Calculating Predicted Incentive Spirometry Values
Important Clarification
Incentive spirometry does not have "predicted values" in the same way diagnostic spirometry does—it is a therapeutic device used to encourage deep breathing, not a diagnostic test with standardized reference equations.
The question appears to conflate two distinct concepts:
- Diagnostic spirometry (which has predicted values based on age, sex, height, and ethnicity)
- Incentive spirometry (a therapeutic breathing exercise device)
Understanding Incentive Spirometry
What Incentive Spirometry Measures
- Incentive spirometry is a feedback system designed to encourage patients to take deep breaths and produce sustained maximal inspiration (SMI), primarily to prevent atelectasis 1
- The device measures inspiratory capacity or inspiratory flow, depending on whether it is volume-oriented or flow-oriented 2
- Volume-oriented devices (like Coach and Voldyne) measure the volume of air inhaled, with studies showing tidal volumes ranging from 947-1035 mL in healthy subjects 2
- Flow-oriented devices (like Triflo II and Respirex) measure inspiratory flow rates rather than volume 2
How to Set Goals for Incentive Spirometry
The goal for incentive spirometry is typically set based on the patient's own baseline inspiratory capacity, not on population-based predicted values.
- Patients should be instructed to achieve their maximum inspiratory volume, which serves as their individualized baseline 3
- Goals are then set to maintain or exceed this baseline volume with each use 3
- In one study, healthy outpatients achieved an average baseline maximal inspiration of 1885 mL, which increased to 2235 mL (16% improvement) over 30 days of consistent use 3
If You Need Predicted Spirometry Values (Diagnostic Testing)
Standard Spirometry Predicted Values
If you are actually asking about diagnostic spirometry predicted values, these are calculated using validated reference equations based on patient demographics.
- The Global Lung Function Initiative (GLI) 2012 reference equations are the current standard for calculating predicted FEV₁, FVC, and FEV₁/FVC ratio 4
- Predicted values require: age, sex, height, and ethnicity as input variables 4
- Results are expressed as percent predicted and z-scores, with the lower limit of normal (LLN) defined as a z-score of -1.64 4
- Spirometry measures only volumes that move in and out through the mouth—it cannot measure residual volume or total lung capacity 5
Key Spirometry Parameters
- FEV₁ (Forced Expiratory Volume in 1 second) and FVC (Forced Vital Capacity) are the primary measured values 4
- FEV₁/FVC ratio should be reported as a decimal fraction, with the percent predicted column left blank to avoid miscommunication 4
- At least three acceptable maneuvers must be performed, with reproducibility within defined limits 4
Common Pitfalls
- Do not confuse incentive spirometry (therapeutic) with diagnostic spirometry—they serve completely different purposes 1, 6
- Incentive spirometry goals should be based on the patient's own maximum effort, not population norms 3
- A reduced vital capacity on diagnostic spirometry does not prove restrictive lung disease—body plethysmography is needed to measure total lung capacity for definitive diagnosis 5
- Proper technique is essential for both diagnostic spirometry and incentive spirometry—patients must be instructed and observed to ensure correct performance 4, 7