Nunn's Chapter 4

Question 1

What are the components of “respiratory system resistance”?

Answer 1

Essentially, all of the “non-elastic” resistances.

1) Frictional resistance to airflow
2) Thoracic tissue resistance to deformation
3) Inertia of gas and tissue (small contribution)
4) Gas compression (small)

Question 2

Difference in work done against elastic and non-elastic components of resistance

Answer 2

Work done against elastic components is stored as potential energy. Work done against resistive components is lost as heat

Question 3

Principles that govern gas flow?

Answer 3

1) Flows from high to low pressure as a function of resistance
2) Resistance depends on whether flow is laminar or turbulent

Question 4

Relationship of driving pressure (P1-P2) to flow in laminar flow

Answer 4

With laminar flow , flow rate is directly proportional to the pressure gradient

Question 5

Formula for laminar gas flow

Answer 5

Flow = ∆P x π x radius ^4 / 8 x length x viscosity

Question 6

Formula for resistance to laminar gas flow

Answer 6

Resistance = 8 x length x viscosity / π x radius ^4

Question 7

What is turbulent flow. How does gas movement differ from that in laminar flow

Answer 7

1) At high flow rates or when lots of airway branchings, orderly flow breaks down
2) A square wavefront replaces thee conical one seen in laminar flow. Therefore, no fresh gas reaches end of tube until volume of gas entering the tube is equal to volume of gas in the tube - unlike laminar flow
3) Frictional forces between tube wall and gas become significant

Question 8

Relationship between driving pressure and flow in turbulent flow? (differences from laminar flow).

Answer 8

1) Driving pressure is equal to square of gas flow rate
2) Driving pressure is proportional to DENSITY of gas and independent of viscosity (opposite of laminar flow).
3) Driving pressure is inversely proportional to 5th power of tube radius (Fanning equation) instead of 4th as in laminar

Question 9

Difficulty to consider when calculating resistance to airflow in non-laminar flow

Answer 9

Resistance is not constant in turbulent flow (unlike laminar, it increases in proportion to the flow rate

Question 10

Methods for estimating resistance to flow in turbulent flow

Answer 10

1) 2 constants method - estimates a constant for both the laminar and exponential components. i.e. ∆P = k1(Flow) + k2(Flow). In normal subjects this is roughly ∆P = 0.24(Flow) + 0.03(Flow)
2) “exponential Methods” - can condense the “2 constants method” into ∆P = k(flow)^n
3) Graphical method - linear or logarithmic graphing

Question 11

1) What is Reynolds Number?
2) Formula?
3) Relevant numbers?

Answer 11

1) Dimensionless quality that predicts nature of gas flow in a long, straight unbranched tube
2) (Linear gas velocity x tube diameter x gas density) / gas viscosity
3) RN < 2000 = mainly laminar, RN> 4000 = mainly turbulent

Question 12

1) What is entrance length?

2) Effect of low Reynolds Number on entrance length, resistance during turbulent flow and establishment of laminar flow

Answer 12

1) Entrance length is distance after entering a straight tube until laminar flow is established EL = 0.03 x diameter x RN
2) Low RN - short entrance length, less resistance during turbulent flow, faster reestablishment of laminar flow

Question 13

Why does vapour density/viscosity ratio matter?

2) Values for O2, N20 mix, HeO2 mix?

Answer 13

02 D/V = 1
70% N20/30%O2 = 1.59 (not good if high resistance airways)
80%He/20%02 = 0.31 - good for high resistance

Question 14

Variability of density and viscosity of respiratory gases?

Answer 14

resp gas viscosity varies little, but density varies widely

Question 15

Is airway resistance normally a major factor in resp system resistance?
Which airways are responsible for most of it?

Answer 15

not usually a major factor, as total cross-sectional area is very large after 8th generation
Overall airway resistance is dominated by the large airway

Question 16

1) where in system does laminar flow start?

2) What prevents laminar flow in large airways?

Answer 16

1) laminar flow begins around 11th generation
2) High velocity in large airways, therefore RN is high. This drops as you move into lung. Frequent branching prevents establishment of laminar flow. In large airways, entrance length is longer than airway length

Question 17

1) clinical implications of turbulent flow in conducting airways?
2) Effectiveness of Helium gases in large and small airway disease?

Answer 17

1) Physical characteristics of airway lining influence frictional resistance more during turbulent flow - so mucus consistency can matter
2) He gases (low RN) are more useful for resistance problems in large airways (i.e. stenosis) versus small airways (asthma).

Question 18

What are components of “tissue resistance” portion of resp system resistance

Answer 18

1) resistance to tissue deformation by viscoelastic forces

Question 19

what are the “time dependent” properties of tissue resistance

Answer 19

resists deformation initially, by this resistance decreases 2-3 sec later

Question 20

What is inertance

Answer 20

Inertia of gas when changing rate or direction or movement. Important only with dense gases (or hyperbaric) or at very high frequencies

Question 21

Effect of lung volume on respiratory resistance?

Answer 21

As lung volume decreases, component sizes shrink, therefore resistance goes up.

Question 22

Describe relationships between lung volume, resistance and maximum expiratory flow

Answer 22

Inverse function between lung volume and airways resistance and a direct relationship between lung vol and max expiratory flow.

Question 23

Graph airways resistance, conductance and lung volume

Answer 23

pic

Question 24

Why use conductance rather than resistance?

Answer 24

Hard to quantify resistance in this setting as it fluctuates and is non-linear

Question 25

What is specific conductance?Useful index of what?

Answer 25

sGaw - conductance relative to lung volume, good index of bronchomotor tone

Question 26

What is closing capacity? What is closing volume?

Answer 26

airways and alveoli in dependent regions of the lung are smaller than elsewhere. As lung vol decreases towards RV, there is a point at which they collapse - this volume is known as Closing Capacity. Closing Volume = CC-RV

Question 27

How does Closing Capacity change with age?

Answer 27

CC is less that FRC in young adults. When supine, FRC=CC at 44yo, and in upright at 66yo

Question 28

Does Closing

Capacity change with body position and FRC?

Answer 28

CC is independent of body position but FRC changes with position

Question 29

What does relationship of closing capacity and FRC matter clinically

Answer 29

When FRC < CC some blood flow goes to unventilated alveoli (shunt) and therefore a-A gradient increases. Important cause of decreased PaO2 with age.

Question 30

What is flow related airway collapse

Answer 30

Can collapse airways if pleural pressure exceeds intra-airway pressure (eg active exhalation).Need 50-70 cm H2O to collapse trachea. Beyond 11th generation, no structural rigidity, so collapse easily.

Question 31

When can reversal of normal transmural pressure gradient occur?

Answer 31

During strongly active expiration

Question 32

What is name of anatomic location where airway and intrathoracic pressure are equal?

Answer 32

Equal Pressure Point

Question 33

What is happening downstream of the equal pressure point?

Answer 33

Thoracic pressure is greater than airway pressure and collapse may occur

Question 34

Effect of lung volume on Equal Pressure Point? Why does this matter?

Answer 34

As lung volume decreases, equal pressure point moves towards the smaller airways, therefore as lung volumes drop, the probability of expiratory flow limitation increases

Question 35

Draw a flow-volume plot showing effort dependent and effort independent flow. (i.e. expiratory flow limitation).

Answer 35

pic

Question 36

what causes “effort independent” portions of the flow volume loops?

Answer 36

air flow rate is limited by airway collapse, not by maximal effort

Question 37

what mechanisms compensate for increased inspiratory resistance?

Answer 37

increased insp muscle effort , with little change in FRC. This is due to immediate feedback from muscle spindle stretching, and delayed feedback from increased CO2

Question 38

what mechanisms compensate for increased expiratory resistance

Answer 38

with small increases in resistance (up to 10 cm) the work is done by INSPIRATORY muscles, who increase FRC so that elastic recoil is sufficient to overcome increased resistance. Conscious patients will use expiratory muscles to overcome increased resistance greater than 10 cmH2O