How do you calculate spirometry results, specifically forced expiratory volume in one second (FEV1) and forced vital capacity (FVC), in a patient with a history of elephantiasis?

How to Calculate Spirometry Results (FEV1 and FVC)

Primary Measurement Method

Record the largest FEV1 and largest FVC values from at least three acceptable forced expiratory maneuvers, even if these maximum values come from different curves. 1

Step-by-Step Calculation Process

Performing the Test

• Have the patient perform a minimum of three forced expiratory maneuvers from maximal inhalation 1, 2
• Continue testing until repeatability criteria are met or a maximum of eight maneuvers is reached 1
• Each maneuver must start from a position of full inspiration with maximal forced expiration 1

Defining Time Zero for FEV1

• FEV1 measurement begins at time zero, which is determined by back-extrapolation from the maximum flow portion of the volume-time curve 2
• The extrapolated volume must be <5% of FVC or <0.15 L (whichever is greater) for an acceptable start 2

Selecting Final Values

• FEV1: Select the single largest FEV1 value (in liters at BTPS) from all usable curves 1, 2
• FVC: Select the single largest FVC value from all usable curves 1
• These maximum values do not need to come from the same maneuver 1, 2

Acceptability Criteria

Each individual maneuver must be:

• Free from artifacts including cough during the first second, glottic closure, early termination, submaximal effort, leaks, or obstructed mouthpiece 2
• Have satisfactory exhalation duration >6 seconds (or >3 seconds for children) or show a plateau in the volume-time curve 2
• Meet the extrapolated volume requirement mentioned above 2

Repeatability Requirements

The test session is complete when:

• The two largest FEV1 values are within 0.150 L of each other AND 2
• The two largest FVC values are within 0.150 L of each other 2

If these criteria are not met, continue testing up to eight total maneuvers 1, 2

Calculating the FEV1/FVC Ratio

• Report FEV1/FVC as a decimal fraction (not as a percentage) 1
• Use the largest FEV1 and largest FVC values to calculate this ratio 1
• Leave the "percent predicted" column blank for the ratio to avoid miscommunication 1

Alternative Denominator

• In some patients, a slow vital capacity (VC) or inspiratory vital capacity (IVC) may provide a larger and more appropriate denominator for calculating FEV1/VC% 1, 2
• VC is often slightly higher than FVC in normal subjects 1

Important Considerations for Elephantiasis or Other Conditions

The calculation method remains identical regardless of underlying conditions including elephantiasis, as spirometry measures lung function mechanics, not peripheral tissue changes. 1

Quality Control Pitfalls to Avoid

• Never reject a spirogram solely based on poor repeatability - document the repeatability issue and leave interpretation to the clinician 1
• Ensure forced expiratory time (FET) is recorded to aid quality assessment 1
• If progressive reduction in FEV1 or FVC exceeds 20% of starting value across maneuvers, terminate testing for patient safety 1

Alternative Measurements

• FEV6 (forced expiratory volume at 6 seconds) can serve as a reliable surrogate for FVC and is more reproducible and less physically demanding 1, 2
• When using FEV6, the ratio should be labeled as FEV1/FEV6 1

Reporting Format

Present numerical values for:

• FEV1 (in liters) 1
• FVC (in liters) 1
• FEV1/FVC ratio (as decimal) 1
• Forced expiratory time 1
• Number of acceptable maneuvers performed 1