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 * r⁴) / (8 * eta * l); eta is viscosity

compliance = V/P = 1/elastance

Ohm’s Law: resistance = P/flow = P/V; resistance is inversely proportional to r⁴

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

(1) xtrathoracic (trachea + main stem bronchi) = 50% of resistance (2.0 cmH₂O/L/sec)
(2) large intrathoracic (bronchi >2mm) = 40% (1.6 cmH₂O/L/sec)
(3) small intrathoracic (<2mm, about 8^th division) = 10% (0.4 cmH₂O/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

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

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