Spirometry and Pulmonary Function Testing

• Spirometry is based on the principles of pulmonary function testing
• Pulmonary function testing involves quantitative testing of various aspects of the respiratory system.
• These aspects include ventilation, strength of respiratory muscles, elastic properties of the respiratory system like compliance, airway resistance, diffusion across the respiratory membrane, pulmonary circulation, and ventilation/perfusion matching.
• The techniques used in clinical practice are research tools to uncover the principles of respiratory physiology.

Utility of Spirometry

• Spirometry measures volume and flow of air during inhalation and exhalation.
• Spirometry is a pulmonary function test.
• Spirometry is used in the diagnosis of pulmonary diseases.
• Spirometry helps optimize treatment, follow-up, and estimate prognosis.
• Spirometry is used in occupational medicine.
• Spirometry is used in preventative programs, like testing smokers for COPD.
• Spirometry is used in research.

Closed System Spirometer

• Volume displacement spirometers are "water spirometers" or "bell spirometers."
• Water seals the closed system with low friction.
• The closed system limits O2 and CO2 exchange, but the short time required for spirometry mitigates this.

Flow Spirometer

• Flow spirometers use an open system.
• Flow is measured using a flow meter or pneumotachometer.
• Volumes are calculated by time integration of flow.
• Measurement principle: The pressure difference (P2>P1) across a constant resistance is directly proportional to flow.

Spirogram

• Spirometric parameters include static values like volume and capacities as well as dynamic values.

Static Parameters - Volumes

• Tidal volume (Vt) is the volume of air breathed in and out during a normal breath without conscious effort.
• Inspiratory reserve volume (IRV) is the additional volume of air that can be inspired with maximum effort after normal tidal inspiration.
• Expiratory reserve volume (ERV) is the additional volume of air that can be expired with maximum effort after normal tidal expiration.
• Residual volume (RV) is the volume of air remaining in the lung following maximum expiration.

Static Volume Values

• Tidal volume (Vt) is approximately 0.5 L.
• Inspiratory reserve volume (IRV) is approximately 3.3 L.
• Expiratory reserve volume (ERV) is approximately 1 L.
• Residual volume (RV) is approximately 1.2 L.

Static Parameters - Capacities

• Inspiratory capacity (IC) is the sum of Vt + IRV.
• Total lung capacity (TLC) is the sum of Vt + IRV + ERV + RV; It is the volume of air in the lungs at peak of maximum inspiration.
• Vital capacity (VC) is the sum of Vt + IRV + ERV and represents the maximum volume of air that can be expired after maximum inspiration.
• Functional residual capacity (FRC) is the sum of ERV + RV, which is the volume of air remaining in the lungs at the end of normal tidal expiration.
• Expiratory capacity (EC) is the sum of Vt + ERV.

Static Capacity Values

• Inspiratory capacity (IC) is approximately 3.8 L.
• Expiratory capacity (EC) is approximately 1.5 L.
• Functional residual capacity (FRC) is approximately 2.2 L.
• Vital capacity (VC) is approximately 4.8 L.
• Total lung capacity (TLC) is approximately 6 L.

Determination of RV: Helium Dilution Method

• Procedure occurs after forceful expiration
• Vspir X C1 = (RV + Vspir) X C2, where:
  • c1 is the helium concentration before examination.
  • c2 is the helium concentration after examination.
  • Vspir is the volume of air in the spirometer.
  • RV is the residual volume.

Factors Affecting Spirometric Parameters

• Gender
• Age
• Height
• Smoking
• Training
• Disease

Spirometric Parameters and Aging

• Lung elastic recoil decreases with age.
• Inspiratory muscle strength decreases with age.

BTPS Correction

• BTPS Correction converts flow and volume measured at ambient conditions to the conditions within the lungs.
• Ambient conditions are called ATP (ambient temperature, pressure).
• Conditions within the lungs are called BTPS (body temperature, pressure, water vapor saturated).
• Temperature and relative humidity inside the spirometer vary with room air conditions.
• ATP is ambient temperature and pressure.
• Spirometry results must be standardized to be comparable under various ambient conditions.
• Reference conditions are those in the lungs, or BTPS (body temperature, pressure, saturated with water vapor).