How should bedside spirometry be performed and interpreted?

Bedside Spirometry Performance and Interpretation

Bedside spirometry should be performed with the patient seated (or standing if preferred), using proper coaching technique to obtain at least three acceptable maneuvers, with the largest FEV1 and FVC values reported when the two best efforts are within 0.150 L of each other. 1

Pre-Test Preparation

Before testing, ensure:

• Patient positioning: Seated or standing (document which is used for serial testing) 2
• No nose clip required for basic spirometry 1
• Proper instruction and demonstration by the technician before starting 1
• Document recent factors that may affect results: respiratory infection, recent bronchodilator use, thoracic/abdominal surgery 2

Performing the Maneuver

Patient Instructions (Critical for Quality)

The patient must:

1. Inhale completely and rapidly to total lung capacity with pause <1 second 1
2. Position neck neutrally (not flexed or extended) 1
3. Seal lips tightly around mouthpiece with no leakage 1
4. "Blast" the air out immediately without hesitation—even 2 seconds of delay drops peak flow by 10% 1
5. Continue exhaling forcefully for at least 6 seconds (3 seconds for children) until no more air can be expelled 1

Critical Coaching Points

Vigorous coaching throughout is essential—use phrases like "keep going" and appropriate body language 1. The technician must observe the patient while occasionally glancing at the tracing to ensure maximal effort 1.

Acceptability Criteria (Within-Maneuver)

Each individual blow is acceptable only if ALL of the following are met 1:

• Good start: Extrapolated volume <5% of FVC or <0.150 L, whichever is greater
• No cough during the first second
• No hesitation or glottis closure causing airflow cessation
• No leak at the mouth
• No obstruction (tongue/teeth blocking mouthpiece)
• Adequate duration: ≥6 seconds or plateau on volume-time curve
• No extra breath taken during maneuver

Repeatability Criteria (Between-Maneuver)

Perform minimum of 3 maneuvers, maximum of 8 1:

• The two largest FVC values must be within 0.150 L of each other
• The two largest FEV1 values must be within 0.150 L of each other
• If not achieved after 3 attempts, continue up to 8 total attempts 1
• If repeatability not achieved in 5 attempts, more attempts unlikely to help 1

Important Caveat

Do not reject results solely based on poor repeatability—document the quality grade and let the interpreter decide clinical utility 1. Poor repeatability may reflect underlying disease, not just technique 2.

Result Selection and Reporting

Report the largest FVC and largest FEV1 from all acceptable maneuvers, even if they come from different curves 1. This differs from selecting a single "best" curve.

Calculate and report:

• FEV1/FVC ratio to identify obstruction (ratio <70% indicates airflow obstruction) 3
• Compare values to predicted normals based on age, height, sex, and ethnicity

Interpretation Framework

Obstructive Pattern

• FEV1/FVC ratio <70% 3
• FEV1 reduced more than FVC
• Changes >200 mL in FEV1 are clinically significant (exceeds measurement variability) 3

Restrictive Pattern (Suspected)

• FEV1/FVC ratio normal or increased (>85-90%)
• Both FEV1 and FVC proportionally reduced
• Requires lung volume measurement to confirm—reduced VC alone does not prove restriction 4

Common Pitfall

When FEV1 and FVC are both reduced with normal ratio, this most frequently reflects incomplete inhalation/exhalation, not true restriction 4. Repeat with better coaching before concluding restrictive disease.

Quality Improvement for Bedside Testing

Bedside spirometry quality is typically lower than laboratory testing. A structured program including technician training, real-time feedback, and central review can improve acceptability rates from 15% to 64% 5:

• Limit testing to a core group of trained operators 5
• Use spirometers that produce printouts for immediate curve review 5
• Implement routine feedback from pulmonary function specialists to operators 5
• Perform daily calibration checks with 3-L syringe 1

Safety Considerations

Stop the test if the patient becomes dizzy—syncope can occur from prolonged interruption of venous return, especially in elderly patients and those with severe airflow limitation 1. Consider performing slow vital capacity maneuvers instead of forced maneuvers in high-risk patients.