Mechanics of Breathing - Part 2 Charles L. Webber, Jr., Ph.D.

Learning Objectives: You should be able to:

• Explain the deceleration/acceleration of air in terms of three air flow profiles that occur during each inspiratory/expiratory half cycle.
• List the four primary factors that affect the prevailing airway resistance centrally and peripherally.
• Diagram how forced expirations cause flow limitations at lung volumes below FRC in exact accordance with a Starling resistor model.
• Name the two major components contributing to the work of breathing and show how each is altered in different disease states.

Rhoades & Tanner Text Readings: Chapter 19, Pages 352-362

Airflow Profiles       Dynamic Compression      Air-Flow Resistance         Work of Breathing       MainMenu

1. Cross-Sectional Area of the Airways

   

   • total cross-sectional area increases exponentially with generation (solid line)
   • forward air flow velocity decreases exponentially with generation (dashed line)

      

    

2. Three Types of Air-Flow Profiles
   • continuity equation
     • flow (mL/sec) = vel (cm/sec) * cross-sect area (cm²) * (mL/cm³)
   • Reynold's Number (N_R) = density * mean velocity * diameter / viscosity
   • turbulent flow for N_R > 3000 (gen. 0-6)
   • laminar flow for N_R < 2000 (gen. 7-17)
   • diffusive flow with cardiogenic mixing for N_R = 0 (gen. 18-23)

Airflow Profiles       Dynamic Compression      Air-Flow Resistance         Work of Breathing       MainMenu

1. Flow-Volume Loops

   

    

   • air flow (L/sec) is plotted against lung volume (% Vital Capacity)
   • inspiratory and expiratory air flows form a closed loop (clockwise rotation)
   • for any lung volume, max inspiratory air flow is effort dependent
   • for high lung volumes, max expiratory air flow is effort dependent
   • for low lung volumes, max expiratory air flow is effort independent

    

2. Dynamic Compression of the Airways
   • the expiratory portion of the maximal flow-volume loop has a scooped- out appearance
   • during active, forced expiration P_pleural can actually exceed P_airway
   • this transmural pressure gradient favors airway compression (Starling resistor effect)
   • greater effort ( P_pleural) results in greater compression ( radius) with no change in air flow
   • compression starts at the equal-pressure point (EPP) in the cartilage-free airways within the lung
   • in disease the weakened airways can actually collapse causing air-trapping behind the blockade
   • lip pursing moves the EPP to the mouth, a psychological relief to the patient

Airflow Profiles       Dynamic Compression      Air-Flow Resistance         Work of Breathing       MainMenu

Four Factors Affecting Air-Flow Resistance

1. Airway Caliber
   • laminar flow () = pressure (P) / resistance (R)
   • slope of line = 1/R = conductance
   • resistance (R) to laminar flow 1/radius⁴
   • airway caliber (secretions, bronchoconstriction): < R and < P

    

2. Air-Flow Profile
   • laminar flow (<) increases linearly with < driving pressure
   • non-laminar flow (<) increases curvilinearly with < driving pressure
   • R_{non-laminar flow} is effectively greater than R_{laminar flow} due to turbulence
   • for any given < P, <_non-laminar < <_laminar
   • or any given <, <P_non-laminar > < P_laminar

    

3. Airway Generation

   

   • in general, regional airway resistance decreases as a function of airway generation
   • in specific, the highest regional resistance is at generation 4
     • medium sized bronchi of short length and frequent branchings highly non-laminar air flow with extreme turbulence

    

4. Lung Volume

   

   • total airway resistance = summation of serial regional resistances
   • R_total decreases hyperbolically with increases in lung volume
     • conductance_total increases linearly with increases in lung volume (dashed line)
     • increases in lung volume cause increases in radius due to tethering of the airways
   • R_total can be partitioned into two components
     • R_peripheral (gen. 7 - gen. 23): low resistance (laminar & diffusive zones)
     • R_central (nose - gen. 6): high resistance (turbulent flow zone)
     • R_central >>> R_peripheral (50% of resistance in nasal passages alone)

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Work of Breathing

1. Components of Work
   • elastic work - work to overcome:
     • lung elastic recoil
     • thoracic cage displacement
     • abdominal organ displacement
   • frictional work - work to overcome:
     • air-flow resistance (major)
     • viscous resistance (lobe friction, minor)
   • inertial work - work to overcome:
     • acceleration and deceleration of air (negligible due to low mass of air)
     • acceleration and deceleration of chest wall and lungs (negligible due to overdamping)

    

2. Graphical Representation of the Major Components of Work

   

   • work = force * distance pressure * volume / 2
   • elastic work area a-b-c-a
   • inspiratory flow-resistive work area a-i-b-a
   • expiratory flow-resistive work area a-b-e-a
   • negative work area a-e-b-c-a (tone on inspiratory muscles during expiratory air flow)
   • total work W_elastic + W_{inspiratory flow-resistive} + W_negative
   • passive recoil of lungs overcomes the work of expiratory flow-resistance

    

3. Work of Breathing in Disease (Fig. 12)
   • restrictive or low compliance diseases (e.g. fibrosis)
     W_elastic + normal W_{flow-resistive} + W_negative = W_total
   • obstructive or high air flow resistance diseases (e.g. asthma)
     normal W_elastic + W_{flow-resistive} + W_negative = W_total

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