Mechanics of Ventilation and the Effect of IPPV on the Lungs Flashcards
Define work of breathing?
Work of Breathing: energy spent on overcoming airway resistance and the elasticity of the chest wall and lungs.*
Work of breathing only occurs during inspiration as expiration is a passive process through elastic recoil.
*These forces are balanced at the end of expiration (FRC)
Draw a pressure volume curve for a normal lung?
Negative pressure on x axis
Volume on y axis
Inspiration is sigmoid shaped and expiration is a concave curve.
What is compliance and how can it be measured using a pressure volume curve?
Compliance: This is the relationship between the volume displaced and the pressure change and can be measured by the gradient of the slope of a pressure volume curve.
What is a normal compliance in healthy lung?
200 ml/cm H2O
What factors increase and decrease compliance?
Increased compliance:
* Age
* Emphysema
Decreased compliance:
* Raised pulmonary venous pressure
* Alveolar oedema
* Fibrotic Lung Disease
What is specific compliance?
Specific compliance is compliance that is normalized by a lung volume, usually FRC. It is used to compare compliance between lungs of different volumes (eg. child and adult)
What is hysteresis?
Hysteresis is the term used to describe the difference between inspiratory and expiratory compliance.
Lung volume at any given pressure during inhalation is less than the lung volume at any given pressure during exhalation.
Define surface tension?
The tension of the surface film of a liquid caused by the attraction of the particles in the surface layer of the liquid, which tends to minimize surface area
What is Laplace’s law?
Laplaces’ law allows us to calculate the pressure exerted by surface tension.
P = (4 x T) / r (radius of the sphere)
From this formula you can see that smaller spheres will generate greater pressures.
What would happen if surfactant was not present in the lung and why?
It would result in alveoli collapsing.
If you consider Laplaces law, P = (4 x T) / r (radius of the sphere)
Smaller spheres will have an increased pressure. In the case of alveoli, there will be differing degrees of inflation and therefore sizes. If surfactant were not present the volume from smaller (less inflated) alveoli would transfer to the larger alveoli to equilibrate the pressure, resulting in alveolar collapse.
How does surfactant work?
Surfactant is a phospholipid containing dipalmitoyl phosphatidyl choline. It is produced by type II alveolar epithelial cells.
Surfactant reduces surface tension by opposing the normal attraction forces of the surface molecules making Laplace’s law obsolete.
In the smaller alveoli with smaller volumes, the surfactant molecules are pushed closer together making the repellent forces greater to give better stability, preventing them from emptying and collapsing at the end of expiration. It also reduces their compliance.
What is Hagen-Poiseuille’s Law?
Q= πΔPr(4)/8ηl
η = viscosity
l = length
r=radius of a tube
What is hagen poiseuilles law expressed in terms of resistance?
R= 8ηl/πr(4)
What are the different types of air flow that occur in the lungs and where do they occur?
Laminar Flow: No disruption between layers of flow. Occurs at low velocity smooth walled airways. The driving pressure (ΔP) is the alveolar – barometric pressure. Resistance is determined by viscosity of the gas rather than its density.
Transitional Flow: some characteristics of both laminar and turbulent flow. Occurs at **bifurcation of airways and in airway narrowing **(reduction in radius)
Turbulent Flow: Chaotic movement of the gas and occurs at high flow rates typically in the larger airways. In this situation the density of the gas (d) is more important than the viscosity (η) in resistance.
What is a physiological advantage of turbulent flow?
The physiological advantage is that turbulent flow will cause particles in the inspired gas to collide with and get trapped in the mucus on the walls of larger airways and not get to the alveoli.
