Lung Volume

See Also

Background

At rest
1. Tidal Volume (TV) represents basal respiratory flow, in and out (similar to gentle ocean tides)
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
Images

Interpretation
Overall Lung Volumes

Vital Capacity (VC)
1. Maximal volume expelled after maximal inspiration
2. VC = ERV + TV + IRV
3. VC = TLC - RV
4. Decreased with low Lung Compliance, high elastic rebound (e.g. Restrictive Lung Disease, pulmonary fibrosis)
5. Also low in Obstructive Lung Disease (e.g. Asthma, COPD), due to limited expiration (high Residual Volume)
Total Lung Capacity (TLC)
1. Volume in lungs at end of maximal inspiration
2. Calculation: VC + RV
3. Normal adult: 4-6 Liters
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
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

Interpretation
Inspiratory Volumes

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)
Tidal Volume (Vt)
1. Volume inspired and expired with each normal breath
2. Tidal Volume is approximately one third of Vital Capacity
3. Analogous to Cardiac Stroke Volume at Rest
4. Minimum volume: 3 ml/kg
5. Normal volume: 6-7 ml/kg (e.g. 500 ml in a 70 kg adult at rest)
6. Typical ventilator Tidal Volume settings: 6-8 ml/kg
Inspiratory Capacity (IC)
1. Maximal volume inspired from resting expiratory level
2. Calculation: IRV + Vt

Interpretation
Expiratory Volumes

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)
Functional Residual Capacity (FRC)
1. Volume remaining in lungs at resting expiratory level
2. Resting position of the respiratory Muscles
  1. Balance between the opposing recoil forces of lung and chest wall
3. Functional Residual Capacity (FRC) is a key point from which other Lung Volumes are measured
  1. Inspiratory Capacity (IC) is measured from FRC to Total Lung Capacity (TLC)
  2. Expiratory Reserve Volume (ERV) is measured from FRC to Residual Volume (RV)
4. FRC Measurements
  1. Body Plethysmography or body box
    1. Based on Boyle's law (Product of Pressure and Volume is constant, at constant Temperature)
    2. Technique
      1. Initial pressures are measured at mouth and in airtight box at rest (volume = FRC)
      2. Patient pants against a closed shutter and pressures are re-measured
        Volume has decreased from FRC to Residual Volume
        Pressure has increased
    3. FRC = Patm x dPb/dPm
      1. Where Patm = atmospheric or barometric pressure
      2. Where dPb = change in airtight box pressure
      3. Where dPm = change in mouth pressure
  2. Nitrogen Washout
    1. Start by rest breathing with normal Tidal Volumes
    2. At, end expiration, inhaled gas changed from room air to 100% oxygen
    3. Total exhaled nitrogen is measured over subsequent exhalations
    4. Volume of gas required to contain expired nitrogen is calculated (air is 78% nitrogen)
  3. Helium Dilution in Closed Circuit
    1. Start by rest breathing with normal Tidal Volumes
    2. At, end expiration, inhaled gas changed from room air to 10% helium in air
    3. Breathing and measurements continue until measured helium concentration reaches steady state
    4. Expired Carbon dioxide is absorbed by soda lime
    5. Used oxygen is replaced in the inspired air
    6. Helium concentration before and after steady state are used to calculate volume of replaced gas
Forced Expiratory Volume in One Second (FEV1)
1. Expiratory rate (FEV1) slowed by Obstructive Lung Disease (Asthma, COPD, chest masses)
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

Interpretation
Dead Space

Total Dead Space (Physiologic Dead Space)
1. PhysiologicDeadSpace = AnatomicDeadSpace + AlveolarDeadSpace
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)
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
4. Ventilation-Perfusion mismatch results in some regions of lung that are over-ventilated and other underventilated
  1. Over-ventilated alveoli (V/Q>1) waste ventilation as blood is saturated with oxygen at a fixed maximum
  2. Alveolar dead space is the total excess ventilation to over-ventilated alveoli (V/Q>1)
Measured Dead Space
1. Vd/Vt = (PaCO2 - PeCO2)/PaCO2
2. Where
  1. Vd = Physiologic Dead Space Volume
  2. Vt = Tidal Volume
  3. PaCO2 = arterial pCO2
  4. PeCO2 = expired CO2 (collected gas from several expired breaths)
    1. Expired CO2 is diluted by Ambient air and regions of excessive ventilation (V/Q>1)
      1. In these cases, PeCO2 will be much lower than PaCO2
3. Interpretation
  1. Vd/Vt = 0.33 in normal lungs (resulting in ~150 ml in a 70 kg adult with TV 450-500 ml)
  2. Vd/Vt may increase to high fractions (0.6 to 0.8) in severe lung disease with V/Q>>1

Resources

Intagliata (2020) Lung Dead Space, StatPearls, Accessed 3/30/2021
1. https://www.ncbi.nlm.nih.gov/books/NBK482501/

References

Rollings (1984) Facts and Formulas, McNaughton & Gunn
Marini (1987) Respiratory Medicine, Williams & Wilkins