Mechanics of Breathing/Compliance

Question 1

Muscles involved in inspiration

Answer 1

• diaphragm: contracts dome of muscle downward during inspiration
• external intercostals: pull ribs forward and outward
• accessory muscles: silent during normal breathing; elevate rib cage during icnreased respiratory load
  
  • sternomastoids
  • scalenes

Question 2

Muscles involved in expiration

Answer 2

• normal breathing = passive; no muscle contraction neccessary
• active expiration is neccesary @ higher minute volumes
  
  • abdominal wall muscles: push diaphragm up w/contraction
  • internal intercostals: pull ribs downward and inward w/contraction

Question 3

Diaphragmatic muscle fxn in disease states

Answer 3

• force generated by diaphragm is a function of length
  
  • max forces @ 130% of resting length
  • tension generated decreases w/decreased length
• COPD diseases: breathing occurs @ increased lung volumes –> diaphragm is more contracted/shorter = mechanical disadvantage

Question 4

Intrapleural pressure definition (P_IP)

Answer 4

• pressure w/in the intrapleural space
• source of P_IP is the intrinsic elastic properties of the lung and the chest wall + lung/chest wall deviate from intrinsic positions if separate
  • ​lungs = more inflated
  • chest wall = more deflated
• ​opposing forces ==> negative P_IP (vacuum-like) force that acts to essentially connect lungs to chest wall

Question 5

Intrapleural pressure during inspiration/expiration

Answer 5

• P_IP is negative throughout breathing cycle
• P_IP = ~ -5 cm H₂O @ end of expiration
• P_IP = ~ -30 cm H_2​O @ end of inspiration
  
  • increased pressure b/c lung tends to recoil towards intrinsic deflated position and exerts force on intrapleural space
• P_L= pressure w/in lung ==> must reach negative values in order to inspire
  
  • P_IPincrease in negativity due to lung inflation allows this to happen

Question 6

Transpulmonary pressure (P_TP) definition

Answer 6

• P_TP = P_L- P_IP

Question 7

Elastic recoil pressure definition

Answer 7

• during expiration ==> lung has inherent tendency to recoil back towards intrinsic equilibrium position
• recoil ==> transient positive pressure = “elastic recoil pressure”

Question 8

Compliance (C) definition

Answer 8

• C = measure of elastic properties of the lung
• C = [V₂ - V₁] / [P₂ - P₁] = “change in volume per unit change in pressure”
• determined by plotting volume of air as function of PTP
• inversely proportional to elasticity (i.e. high compliance = low elasticity)

Question 9

Hysteresis definition

Answer 9

• compliance differs w/inspiration and expiration
• greater change in P_TP pressure to produce a given volume change during inspiration
• property of all elastic structures
• due to combo of elastic properties and surface forces on pulmonary cells

Question 10

Impacts of abnormal lung compliance

Answer 10

• fibrosis ==> low compliance ==> difficult inspiration
• emphysema ==> loss of elastic tissue ==> loss of elastic tissue/high compliance ==> difficult expiration

Question 11

Abnormal chest wall impacts on breathing

Answer 11

• chest wall must move properly in order to breath properly
• abnormalitys ==> reduction in volume change during breathing (tidal volume)
• reduced tidal volume ==> reduced air flow in lungs