Spirometry Values and Measurements

• Forced Vital Capacity (FVC)

  • Definition: The total amount of air that can be forcibly exhaled from the lungs after taking the deepest breath possible.
  • Also referred to as Slow Vital Capacity (SVC), where it is noted that DC (Dynamic Capacity) equals SCC (Static Capacity).

• Forced Expiratory Volume in 1 second (FEV1)

  • Definition: The volume of air that can be forcibly exhaled in the first second of a forced expiration.

• Peak Flow Measurement

  • Defined as maximum expiratory flow rate, usually measured in liters per minute.

• Types of Measurements

  • Some measurements are related to flow, while others relate to volumes.
  • Instrumentation: Different spirometers utilize distinct measurement technologies.

Types of Spirometers

• Volume Displacing Spirometer

  • Also known as Water Seal Spirometer or Spedwell Spirometer.
  • Recognized as the gold standard for measuring lung volumes accurately.
  • Consists of a water column; patient exhales, pushing the device out of the water, and inhales pulling it back down while graph paper records the results.
  • Challenges: Difficult to calibrate and large, leading to its decline in use in modern settings.

• Flow Sensing Spirometer

  • Known as a Pneumotachometer.
  • This technology measures air flow in liters per second and converts it to volume using the relationship:
    \text{Volume} = \text{Flow} \times \text{Time}

• Turbine Flow Spirometer

  • Commonly used in clinical settings, specifically referred to as the Wright Respirometer.
  • Function: Measures the flow of air in milliliters and liters; typically used at the bedside.
  • Digital Spirometers: Modernized version for easier reading and use.

Spirometry Techniques

• Collecting Vital Capacity

  • Patients with neuromuscular diseases may have their vital capacity measured daily to track muscle weakness effects on ventilation.
  • Careful instruction during the procedure is necessary to ensure adequate performance without forcing exhalation.

• Instructions for Spirometry:

  • Patients should inhale deeply until they cannot do so anymore and then exhale steadily to prevent overwhelming the spirometer.
  • Forceful breathing can damage certain spirometers (specifically the turbine types) due to pressure.

• Challenging Measurements and Codes:

  • For patients nearing 10 ml/kg vital capacity indicative of respiratory failure can indicate the need for ventilation support.

Bedside Parameter Measurements

• Minute Volume Measurement

  • Measure the total volume of air exchanged in a minute by having the patient breathe normally for a set time frame.
  • Tidal Volume Calculation:
  • If the respiratory rate (RR) is divided by minute volume (MV), average tidal volume can be derived.

• Repeatability of Forced Vital Capacity

  • FVC measurements must be repeated at least three times, where two measurements should be within 150 ml of each other, with the highest recorded.
  • Importance in verifying accuracy of measurements.

Peak Flow Meters

• Purpose: Used by asthma patients to monitor lung function at home.

  • Detects subtle declines in lung function before symptoms manifest.
  • Usage Instructions: Patients blow into the meter, which measures their peak flow rate.

• Asthma Action Plan

  • Zone system:
  • Green Zone: >80% of personal best
  • Yellow Zone: 50-80% of personal best, indicating caution and potential rescue inhaler use.
  • Red Zone: <50% of personal best requiring immediate medical attention.

• Recording Methodology:

  • Patients should track their peak flow readings over a two-week period and work with a physician to establish an action plan based on their personal best values.

Detailed Spirometry Graphs

• Flow Volume Loop and Time Volume Curve

  • Two graphs generated alongside numerical measurements during a spirometry test.
  • Flow Volume Loop: Measures air flow during inhalation and exhalation, visualized as a loop.
  • Time Volume Curve: Measures air volume against time, indicating total lung volume occupancy over time.

• Identifying Obstruction and Restrictions:

  • For obstructive patients, there is a longer expiratory duration indicated by the curves.

• Artifact and Acceptability:

  • Acceptable results require smooth curves without artifacts like hesitation or leaks; minimum of three acceptable maneuvers to report valid results.

Summary of Expected Clinical Practices for Spirometry

• Proper instructions, repeated measurements, and monitoring patient effort are critical for accurate spirometry testing.
• Awareness of parameter thresholds indicating possible clinical intervention is essential for patient management in respiratory care.