Respiratory mechanics Flashcards
Compliance: define compliance and elastance and explain the relationship between them Surfactant: explain the role of surfactant in ventilation and lung structure Resistance: explain the concept of airway resistance in the lung, and explain it effects, and is affected by, changes in pulmonary function Mechanics: explain the mechanical relationship between the chest wall, pleural membranes and the lung; distinguish the mechanical forces involved in tidal and maximal ventilation, and fluid- and
What is compliance?
Tendency to distort under pressure (change in volume or pressure).
What is elastance?
It is the inverse of compliance. The tendency to resist change and recoil to original volume (change in pressure or volume).
Where is the airways surfactant produced? Contents?
Secreted by TII pneumocytes. Contains 80% polar phospholipids, 10% protein and 10% non-polar lipids.
What is the role of surfactant in respiratory mechanics?
Prevent collapse of small airways and alveoli (caused by surface tension forces), by increasing compliance to reduce the work of breathing.
What happens to resistance for each generation of the airways?
Increases as airways narrow, peaking at GENERATION 4, then decreasing.
How is resistance calculated – what law, and the calculation?
Poiseuille’s Law: Resistance = 8nl/πr^4 n = viscosity; l = length; r = radius.
What happens to alveolar pressure-volume curves in tidal volume?
Red = change in volume in the lungs. Blue = alveolar pressure. Before you breathe in, chest expands, so Palv decreases. This pressure gradient results in movement of air into the lungs, so there is a change in volume. When alveolar pressure and pressure in the air outside are equal, this movement stops, and you reach a peak in the change of volume in the lungs – this is tidal volume.
What is ambient pressure?
The pressure of the surrounding medium, such as a gas or liquid, in contact with the object.
What are respiratory gases measured in?
cm/H2O.
What is the tendency of movement of the chest wall and lungs, and when do they balance?
The chest wall has a tendency to spring outwards, and the lung has a tendency to recoil inwards. These forces are in equilibrium at end-tidal expiration (functional residual capacity, FRC) which is the NEUTRAL POSITION of the intact chest.
How does inspiration occur in relation to the chest wall relationship described?
Inspiratory muscle effort + chest recoil > lung recoil. Results in inspiration.
How does expiration occur in relation to the chest wall relationship described?
Chest recoil < lung recoil + expiratory muscle effort.
What are the pleural membranes of the lung?
Lung surrounded by visceral pleura. Chest wall covered inside by parietal pleura.
What is a haemothorax and pneumothorax, in relation to the pleura and its cavity?
Haemothorax = intrapleural (within the pleura) bleeding. Pneumothorax = perforated chest wall or punctured lung lead to accumulation of air.
What is negative and positive pressure breathing?
NEGATIVE: Palv < Patmospheric causes air movement. This is physiological. POSITIVE: Palv > Patmospheric meaning air is forced into the lungs. E.g. mechanical ventilation, CPR breathing and fighter pilots.
What happens to the pressure-volume relationship of the lungs, and what is the purpose?
At middle of lung volumes, small change in pressure leads to large change volume. At end lung volumes (extra inhalations and exhalations), same change in pressure leads to smaller changes in volume: This prevents the lungs from over-filling or fully emptying – because it would have to require large and more forcible pressure changes. This curve is a product of an independent chest wall and lung.
What happens to lung pressures in inspiration and expiration?
- AT THE NORMAL TIDAL VOLUME, there is normal inwards and outwards movement of air and pleural pressure is -5. PTP and PTT both equal: PTP = 5, PTT = -5. PRS is 0.
- GREATER FORCE OF EXPIRATION: the aim for expiration is to produce a positive pressure inside the lungs to drive outward air movement. SO, external forces increase i.e. muscles contract. This pushes the pleura together, INCREASING pleural pressure. PTP and PTT are lower because of this increased pleural pressure, but equal: PTP = 2 and PTT = -2. PRS is always 0.
- GREATER FORCE OF INSPIRATON: chest wall pulling at pleura, so pleural pressure DECREASES to -8. PTP = 8, PTT = -8 and PRS = 0.
PTP: Transpulmonary pressure = Palv – Ppl (intrapleural space). Should be -5cmH2O at rest.
PTT: Transthoracic pressure = Ppl – Patm. Should be 5cmH2O at rest.
PRS: Trans-respiratory pressure = Palv – Patm. It is the most important because if positive, air will flow in; negative means air will flow out. Should be 0cmH2O at rest.
How does a pressure volume curve differ in restrictive and obstructive airway diseases?
Restrictive: less because decreased vital cavity. Stretched widely because it takes MUCH MORE EFFORT taking air in, so inhalation and exhalation tend to require larger pressure differences – because of REDUCED COMPLIANCE. Obstructive: higher volumes, but tissue is MORE COMPLIANT, so less pressure required to draw air in and out.
How is volume change and intrapleural pressure related to pressure in the alveoli and flow rate?
If volume change curve is superimposed (and turned upside down,) onto pleural pressure graph, the differences in pressure give us the shapes of the alveolar pressure and flow rate graphs for inspiration and expiration. This means that volume change and intrapleural pressure are both linked – and these two are what produces ventilation.
What is the effect of filling lungs with water on compliance?
Ventilating lungs with water (I know, this is bad) makes the lungs MORE COMPLIANT i.e. requires less pressure to fill with same volume e.g. filling balloon with water doesn’t require us to put tap on full blast because it actually fills quite easily.
How does resistance change as airway travels down the airways?
AS you move down the airways, the diameter of them gets smaller, so resistance increases (because of Poiseuille’s law). However, for each airway generation, number of airways increases exponentially. This means that although diameter gets smaller, cumulative diameter gets larger for each airway generation. Therefore, resistance peaks at generation four, then decreases.
What happens to conductance and resistance as lung volume increases?
As volume of lungs increases, conductance increases linearly. Resistance decreases exponentially – since larger lung volume indicates dilating airways, so less resistance.
How does transpulmonary pressure change at each stage of inspiration? [This has kind of already been covered.]
AT REST (NO AIR MOVEMENT): Transpulmonary pressure = +5; intrapleural pressure = -5: Originates from the recoil of the chest and the lung that pulls on the pleural space. INSPIRATION: pulls apart pleural space, so decreased intrapleural pressure.
What happens to the intrapleural pressure and transpulmonary pressure during hard expiration? What does this mean for normal structures but why do our lungs prevent this?
Expiration is so hard, and so much pressure is pushed onto the intrapleural space, that intrapleural pressure skyrockets. While this does produce a high positive pressure inside the lungs, it can become so high in hard expiration that the airway is occluded. Note the intrapulmonary pressure.
THIS NEVER HAPPENS: because trachea and bronchi have cartilaginous structures which prevent inversion.
