### F3viii 21B12 / 16B06: Airway resistance

## 21B12: Exam Report

### Explain the physiological factors that affect airway resistance

**31% of candidates passed this question.**

It was expected candidates cover the breadth of the factors that affect airway resistance. Generally, as a concept the type of flow (laminar vs turbulent) was answered well by most candidates, however many failed to mention the other factors that affect airway resistance. Airway diameter as a primary determinant of airway resistance was commonly omitted. Better answers which covered the factors affecting airway diameter classified them broadly and included examples such as physical compression/external obstruction, broncho-motor tone and local cellular mechanisms. Some answers did not explain these factors in enough detail and often with incorrect facts.

## 16B06: Exam Report

### Describe the factors that affect airways resistance.

**47% of candidates passed this question.**

Candidates who used a structured approach of using formulae that describe resistance fluid flow scored well. Poiseuille’s law describes the determinates of resistance to laminar fluid flow and provides a useful answer structure. The most common mistakes were confusion between resistance and compliance as well as failure to describe turbulent as well as laminar flow.

# F3viii / 21B12 / 16B06: Describe factors that affect airway resistance.

## Facts

- Normal R
_{AW}= 0.5 – 2cm H_{2}O/L/sec - Majority R
_{AW}in bronchi gen 7 – 8- Although further airway generations are smaller in diameter, their total cross-sectional area is huge

- Respiratory System Resistance is due to:

- TISSUE RESISTANCE
- AIRWAY RESISTANCE

### Tissue Resistance

- 20% of Respiratory System Resistance
- Due to friction of lung parenchyma as the tissues slide over each other

### Airway Resistance

- The forces which have to be overcome for gas to flow in the airways
- Influenced by:
- Properties of inspired gas
*(viscosity + density)* - Type of flow
*(influenced by Reynold’s No.)* - Airway Calibre
- Lung volume

- Properties of inspired gas

## 1) Properties of Inspired Gas

- Behaviour of gas in flow depends on 2 intrinsic properties of the gas:

- DENSITY = the ratio of mass to volume
- VISCOSITY = internal property of fluid which is a measure of resistance to flow

- Density & viscosity of gas are related to Airway Resistance (R
_{AW}) through the HP equation & Reynold’s No.

## 2) Type of Flow

**Flow**= the volume of fluid*(gas or liquid)*passing a point in unit time

### Laminar Flow

- Particles of fluid flow in parallel lines
- Flow greatest at centre
- Described by Hagen-Poiseuille equation

*V = flow*

*h = viscosity*

*L = length*

**Viscosity dependent***(density independent)*- Very efficient because ∆P varies
**directly**with flow - Present in smooth tubes with low flow rates
*g. anaesthetic circuit*

### Turbulent Flow

- Disorganised air flow moving in the same direction
- More likely at high flow rates
**Density dependent***(viscosity independent)*- Flow is not described by HP equation ∴
**Fanning Equation**

- It is
**less**efficient because ∆P varies directly with V^{2} - For a given flow, the necessary driving P is greater
- The driving P is also proportional to the 5
^{th}power of tube radius

### Transitional Flow

- Flow is neither fully turbulent nor fully laminar
- There is a gradual transition from laminar → turbulent

## Reynold’s Number

- A dimensionless number used to determine if flow is going to be laminar or turbulent

**Low (<2000)**= laminar flow because viscous forces predominate = smooth, consistent fluid motion**High (>2000)**= turbulent flow = random eddies, vortices, flow fluctuations = dominated by inertial forces

## 3) Airway Calibre

- All types flow affected by airway calibre
- ↓radius of airway = ↑R
_{AW} - Calibre affected by:
*Intraluminal factors*- Tumor
- Foreign Body

*Intramural factors:*- Mainly affected by bronchial smooth m. tone
*Drugs*- Adrenergic = bronchodilation
- Cholinergic = bronchoconstriction

*Innervation*- Parasymp = bronchoconstriction
- Symp = poorly innervated lung

*MC products*- Bronchoconstriction

*Extraluminal factors*- Kinking ETT

## Measurement of Airway Resistance

- P1 = mouth pressure → measured with manometer
- P2 = alveolar pressure → body plethysmography
- Body plethysmograph measures total volume of gas in lungs
- Uses:
**BOYLE’S LAW: P x V is constant (at constant temp)**

∴P1V1 = P2(V1 – ∆V)

- ∴to deduce alveolar P:
- On INSP ↑alveolar V & ↓alveolar P
- ∆P in the body plethysmograph box
- Can calculate the ∆ volume & deduce the alveolar P

- ∴to deduce alveolar P:
**Flow rate**→ measured with Pneumatochograph- Flow is derived from the pressure difference calculated against a known small resistance

*If you blow into mouthpiece (with known Resistance) P1 & P2 are deduced and Flow can be calculated*

Author: Krisoula Zahariou