Flashcards in S2: Mechanics of Breathing II Deck (24):
How can lung function be investigated?
- Lung volumes
- Lung compliance
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
- Healthy BMI
- Healthy lung tissue
- Shorter individuals
- 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
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
- 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
- Contraction of smooth muscle
- Excess mucus secretion
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
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