Overview
This lecture provides a detailed review of pulmonary function testing (PFT) principles, key values, test interpretation, and exam-style practice questions to prepare for the TMC exam.
Pulmonary Function Test Interpretation
- Reduced FEV1, FVC, and TLC with a normal FEV1/FVC ratio indicate restrictive lung disease.
- Severely reduced FEV1/FVC ratio signifies obstructive lung disease (e.g., chronic asthma).
- Both reduced FEV1/FVC ratio and reduced FVC suggest combined obstructive and restrictive disease.
- Classic obstructive diseases show decreased expiratory flow rates and often increased TLC due to air trapping.
Key PFT Values and Norms
- FEV1/FVC ratio: Normal is 70–83%; less than 70% suggests obstruction.
- Vital capacity (VC): Normal range 4,000–5,000 mL or 65–75 mL/kg in healthy adults.
- Peak expiratory flow: Normal in adults is 6–12 L/sec.
- To measure minute ventilation: rate × tidal volume.
PFT Procedures and Trouble-Shooting
- A difference between slow VC and FVC implies air trapping (common in obstructive diseases).
- Advise patients to "blast air out faster" if peak flow is delayed, or "don’t hesitate" if there is a start hesitation.
- Consistent poor effort or variable results indicate patient cooperation issues.
- FEV1 increase ≥15% post-bronchodilator confirms reversible obstruction (e.g., asthma).
- Suction patient if a "sawtooth" pattern is seen on the flow-volume loop during mechanical ventilation.
Devices and Measurements
- Simple spirometry cannot measure residual volume (RV); use body plethysmography for RV.
- Pressure manometer is used to measure maximum inspiratory/expiratory pressure (MIP/MEP).
- MIP is the best test for respiratory muscle strength and weaning readiness.
- For most tests, patient’s height, gender, and age are required to determine predicted values.
Special Considerations
- Lung diffusion tests are affected by recent smoking.
- Methylcholine challenge: ≥20% decrease in FEV1 = positive test for airway hyperreactivity.
- Flow-volume loops: Tall/skinny indicates restrictive disease; short/wide suggests obstruction.
- Asthma diagnosis relies on flow-volume loops, bronchodilator response, and bronchoprovocation—not lung diffusion alone.
Key Terms & Definitions
- FEV1 — Forced expiratory volume in 1 second.
- FVC — Forced vital capacity.
- TLC — Total lung capacity.
- VC — Vital capacity; max exhaled after max inhalation.
- IRV — Inspiratory reserve volume; extra air inhaled after normal inspiration.
- MIP/MEP — Maximum inspiratory/expiratory pressure.
- Peak Expiratory Flow — Fastest airflow during forced exhalation.
Action Items / Next Steps
- Review normal PFT values and interpretation patterns.
- Practice PFT calculations and troubleshooting poor test effort/results.
- Study device uses and procedure steps for bedside PFTs.
- Complete additional practice questions from the TMC test bank as assigned.