Mechanics Of Breathing II Flashcards
What are four ways in which lung function can be investigated?
1) LUNG VOLUMES: what total volume of air can an individual breathe in/out?
2) VENTILATION: what volume of fresh air reaches respiratory surfaces over a given time?
3) LUNG COMPLIANCE: how much force is required to overcome the recoil of the lungs?
4) AIR FLOW: at what rate can air be moved between the lungs and the atmosphere?
What does the total level of ventilation depend on?
It depends on the volume of air inspired and the frequency of breathing.
To find minute volume (mL), we multiply the tidal volume by the frequency of breaths.
Describe how alveolar air has a mixture of ‘fresh’ and ‘stale’ air, and how we would calculate the volume of ‘fresh’ air reaching respiratory surfaces.
Total ventilation doesn’t perfectly reflect the volume of air reaching respiratory surfaces, as some of the fresh air is required to occupy the dead-space, since the volume of the respiratory system is greater than zero (ie. not collapsed) at the end of expiration.
Therefore, the alveoli (where gas exchange takes place) is a mixture of ‘fresh’ and ‘stale’ air.
This means that if we want to calculate the volume of air reaching respiratory surfaces (alveolar ventilation), we need to take into account the volume of the anatomical dead-space. This is typically around 150ml in adults.
How do we calculate alveolar ventilation?
Va = (Vt - Vd) x f
Va is alveolar minute volume, Vt is tidal volume, Vd is dead-space volume and f is frequency.
Vt - Vd represents the volume of fresh air entering the alveoli in each breath.
What is lung compliance? What is static and dynamic compliance?
The relationship between intrapleural pressure and lung volume changes is termed lung compliance, and essentially describe the degree of force required to expand the lungs.
Compliance (Cl) = ΔVolume/ ΔPressure
Static compliance is the measurement taken whilst there is no airflow. Dynamic compliance is the measurements taken during the movement of air.
How can we graphically visualise lung compliance?
The relationship can be demonstrated by a graph of lung volume versus intrapleural pressure, with lung compliance as the gradient of the curve.
For static compliance, the steepest part of the curve is used, whereas for dynamic compliance, the gradient between the end-tidal inspiratory and end-tidal expiratory points is used.
If there is increased compliance, the gradient will be steeper. If there is decreased compliance, the gradient will be less steep.
How is lung compliance affected by disease?
LOWER COMPLIANCE CAUSED BY:
scoliosis
muscular dystrophy
obesity
NRDS
fibrosis
HIGHER COMPLIANCE CAUSED BY:
COPD
emphysema
Describe the rate at which air can pass through the airways (ie. airflow).
To get from the atmosphere to gas exchange surfaces, air passes through the airways. As it does so, it generates resistance as it comes into contact with the airway surface. Resistance is an opposing force that acts to reduce the flow of a gas or fluid through a conduit (pipe).
The more resistance present, the lower the rate of air flow.
Expand on the two laws/equations that govern the rate of air flow.
OHM’S LAW:
Air flow (V) = ΔPressure (P) / Resistance (R)
This states that the more the resistance, the less the airflow, unless the pressure gradient is increased to compensate.
HAGEN-POISEUILLE EQUATION:
Resistance ∝ 1 / radius^4
This states that as an airway’s radius decreases, the resistance increases (and the airflow decreases) dramatically.
How does the pattern of airflow affect the airflow?
The pattern of airflow can increase the resistance, which would negatively impact airflow.
Where airflow changes from a linear to a turbulent pattern, increased airway resistance is generated because the airflow is no longer unidirectional and momentum is lost. Turbulence can occur when there is a sudden decrease in luminal area, such as in obstructed airways.
The vibration generated by the turbulent airflow is responsible for the wheezing sound produced in patients with obstructed airways.
Describe how loss of airway patency due to degradation of structure can cause airway obstruction.
In healthy alveoli, elastin in the surrounding alveoli provides radial traction to splint bronchioles against positive Palv.
In alveoli with COPD, there isn’t radial traction (due to the absence of elastin).
Without radial traction, the bronchioles collapse.
How can the level of airway obstruction be investigated?
We can visualise it using a spirometry graph demonstrating forced vital capacity (FVC) and forced expiratory volume in one-second (FEV1) values.
If it is OBSTRUCTIVE: the FEV1/FVC <70% (for example, asthma, with increased resistance)
If it is RESTRICTIVE: FEV1/FVC >80% (for example, fibrosis, with decreased compliance)
