Lung Volume

Aka: Lung Volume, Inspiratory Reserve Volume, Tidal Volume, Inspiratory Capacity, Expiratory Reserve Volume, Residual Volume, Total Lung Capacity, Minute Ventilation, Respiratory Minute Volume, Respiratory Dead Space, Anatomic Dead Space, Physiologic Dead Space

1. See Also
   1. Pulmonary Function Test
   2. Diffusing Capacity
   3. Inhalation Challenge Test
   4. Exercise Spirometry
   5. Functional Residual Capacity
   6. Vital Capacity
   7. Peak Expiratory Flow Rate (PEFR)
   8. Spirometry Lower Limit of Normal (PFT LLN)
   9. Obstructive Lung Disease
   10. Restrictive Lung Disease
2. Background
   1. At rest
      1. Tidal Volume (TV) represents basal respiratory flow, in and out (similar to gentle ocean tides)
   2. With exertion
      1. Vital Capacity (IRV and ERV, in addition to TV) is recruited to allow for deeper breaths
         1. Analogous to Cardiac Stroke Volume, which increases with exertion
         2. May increase with Exercise from 500 ml to 4200 ml per breath (TV+IRV +ERV) in 70 kg adult
         3. In Obstructive Lung Disease (Asthma, COPD), taking deep breaths is primary compensation
            1. Increasing Tidal Volume with deep breaths is less work than increasing Respiratory Rate
      2. When Vital Capacity is insufficient to support exertion, Respiratory Rate (RR) increases
         1. Respiratory Rate is analogous to Heart Rate in compensation for exertion
         2. May increase with Exercise, from 12 to 36 breaths per minute with strenuous Exercise
         3. Increased Respiratory Rate is primary compensation in high lung elastic rebound
            1. Increasing Respiratory Rate is less energy for these patients, then taking deep breaths
      3. Minute Ventilation may increase with Exercise from 6 L/min to over 150 L/min
   3. Images
      1. 
3. Interpretation: Overall Lung Volumes
   1. Vital Capacity (VC)
      1. Maximal volume expelled after maximal inspiration
      2. VC = ERV + TV + IRV
      3. Decreased with low lung compliance, high elastic rebound (e.g. Restrictive Lung Disease, pulmonary fibrosis)
      4. Also low in Obstructive Lung Disease (e.g. Asthma, COPD), due to limited expiration (high Residual Volume)
   2. Total Lung Capacity (TLC)
      1. Volume in lungs at end of maximal inspiration
      2. Calculation: VC + RV
      3. Normal adult: 4-6 Liters
   3. Minute Ventilation (MV, Respiratory Minute Volume)
      1. Volume of breath inspired (and expired) per minute
      2. MV = TidalVolume x RespiratoryRate
      3. Typical resting Minute Ventilation in a 70 kg male is 6 liters/min = 500 ml TV x 12 breaths/min
   4. Alveolar Ventilation
      1. Volume of inspired air entering the alveoli per minute
      2. Similar to Minute Ventilation but subtracts alveolar dead space
         1. Aveolar dead space is 30-33% of Minute Ventilation in normal lungs (primarily Anatomic Dead Space)
         2. In a healthy lung, alveolar ventilation is typically two thirds of Minute Ventilation
         3. In very severe, end-stage COPD, alveolar ventilation may be as low as 20% of Minute Ventilation
      3. AV = (TidalVolume - AnatomicDeadSpace) x RespiratoryRate
         1. Normal adult example = (500-150) * 16 = 5600 ml
4. Interpretation: Inspiratory Volumes
   1. Inspiratory Reserve Volume (IRV)
      1. Additional volume that could be inspired above normal inspiration (or Tidal Volume)
      2. Maximal inspired volume from end-tidal inspiration
      3. IRV may be as high as 40-43 ml/kg or 2800 ml in a 70 kg adult)
   2. Tidal Volume (Vt)
      1. Volume inspired and expired with each normal breath
      2. Analogous to Cardiac Stroke Volume at Rest
      3. Minimum volume: 3 ml/kg
      4. Normal volume: 6-7 ml/kg (e.g. 500 ml in a 70 kg adult at rest)
      5. Typical ventilator Tidal Volume settings: 6-8 ml/kg
   3. Inspiratory Capacity (IC)
      1. Maximal volume inspired from resting expiratory level
      2. Calculation: IRV + Vt
5. Interpretation: Expiratory Volumes
   1. Expiratory Reserve Volume (ERV)
      1. Additional volume that could be expired in addition to normal exhalation (or Tidal Volume)
      2. Maximal expired volume from end-tidal inspiration
      3. Normal: 25% of Vital Capacity (up to 15 ml/kg or 1050 ml in a 70 kg adult)
   2. Functional Residual Capacity (FRC)
      1. Volume remaining in lungs at resting expiratory level
   3. Forced Expiratory Volume in One Second (FEV1)
      1. Expiratory rate (FEV1) slowed by Obstructive Lung Disease (Asthma, COPD, chest masses)
   4. Residual Volume (RV)
      1. Volume remaining in lungs after maximal expiration
      2. Normal adult: 1.0 to 2.4 Liters
      3. Includes Respiratory Dead Space (anatomic and physiologic)
      4. Also includes healthy respiratory zone (alveoli to Bronchioles) that contribute to gas exchange
         1. Allows for passive CO2 and O2 diffusion in gas remaining in aveoli and Bronchioles between breaths
         2. Contrast with Physiologic Dead Space in diseased respiratory zone that cannot gas exchange
      5. RV is not measureable with standard Spirometry/PFTs
         1. Requires advanced testing (e.g. nitrogen washout, helium dilution, body plethysmography)
         2. Increased Residual Volume may also be suggested by barrel chest, hyperexpansion on CXR
   5. Dead Space
      1. Total Dead Space (Physiologic Dead Space)
         1. PhysiologicDeadSpace = AnatomicDeadSpace + AlveolarDeadSpace
      2. Anatomic Dead Space
         1. Volume of air within the nose, trachea and Bronchi that does not contribute to gas exchange
         2. Present in all patients regardless of health, representing 30-33% of Tidal Volume (e.g. 150 ml in 70 kg adult)
      3. Alveolar Dead Space
         1. Volume of air within the respiratory zone (alveoli to Bronchioles) that does not contribute to gas exchange
         2. In healthy patients, alveolar dead space is minimal (<5 ml)
         3. In diseased lung (e.g. COPD) or edematous lung (e.g. CHF, ARDS), alveolar dead space may be large
            1. Severe disease total dead space may be as large as 50-80% of Tidal Volume
6. Resources
   1. Intagliata (2020) Lung Dead Space, StatPearls, Accessed 3/30/2021
      1. https://www.ncbi.nlm.nih.gov/books/NBK482501/
7. References
   1. Rollings (1984) Facts and Formulas, McNaughton & Gunn
   2. Marini (1987) Respiratory Medicine, Williams & Wilkins