Ventilation

Question 1

Learning outcomes

Answer 1

• Describe the role of the work of breathing in the symptom of breathlessness
• Describe the muscles and movements of breathing
• Define lung compliance/elastic recoil of the lungs
• Describe the role of elastic recoil of the lungs in breathing
• Define airway resistance
• Describe the roles of lung compliance and airway resistance in the work of breathing

Question 2

Breathing and breathlessness

Answer 2

Inspiration: muscles contract and do work
Expiration: passive in quiet breathing in health

Breathlessness: usually more work being done by the respiratory muscles- higher load than normal

Question 3

Structure of respiratory system

Answer 3

• R and L lung contain million of alveoli, site of gas exchange in the lungs
• Air is conducted from atmosphere to alveoli by system of tubes (airways: not inclusive of alveoli): through nose/mouth individually (muscular soft pallete will close other passage)
• Trachea- some inside and some outside thoracic cavity, branches into left and right main bronchi
• 15 generations > alveoli
• Lungs contained in thoracic cage: ribs, IC muscles: below lungs 2 hemidiaphragms
• Made of striated skeletal (voluntary) muscle: somatic nerve supply : IC- IC nerves, Diaphragm phrenic nerves
• Pleural fluid in PC between parietal and visceral pleura- serous fluid, reduce friction

Question 4

Lung volume

Answer 4

Spirometer
Minute volume = Tidal Volume ×Respiratory Frequency
-Inspiratory reserve volume- max volume of air above normal inspiration
-Tidal volume- volume of air moved
-Expiratory reserve volume- Maximal volume expired after expiration- cannot fully remove air
-Residual volume- volume left in lungs after ERV
(not to be confused with dead space- areas of lungs where gas exchange cannot occur)
Vital capacity- TV+IRV+ERV
Total lung capacity- max vol of air in full expiration
IRV+TV+ERV+RV

Functional residual capacity- volume left in lungs after quiet expiration (ERV+Res.V)
Gas composition doesn’t change much in alveoli in exp/inspiration- oxygen levels mainly uniform for blood oxygenation

Question 5

Use of dilution to measure lung volumes

Answer 5

Vital capacity of helium mixture inhaled (RV)- after inhalation and mixing with the residual volume, the helium concentration is diluted (RV+VC)
Helium is not taken up by blood or lost from alveoli
The dilution of the helium enables the residual volume to be obtained

Question 6

Compliance/elastance

Answer 6

• Lung compliance is the increase in lung volume per unit increase in distending pressure measured statically
• Lung elastance/ elastic recoil is the reciprocal of lung compliance (pressure needed to inflate lung by 1l vol)

Question 7

Roles of compliance/ elastance

Answer 7

•In healthy quiet breathing, the work of breathing is done during inspiration
–Most work is against lung elastance
–Work is also done overcoming resistance of the airways to air flow. This is small in health but can be very significant in diseases of the airways.

•In healthy quiet breathing, no work is done during expiration. (Alveoli stretched, have elastic recoil energy which can push alveolar air out, overcoming airway resistance) The muscles relax and the energy stored in lung elastic recoil is used to drive the air out.

Question 8

Airway resistance

Answer 8

Ohm’s law
–electricity V = I ×R
–Air flow P = F ×AWR
–or AWR = P/FP
•Airway resistance (AWR) is the pressure required to drive 1 unit air flow through the conducting airways
- Pleural pressure overcomes elastic recoil of alveoli and airway resistance

Ppl-Patmos= (Ppl–PAlv) (across alv walls)+ (PAlv-Patmos)(across airways)

Question 9

Respiratory cycle- see lecture 46

Answer 9

Alveolar pressure drops in inspiration, increases in expiration- at atmospheric (0) at start, end of inspiration and end of expiration
Flow- less negative pressure in inspiration (flow into lungs), less positive in expiration
Pleural pressure- negative throughout cycle:so, if leak occurs air will flow from atmosphere into the PC, allows collapsed lung (pneumothorax)
Negative pressure at end of quiet expiration needed to prevent this: generated by elastic recoil of the ribcage
( lungs trying to collapse, ribcage trying to expand- forces equal)
Lung volume- end of exp +FRC: IPP similar shape but not as deep due to pressure generated by pleural cavity

Question 10

Work of breathing

Answer 10

•Muscles
•Load on muscles
–Mechanical factors in lung
•elastic properties of alveolar walls
•resistance to airflow through airway

–Minute volume
WORK = Pressure×Volume(WORK = Force × Distance)

Question 11

Dyspnoea

Answer 11

•Common respiratory complaint is breathlessness (dyspnoea)
•Complex sensory experience, commonest cause is increased work of breathing
•Pulmonary causes of increased work of breathing
–‘stiff’ lungs (low compliance)
–narrowed airways (increased airway resistance)
–increased ventilation (minute volume)