Respiratory mechanics

Question 1

What is compliance?

Answer 1

The tendency to distort under pressure (change in volume/pressure)

Question 2

What is Elastance?

Answer 2

The inverse of compliance - tendency to resist change and recoil to original volume (change in pressure/volume)

Question 3

What is surfactant secreted from?

Answer 3

Secreted by TII pneumocytes (80% polar phospholipids, 10% protein and 10% non-polar lipids)

Question 4

What is the role of surfactant?

Answer 4

To prevent collapse of small airways and alveoli, increasing compliance to reduce the work of breathing

Question 5

Describe resistance in the airways:

Answer 5

Increases as airway narrows, peaking at generation 4, but then decreasing

Question 6

Describe airway conductance:

Answer 6

As lung volume increases airways get larger linearly and resistance decreases exponentially

Question 7

Describe the pressures in collapsible tubes:

Answer 7

Transmural pressure = inside - outside; increases during inspiration (+5 to +8) preventing airway collapse

Question 8

Describe pressures during:
Pre-inspiration
Mid-inspiration
End-inspiration

Answer 8

Pre-inspiration: at FRC, pressure in lungs and atmosphere is 0 so no-airflow; intrapleural space at -5 due to recoil

Mid-inspiration: intrapleural tension increased to -8 cmH2O, creating pressure gradient for air to move in

End-inspiration: intrapleural tension still at -8, but lung pressure and atmospheric are at 0 cmH2O

Question 9

Describe pressures during hard expiration:

Answer 9

Creates bigger positive pressure in intra-pleural space; transmural pressure now surpassed collapsing pressure and airway should collapse - cannot maintain pressure and cartilage splinting prevents airway collapse

Question 10

What is the chest wall relationship with the lungs?

Answer 10

Chest wall has tendency to spring outwards while lung recoils inwards; forces are in equilibrium at end-tidal respiration (neutral position of intact chest)

Question 11

Describe the strengths of chest and lung recoil during inspiration and expiration:

Answer 11

Inspiratory muscle effort + chest recoil > lung recoil results in inspiration

Expiratory muscle effort + lung recoil > chest recoil results in expiration

Question 12

Describe the pleural membranes:

Answer 12

Lungs surrounded by visceral pleura, and chest wall covered in parietal pleura, with pleural cavity and fluid between the two (fixed volume) - double folded layer to allow two surfaces to work together

Question 13

Describe the basic muscle movements that allow ventilation:

Answer 13

The diaphragm contracts to flatten, while ribs move upwards and outwards to increase lung volume

Question 14

What are the basic pressure differences that allow ventilation?

Answer 14

Muscle movements cause:

Palv < Patm causing inspiration; recoil causes Palv > Patm so air is expelled

Question 15

What are the three compartments involved in resp?

Answer 15

Palv
Patm
Ppl

Question 16

Describe the pressures of the three compartments at rest (in cmH2O)

Answer 16

Palv = 0 at rest

Patm = 0 at rest

Ppl = -5 at rest

Question 17

What are the three transmural pressures?

Answer 17

P(TT) Transthoracic
P(TP) Transpumonary
P(RS) Respiratory

Question 18

What should the transmural pressures be at rest?

Answer 18

PTT intrapleural space - atmosphere (transthoracic) should be -5 cmH2O

PTP intra-alveolar - intrapleural (transpulmonary) should be 5 cmH2O

PRS intra-alveolar - atmosphere (respiratory system) should be 0 cmH2O

Question 19

What are pressure/volume relationships?

Answer 19

A sigmoid shaped graph where at no external pressure (from the intercostals and diaphragm), a small change in pressure results in a large change in volume, and at extremes a large change in pressure is required to effect a change in volume (uncomfortable)

Question 20

What is normal pleural pressure?

Answer 20

approx. -5cmH2O

Question 21

What happens to pleural pressure on forced exhalation?

Answer 21

Inward muscle force is larger than the outward recoil force, leading to an increase in pleural pressure to -2 cmH2O

Question 22

What happens to pleural pressure on forced inhalation?

Answer 22

Outward muscle force is larger than the inward recoil force, leading to the pulling apart of the pleura, increasing the negative pressure to -8cmH2O

Question 23

What is the distribution of muscle effort in normal tidal breathing?

Answer 23

75% due to diaphragm contraction and 25% external intercostals with passive recoil

Question 24

What is maximal ventilation?

Answer 24

Use of accessory muscles and internal intercostals to effect a much larger change in volume and hence gas exchange

Question 25

Describe ventilation with fluid filled lungs:

Answer 25

water-water interface increases compliance, so fluid filled lungs expand under greater pressure