MC

Pulmonary Function Testing

Pulmonary Function Testing (PFT)

Objectives of Pulmonary Function Testing

This chapter aims to enable you to:

  • List the three primary categories of pulmonary function tests.

  • State the main purposes for performing pulmonary function testing.

  • Describe the characteristic pathophysiologic patterns associated with both obstructive and restrictive lung diseases.

  • Define the term "spirometer" and enumerate the parameters it can measure.

  • Identify and describe the four general principles guiding pulmonary function tests.

  • List and describe the specific measurements that indicate pulmonary mechanics.

  • Describe the purpose and technique involved in the bronchial challenge test.

  • List and describe the four distinct lung volumes and four distinct lung capacities measurable by pulmonary function testing.

  • Describe the purpose and specific techniques utilized to measure the diffusion capacity of the lung.

  • Accurately interpret pulmonary function reports.

Purposes of PFT

Pulmonary Function Tests (PFTs) serve several critical clinical and research purposes:

  • Identify and Quantify Changes: To detect and measure alterations in pulmonary function caused by disease processes.

  • Evaluate Therapy Effectiveness: To assess how well various therapeutic interventions are working to improve lung function.

  • Epidemiological Surveillance: To monitor populations for the prevalence and incidence of pulmonary diseases.

  • Assess Postoperative Risk: To evaluate a patient's risk for developing pulmonary complications following surgical procedures.

  • Determine Pulmonary Disability: To ascertain the degree of impairment in lung function for disability assessment or legal purposes.

Pathophysiologic Patterns in Lung Disease

There are two primary categories of pulmonary disease, often distinguished by PFTs:

  • Obstructive Lung Disease:

    • Primary Abnormality: Characterized by an increased resistance to airflow within the airways.

    • Examples: Conditions like asthma, COPD (emphysema, chronic bronchitis).

  • Restrictive Lung Disease:

    • Primary Problem: Defined by a decrease in lung compliance and reduced lung volumes.

    • Examples: Conditions like pulmonary fibrosis, scoliosis, neuromuscular diseases affecting respiratory muscles.

  • Mixed Patterns: It is possible for some pulmonary diseases to present with characteristics of both obstructive and restrictive disease.

PFT Equipment and Principles of Measurement

PFT Equipment

Measuring devices in PFT fall into two general types:

  • Volume-Measuring Devices: Known as spirometers.

  • Flow-Measuring Devices: Commonly referred to as pneumotachometers.
    Every measuring device possesses specific characteristics that define its performance:

  • Capacity: The maximum value it can measure.

  • Accuracy: How close a measurement is to the true value.

  • Error: The difference between the measured and true value.

  • Resolution: The smallest change in value it can detect.

  • Precision: The reproducibility of measurements.

  • Linearity: The consistency of accuracy across the entire measurement range.

  • Output: The way the measurement data is presented.

General Principles of Measurement

Most pulmonary function laboratories are structured around three main components. All three are essential for comprehensive assessment, particularly when the goal is to identify the presence and degree of pulmonary impairment:

  • Spirometry for Airway Mechanics: Involves performing spirometry to measure how efficiently air moves in and out of the lungs.

  • Measurement of Lung Volumes and Capacities: Determines the various amounts of air the lungs can hold.

  • Measurement of Diffusion Capacity: Assesses the efficiency of gas exchange across the alveolar-capillary membrane.

Tests of Pulmonary Mechanics (Spirometry)

These measurements evaluate the ability of the lungs to move large volumes of air quickly through the airways. They are crucial for detecting obstructive patterns.

  • Forced Vital Capacity (FVC):

    • The most common test of pulmonary mechanics.

    • It is an effort-dependent maneuver requiring careful patient instruction and cooperation.

    • Many measurements are derived from the FVC maneuver.

    • Validity Requirement: To ensure valid results, each patient must perform at least three acceptable FVC maneuvers.

  • Forced Expiratory Volume in 1 second (FEV1):

    • The volume of gas exhaled during the first one second of the FVC maneuver.

  • FEV1/FVC Ratio:

    • Calculated by dividing the largest recorded FEV1 by the largest recorded FVC (FEV1 / FVC).

  • Forced Expiratory Flow from 200 to 1200{ mL} (FEF_{200-1200}):

    • Represents the average flow rate early in the FVC maneuver, specifically over the initial 1000{ mL} of exhaled air after the first 200{ mL} .

  • Forced Expiratory Flow from 25 to 75 (FEF_{25-75}):

    • A measure of the average flow during the middle 50 ext{%} of the FVC maneuver.

  • Maximum Voluntary Ventilation (MVV):

    • Another effort-dependent test where the patient is asked to breathe as deeply and as fast as possible for 12 seconds.

    • Reflects:

      • Patient effort and motivation.

      • The function and strength of respiratory muscles.

      • The ability of the chest wall to expand and contract.

      • The patency (openness) of the airways.

Significance of Pulmonary Mechanics Results

FEV1 and FEV1/FVC

  • Normal FEV1: Approximately 4.7 ext{ L} for an average 20-year-old man.

  • FEV1 Reduction: Reduced in both obstructive and restrictive lung diseases, making it a general indicator of impairment.

  • FEV1/FVC Ratio:

    • Normal: Should be at least 70\% (FEV1/FVC)

    • Obstructive Disease: Reduced below 70 % (typically <0.70) due to increased airway resistance, trapping air and slowing exhalation.

    • Restrictive Disease: Usually normal, as both FEV1 and FVC are proportionally reduced due to decreased lung volumes, maintaining the ratio.

  • Other Expiratory Flow Measures (FEF{200-1200}, FEF{25%-75%): Also reduced when obstructive disease is present, further indicating compromised airflow.

Maximal Voluntary Ventilation (MVV)

  • Normal MVV: For males, it is typically around 160 to 180{ L/min} , with slightly lower values in females.

  • Obstructive Lung Disease: Reduced in patients with moderate to severe obstructive disease due to increased airway resistance and compromised respiratory muscle endurance.

  • Restrictive Lung Disease: May be normal or slightly reduced in patients with restrictive disease, depending on the severity and impact on respiratory muscle strength.

  • Other Factors: Reduced MVV can also be observed in undernourished patients, reflecting overall muscle weakness.

Lung Volumes and Capacities

Lung volumes are discrete measurements, while lung capacities are combinations of two or more volumes.

Lung Volumes

  • Tidal Volume (VT): The volume of air inhaled or exhaled with each normal breath. Normal is typically 500{ to }700{ mL} . Measurement alone is generally not very helpful in PFT interpretation.

  • Inspiratory Reserve Volume (IRV): The maximum volume of air that can be inhaled beyond a normal tidal inhalation.

  • Expiratory Reserve Volume (ERV): The maximum volume of air that can be exhaled beyond a normal tidal exhalation.

  • Residual Volume (RV): The volume of air remaining in the lungs after a maximal exhalation. This volume cannot be exhaled.

Lung Capacities

  • Total Lung Capacity (TLC): The maximum volume of air that the lungs can hold after a maximal inspiration. (TLC = VC + RV or TLC = VT + IRV + ERV + RV).

  • Inspiratory Capacity (IC): The maximum volume of air that can be inhaled after a normal tidal exhalation. (IC = VT + IRV).

  • Functional Residual Capacity (FRC): The volume of air remaining in the lungs at the end of a normal tidal exhalation. (FRC = ERV + RV).

  • Vital Capacity (VC): The maximum volume of air that can be exhaled after a maximal inspiration. (VC = VT + IRV + ERV or VC = IC + ERV). Normal adult VC is about 4.8 ext{ L}; results vary with age, gender, height, and ethnicity.

Techniques for Measuring Residual Volume (RV)

Since RV cannot be exhaled, indirect methods are used to measure it and, consequently, FRC and TLC.

  • Helium Dilution:

    • Principle: Based on the fact that a known amount of helium (an inert gas) introduced into a closed system will be diluted by the volume of gas already in the patient's lungs (specifically, the FRC). The final concentration allows calculation of the initial unknown volume.

    • Assumes complete mixing of gases.

  • Nitrogen Washout:

    • Principle: Based on the fact that approximately 79%{%of the RV (and FRC) is nitrogen. The patient breathes 100 %oxygen, washing out the nitrogen from the lungs into a collection system.

    • Calculation: Total volume of nitrogen exhaled divided by 0.79 equals the FRC (and from FRC, RV can be calculated).

  • Body Plethysmography (Body Box):

    • Principle: Applies Boyle's Law (P1V1 = P2V2). The patient sits in an airtight chamber. During a respiratory maneuver (e.g., panting against a closed shutter), changes in pressure and volume within the box and at the mouth are measured.

    • Advantages: Measures all gas within the thorax, including trapped gas, which may be underestimated by gas dilution methods in obstructive disease.

Significance of Lung Volume and Capacity Results

  • TLC, FRC, and RV:

    • Obstructive Disease: These volumes generally increase (hyperinflation) as air trapping occurs due to airway obstruction.

    • Restrictive Impairment: These volumes generally decrease as the lungs become stiffer or lung expansion is limited.

  • Normal Tidal Volume (VT): Typically 500 to 700{ mL} . Measurement alone is generally not helpful for diagnostic purposes.

  • Normal Total Lung Capacity (TLC): Approximately 6{ L}. A significant deviation (e.g., less than 80% of predicted normal) suggests restrictive disease if other spirometric indices are normal.

  • Normal Vital Capacity (VC): About 4.8{ L} in the adult. Results for VC, like other volumes, vary significantly with age, gender, height, and ethnicity, thus requiring comparison to predicted normal values.

Diffusing Capacity of the Lung (DLCO)

  • Purpose: Measures the efficiency of gas transfer from the alveoli into the blood. Most PFT labs use carbon monoxide (CO) for this measurement because its uptake is diffusion-limited rather than perfusion-limited.

  • Reporting: Results are typically reported in {{mL}}/{min}/{mmHg} .

  • Reduction: DLCO results can be low in both obstructive and restrictive lung diseases.

  • Common Causes of Reduced DLCO:

    • Emphysema: Leads to a reduced surface area for gas exchange due to alveolar wall destruction.

    • Pulmonary Fibrosis: Causes a thickened alveolar-capillary (A-C) membrane, increasing the distance gases must travel.

Interpreting the PFT Report

Interpreting PFT results involves a systematic approach, often starting with key ratios and then assessing volumes and diffusion capacity.

  1. Start with the FEV1/FVC Ratio:

    • Reduced (<70% or <0.70): Strongly indicative of obstructive lung disease. This ratio is the primary indicator for obstruction.

    • Normal ( ext{>70% or ext{>0.70}}): Rules out significant obstructive disease. Proceed to assess lung volumes for restriction.

  2. Assess Total Lung Capacity (TLC) for Restrictive Disease (if FEV1/FVC is normal):

    • TLC < 80% of Predicted Normal and FEV1/FVC is Normal: Suggests restrictive lung disease. The reduced TLC indicates smaller lung volumes.

  3. Evaluate Diffusion Capacity (DLCO):

    • DLCO < 80% of Normal: Indicates a diffusion defect.

    • Possible Causes of Diffusion Defect:

      • Reduced Surface Area: Often associated with conditions like emphysema (destruction of alveolar walls).

      • Thickened Alveolar-Capillary Membrane: Characteristic of conditions like pulmonary fibrosis, where the membrane becomes scarred and thickened, impairing gas exchange.

Measurement

Normal*

Obstructive

Restrictive

VT

500{ mL}

N or {↑}

N

IRV

3.10{ L}

N or {↓}

{↓↓}

ERV

1.20{ L}

NO-{↓↓}

{↓↓}

RV

1.20{ L}

{↑↑}

{↓↓}

IC

3.60{ L}

N or {↓}

{↓↓}

FRC

2.40{ L}

{↑↑}

{↓↓}

TLC

6.00{ L}

{\uparrow}

{↓↓}

FVC

4.80{ L}

{↓↓}

{↓↓}

FEV1

4.20{ L}

{↓↓}

{↓↓}

FEV1/FVC

>70\%

N or {↑}

FEF_{200-1200}

8.5{ L/sec}

{↓↓}

N or N-{↓}

FEF_{25%-75%}

4.5{ L/sec}

{↓↓}

N or N-{↓}

PEFR

9.5{ L/sec}

N or {↓}

N or N-{↓}

FEF_{25%}

9.0{ L/sec}

{↓↓}

N or N-{↓}

FEF_{50%}

6.5{ L/sec}

{↓↓}

N or N-{↓}

FEF_{75%}

3.5{ L/sec}

{↓↓}

N or N-{↓}

MVV

160{ L/min}

{↓↓}

N or N-{↓}

DLCO

40{ mL/min/mmHg}

N or {↓}

N or N-{↓}

DLCO/VA

6.6{ mL/min/mmHg/L}

N or {↓}

N-{↑}

*Values are approximate for a 20-year-old, 70-kg male.

  • VA (Alveolar Volume): A term often used in conjunction with DLCO to provide a measure of diffusion relative to the lung volume that is participating in gas exchange.