The Flow Axis in Spirometry
In spirometry, the flow axis represents the rate of airflow (measured in liters per second, L/s) during breathing maneuvers and is displayed on the vertical axis of a flow-volume curve. 1, 2
Technical Specifications of the Flow Axis
The flow axis in spirometry has specific standardized requirements:
Scale factors:
- For instrument displays: 0.200 L/s per 2.5 mm (2.5 mm·L⁻¹·s⁻¹)
- For hardcopy graphical output: 0.100 L/s per 5 mm (5 mm·L⁻¹·s⁻¹) 1
Aspect ratio: The correct aspect ratio for a flow versus volume display is two units of flow per one unit of volume 1
Flow range: Spirometers must be capable of measuring flows in the range of 0-14 L/s 1
Accuracy requirements: For instantaneous flows (except PEF), accuracy should be within ±5% of reading or ±0.200 L/s, whichever is greater 1
Clinical Significance of the Flow Axis
The flow axis provides critical information about respiratory function:
Airway obstruction assessment: The shape of the flow-volume curve, particularly the expiratory portion, helps identify and localize airway obstruction 3
Diagnostic utility: When combined with volume measurements (on the horizontal axis), the flow-volume loop becomes a powerful tool for diagnosing various respiratory conditions 3
Quality control: The flow-volume curve provides a means for quality assessment during the initial portion of the Forced Vital Capacity (FVC) maneuver 1
Flow Parameters Measured
Several important flow parameters are measured and displayed on the flow axis:
Peak Expiratory Flow (PEF): The maximum expiratory flow achieved during forced expiration, with accuracy requirements of ±10% of reading or ±0.30 L/s, whichever is greater 1, 2
Forced Expiratory Flow between 25% and 75% of FVC (FEF25-75%): The mean forced expiratory flow during the middle portion of the FVC, used to assess small airway function 1, 2
Instantaneous flows: Various flow rates at specific points during the expiratory maneuver 1
Calibration and Quality Control
Proper calibration of the flow axis is essential for accurate measurements:
Flow linearity testing: Should be performed weekly, testing at least three different flow ranges 1
Resistance requirements: Mean resistance at flow rates of 3.3,6.7, and 10 L/s must be <2.5 cmH₂O·L⁻¹·s⁻¹ 1
Validation: Flow measurements should be evaluated using computer-driven mechanical syringes or equivalent systems to test the range of exhalations likely to be encountered in clinical practice 1
Common Pitfalls and Caveats
Temperature effects: Changes in temperature can affect flow measurements, as gas viscosity (which impacts pneumotachometer signals) increases with increasing temperature 1
Water condensation: Condensation within or on the surfaces of a flow sensor may alter its calibration 1
Cooling of expired gas: It's often assumed that expired gas doesn't cool as it passes through the flow sensor, but this may not be true, particularly with unheated flow sensors 1
Filter effects: Placing a filter in front of the flow sensor can increase error as the flow sensor is located further from the mouth, allowing more cooling to occur 1