S2: Mechanics of Breathing II Flashcards Preview

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Flashcards in S2: Mechanics of Breathing II Deck (24):

How can lung function be investigated?

- Lung volumes
- Ventilation
- Lung compliance
-Air flow


What is lung volume?

Total volume of air an individual can breathe in/out


Factors affecting lung volume

Large FVC (forced vital capacity) :
- Taller individuals
- Men
- Healthy BMI
- Healthy lung tissue

Small FVC:
- Shorter individuals
- Women
- Obese
- Restrictive lung disease - fibrosis and emphysema


What is ventilation?

What is the equation for ventilation?

What volume of fresh air reaches respiratory surfaces over a given time.

- Ventilation depends on volume (depth) and rate of breathing

V. = Vt x f

Vt = tidal volume (mL) the volume of air inhaled in each breath

f= frequency


What does alveolar air contain?

Alveolar air contains a mixture of fresh and stale air


What is alveolar ventilation?


Alveolar ventilation corrects for the volume of inspired air which doesn’t take part in gas exchange (due to the dead space)

V.A = (VT - VD) x f

V.A = Alveolar minute volume (mL) the total volume of fresh air entering the alveoli across all breaths over one minute
VT - VD = The volume of fresh air entering the alveoli in each fresh breath
VD = Dead space volume (mL) the volume of air remaining in the respiratory system at the end of expiration


What is Lung Compliance?

How much force is required to overcome the recoil of the lungs.

A more compliant lung needs less pressure to force open. If lungs are stiff and hard (low compliance), a certain amount of pressure may not be enough to change volume


What is Transpulmonary Pressure?

Difference with alveoli and pleural cavity which equals amount of force generated to pull the lungs


Equation of lung compliance

Lung compliance is the relationship between transpulmonary pressure and lung volume

Compliance (CL) = Change Volume/ Change Pressure


What is Static and Dynamic Compliance?

Static compliance = incrementally increasing lung volume of person (parts of breath increasing volume)
Dynamic compliance = Inhalation and Exhalation (one big breath taking in all the volume all at once)

A graph is plotted with lung volume against intrapleural pressure and the gradient is worked out for the compliance


Factors affecting lung compliance

1. Chest wall mechanics- scoliosis muscular dystrophy + obesity decrease CL

2. Alveolar surface tension- NRDS (surfactant) increase CL

3. Elastin fibres- Fibrosis decreases CL + COPD increases CL


How does emphysema and fibrosis affect compliance?

- Elastin degradation increases compliance
- Lungs are hard to recoil back into normal shape

- Fibrosis causes scarring and deposition of collagen
-Stiff lung
- Decreases compliance


What is Airflow?

At what rate can air be moved between the lungs and atmosphere


What is FEV1, FVC and 100 x FEV1/FVC?

FEV1 = Forced expiratory volume in 1 second
FVC= Forced vital capacity

100 x FEV1/FVC = % of total lung capacity of an individual can exhale in the first second (<80% is indicative of obstructive airways disease)


What is Ohms law?

Airflow (V) = Change Pressure (P)/Resistance (R)

More resistance= less airflow
Unless pressure gradient is increased to compensate


What is the Hagen-Poiseulle Equation?

Resistance (R) = 1/r4

As an airways radius decreases, the resistance increases (and the airflow decreases) dramatically


What is airflow proportional to?

Airflow is proportional to the size of the airway lumen

- Hagen Poiseulle equation

Increased luminal area decreases resistance and increases air flow


Compare healthy airway and asthmatic airway

Asthmatic airway:
- Contraction of smooth muscle
- Excess mucus secretion
- Oedema/swelling

The overall effect decreases luminal area compared to healthy airway. This increases airway resistance which decreases airflow.


How does airway resistance affect airflow?

Obstruction changes the pattern of airflow producing turbulent flow due to increasing resistance. The wheezing sound is caused by the vibration of air due to the turbulent flow.


Give an example of airway obstruction

· Loss of airway patency due to degradation of structure can cause airway obstruction
· Patency=state of being open
Healthy: elastin in surrounding alveoli provides radial traction to splint bronchioles against positive Palv

COPD: without radial traction, bronchioles collapse. There is therefore obstruction


Name two things that measure level of airway obstruction

- Peak flow
- Spirometry


FEV1/FVC cut off for obstructive and restrictive

- FEV1/FVC < 70%
e.g. Asthma increasing resistance

-FEV1/FVC > 80%
e.g. Fibrosis decreasing compliance


What is the airflow like in upper and lower airway?

· Upper and lower airway contribute 50% each to total airway resistance
· Upper airway- lots of turbulent flow
. Lower airway- flow becomes laminar


Sites of resistance within the respiratory tree

With increasing branching, airways get more numerous and shorter and contribute less to resistance.

e.g. segmental bronchi