l14 - cardio

define compliance

a measure of the elastic property of the lung

• The magnitude of the change in lung volume produced by the given change in transpulmonary pressure

what is complinace inverse of

stiffness

why is this not a linear curve?

this is because compliance is not constant

• The lungs are harder to expand at very low and very high volumes, but easier to expand at intermediate volumes > so it is not the same with every lung volume

• low volume: harder to expand

• middle: easiest to expand

• high volume: harder to expand again

what determines lung compliance?

1. the elasticity of the lung

2. surface tension at the fluid gas interface

if a lung is more elastic what does this mean?

it means it is more stiff so therefore less compliant

name 2 conditions involved with elasticity/compliance of the lungs

1. emphysema

2. fibrosis (e.g. pulmonary fibrosis)

what is fibrosis?

this is when there is fibrotic scarring in the lung, which makes it difficult to expand as it is more elastic

• this is when more energy is required to expand the lung

• the same change in transpulmonary pressure is not enough to produce the same change in volume

what happens to someones compliance curve in a Tp vs volume graph when someone has fibrosis

the curve is shifted downwards and rightwards

• blue line is representative of the extra energy required

is fibrosis restrictive or obstructive?

restrictive

what is emphysema?

this is where the alveoli become damaged, so the walls of the alveoli fuse and essentially become one big airspace

• this makes the lungs flaccid/floppy

• making them EXTREMELY COMPLAINT, with extremely little elasticity

what is the issue with emphysema?

for the same transpulmonary pressure, a much bigger increase of lung volume occurs

• but this makes expiration much harder, because usually the lung relies on recoil to exspire, but an ephysema lung lacks this recoil

• so a lot more energy is required to expire (has a lot of trouble returning to its pre-inspiration state)

what happens to someones compliance curve in a Tp vs volume graph when someone has emphysema

shifted upwards and left

is emphysema restrictive or obstructive

obstructive

whats the difference between an obstructive vs restrictive lung condition?

obstructive lung disease makes it hard to exhale air, while restrictive lung disease makes it hard to fully expand the lungs to inhale air

• both cause shortness of breath though

what is the second factor which effects lung compliance

surface tension

the inside of each alveoli has a very thin layer of fluid lining it that contains water molecules

• these H2O molecules are very attracted to each other, so they pull together and resist expansion of the alveoli

• this promotes alveolar collapse (especially in smaller alveoli)

why is surface tension a problem during breathing?

during inspiration our lungs are already working to overcome the elasticity of the lung, if we had lots of surface tension this would make it a lot harder

• so it would lower compliance a lot

how do we overcome the surface tension problem of alveoli

we have type 2 alveolar cells (pneumocytes) which produce surfactant

• this surfactant is responsible for overcoming surface tension

what is surfactant composition

proteins + lipids

• MAIN component = phospholipids

how does surfactant work?

the surfactant (primarily phospholipid) inserts itself at the gas-fluid interface and gets in between the water molecules

• this decreases the attractive forces between the H2O molecules

• this makes the alveoli easier to expand, and increases lung compliance

type 1 vs type 2 alveolar cells?

type 1 = the alveolar cells involved in gas exchange

type 2 = the alveolar cells which produce surfactant

why is surfactant especially significant for smaller alveoli?

small alveoli naturally are more vulnerable to collapsing (due to laplaces law)

• so a tiny alveolus has to fight a stronger inward collapsing force then a large alveoli would

• so surfactant is very important for small alveoli specifically to prevent their collapse

what is Law of Laplace?

for the same surface tension, a small radius will mean a much higher pressure across that alveoli wall

• too high of a pressure will lead to collapse

does surfactant work more strongly on smaller alveoli?

yes, because the molecules are closer together in a small alveoli, surfactant will work stronger causing the pressure in a smaller alveoli to be equal to that of a bigger alveoli

what is significant about compliane in premature infants?

premature infants (prior to 26 weeks) tend to lack surfactant

• so this can result in respiratory distress syndrome (RDS)

whats the treatment for a premature infant with RDS?

treatment for respiratory distress syndrome

assisted ventilation and administration of synthetic surfactant via the infants trachea

what does the work of breathing overcome

1. elastic properties of the lung > how hard it is to expand the lung

• compliance of the lung

• surfactant

2. airway resistance

• how hard is it to move air in and out of the airways (dependent on friction)

what flow is airflow during restful breathing

laminar flow

what is airway resistance determined by

Poiselles law

• main component is radius

• resistance is inversely proportional to r^4

so e.g half radius will increase resistance by 16

what are the main contributing airways of resistance?

bronchi > occurs mainly within the first 6 generations of the airway

SPECIFICALLY THE MEDIUM SIZED BRONCHI

howcome the bronchi have the most resistance if they are not the vessel with the smallest radius?

because bronchi are arranged in SERIES, so the air only has a very limited number of pathways it can rely on

• whereas as the airway keeps branching into e.g. the bronchioles, these airways are arranged in parallel > so the air can split and travel through many different airways at once

• so lots more CSA compared to the bronchi

hy is measuring airway resistance a poor test for small-airway obstruction?

Because there are many small bronchioles in parallel, so narrowing of some bronchioles may cause only a small change in total airway resistance.

• meaning it can be impossible to pick up on

what tends to happen to airway resistance as the lungs volume increases?

airway resistance gets SMALLER

why does airway resistance decrease during inspiration? and whats the name for this

because when you breathe in, the alveoli expand > alveoli are physically attached to the adjacent airways

• so when alveoli expand, they pull on the airways, making them wider

• bigger radius = SIGNIFICANTLY less resistance

• this outward pulling is called RADICAL TRACTION

when is resistance low vs high

low resistance = inspiration > due to radial traction

high resistance = expiration > due to lack of radial traction

what is dynamic airway compression?

this is the narrowing of SMALL airways during expiration due to increased INTRATHORACIC pressure around them

• so small airways are very collapsible

• this narrowing increases resistance and limits airflow

does dynamic airway compression occur in healthy people

yes, but only during FORCED expiration

why doesnt dynamic compression occur in larger airways. why only small airways?

because larger airways have more support

• e.g. trachea has lots of cartilage, so it will not be compressed

when is dynamic compression worse?

in people with EMPHYSEMA

why? because emphysema means less elastic tissue in the lungs, so they reduced elastic recoil

• AND they have loss of radial traction

• so this makes the airway MUCH more collapsible > meaning this collapse/and resistance can happen even during NORMAL restful expiration, not just forceful expiration

whats the consequence of dynamic compression in someone with emphysema

reduced exercise ability and air trapping

• the airway collapses, so air cannot get out, and remains trapped in the airways

how do patients counteract dynamic compression?

pursed lip breathing > so breathing out of partly closed lips

• this helps because it increases intrathoracic pressure SLOWLY

• **remember that it is the ^^^ of intrathoracic pressure which causes the airway collapse, so by breathing out of pursed lips, we slow this increase

• so it helps to SPLINT the airway open

whats are some different diseases that INCREASE airway resistance?

ALL OBSTRUCTIVE

1. asthma > bronchial smooth muscle contracts (bronchoconstriction)

2. chronic obstructive pulmonary disease (COPD)

A. emphysema > loss of elastic tissue = loss of radial traction = airway collapse = increased resistance

B. chronic bronchitis > bronchi are chronically inflamed > increased resistance

what are other non health related causes which can effect airway resistance

differences in viscosity and density of the gas you are breathing in

• e.g. during scuba driving the density and viscosity of air can be effected > gas density rises

how do we measure/visualise how much work out lungs do during a breath?

a pressure-volume loop

intrapleural pressure VS. lung volume

loop is representative of one full breath in AND out and work is shown as AREAS on the graph

explain whats happening during the inspiration and expiration curve on this graph

during inspiration, intrapleural pressure becomes MORE negative and lung volume increases

during expiration, intrapleural pressure becomes LESS negative and lung volume decreases

what does a WIDER volume-pressure loop mean

more pressure is needed to move the same volume of air

what is this graph showing?

yellow > this is showing the work required to overcome elastic resistance

• 0ABCD

green > this is showing the work required to overcome non-elastic resistance (aka airway resistance)

• AECF

yellow + green = work of respiration

dark green = inspiration

lighter green = expiration > this is stored energy in the stretched lung tissue

what is work of respiration on a pressure volume loop

0AECD

this is showing the work required to overcome BOTH elastic resistance and non-elastic resistance

what shift occurs to the pressure-volume loop in restrictive conditions

e.g. when a lung becomes fibrotic (pulmonary fibrosis or fibrotic lung disease)

the whole pressure volume loop shifts RIGHT

what work (area) changes occur in the pressure volume loop in restrictive conditions

“C” has been shifted further right

• so the work to overcome ELASTIC resistance increases (bc compliance ^^)

• non elastic resistance stays the same > so the green does not change

• so to move the same amount of air, the work of respiration area increases (0AECD)

what shift occurs to the pressure volume loop in obstructive conditions

work to overcome NON-elastic conditions increases

• can see that specifically the work during EXPIRATION (ABCF) has increased substantially

• you can see that more work is required during both inspiration and expiration

• BUT work to overcome elastic resistance has remained the same

what happens to expiratory pressure in obstructive lung disease

expiratory pressure = the pressure your body generates to push air out during expiration

• it becomes more positive during expiration in obstructive lung disease

• normally expiration is passive so do not need lots of pressure, but in obstructive disease, airways have more resistance so we need the assistance of accessory expiration muscles to make the pressure within the chest more positive, so that air wants to move OUT

what consequences occur in a RESTRICTIVE v OBSTRUCTIVE lung disease?

RESTRICTIVE:

• lung compliance is DECREASED

• so more work is required to overcome elastic resistance > a more negative intrapleural pressure is required to move the same amount of air

• no change in airway resistance > so no work needed for non elastic resistance

OBSTRUCTIVE:

• airway resistance is INCREASED

• so more work is required to overcome non elastic resistance > a more positive expiratory pressure is required so energy is now needed

• lung compliance is INCREASED

• so approximately the same amount of energy is required to overcome elastic resistance

• BUT, a more negative intrapleural pressure is required to move the same amount of air

in obstructive airway diseases the lungs are MORE compliant. howcome we still need a mroe negative intrapleural pressure then?

Because the lung tissue is easy to stretch (due to the increased compliance), but the airways are harder to keep open as they are more prone to collapse and air has more resistance moving through them. A more negative intrapleural pressure helps expand the lungs and pull the small airways open, so air can flow more easily.

why does chronic emphysema increase airway resistance?

chronic emphysema = obstructive disease

because the loss of elastic tissue reduces radial traction > so small airways are NOT held open as well