General Respiratory Mechanics I & II Questions

Question 1

What is IP Pressure?

Its’ function?

Answer 1

The pressure within the pleural cavity - should be negative. Created by the negative surface tension exerted by the pleural fluid. This pressure resists the separation of the pleura, thus holding the lungs and thoracic cage together (so that when cage expands, so do the lungs, and vice versa)

Question 2

What is TP pressure?

Answer 2

Palv - Pip

Essentially, can be thought of as the ‘balanbe’ between opening and collapsing forces.

The greater the lung volume, the higher the TP - this is a determinant

Question 3

Pressures ‘between breaths’ at end expiration

Answer 3

Palv = 0 (no flow)

Pip = -4 (Minor inward/outward forces, that are temporarily in equilibrium. Thoracic cage is at its’ resting position)

Ptp = 4 (Lower lung volume)

Question 4

Pressures during mid-inspiration

Answer 4

Patm = -1 (Favouring inward flow)

Pip = -6 (Opening forces are outweighing collapsing forces - increased outward force accounts for more negative pressure)

Ptp = 5 (Moderate lung volume)

Question 5

Pressures at end-inspiration

Answer 5

Palv = 0 (No flow)

Pip = -7 (Opening and collapsing forces are transiently in equilibrium as collapsing forces of lung become greater with higher lung volumes)

Ptp = 7 (Greater lung volume)

Question 6

Pressures during mid-expiration

Answer 6

Palv = 1 (Favouring outwards flow)

Pip = -5 (Collapsing forces have outweighed the opening forces, inspiratory muscles have relaxed)

Ptp = 6 (Lung volume is again decreasing)

Question 7

What is Boyle’s Law?

Answer 7

P1 V1 = P2 V2

The basis of breathing

Question 8

What are the components compliance?

When is lung compliance generally greatest?

Answer 8

C = change in lung V / change in Ptp

Elasticity of the lung tissue (ability to stretch)

Surface Tension of alveoli (most powerful at high lung volumes) - resisted by surfactant at lower lung volumes

Elastic properties of the thoracic cage - its ability to expand

Usually greatest over the middle ranges of lung volumes: difficult to start it off at lower lung volumes, and at max volumes there isn’t much stretch left

2.3-2.8L

Question 9

What effect would increased or decreased pulmonary compliance have on Ptp?

Answer 9

Increased compliance: high volumes will be achieved at lower Ptp (e.g. emphysema)

Decreased compliance: Higher Ptp are required to achieve small increases in lung V (e.g. restrictive lung disease)

Question 10

What is ‘hysteresis’?

Answer 10

The difference between the pressure/volume curve during inhalation and exhalation.

It demonstrates that a higher pressure is required to maintain a given volume during inspiration, compared to during exhalation.

Question 11

What contributes to hysteresis?

Answer 11

The Lung: Elastic recoil, stretchiness, collagen preventing over-stretch

Chest wall: Elastic properties, where it reaches its natural position (2/3 total lung volume) and collapsing forces

Surface tension: Powerful at higher lung volumes, resisted by surfactant at lower lung volumes

Question 12

Describe the significance of the lung volumes over which normal quiet breathing occurs

Answer 12

Tidal volume during normal quiet respiration = ~500mls

Usually occurs at 2.3-2.8 L

This is where compliance is greatest - where WOB is the lowest, due to the specific balance of all factors contributing to the WOB, and all factors contributing to pulmonary compliance

Question 13

Ventilation is affected by which major concepts?

Answer 13

Lung Compliance and Airway Resistance

contribute the most to WOB

Question 14

What governs ‘flow’?

Answer 14

Airway resistance and Pressure gradient

Boyle’s law = P1 V1 = P2 V2

Flow = Pressure gradient / Airway Resistance

Question 15

What is Airway Resistance?

How is it expressed as an equation?

When does it occur?

Answer 15

‘Drag’ as air flows through the airways

Poisuelle’s Law” R = (8 x L x n) / (pie)r(to the power of 4)

I.e.(8 x length of tube x gas viscosity) / pie x radius of tube, to the power of 4

Only occurs when there is airflow

Question 16

What is Poisuelle’s Law?

Answer 16

Of Resistance

R = (8 x l x n) / pie x r, to the power of 4

Question 17

Where is resistance greatest? Why?

Answer 17

Fist 4-7 divisions

Total cross sectional area increases with increasing divisions

Larger airways tend to be longer, which increases resistance

Flow rates (velocity) tend to be higher in larger airways, which increases resistance

Question 18

What happens to velocity through the respiratory tree?

Answer 18

Greatest in the trachea

Velocity decreases with increasing cross-sectional area

Question 19

What are the determinants of Airway Resistance? How does this relate to resistance throughout the breathing cycle?

Answer 19

Determinants include:
Lung Volume
Elastic Recoil - dynamic airway compression
Bronchial Smooth Muscle Tone

Higher Lung volumes = Lower Resistance. Mainly attributable to the greater distension of smaller airways, thus decreases in resistance. *Larger airways, with cartilage etc, are not as prone to large changes in distension with increasing lung volumes

Resistance is then greatest upon expiration, as the smaller airways tend to collapse

Question 20

What is the Bernoulli Principle?

Answer 20

States that pressure will decrease as air moves towards the mouth, because the air is moving from an area of high pressure, to an area of relatively lower pressure, reaching 0 as it reaches the mouth

Question 21

What is Dynamic Airway Compression?

Answer 21

Bernoulli Principle

Dynamic compression occurs at the point (of air exiting during expiration) where Ptp exceeds Palv.

Bernoulli principle states that Palv will gradually decrease from its initial value (+) to 0 as it reaches the mouth.

In normal individuals, during normal quiet breathing, the point at which Ptp and Palv equilibrate is usually in the larger, cartilagenous airways - thus, dynamic airway compression doesn’t occur.

Compression of smaller airways only occurs in health people during forced expiration.

In lung disease states, such as emphysema, the damage to the elastic fibres of the parenchyma etc weakens the tethering effect that normally holds the smaller airways open during expiration. Thus, dynamic airway compression can occur in the smaller airways during normal quiet breathing

Question 22

Describe the relationship between certain lung diseases and airway calibre, and how this affects breathing

Answer 22

Emphysema: destruction of lung parenchyma causes loss of traction in smaller airways. Thus, they tend to collapse during expiration (dynamic airway compression). Thus, inhaling is easier, but exhaling is difficult.

Fibrosis: Thickening/hardening of airways can cause excessive radial traction, so small airways do not collapse during expiration. However, the fibrotic parenchyma does not expand easily. Thus, inhalation is difficult, but exhalation is easier

Question 23

How can WOB be expressed?

Answer 23

WOB = change in volume / change in pressure

I.e. the WOB relates to the amount of energy required to create the specific pressure change to cause the desired/required change in volume

Question 24

WOB components and percentages:

Exacerbating Conditions for each?

Answer 24

Elastic Resistance = 65%
Exacerbated by restrictive lung conditions

Airflow Resistance = 28%
Exacerbated by obstructive conditions

Friction between pleural surfaces = 7%
Exacerbated by diseases involving the pleura

Question 25

How does ventilation pattern affect WOB?

How can ventilation pattern be expressed?

Answer 25

Ventilation = Vt x f

(volume of breath x frequency)

Deep Breathing = Increased WOB due to elastic recoil

Fast Breathing = Increased WOB due to airway resistance

Question 26

What are the limits of ventilation due to WOB?

Answer 26

Limits to ventilation occur when the O2 required to support WOB cannot be provided by added ventilation

In normal individuals, this does not occur until VERY high levels of ventilation are reached

In emphysemic patients, this may occur at rest. Thus, maintaining O2 may require supplemental O2 so that they can continue to breathe

Question 27

What is ‘total ventilation’?

Answer 27

Minute volume in L/min

= f x tidal volume

Question 28

Dead space?

Answer 28

150-200ml normally. May increase slightly with large inspirations as bronchi pulled apart slightly be parenchyma

Question 29

What is physiological dead space?

Answer 29

Anatomical dead space + Alveolar dead space.

IN normal people, anatomical and physiological dead space should be the same

Question 30

Regional differences in ventilation, and regional differences in Pip*

Answer 30

…

Question 31

Why is gas trapping more likely to occur at base of the lung?

Answer 31

Because Pip at base is LESS negative - thus tends to equilibrate with Palv more easily