1: Respiratory Mechanics Practice Questions Flashcards
• Describe the process of inhalation and exhalation. Account for the movement of air into and out of the lungs, and indicate the general changes in alveolar and Intrapleural pressure during inspiration and expiration
Largely, inhalation and exhalation are dependent taking advantage of Boyle’s Law: P1 x V1 = P2 X V2
This law shows that volume and pressure are inversely related. At a constant temperature, pressure decreases as volume increases, and pressure increases as volume decreases.
Thus, the movement of air in and out of the lungs is dependent on creating a pressure gradient between Palv and Patm. This is achieved by increasing (promotes air in) and decreasing (promotes air out) the thoracic volume.
At the end of expiration, there is no air-flow. Thus Palv = 0 (equal to atmospheric air). The TP pressure is around 4: the IP pressure is around -4, because of the opposing inward and outward forces of the elastic recoil of the lung + surface tension of alveoli (collapsing forces) and the elastic recoil of the chest wall (expanding force).
Increasing thoracic volume: Mid-inspiration
During normal, quiet, breathing, inspiratory muscles (chiefly the diaphragm and external intercostals) contract, increasing the thoracic volume. This process is aided by the elasticity of the thoracic cage which wants to swing outwards (its natural position sits at 2/3 total lung capacity).
The increase in thoracic volume results in a reduction of Palv (-1) which promotes inward flow of air. At this point, TP pressure is 5 (because IP pressure is about -6 because with increasing lung volumes, there becomes a greater inward force).
End-Expiration: At the end of expiration, there is again no air movement. Thus both Palv and Patm are 0. Because the lung is full of air, there is an more negative surface tension, resulting in a IP pressure of ~-7. Thus TP = 7. At this point, the inward and outward forces are temporarily equal (although just about to tip in favour of expiration). However, the combined inward and outward forces at end of inspiration are greater than at the end of expiration. Thus why there is a much greater TP pressure.
Mid-Expiration:
During normal, quiet breathing, expiration is a passive process dependent on the relaxation of respiratory muscles, elastic recoil of the lung, and surface tension in the alveoli. D To create an outward flow of air, the thoracic cavity decreases in size, raising Palv to ~1 (atmP=0). IP pressure lowers to around -5, thus TP pressure is around 6.
Overall, TP pressure increases with lung volume.
What is Compliance?
What are the factors affecting compliance?
What can reduce compliance?
C = (change in volume) / (change in pressure). I.e. increased compliance means a lesser change in pressure is required to reap a given change in volume.
Elasticity of the lung (reduced in restrictive lung diseases), collagen resisting over-stretch, elastic tissue allowing stretch
Surface tension of alveoli (collapsing force) which is resisted by surfactant at lower volumes.
Elasticity of the chest wall, its’ natural position at 2/3 lung capacity
Things that reduce compliance: fibrosis of lung tissue, scars, burns to chest, musculoskeletal problems, neuromuscular problems, obesity, posture, ossification of costal cartilages
How does compliance affect breathing?
Compliance is greatest throughout mid-range of lung volumes. Normal tidal volume of 500mls usually takes place between 2.5-3L. This is the volume over which the least amount of pressure change is required to reap a given change in volume. Thus, this is the volumes over which breathing costs the least amount of energy (decreased WOB).
Factors that affect WOB: decreased compliance, and increased airway resistance.
Airway resistance is greatest when breathing in and out quickly - airway resistance = drag as air flows through. The more air in and out, the more drag, thus the more WOB due to airway resistance.
Very deep breaths in and out contribute more WOB due to overcoming elastic resistance: i.e. the effort involed in maximally expanding and collapsing the thoracic cavity with each breath.
Normal quiet breathing normally sits at around 12-16 breaths per minute, where the combined WOB due to airway resistance and elastic resistance is lowest
