration
Hysteresis
Lung volume during expiration always greater for a given pressure than during inspiration
On which part of PV curve does tidal breathing usually occur
The steepest pert of PV curve as this is where compliance is greatest so minimises work of breathing
Changes in compliance at different parts of the lung
Compliance at base of lung is better than apex of lung
Volume at base is lower due to gravitational effects but ventilates better
Factors which decrease lung compliance
Extremes of lung volumes
Atelectasis
Kyphoscoliosis
Vascular engorgement
Lung fibrosis
Pulmonary oedema
Factors which increase lung compliance
Surfactant
Old age
Emphysema
What causes surface tension
Forces of attraction between molecules at the gas / fluid interface
Action of surface tension
Collapse down the alveoli
Smaller radius of alveolus, the greater the pressure collapsing the sphere
Laplace’s Law
P = 2T / R
P = Pressure
T = Tension
R = Radius
Refers to collapsing pressure of alveoli
Why does saline filled lung have greater compliance than air filled lung
Gas/fluid interface is removed and therefore surface tension is removed
Composition of surfactant
Phospholipid dipalmitoylphosphatidylcholine (DPPC), protein and carbohydrate
Production of surfactant
Produced by Type II pneumocytes
From free fatty acids extracted from blood
Factor which can impact surfactant production
Lack of blood flow can affect surfactant production as it uses free fatty acids extracted from blood
Functions of surfactant
Increases compliance
Preventing transudation of fluid into alveoli (pulmonary oedema)
Stabilising alveoli - preventing collapse
How does surfactant increase compliance of alveoli
Profoundly reduces surface tension by disrupting attractive forces
DPPC have hydrophilic heads and hydrophobic tails
Hydrophilic ends line up in alveoli and repel each other
When is surfactant most effective at reducing surface tension and why
At lower volume / smaller radius of alveoli as repulsive forces between DPPC molecules is greater
Action of surfactant to stabilise alveoli - diagrams
How does surfactant reduce transudation of fluid into the alveoli
By reducing surface tension
Surface tension tends to draw fluid into the alveolus from the capillary
Work of breathing definition
Effort required to overcome:
- elastic forces in the lung
- resistance force from air flow and viscous resistance of tissue moving over tissue
Elastic forces of lung in terms of energy
Energy stored as potential energy during inspiration and utilised during expiration
Resistance force of air flow and viscous resistance of tissue on tissue in terms of energy
Extra energy is dissipated as heat during quiet breathing where expiration is passive from elastic recoil portion of energy stored
How to measure work of breathing
Area under a pressure-volume curve
Work done equation (used for work of breathing) and derivation
Work done = Change in pressure x Change in volume
Derivation in picture shown
Proportion of work done during inspiration for spontaneous breathing
65% total work done during inspiration to overcome elastic forces
Stored as potential energy
Proportion of work done during expiration for spontaneous breathing
35% total work done during expiration to overcome resistance forces
(28% airway resistance, 7% viscous tissue resistance)
Extra energy dissipated as heat
Effect of RR on work done
As RR increases, work against resistance forces increases
Effect of tidal volume on work done
As VT increases, work against elastic tissues increases
Optimal RR for a given minute ventilation to minimise total work done
RR 14 - 16
Effect of obstructive lung defects on optimal RR and VT to minimise work done
Obstructive lung disease increases work of resistance
Therefore lower RR and higher VT minimise work
Effect of restrictive lung defects on optimal RR and VT to minimise work done
Restrictive lung disease increases elastic work
Therefore higher RR and lower VT minimise work
Factors which increase work of breathing
Anything that increases the area under the pressure-volume curve:
Larger tidal volumes
Reduced compliance
Obstructive defects
Exercise - increases VT and RR
How does GA increase work of breathing in spontaneously breathing patients
Reduced FRC - lung compliance decreased
Narrow ETT and circuits - increased airway resistance
