30 - Respiratory Mechanics Flashcards
Muscles of inspiration
• Inspiration:
o Diaphragm
Main muscle (75% of change in intrathoracic volume)
o External Intercostal
Normally contribute very little but used when difficulty in breathing is experienced = dyspnea sign
o Scalene and SCM
Raise 1st and 2nd ribs
Used when difficulty in breathing is experienced = dyspnea sign
Muscles of expiration
• Expiration:
o Internal Intercostal
Normally contribute very little but used when difficulty in breathing is experienced = dyspnea sign
o Abdominal Wall muscles
Most important
Forces diaphragm up
o Passive process typically. Due to relaxation of diaphragm and intrinsic recoil of lung tissue.
• Pleural pressure:
o Pressure of pleural fluid
o Normally a negative pressure.
o At the beginning of inspiration, pleural pressure is the amount of suction needed to hold the lungs open.
o During inspiration, the chest wall expands, pulling the lungs outward creating a larger negative pressure.
o Expiration events are reversed.
• Alveolar pressure:
o Pressure of air inside alveoli.
o During inspiration this pressure drops to about -1 cm (below atm pressure).
o During expiration it raises to about 1 cm (above atm pressure).
• Transpulmonary pressure:
o Difference between alveolar and pleural pressure.
o Measure of the elastic forces in the lungs that tend to collapse lungs at each instant of respiration (recoil pressure)
• Pressure gradient for normal respiration
o As soon as lowering alveolar pressure below atmospheric pressure has surpassed the amount of force needed to counteract airflow resistance by the airway, inspiration of air can happen.
o And vice versa for expiration.
3) Describe the sequence of events in normal breathing
Inspiration
Inspiration:
o Brain initiates inspiratory effort
o Nerves carry message to muscles
o Diaphragm contracts
o Thoracic volume ↑ with chest wall expansion
o Pleural pressure ↓
o Transmural pressure gradient ↑
o Alveoli expand increasing alveoli elastic recoil
o Alveolar pressure ↓(establishing a pressure gradient)
o Air flows into alveoli until pressure is equal to atmospheric pressure
3) Describe the sequence of events in normal breathing
Expiration
Expiration:
o Brain ceases inspiratory command
o Muscles relax
o Thoracic volume ↓
o Pleural pressure ↑
o Transmural pressure gradient ↓
o Alveolar elastic recoil return alveoli to pre-inspiratory volumes
o Alveolar pressure ↑ (establishing pressure gradient)
o Air flows out of alveoli until pressure is equal to atmospheric pressure
• Lung Compliance
o Lung compliance is the extent to which the lungs will expand for each unit increase in transpulmonary pressure
o Lung compliance reflects the distensibility of the lung
The lungs distend easily at low lung volume
At high lung volume, there is an increase in transpulmonary pressure, which results in a small increase in volume
o Compliance is the inverse of elastic recoil *****
• Hook’s Law
o With an elastic structure, an ↑ in length or volume varies directly with an ↑ in the force or pressure until the elastic limit is reached
Determinants of compliance
o Elastic force of lung (elastin or collagen) and elastic forces of surface tension
5) Understand the hysteresis between the inspiratory and expiratory curves in a pressure-volume curve
• The hysteresis is due to surface tension at liquid-air interface of air-filled lung – Liquid molecules are stronger than the force between liquid-air
• Inspiration:
o Liquid molecules closest together will need to break intermolecular forces to inflate lung
• Expiration:
o There is no need to break intermolecular forces
• Compliance curves are determined by intrinsic compliance of lung and surface tension at liquid-air interface
6) Understand the concept of combined lung and chest compliance
• (1/ Total compliance ) = (1/lung compliance) + (1/ chest wall compliance)
o Functional residual capacity is the equilibrium between the two compliances
o When volume is less than FRC…
Chest wall expanding force is greater
o When volume is greater than FRC…
Lung collapsing force is greater
o When volume is extremely higher than FRC…
Both lung and chest wall want to collapse
7) Understand the role and mechanism of action of surfactant
• Synthesized by type II cells, surfactant decreases surface tension preventing atelectasis following expiration. Its main component is lecithin.
• Mechanism:
o Align on alveolar surface (hydrophobic portions attracted to each other and hydrophilic repelled)
o Intermolecular forces between surfactant molecules break up attracting forces between liquid molecules lining alveoli
o This decreases surface tension and collapsing pressure
Law of Laplace
• Law of LaPlace:
o P = 2T/r
o P: pressure tending to collapse alveolus
o T: alveolar surface tension
o r: alveolar radius
8) Know the use of lecithin/sphingomyelin ratio in clinical settings
- Ratio use in amniotic fluid as measure of fetal lung maturity.
- Fetal lung synthesizes mostly sphingomyelin
- In 3rd trimester, type II cells convert stored glycogen FA dipalmitoyllecithin (lecithin detectable in 30-32nd week)
- L/S ratio less than 2 is a potential RDS
- L/S ratio = 1.5 than 80% chance RDS
9) Explain the pathophysiology of Infantile Respiratory Distress Syndrome (IRDS)
- Newborns with ineffective lecithin at high risk
- Glucocorticoids could be used during pregnancy to stimulate fetal lung growth
- IRDS patients need both glucocorticoid and + end expiratory pressure (PEEP)
10) Understand the mechanisms of changes in lung compliance in emphysema
Emphysema (↑ lung compliance)
o Loss of elastic fibers in lungs which increases compliance (inverse relationship).
o This correlates with a steeper slope in the pressure-volume curve.
o Collapsing force for lungs is decreased.
o This results in a new, higher FRC (volume is added to lungs to increase collapsing force).
o Patients breath at ↑ lung volumes and have barrel-shaped chest.
10) Understand the mechanisms of changes in lung compliance in pulmonary fibrosis
Lung Fibrosis (↓ lung compliance)
o Stiffening of lung tissue.
o This will decrease the slope of the pressure-volume curve.
o At original FRC, the tendency for lungs to collapse is greater than tendency for them to expand.
o This means we have to lower the FRC.
o Also occurs in kyphosis and pectus excavatum.
11) Know the key factors that determine resistance to airflow in the lungs. Understand the mechanism in relevant examples of altered airway resistance.
R = (8 n l) / pi x r^4
R = airway resistance n = viscosity of air l = length of airway r = radius of airway
• Radius of airway is most influential factor determining airflow resistance.
