0825 - Mechanics of Lung and Chest Wall Flashcards
What are the three different types of airflow conditions?
Laminar - ‘Straight line’ flow that can be considered like a series of progressively smaller liquid cylinders - the innermost cylinder moves fastest, the outermost cylinder does not move at all. Occurs in low-flow velocity in small pipes. Flow is proportional to pressure - very efficient.
Turbulent - Is disrupted by unpredictable eddies and vortices. Greatly increased resistance to flow. Occurs in high velocity and large pipes. Flow is proportional to SQRT pressure - very inefficient.
Transitional - Flow switches between laminar and turbulent due to bifurcations in the airway. Becomes turbulent at bifurcations, then resolves, then turbulent at next bifurcation.
Identify determinants of airway resistance (RAW)
RAW=(8 x viscosity x length)/(pi x radius^4)
Viscosity is typically constant.
40% of RAW is visco-elastic - dynamic parameter that depends on flow rates, largely determined by laminar and turbulent flow. Tissue resistance (friction between elastic fibres) and a little bit of inertia too.
60% is elastic resistance - static parameter that depends on how well inflated you are.
Lung volume is the key determinant, and resistance is much greater on expiration than inspiration (alveolar pressure becomes positive, shrinking bronchioles, and whole lung becomes smaller, reducing bronchiole radius).
Identify two ways to modulate airway resistance
Innervation - Parasympathetic induces bronchial constriction and increased mucus production.
SY - β2 action leads to smooth muscle relaxation, reduced mucus secretion.
What is static and dynamic compliance?
Static compliance - compliance of tissue at a fixed volume (and thus no flow).
Dynamic compliance - compliance of tissue when both flow and volume are changing. In both inspiration and expiration, compliance is smaller at the beginning than at the end.
Static is more important - essentially represents the average of dynamic compliance over the cycle.
How do you measure static compliance?
Need valve and spirometer.
Operator taps off the flow after breathing out, and oral pressure becomes equal to the alveolar pressure. Do that over repeated volumes to get a series of total compliance measurements - will be linear over a large range. But clinically need to separate lung and thorax compliance (emphysema vs scoliosis). 1/total compliance = 1/Chest wall + 1/Lung compliance. Can get chest wall compliance by measuring the volume change if intrapleural pressure is known - then it’s just maths. Very involved process simply because you need intrapleural pressure.
Within TV, it will be approx 0.1L/cm H2O.
How do you measure dynamic compliance?
Need a plethysmograph. Develop a PV loop over the cycle which is a hysteresis (every pressure has 2 volumes and vice versa). Lentil shape - compliance is smaller at the beginning than the end for both inspiration and expiration.
How does compliance change in disease?
Emphysema, asthma, and COPD characterised by high compliance and a wide hysteresis - need to do lots of work to make up for lost recoil/increased airway resistance (obstructive).
Fibrosis and respiratory distress syndrome characterised by low compliance - harder to breath in, but easier to get air out (restrictive)
Why is a small lung volume harder to inflate than a larger
To inflate a lung, recoil pressure must be overcome. As Laplace’s Law states that recoil pressure = (2xTension)/radius, the smaller the radius, the greater the recoil pressure. Thus, a large lung volume (large radius) is considerably easier to inflate than a small one.
How do surfactants increase compliance?
Surfactants increase compliance by reducing surface tension. If the alveolus expands slowly, surfactants move from being micelles to being distributed around the alveolus. This lowers alveolus surface tension, increasing compliance.
If the alveolus expands quickly or contracts, there is a lower concentration of surfactants, thus more surface tension and less compliance.
Why does airway resistance increase massively in COPD?
COPD narrows the radius of the distal airway. Given the airway resistance equation has a denominator of pi x radius^4, even a small decrease in radius will massively reduce the denominator, increasing airway resistance.
RAW=8 x viscosity x length/pi x radius^4
