L04 - pk2 Pressure and Compliance

environmental and genetic factors affecting lung volume

• Height (taller > smaller)

• Sex ( males > females)

• Age

• Race ( European>Asian)

• Respiratory Disease

Boyle’s law

altering the thoracic volume leads to a change in alveolar volume which results in air flow

diaphragm and internal/external intercostal muscles during inspiration

• external intercostals

diaphragm and internal/external intercostal muscles during expiration

• internal intercostals

respiratory muscles do work to: (2)

• to stretch the elastic components of the respiratory system

• to overcome the resistance to flow

key indicator of lung function

vital capacity

accessory muscles of forced inspiration

• Sternocleidomastoids

• Scalenes

• External intercostals

Contraction of these muscles results in an increase in the lateral and anterior-posterior dimension of the thoracic cage -> greater decrease in P_A

accessory muscles of forced expiration

• Internal intercostals

• Abdominal muscles

  • abdominus rectus

  • abdominal obique muscles

    • external

    • internal

  • transversus abdominis

oxygen consumption by respiratory muscles at rest

Respiratory muscles use 5% of oxygen consumption at rest

what is between the parietal and visceral pleura

pleural space containing a small amount of cohesive, lubricating and nonexpanding, pleural fluid.

how do the lungs adhere to the chest wall (simple version)

Each lung has its own pleural covering

Each lung acts separately from each other in terms of this coupling

Lung adheres to the chest wall via negative pressure in the intrapleural space.

Lung-thorax coupling

Each lung acts separately from each other in terms of this coupling

Lung adheres to the chest wall via negative pressure in the intrapleural space.

Negative pressure (lower than atmospheric pressure) is created in the pleural space:

• At every lung volume healthy lung tends to recoil inwards (collapse) and at most lung volumes the thoracic cage tends to recoil outwards and pull away from the lungs.

  • [Explanation: As a result, they are trying to increase the volume of the pleural cavity which in turn results in a decrease intrapleural pressure → negative]

• This creates negative intrapleural pressure (which is exerted on the pleural fluid) which allows for mechanical coupling of the lungs and thorax (the visceral & parietal pleura push against one another)

• Therefore, if the chest wall pulls outwards the lung will come with it

Explain the following image in terms of lung-thorax coupling

Actual touching between opposing pleurae does not occur because of repulsive forces between several layers of phospholipids adsorbed on mesothelial surfaces carrying charges of the same sign

What is occurring at FRC

At FRC, when respiratory muscles are relaxed, the outward and inward recoils are equal and opposite - there is a negative intrapleural pressure but the respiratory system is at equilibrium

Explain what is happening at t=0

comment on lung volume, air flow, intrapleural pressure

• Lung volume = FRC

• Air flow = 0 as alveolar pressure is 0 relative to barometric pressure

• Negative intrapleural pressure -> allows for coupling of lungs and thorax (-0.5kPa)

explain what is happening at the 2nd line

Half way through inspiration

• peak inspiratory air flow → increase in lung volume

  • Air flow is at its peak here as we have the greatest difference between alveolar pressure and barometric pressure

• Intrapleural pressure becomes more negative during inspiration:

  • The chest wall moves outwards and the lungs recoil pressure becomes greater → as a consequence, this makes intrapleural pressure more negative (coupling lung more strongly to chest wall)

explain what is happening at the 3rd line t=2

• Air flow = 0

  • as alveolar pressure is now equal to barometric pressure

• Max lung volume

• Intrapleural pressure is now at its most negative:

  • As Lung is at its greatest recoil → greatest volume and greatest tendency to collapse.

explain what is happening at the 4th line

half way through expiration

• Peak expiratory air flow → results in an decreasing in lung volume

  • Air flow is at its peak here as we have the greatest difference between alveolar pressure and barometric pressure

• Intrapleural pressure also rises back to resting FRC value

explain what is happening at the 5th line

• Lung volume = FRC

• Air flow = 0 as alveolar pressure is 0 relative to barometric pressure

1. Airflow profile follows PA

2. During expiration, both Ppl and PA rise

3. PA is always > Ppl

4. In quiet breathing, Ppl is always negative whilst PA is negative in insp and positive in exp

5. Airflow ceases when PA is zero

6. At high ventilation, Ppl and PA changes are increased.

7. Ppl can be +ve in forced expiration (e.g. +8kPa in coughing/sneezing)

• due to increased compression into pleural space

what is compliance

and what is meant by high compliance

A measure of the ‘distensibility’ of an elastic structure

High compliance = lungs can stretch easier for smaller pressure changes. High change in volume, for a fixed change in pressure

Elastic recoil of the lungs brings it back

how to measure compliance

equation and conditions

In order to measure the compliance of lung or Thoracic cage we need to remove air way and air flow resistance.

Therefore, compliance is measured during static manoeuvres – when air is not moving.

distending pressure calculation

Distending pressure = P_in - P_out

Positive Distending pressure: P_in > P_out so this the structure is being distended  

Negative distending pressure: P_in < P_out so the structure is being compressed.

calculating distending pressure for the lungs

• Pressure in = P_A

• Pressure out = P_pl

  • Therefore, distending pressure is P_A- P_pl

calculating distending pressure for the thoracic cage

• Pressure in = P_pl

• Pressure out = P_B

  • Therefore, distending pressure is P_pl - P_B 

thoracic distending pressure

lung distending pressure

can lung volume reach 0

It is not possible for lung volume to reach 0 due to the presence of the residual volume.

lung and thoracic compliance curve

explaining the lung compliance curve

• As distending pressure increases the volume of the lungs also increases

• This continues to TLC – where we reach elastic limit of the lungs is reached.

• NB: A lung volume of 0L occurs when there is no distending pressure.

• This is the equilibrium volume of the lung (=0L)

  • Equilibrium volume of structure is found at zero distending pressure – this is where elastic structure would like to be is where there is not distending pressure on it.  Where elastic structure is attempting to recoil towards.

explaining the thoracic cage compliance curve

• At high volumes the thorax is being distended.

  • If we take a deep breath in to TLC at this point we stretch thoracic cage beyond its equilibrium volume so it will now have the tendency to collapse.

• The chest walls equilibrium volume is around 5L.

  • When we are breathing in the chest wall is moving towards where it wants to go (being helped) but after large inspiration chest wall wants to collapse. (at the point where it crosses the y axis)

specific compliance and its importance

specific compliance = compliance/FRC

Lungs of a baby and adult have similar compliances but different specific compliances

Flatter lung = low compliance

at what point is the equilibrium volume reached

Equilibrium volume found at zero distending pressure

Only thoracic cage compliance curve crosses zero distending pressure

Lung compliance lowest towards TLC

• if you take a deep breath in and try to breathe again it is very difficult to breathe.

Normal compliance ca. 1.5L.kPa-1

Lung and thoracic cage compliance similar around FRC

• slope of line is identical → so chest wall and lung are similar in there stretchiness.

Compliance (slope) greatest around FRC

why is this convenient

• This is convenient as this is where the region where we breathe

• So this is the region that is easiest to breathe – requires the least amount of energy.

why is total system compliance considered

The lungs lie within the thoracic cage and both need to be stretched together during inspiration - so we need to consider the ‘total’ system compliance.

calculating total system compliance

what is FRC in terms of distending pressure

why does a pneumothorax occur

Occurs if the lung perforates or there is a penetrating wound of the chest wall, air enters and fills the pleural space from the lung or from the atmosphere.

Distending pressures lost

Ppl = PB = 0 kPa

The lung collapses & the thoracic cage expands with both structures moving towards their respective equilibrium volumes

result of pneumothorax on coupling

As a result, breathing becomes uncoupled (lungs no longer move as chest wall moves) → thoracic cage can still move if innervation intact but breathing is painful, difficult or impossible.

types of pneumothorax (4)

• primary spontaneous

• secondary

• traumatic

• tension

primary spontaneous pneumothorax

signs/symptoms + treatment

most common

Occur often in tall thin young individuals

Small blebs on lung → ruptures → internal lung perforations

outpatient treatment

seconadary pneumothorax

associations + treatment

Associated with respiratory disease (COPD, asthma)

requires intercostal tube drainage

traumatic pneumothorax

cause + treatment

Following blunt or penetrating chest trauma or mechanical ventilation

Requires intercostal drainage

Tension Pneumothorax

cause and result

Mechanical ventilation or trauma

“Mediastinal shift” - shock

• movement of heart & trachea due to air rushing in – great veins are blocked off

Tension Pneumothorax

treatment

Medical emergency - immediate drainage with 14-G needle

• allows air to escape from pleural space

• so much air flowing into pleural space; wound acts as a valve preventing outflow of air