Obstructive flow

Question 1

Learning outcomes

Answer 1

• Define the airways
• Relate the dimensions of an airway to resistance to flow
• Recognise the differences between laminar and turbulent flow
• Describe the relative contributions of parts of the respiratory tract to airway resistance
• Describe factors which can increase airway resistance
• Explain the actions of parasympathetic and other nerves to airways
• Explain the action of adrenaline, selective adrenoreceptor agonists and phosphodiesterase inhibitors on the airways and bronchial smooth muscle cells
• Explain the action of the mucociliary escalator and coughing
• Describe the defence action of alveolar macrophages and lymphatics

Question 2

Airway resistance (AWR)

Answer 2

AWR= pressure in alveoli/flow or

Gaw (airway conductance) = flow/pressure in alveoli

Question 3

Laminar flow through a tube

Answer 3

Flow =(P1-P2)×p×R^4/ 8×h×L
Poiseuille’s law
P1 and p2 are pressure gradient, driving flow, r is radius of tube, l is distance between p1 and p2

Flow = (P1-P2)/ Resistance
Ohm’s Law

Question 4

Laminar vs tubular flow

Answer 4

Laminar flow e.g normal flow through a tube
Increased velocity coming medially from sides of tube
All flow parallel to walls- SILENT

Turbulent flow will flow anywhere around walls- not as efficient as laminar flow, resistance increases from L to T
Vibrate wall of tube, which creates a wheezing sound

Question 5

The relative contributions of parts of the respiratory tract to airway resistance

Answer 5

Change in CSA to trachea to alveoli- increased CSA along the 15 gen of branches, faster at trachea, slows down until stationary at alveoli

Question 6

Obstructive ventilatory defect

Answer 6

Spirometry- full breath in, rapidly breathed out, continue to squeeze air out- volume expired/ time taken for expiration

• FEV1 reduced- compared to predicted value based on patient’s background, age, height, sex- recorded as percentage of expected
• FVC reduced but not as much- forced vital capacity, fully in > fully out
• FEV1/FVC reduced below 0.70

Obstructive-
↓FEV1/FVC
/↑AWR

Restrictive-
↓FEV1↓FVC

─or↑ FEV1 FVC/
↓C

Question 7

Dypsnoea

Answer 7

In obstruction, increased load on respiratory muscles caused by increased airway resistance
Also there is hyperinflation so more work against elastance

Question 8

Factors that can increase airway resistance

Answer 8

Intra/extrathoracic airways
Intrathoracic airways compressed in forced expiration
Extrathoracic airways compressed in inspiration
Airways collapsing on inspiration during sleep due to tension lost in skeletal muscle of pharynx- Obstructive sleep apnoea, leads to snoring
this relaxation can cause complete occlusion of airways, leading to insufficient breathing and waking up