Nunn's Chapter 4 Flashcards
(38 cards)
What are the components of “respiratory system resistance”?
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)
Difference in work done against elastic and non-elastic components of resistance
Work done against elastic components is stored as potential energy. Work done against resistive components is lost as heat
Principles that govern gas flow?
1) Flows from high to low pressure as a function of resistance
2) Resistance depends on whether flow is laminar or turbulent
Relationship of driving pressure (P1-P2) to flow in laminar flow
With laminar flow , flow rate is directly proportional to the pressure gradient
Formula for laminar gas flow
Flow = ∆P x π x radius ^4 / 8 x length x viscosity
Formula for resistance to laminar gas flow
Resistance = 8 x length x viscosity / π x radius ^4
What is turbulent flow. How does gas movement differ from that in laminar flow
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
Relationship between driving pressure and flow in turbulent flow? (differences from laminar flow).
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
Difficulty to consider when calculating resistance to airflow in non-laminar flow
Resistance is not constant in turbulent flow (unlike laminar, it increases in proportion to the flow rate
Methods for estimating resistance to flow in turbulent flow
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
1) What is Reynolds Number?
2) Formula?
3) Relevant numbers?
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
1) What is entrance length?
2) Effect of low Reynolds Number on entrance length, resistance during turbulent flow and establishment of laminar flow
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
Why does vapour density/viscosity ratio matter?
2) Values for O2, N20 mix, HeO2 mix?
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
Variability of density and viscosity of respiratory gases?
resp gas viscosity varies little, but density varies widely
Is airway resistance normally a major factor in resp system resistance?
Which airways are responsible for most of it?
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
1) where in system does laminar flow start?
2) What prevents laminar flow in large airways?
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
1) clinical implications of turbulent flow in conducting airways?
2) Effectiveness of Helium gases in large and small airway disease?
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).
What are components of “tissue resistance” portion of resp system resistance
1) resistance to tissue deformation by viscoelastic forces
what are the “time dependent” properties of tissue resistance
resists deformation initially, by this resistance decreases 2-3 sec later
What is inertance
Inertia of gas when changing rate or direction or movement. Important only with dense gases (or hyperbaric) or at very high frequencies
Effect of lung volume on respiratory resistance?
As lung volume decreases, component sizes shrink, therefore resistance goes up.
Describe relationships between lung volume, resistance and maximum expiratory flow
Inverse function between lung volume and airways resistance and a direct relationship between lung vol and max expiratory flow.
Graph airways resistance, conductance and lung volume
pic
Why use conductance rather than resistance?
Hard to quantify resistance in this setting as it fluctuates and is non-linear
