• Spirometry measures lung volumes (y-axis) as a function of time (x-axis).

  • Flow rates are determined from spirometry by measuring volume achieved over a given time period (y/x values).

    

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    FIGURE 1. Spirometry and lung volumes.

• Flow volume loops are a graphic representation of inspiratory and expiratory air flow (y-axis) as a function of lung volume (x-axis). It measures a complete respiratory cycle in a clockwise manner from forced maximal expiration to maximal inspiration.

  • Components of maximum air flow depend on volumes.

    

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    FIGURE 2. Normal flow volume loop.

  • Volume (x-axis): Movement along the x-axis from left to right indicates decreasing lung volumes.
  • Expiration (positive y-axis): Curve moves from high to low lung volumes (left to right on x-axis). There is a rapid rise to peak expiratory flow (PEF) followed by a linear fall as lung volumes decrease, airways narrow, and resistance increases.
  • Inspiration (negative y-axis): Curve moves from low to high lung volumes (right to left) in a symmetrical saddle shape.

• Lung volumes (Figure 1): Measurements obtained with spirometry
  • Tidal volume (TV): Volume of air measured with passive ventilation.
  • Inspiratory reserve volume (IRV): Volume of air measured from end tidal volume inspiration to maximal inspiration.
  • Expiratory reserve volume (ERV): Volume of air measured from passive expiration to maximal forced expiration.
  • Residual volume (RV): Volume of air that remains in the lungs after maximal expiration; unable to measure with spirometry.
• Lung capacities (Figure 1): Represents the sum of two or more lung volumes on spirometry.
  • Functional vital capacity (FVC): Volume of air forcibly exhaled after maximal inspiration (does not include RV).
    • FVC = IRV + TV + ERV
  • Functional residual capacity (FRC): Volume of air present in the lungs at the end of passive expiration.
    • FRC = ERV + RV
  • Total lung capacity (TLC): Volume of air contained within the lungs following maximal inspiratory effort.
    • TLC = FVC + RV
• Lung flows (Figure 2):
  • PEF: Maximum rate of respiratory gas flow during expiration (peak of expiratory curve). Following PEF, the slope of the expiratory curve is nearly linear. It is a function of airway diameter, patient effort, and expiratory muscle strength
  • Maximal expiratory flow at 50% FVC (MEF₅₀): Flow value on the positive y-axis at the midpoint of the x-axis.
  • Maximal inspiratory flow at 50% FVC (MIF₅₀): Flow value on the negative y-axis at the midpoint of the x-axis.
  • Under normal conditions, MEF₅₀ is only slightly less than MIF₅₀ (secondary to dynamic compression resulting in reduced flow).
  • Under pathophysiologic conditions, the discrepancy between the MEF₅₀ and MIF₅₀ can be significant.
• Comparison of flow-volume curves performed during resting ventilation with those during forced inspiration and expiration should demonstrate significant ventilatory reserves in normal subjects (1)[B].

• Obstructive lung disease:

  • Asthma, chronic bronchitis, bronchiectasis, COPD

    

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    FIGURE 3. Obstructive lung disease.

  • Curves shift leftward on the x-axis secondary to hyperinflation. "Air trapping" results in higher lung volumes and RV.
  • PEF and MEF₅₀ are significantly reduced, and MEF₅₀ < MIF₅₀.
  • "Scalloping" of the expiratory loop (positive y-axis) indicates reduced expiratory flow at all lung volumes (no longer linear).
  • Since inspiratory flow (not expiratory flow) is primarily increased during exercise, patients with obstructive disease experience increased end-expiratory lung volumes (hyperinflation) with exertion. This may be in addition to hyperinflation present at baseline.
• Restrictive lung disease:

  • Fibrosis, scoliosis, obesity

    

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    FIGURE 4. Restrictive lung disease.

  • Curves shift rightward on the x-axis secondary to a decreased TLC; all lungs volumes are reduced.
  • PEF, MEF₅₀, and MIF₅₀ are all reduced; flow reduction is congruent over the inspiratory and expiratory loops and MEF₅₀ ~ MIF_50. Additionally, PEF is not as diminished or affected compared to obstructive disease.
• Fixed upper airway (extrathoracic) obstruction:

  • Tracheal stenosis, goiters

    

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    FIGURE 5. Fixed extrathoracic obstruction.

  • Volumes (x-axis) are not affected.
  • Flow is decreased throughout the respiratory cycle at all lung volumes.
  • Curves are flattened on both the inspiratory and expiratory curves, MEF₅₀ ~ MIF₅₀.
• Variable (nonfixed) upper airway (extrathoracic) obstruction:

  • Vocal cord dysfunction, obstructive sleep apnea, tracheomalacia

    

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    FIGURE 6. Variable extrathoracic obstruction.

  • Volumes (x-axis) are not affected.
  • Flow is decreased with inspiration but does not affect expiration (MEF₅₀ > MIF₅₀). PEF is unchanged.
  • The inspiratory curve is flattened.
  • Insensitive test for upper airway obstruction: The tracheal lumen circumference must be decreased to less than 8 mm (80% reduction in tracheal area) before abnormalities are detected (2)[B].
• Variable (non-fixed) intrathoracic obstruction:

  • Endobronchial tumors or bronchial compression by other malignancies (anterior mediastinal masses)

    

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    FIGURE 7. Variable intrathoracic obstruction.

  • Volumes (x-axis) are unchanged.
  • Flow is decreased during expiration but does not affect inspiration (MEF₅₀ < MIF₅₀); PEF is significantly reduced.
  • The expiratory curve is markedly affected.

• Flow-volume loops can aid in the diagnosis and monitoring of the progression of chronic lung diseases.
• Flow volume loops provide an alternative to advanced imaging modalities that are not immediately available or feasible in the operative room and intensive care unit.
  • Asthma, COPD exacerbations
  • Endobronchial intubations
• Vocal cord dysfunction (Figure 6): Flow volume loops are commonly utilized to confirm the diagnosis (3)[B].
• Anterior mediastinal mass (Figure 7): In addition to the clinical history and radiological imaging, flow-volume loops provide a tool to measure the "functional" effects of a mass. Abnormalities in the expiratory loop may be accentuated in the supine position and may indicate the potential for complete airway obstruction with anesthetic induction and muscle relaxation.

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

• Spirometry
• Anterior Mediastinal Mass
• Asthma

Sylvia H. Wilson , MD

Ilka Theruvath , MD, PhD