• Pulmonary function tests (PFTs) measure lung volumes, airflow, and diffusing capacity of gas across the alveolar–capillary membrane (DLCO).

• Lung volumes and capacities refer to the volume of air associated with different phases of the respiratory cycle.

  

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  FIGURE 1. Lung volumes and capacities.

• Lung volumes are directly measured. They include:
  • Tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and residual volume (RV).
• Lung capacities are composed of one or more lung volumes. They include:
  • Inspiratory reserve capacity (IRC), vital capacity (VC), total lung capacity (TLC), and functional residual capacity (FRC).

• forced expiratory spirometry measures expiratory flow and volume while the patient forcefully exhales after maximal inhalation.

  

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  FIGURE 2. forced expiratory spirometry.

• TV: Volume of air inspired with passive breathing.
  • Normal TV: ~6–8 mL/kg
  • TV decreases with decreased lung compliance and/or reduced ventilatory muscle strength.
• IRV: Maximal volume of air that can be additionally inspired after a normal tidal inspiration.
  • Normal IRV: ~1.9–3.3 L
• ERV: Maximum volume of air forcibly expired after a normal tidal expiration.
  • Normal ERV: ~0.7–1 L
• RV: Volume of air remaining in the lungs after forceful expiration. It cannot be exhaled, and hence cannot be measured on routine spirometry.
  • Normal RV: ~1 L
• Inspiratory capacity (IC): Volume of air inspired with maximum lung expansion after resting expiration.
  • IC = IRV + TV (normal ~2.5 - 3.5 L)
  • Reduced IC can signify extrathoracic airway obstruction.
• FRC: Volume of gas remaining in the lungs after passive expiration.
  • FRC = ERV + RV (normal ~1.8 - 2.3 L)
  • FRC influences the ventilation–perfusion relationship within the lung.
  • Reduced FRC leads to an increase in venous admixture and arterial hypoxemia.
  • Indirect determination of FRC (because RV cannot be measured on spirometry).
    • Equilibration method: FRC is calculated by the equilibration of a tracer gas (helium) in a closed-circuit system. Using a spirometer and starting at FRC, the patient breathes a given concentration of helium and oxygen. Helium equilibrates between the patient's lungs and the spirometer. Since the system is closed, FRC can be determined by (Concentration1 × Volume1) = (Concentration2 × Volume2).
    • Washout method: FRC is calculated from the washout of a tracer gas (nitrogen) within the lungs. The patient breathes 100% oxygen for several minutes to "wash out" alveolar nitrogen. Nitrogen fractional concentration and expired volume is measured over 7 minutes. The initial and final alveolar nitrogen fractional concentrations and the nitrogen concentration in the collected expirate are used to calculate FRC.
    • Plethysmography: The patient sits in a box with a pressure transducer and breathes through a mouthpiece. The mouthpiece is connected to an electronic shutter and a differential pressure pneumotachometer. Mouth pressure and box pressure changes are measured during tidal breathing and panting maneuvers. The differential pressures are used to calculate thoracic gas volume and FRC.
• VC: The maximum volume of air that can be exhaled after maximal inspiration.
  • VC = IRV + TV + ERV (normal ~60 mL/kg)
  • VC correlates with the ability for deep breathing and effective coughing.
• TLC: The volume of air within the lungs after maximum inspiratory effort.
  • TLC = IRV + TV + ERV + RV (normal ~ 5 L)
• forced vital capacity (FVC): The volume of air exhaled with maximal expiration after maximal inspiration.
  • FVC normally equals VC.
  • FVC measurements are dependent on patient effort and cooperation.
• forced expiratory volume in 1 second (FEV₁): The volume of air exhaled forcefully in the first second of an FVC maneuver.
  • Measure of flow (volume expired over time).
  • Dependent on patient effort. Low FEV₁ may be a result of inadequate effort, not lung disease.
• FEV₁/FVC: Normalizes FEV₁ measurements to the patient's individual lung volume.
  • Normal FEV₁/FVC 0.75; the volume of air that is expired in 1 second should be 75% of the forced vital capacity.
• forced midexpiratory flow rate (FEF_25-75%): Exhaled volume between 25% and 75% of FVC.
  • formerly called the maximum midexpiratory flow rate (MMFR).
  • Less effort dependent than FEV₁ or FVC, but may be reduced by submaximal inspiration (decreased FVC) or markedly decreased expiratory effort.
  • Normal values vary widely due to the dependence on FVC. Both the absolute value (normal ~ 2–5 L/sec) and the percentage of predicted value (normal ~ 100% ± 25%) are recorded.
• Diffusing capacity (DLCO): Measures the diffusion of carbon monoxide (CO) across the alveolar–capillary membrane. Diffusion capacity is determined by measuring the partial pressure difference between inspired and expired CO.
  • DLCO reflects the diffusing capacity of physiologic gases, such as oxygen and carbon dioxide.
• Maximum voluntary ventilation (MMV): Largest volume that can be breathed in 1 minute by voluntary effort.
  • MMV records the patient breathing as rapidly and deeply as possible for 10 seconds by a pneumotachograph and the results are extrapolated to 1 minute.
  • Best indicator of ventilatory muscle endurance.

• Obstructive lung disease: Asthma, chronic bronchitis, emphysema, bronchiolitis, and chronic obstructive pulmonary disease (COPD).
  • Diminished expiratory airflow from collapse of small airways.
    • Decreased FEV₁/FVC and FEF_25-75%.
  • Airway collapse during expiration causes air trapping with subsequent hyperinflation.
    • Increased TLC, FRC, RV.
• Restrictive lung disease: Intrinsic and extrinsic pulmonary processes.
  • Intrinsic (loss of alveolar airspace): Pulmonary fibrosis, sarcoidosis, or pneumonia.
  • Extrinsic (reduction of lung space): Scoliosis, disorders of thoracic cage, pleural effusion, pneumothorax, or neuromuscular disorders.
  • Smaller lung volumes along with normal or decreased expiratory flow (decreases to a lesser extent than volume).
    • Normal or increased FEV₁/FVC and FEF_25-75%
  • Reduction of all lung volumes, especially TLC, VC, and RV.
• Compromise of alveolar capillary unit: Loss of lung parenchyma (emphysema or COPD), thickening of the alveolar–capillary membrane (interstitial lung disease or pulmonary fibrosis), or pulmonary vascular disease.
  • Reduction of DLCO

• Patients undergoing general anesthesia and surgical procedures (particularly thorax and upper abdomen) develop changes in pulmonary function that may contribute to postoperative pulmonary complications.
  • VC is reduced up to 25–50% of preoperative values after open thorax or upper abdominal procedures and remains depressed for 10–14 days.
  • FRC decreases by 10–15% in the supine position.
• The primary goal of preoperative PFTs is to recognize patients at high risk of postoperative pulmonary complications.
  • Reduced FVC values (<15 mL/kg) are associated with postoperative pulmonary complications (1).
    • Most commonly seen in quadriplegics or severe neuromuscular disease.
    • Reduced FVC is associated with ineffective cough and development of atelectasis.
• PFTs provide some predictive benefit in patients undergoing lung resection.
  • Predicted postoperative FEV₁ calculated by multiplying the preoperative FEV₁ by the percentage of lung tissue expected to remain after resection. An increased risk of perioperative pulmonary morbidity is associated with a predicted postoperative FEV₁ <30% (2).

• IC = TV + IRV; where IC is inspiratory capacity, TV is tidal volume, and IRV is inspiratory reserve volume
• FRC = ERV + RV
• VC = IRV + TV + ERV; where VC is vital capacity, ERV is expiratory reserve volume, and IRV is inspiratory residual volume
• TLC = VC + RV; where TLC is total lung capacity, VC is vital capacity, and RV is residual volume

Ilka Theruvath , MD, PhD

Sylvia H. Wilson , MD