Pulmonary Function Testing
Pulmonary Function Testing (PFT)
Purpose of PFT: Used to evaluate pulmonary causes of dyspnea, differentiate between obstructive and restrictive pulmonary disorders, evaluate effectiveness of therapies, follow course of a particular disease, and assess preoperative status.
Major Components of PFT
Lung Volumes and Capacities
Various lung volume measurements including tidal volume, inspiratory capacity, expiratory reserve volume, residual volume, and total lung capacity.
Forced Respiratory Flow Rate Measurements
Assess the efficiency of the airflow during breathing.
Pulmonary Diffusion Capacity Measurement
Evaluates gas exchange across the alveolar-capillary membrane.
Tests of Respiratory Muscle Strength
Evaluates the strength and function of respiratory muscles.
Cardiopulmonary Exercise Testing
Assesses the heart and lung function under physical exertion.
Measurement Techniques
Spirometry: Directly measures volumes in the lungs using a spirometer.
Measurements include:
Tidal Volume (Vt)
Inspiratory Reserve Volume (IRV)
Expiratory Reserve Volume (ERV)
Residual Volume (RV)
Vital Capacity (VC)
Total Lung Capacity (TLC)
Indirect Measurement Methods
Residual Volume Measurement
Indirectly assessed using:
Closed Circuit Helium Dilution Test: Patient rebreathes a known volume of helium to measure dilution and calculate lung volumes.
Open Circuit Nitrogen Washout Test: The patient inhales pure oxygen; nitrogen is then washed out of the lungs to measure total lung capacity.
Body Plethysmography: Measures lung volume by determining how much air is displaced in a sealed chamber.
Procedures
Closed Circuit Test:
Helium concentration is measured before and after the patient rebreates; the change in concentration gives lung volume estimates.
Usually longer in patients with severe obstructive disease.
Body changes monitored by the system ensure accurate gas concentrations.
Open Circuit Test:
The patient inhales pure 100% O2 for about 7 minutes.
Concentration of nitrogen is monitored to assess the time needed to wash out nitrogen from lungs.
When nitrogen concentration drops to approximately 1.5%, forced expiration is measured to assess FRC.
Mathematical Calculations in Tests
Volume Calculation:
The total volume of gas can be determined by [ V_1 C_1 = V_2 C_2 ]
Rearranged to solve for V2 (the unknown volume).
FRC Calculation:
Using nitrogen concentrations and expired volume to find the functional residual capacity (FRC).
Evaluates the efficiency of gas exchange in the lungs.
Pulmonary Function Testing (PFT)
Purpose of PFT: PFTs are essential diagnostic tools used to evaluate pulmonary causes of dyspnea (shortness of breath). They help clinicians differentiate between obstructive lung diseases (e.g., Asthma, COPD, Emphysema) and restrictive lung disorders (e.g., Pulmonary Fibrosis, Sarcoidosis, Chest Wall Deformities). Additionally, they are used to:
Evaluate the effectiveness of therapeutic interventions, such as bronchodilators or steroids.
Monitor the progression of chronic respiratory diseases.
Assess preoperative risk, particularly for patients undergoing thoracic or upper abdominal surgeries.
Screen for occupational lung disease exposure.
Major Components of PFT
Lung Volumes and Capacities
Tidal Volume (V_T): The amount of air inhaled or exhaled during a normal, resting breath.
Inspiratory Capacity (IC): The maximum amount of air that can be inhaled after a normal tidal expiration (IC = V_T + IRV).
Expiratory Reserve Volume (ERV): The maximum amount of air that can be exhaled after a normal tidal expiration.
Residual Volume (RV): The volume of air remaining in the lungs after a maximal exhalation; this cannot be measured directly by spirometry.
Functional Residual Capacity (FRC): The volume of air remaining in the lungs at the end of a normal tidal expiration (FRC = ERV + RV).
Vital Capacity (VC): The total amount of air that can be exhaled after a maximal inspiration (VC = V_T + IRV + ERV).
Total Lung Capacity (TLC): The sum of all lung volumes; the maximum amount of air the lungs can hold (TLC = VC + RV).
Forced Respiratory Flow Rate Measurements
Forced Vital Capacity (FVC): The total volume of air that can be forcibly exhaled from the lungs after taking the deepest breath possible.
Forced Expiratory Volume in 1 Second (FEV_1): The volume of air exhaled during the first second of the FVC maneuver. It is a key indicator of airway patency.
FEV_1/FVC Ratio: Used to distinguish obstructive from restrictive patterns. A ratio less than 70\% usually indicates an obstructive defect.
Pulmonary Diffusion Capacity Measurement (DL_{CO})
Evaluates the ability of the lungs to transfer gas (usually Carbon Monoxide) from the inspired air to the red blood cells in the pulmonary capillaries. It assesses the integrity of the alveolar-capillary membrane.
Tests of Respiratory Muscle Strength
Measures Maximal Inspiratory Pressure (MIP) and Maximal Expiratory Pressure (MEP) to evaluate for neuromuscular weakness affecting respiration.
Cardiopulmonary Exercise Testing (CPET)
A comprehensive assessment of the heart, lungs, and muscle metabolism during controlled physical exertion, typically on a treadmill or cycle ergometer.
Measurement Techniques
Spirometry: The most common PFT. It directly measures volume and flow rates during inhalation and exhalation.
Directly Measured: V_T, IRV, ERV, and VC.
Indirectly Measured: RV, FRC, and TLC (require specialized techniques because air remains in the lungs even after maximal effort).
Indirect Measurement Methods
Residual Volume (RV) Measurement: Necessary to calculate TLC and FRC.
Closed Circuit Helium Dilution Test: Based on the principle that if a known volume and concentration of helium are added to the lungs, the final concentration will allow for the calculation of the lung volume. Helium is used because it is inert and poorly soluble in blood.
Open Circuit Nitrogen Washout Test: The patient breathes 100\% oxygen, causing the nitrogen (N2) normally resident in the lungs to be "washed out" into a collection system. Since air is approximately 78\% to 79\% nitrogen, the total volume of {N2} collected allows the calculation of FRC.
Body Plethysmography (The "Body Box"): Uses Boyle’s Law (P1V1 = P2V2) at a constant temperature. The patient sits in an airtight chamber and performs panting maneuvers against a closed shutter. Changes in pressure inside the box and at the airway are used to calculate the Total Thoracic Gas Volume (VTGV), which includes gas trapped behind closed airways.
Procedures and Considerations
Closed Circuit Test:
Initial Helium (He) concentration is set (C1), and the system volume is known (V1). After equilibration, the final concentration (C_2) is measured.
In patients with severe obstructive disease (like emphysema), equilibration takes longer (up to 10-15 minutes) due to uneven gas distribution.
Open Circuit Test:
Usually takes about 7 minutes for healthy individuals.
If the patient has a significant "leak," or if they have severe air trapping, the test may take longer or yield inaccurate (underestimated) results.
When the {N_2} concentration drops below 1.5\%, the test is concluded.
Mathematical Calculations in Tests
Conservation of Mass Equation:
The total volume of gas is determined by:
V1 C1 = V2 C2To solve for the unknown volume (V2), which represents the system plus the patient's FRC: V2 = \frac{V1 C1}{C_2}
Boyle’s Law in Plethysmography:
As the patient breathes against the shutter, the change in lung volume (\Delta V) is proportional to the change in pressure (\Delta P), allowing for the measurement of the total gas volume in the thorax, which is the most accurate way to measure FRC in patients with obstructive diseases.