Airway Mechanics

Resistance to Flow

- Airway Resistance
- P
_{av }- P_{pl }= P_{l }= transpulmonary pressure (across alveoli) – same as elastic recoil pressure - Pouisilles eqation—flow = (pressure * pi * r
^{4}) / (8 * eta * l); eta is viscosity - compliance = V/P = 1/elastance
- Ohm’s Law: resistance = P/flow = P/V; resistance is inversely proportional to r
^{4 } - low radius causes very high resistance
- dichotomous branching tubes – trachea Þ main stem bronchi Þ lobar bronchi Þ segmental bronchi Þ subsegmental Þ etc.
- each div. produces two smaller tubes, but caliper of each daughter airway is more than half parent
- 23 divisions in all, so total cross sectional area at end is substantially larger than beginning
- by Poiselles Law –
*resistance increases exorbitantly near top of airway* - additionally: top of airway is turbulant flow (Reynold’s number sufficiently high)
- pressure proportional to square of flow – result: resistance increased still further
- Distribution of Resistance
- three regions of decreasing resistance (total = 4.0 cmH
_{2}O/L/sec) - (1) xtrathoracic (trachea + main stem bronchi) = 50% of resistance (2.0 cmH
_{2}O/L/sec) - (2) large intrathoracic (bronchi >2mm) = 40% (1.6 cmH
_{2}O/L/sec) - (3) small intrathoracic (<2mm, about 8
^{th}division) = 10% (0.4 cmH_{2}O/L/sec) - diseases (COPD, emphysema, etc.) affect small airways, but resistance here is so small that total resistance changes very little (if small airway resistance doubles, total resistance increases to 4.4, or by only 10%)
- Effects of Lung Volume on Airway Caliper
- airways are not rigid—transmural pressure (across wall of airway) = P
_{airway}– P_{pl}contributes to caliber - true of interthoracic airways too; elasticity of lung tethers these open
- at large open volume, P
_{av}= P_{airway}= 0 (atmospheric), and airways are held open by negative P_{pl } - during loss of elasticity (emphasema), a less negative P
_{pl}will be required - as volume increases, P
_{pl}becomes more negative and elastic recoil force becomes greater so caliber increases - increasing volume produces increased caliber, decreased resistance, increased conductance

Flow-Volume Relationships

- inspiration
- flow rate independent of P
_{pl}; can increase flow rate by increasing change in P_{pl}(by inspiratory muscles) - expiration
- maximum velocity is dependent upon P
_{pl}, specifically lower at less negative P_{pl}(i.e., smaller volume) - during exhalation, flow depends on volume and decreases linearly from 75% to 25% volume
- cannot increase flow rate by use of expiratory muscles
**the flow-volume loop –**nonsymmetrical for inspiration and expiration

__Determinants of Expiratory Flow Rates__

- increasing P
_{pl}increases flow rate to a certain maximum velocity at given volume - expiratory muscles control P
_{pl}, so maximal flow rate independent of effort (but P_{l}does affect it) - Equal Pressure Point Theory
- P
_{pl}same throughout thoracic cavity - during maximal flow, internal pressure (P
_{av}or P_{airway}) is decreasing toward top of airway (to effect flow) - at some point, P
_{pl}= P_{airway}; this is the**equal pressure point** - above the equal pressure point, P
_{airway}< P_{pl}—pleural pressure can compress airway, increasing resistance - this limits flow to a certain maximum velocity
- lung elastic recoil (P
_{l}) can still increase flow rate, and is the primary determinant of maximum flow rate - increasing recoil will increase P
_{av}without increasing upper airway transmural pressure