eLFH - Respiratory Physiology Part 2

Question 1

Compliance definition

Answer 1

Volume change per unit change in pressure

Question 2

Compliance equation

Answer 2

C = V / P

Compliance = Volume / Pressure

Question 3

Units of compliance

Answer 3

ml/cmH2O

or

L/kPa

Question 4

Measurement of compliance

Answer 4

Measured on pressure-volume (PV) graph

Gradient of line represents degree of compliance
Steeper gradient = greater compliance and easier for lungs to expand

Question 5

Values for lung compliance

Answer 5

1.5 - 2 L/kPa

Question 6

Values for chest wall compliance

Answer 6

1.5 - 2 L/kPa

Question 7

Values for total thoracic compliance

Answer 7

0.75 - 1 L/kPa

Question 8

How to add compliances together

Answer 8

Question 9

Static compliance definition

Answer 9

Lung compliance when gas flow has ceased

Question 10

Dynamic compliance definition

Answer 10

Lung compliance during the respiratory cycle while gas flow is ongoing

Question 11

Which is higher, static compliance or dynamic compliance

Answer 11

Static compliance usually higher

There is time for pressure and volume to equilibrate

Question 12

Why doesn’t Pressure-Volume (PV) curve start at zero

Answer 12

Lungs are never completely collapsed so always some volume present

Question 13

Why are PV curves different for inspiration and expiration

Answer 13

Hysteresis

Lung volume during expiration always greater for a given pressure than during inspiration

Question 14

On which part of PV curve does tidal breathing usually occur

Answer 14

The steepest pert of PV curve as this is where compliance is greatest so minimises work of breathing

Question 15

Changes in compliance at different parts of the lung

Answer 15

Compliance at base of lung is better than apex of lung

Volume at base is lower due to gravitational effects but ventilates better

Question 16

Factors which decrease lung compliance

Answer 16

Extremes of lung volumes
Atelectasis
Kyphoscoliosis
Vascular engorgement
Lung fibrosis
Pulmonary oedema

Question 17

Factors which increase lung compliance

Answer 17

Surfactant
Old age
Emphysema

Question 18

What causes surface tension

Answer 18

Forces of attraction between molecules at the gas / fluid interface

Question 19

Action of surface tension

Answer 19

Collapse down the alveoli

Smaller radius of alveolus, the greater the pressure collapsing the sphere

Question 20

Laplace’s Law

Answer 20

P = 2T / R

P = Pressure
T = Tension
R = Radius

Refers to collapsing pressure of alveoli

Question 21

Why does saline filled lung have greater compliance than air filled lung

Answer 21

Gas/fluid interface is removed and therefore surface tension is removed

Question 22

Composition of surfactant

Answer 22

Phospholipid dipalmitoylphosphatidylcholine (DPPC), protein and carbohydrate

Question 23

Production of surfactant

Answer 23

Produced by Type II pneumocytes
From free fatty acids extracted from blood

Question 24

Factor which can impact surfactant production

Answer 24

Lack of blood flow can affect surfactant production as it uses free fatty acids extracted from blood

Question 25

Functions of surfactant

Answer 25

Increases compliance Preventing transudation of fluid into alveoli (pulmonary oedema) Stabilising alveoli - preventing collapse

Question 26

How does surfactant increase compliance of alveoli

Answer 26

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

Question 27

When is surfactant most effective at reducing surface tension and why

Answer 27

At lower volume / smaller radius of alveoli as repulsive forces between DPPC molecules is greater

Question 28

Action of surfactant to stabilise alveoli - diagrams

Answer 28

Question 29

How does surfactant reduce transudation of fluid into the alveoli

Answer 29

By reducing surface tension Surface tension tends to draw fluid into the alveolus from the capillary

Question 30

Work of breathing definition

Answer 30

Effort required to overcome: - elastic forces in the lung - resistance force from air flow and viscous resistance of tissue moving over tissue

Question 31

Elastic forces of lung in terms of energy

Answer 31

Energy stored as potential energy during inspiration and utilised during expiration

Question 32

Resistance force of air flow and viscous resistance of tissue on tissue in terms of energy

Answer 32

Extra energy is dissipated as heat during quiet breathing where expiration is passive from elastic recoil portion of energy stored

Question 33

How to measure work of breathing

Answer 33

Area under a pressure-volume curve

Question 34

Work done equation (used for work of breathing) and derivation

Answer 34

Work done = Change in pressure x Change in volume Derivation in picture shown

Question 35

Proportion of work done during inspiration for spontaneous breathing

Answer 35

65% total work done during inspiration to overcome elastic forces Stored as potential energy

Question 36

Proportion of work done during expiration for spontaneous breathing

Answer 36

35% total work done during expiration to overcome resistance forces (28% airway resistance, 7% viscous tissue resistance) Extra energy dissipated as heat

Question 37

Effect of RR on work done

Answer 37

As RR increases, work against resistance forces increases

Question 38

Effect of tidal volume on work done

Answer 38

As VT increases, work against elastic tissues increases

Question 39

Optimal RR for a given minute ventilation to minimise total work done

Answer 39

RR 14 - 16

Question 40

Effect of obstructive lung defects on optimal RR and VT to minimise work done

Answer 40

Obstructive lung disease increases work of resistance Therefore lower RR and higher VT minimise work

Question 41

Effect of restrictive lung defects on optimal RR and VT to minimise work done

Answer 41

Restrictive lung disease increases elastic work Therefore higher RR and lower VT minimise work

Question 42

Factors which increase work of breathing

Answer 42

Anything that increases the area under the pressure-volume curve: Larger tidal volumes Reduced compliance Obstructive defects Exercise - increases VT and RR

Question 43

How does GA increase work of breathing in spontaneously breathing patients

Answer 43

Reduced FRC - lung compliance decreased Narrow ETT and circuits - increased airway resistance