Back Extrapolated Volume on Spirometry
Back extrapolated volume (BEV) is the volume of air exhaled before the "time zero" point in spirometry, calculated by tracing back from the steepest slope on the volume-time curve to establish an accurate starting point for FEV1 measurement.
Definition and Purpose
Back extrapolated volume is a critical quality control parameter in spirometry that:
- Determines the true start of forced expiration (time zero) for accurate timing of FEV1 and other timed measurements
- Identifies hesitation or poor effort at the beginning of the maneuver
- Ensures that FEV1 comes from a maximal effort curve
Technical Aspects
Calculation Method
- For manual measurements: Traces back from the steepest slope on the volume-time curve 1
- For computerized measurements: Uses the largest slope averaged over an 80-ms period 1
Quality Control Standards
- For an acceptable spirometry maneuver, BEV must be:
- Less than 5% of the FVC OR
- Less than 0.150 L, whichever is greater 1
Display Requirements
- Volume-time display should include at least 0.25 s (preferably 1 s) before exhalation starts
- This pre-exhalation period is necessary for calculating BEV and evaluating initial effort
- Time zero (as defined by BEV) must be presented as the zero point on graphical output 1
Clinical Significance
Impact on Measurement Accuracy
- Excessive BEV indicates a hesitant or poor start to the forced expiration
- A large BEV can lead to underestimation of FEV1 and other timed parameters
- Spirometry equipment should provide rapid feedback to the technician when start criteria are not met 1
Quality Assurance
- Visual inspection of the flow-volume curve can supplement BEV as a measure of satisfactory test start
- Peak expiratory flow (PEF) should be achieved with a sharp rise and occur close to the start of exhalation 1
- Modern spirometers should display the BEV value for immediate quality assessment 1
Common Pitfalls and Solutions
- Hesitant starts: If a maneuver has an obviously hesitant start, the technician should terminate the trial early to avoid unnecessary prolonged effort 1
- Excessive BEV: Indicates need for better coaching on explosive start to exhalation
- Inconsistent technique: Using the same maneuver type (FVC vs. peak flow) is important for consistent results 2
- Over-reliance on numerical criteria: Visual inspection by trained personnel remains the gold standard and should not be replaced by numerical criteria alone 3