Pulmonary function Volume and Ventilation Charles L. Webber, Jr., Ph.D.

Learning Objectives: You should be able to:

• Describe how respiratory muscles cause air to move into and out of the lungs on a cyclic basis.
• Draw a typical spirometer trace identifying the four primary lung volumes and four lung capacities.
• Plot simultaneous pressure fluctuations in intrapleural and alveolar spaces as functions of time during eupnea and exercise breathing.
• Draw pulmonary function spirograms expected from patients with low pulmonary compliance or high airway resistance.

Respiratory Movements          Lung Volumes               Pressure/ Volume          PFT        MainMenu

Respiratory Movements of the Thoracic Cage

1. Basic Concepts
   • inflation and deflation of the lungs constitute a cyclic, repetitive process
   • lung configuration varies with chest shape
   • chest shape varies with posture and muscles of breathing
   • there is no air within the intrapleural space
   • visceral and parietal pleura membranes are lubricated with serous fluid (7-14 mL)

    

2. Muscles of Breathing
   • inspiratory muscles (cause active increases in lung volume)
     
     1° diaphragm
     2° external intercostals
     3° sternocleidomastoid and other accessory muscles
   • expiratory muscles (cause active decreases in lung volume)
     
     1° abdominal rectus
     2° internal intercostals

    

3. Sequence of Events
   • inspiration:
     
     I muscles > V_chest > P_pl & V_lung > P_alv > air into lungs above FRC

      

   • expiration
     
     I muscles > V_chest > P_pl & V_lung > P_alv > air out of lungs to FRC

      

     
     E muscles > V_chest > P_pl & V_lung > P_alv > air out of lungs below FRC

Respiratory Movements          Lung Volumes               Pressure/ Volume          PFT        MainMenu

Lung Volumes and Capacities

1. Spirometry and Spirogram 

   

    

2. Four Non-Overlapping Lung Volumes
   • IRV (inspiratory reserve volume) = 3000 mL
   • V_T (tidal volume) = 500 mL
   • ERV (expiratory reserve volume) = 1100 mL
   • RV (residual volume) = 1200 mL

    

3. Four Overlapping Lung Capacities
   • IC (inspiratory capacity) = 3500 mL
   • FRC (functional reserve capacity) = 2300 mL
   • VC (vital capacity) = 4600 mL
   • TLC (total lung capacity) = 5800 mL

    

4. Measurement of Functional Residual Capacity (FRC)
   • body plethysmography (overestimation of FRC in disease)
   • nitrogen washout
   • helium dilution (underestimation of FRC in disease)

    

5. Lung Zones
   • conducting zone (dead space) = 150 mL
   • respiratory zone ("live" space) = 2150 mL at FRC
   • transitory zone (dead/live mix) = partitioned into above two zones

    

6. Dead Spaces (V_D)
   • anatomical V_D space (Fowler's technique)= 150 mL
   • physiologic V_D (Bohr's technique) = 160 mL
   • alveolar V_D (non-functional) = 10 mL
   • instrumental V_D = volume of external tubing connected to patient

Respiratory Movements          Lung Volumes               Pressure/ Volume          PFT        MainMenu

Pressure-Volume Relationships

1. Intrapleural Pressure (P_pl) 

   

   • experiment a: insert a pressure measuring device into intrapleural space
   • experiment b: cut a hole in chest wall to view lung (pneumothorax)

      

   • drawn conclusions:
     • pressure within the intrapleural space is negative (sub-atmospheric)
     • lung & chest wall are coupled elastic structures with different equilibrium positions
     • at FRC the chest wall expands the lung whereas the lung pulls the chest wall in
     • P_pl is negative due to forces attempting to expand intrapleural space
     • pneumothorax destroys the important coupling between the chest wall and lung

    

2. Alveolar Pressure (P_alv)
   • P_alv is the pressure within the alveolar space patent to the atmosphere (P_atm)
   • when P_alv < P_atm air flows into the lung (inspiration)
   • when P_alv > P_atm air flows out of the lung (expiration)
   • when P_alv = P_atm air there is no air flow
   • magnitude and direction of air flow (L/sec) P_alv

    

3. Pulmonic Pressure (P_pul)
   • P_pul is the pressue in the larger airways between the alveoli and atmosphere
   • P_pul faithfully follows changes in P_alv, but is of lower amplitude than P_alv
   • magnitude and direction of air flow (L/sec) P_pul>

    

4. Pressure-Volume Changes During One Respiratory Cycle
   • pictorial representation 

   

   • graphical representation

     

     • static (t = 0, 2 & 4 sec) and dynamic (t = 1 & 3 sec) phases
     • air flow occurs only when there are dynamic changes in P_pl
     • air flow cannot occur when there is no P_atm - P_pl gradient

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Pulmonary Function Testing

1. Definitions
   • FVC (forced vital capacity) = 4600 mL
   • FEV₁ (forced expiratory volume in one second) = 3700 mL
   • FEF_0.2-1.2L ( peak forced expiratory flow) = 5 L/sec
   • FEF_25%-75% ( average forced expiratory flow) = 5 L/sec
   • FEV₁/FVC (fraction of VC expelled in first second) = 80%

    

2. Diseases
   • FVC in both obstructive ( R) and restrictive ( C) diseases
   • RV in obstructive, but not restrictive diseases
   • FEF_25%-75% and FEF_0.2-1.2L in obstructive, but not restrictive diseases
   • FEV₁/FVC in obstructive disease (< 75%)
   • FEV₁/FVC in restrictive (> 85%)

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