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📋 PULMONARY FUNCTION TEST (PFT)

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INTRODUCTION

Lung function tests are useful in assessing the functional status of lungs / respiratory system, both in physiological & pathological condition.

LFT are based on measurement of volume of air breathed in and out in quiet breathing & forced breathing.

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TYPES OF LFT

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MEASUREMENTS

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(A) SPIROMETRY

• Apparatus of measuring volume of air inspired and expired by the lungs.
• Measures ventilation, movement of air into and out of the lungs.
• Spirogram will identify two different types of abnormal ventilation patterns: obstructive and restrictive.
• It gives an indication of underlying lung damage by comparing results with predicted normal values.
• In order to compare results the predicted normal value, the patient's age, gender, height, weight & ethnicity are entered into spirometer data prior to each test.

TYPES: John Hutchinson - inventor of spirometer and originator of term Vital Capacity (VC)

1. Traditional
2. Electronic computerised spirometer

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WATERSEAL SPIROMETER (Traditional)

• One type is the waterseal spirometer, which consists of a hollow drum floating over a chamber of water and counterbalanced by weights so that it can freely move up and down.
• Inside the drum is mixture of gases, usually oxygen and air. Leading from hollow space in drum to the outside is a tube that has mouthpiece through which patient breathes.
• As one inhales & exhales through the tube, the drum rises & falls, causing a needle to move on nearby rotating chart. The tracing recorded on chart is called spirogram.

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COMPUTERISED SPIROMETER

• It does NOT contain a drum or water chamber. Subject has to respire into a sophisticated transducer, which is connected to the instrument by means of cable.

★ By using simple spirometer, respirometer or computerised spirometer, not all lung volumes and lung capacities can be measured. → All lung volumes & capacities CANNOT be measured.

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INDICATIONS OF SPIROMETRY

A major advantage of spirometry is that it enables you to detect COPD before symptoms become apparent.

INDICATIONS:-

• Diagnostic
• Monitoring
• To evaluate symptoms, signs, abnormal lab tests
• To measure effects of disease on pulmonary function
• Screen individuals at risk of having pulmonary disease (e.g. smokers)
• Routine physical examination:
  • Assess pre-op risk
  • Assess prognosis
  • Assess health status before enrolment in strenuous physical activity

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CONTRAINDICATIONS OF SPIROMETRY

• Pneumothorax
• ↑ ICP (increased intracranial pressure)
• ↑ Risk of syncope
• Chest pain, abdominal, thoracic or eye surgery within the previous 8 weeks
• Nausea, diarrhoea, vomiting

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USES OF SPIROMETRY

• To measure FVC & its derivatives (FEV₁, FEF 25-75%)
• Peak expiratory flow rate (PEF)
• Maximum voluntary ventilation (MVV)
• Slow VC
• Pre & post bronchodilator uses

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PROCEDURE

Preparation:

• Explain purpose of test and demonstrate the procedure.
• Record and enter patient's age, height, gender, ethnicity (non-caucasians have lower predicted value)
• Note what type (short acting / long acting) & when bronchodilator was last used.
• Have patient sitting comfortably
• Suggest the patient loosen any tight clothing
• Ask patient to empty bladder beforehand if needed

Precautions:

• Contraindications (as listed above)

Instructions:

• Maximum inspiration - "Big breath in!"
• Put the mouthpiece in your mouth with lips well around - "make a good seal with your lips"
• Blow or blast the air out as hard and fast and as long as possible - "Blow out"
• Provide encouragement - "Keep blowing, keep blowing until you feel your lungs are empty."

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INTERPRETATIONS

Obstructive Pattern:

• FVC ↓ or ↓ (Forced vital capacity)
• FEV₁ ↓ (Forced expired in 1 sec)
• FEF 25-75% ↓ (Forced exp flow in middle half)
• FEV₁/FVC ↓
• TLC normal or ↑

Restrictive Pattern:

• FVC ↓
• FEV₁ ↓
• FEF 25-75% Normal or ↑
• FEV₁/FVC normal to ↑
• TLC ↓

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FEV₁/FVC RATIO

• Reduced in obstructive disorders

• 75% - Normal

• 60% to 75% - Mild obstruction
• 50% to 59% - Moderate obstruction
• <49% - Severe obstruction

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MEASUREMENTS OF VOLUMES

TLC, RV, FRC - measured using:

• Nitrogen washout
• Helium dilution
• Total body plethysmography

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(C) HELIUM DILUTION METHOD

Patient breathes in and out of a spirometer filled with 10% Helium and 90% O₂ till concentration equilibrium. In spirometer & lung becomes same (equilibrium). As no helium is lost (as it is insoluble in blood).

C₁ × V₁ = C₂ (V₁ + V₂)

V₂ = V₁(C₁ - C₂) / C₂

Where:

• V₁ = Volume of spirometer
• V₂ = FRC
• C₁ = Conc. of He in spirometer before equilibrium
• C₂ = Conc. of He in spirometer after equilibrium

BEFORE EQUILIBRATION: Initial concentration of helium = 15% AFTER EQUILIBRATION: Final concentration of helium = 10%

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(B) TOTAL BODY PLETHYSMOGRAPHY

• Subject sits in airtight box. At the end of normal exhalation - shutter of mouthpiece closes and patient is asked to make deep respiratory efforts. As subject inhales - expands gas volume in lung so lung volume increases and box volume decreases.
• It is based on Boyle's law of Gas, which states that the volume of a sample of gas is inversely proportional to pressure of that gas at constant temperature.
• Detects volume changes during different phases of respiration.
• After normal breathing for few minutes, the subject breathes rapidly at maximum forces.
• During maximum respiration, lung volume decreases very much. But volume of gas in chamber increases with decrease in pressure.
• By measuring volume & pressure changes inside the chamber:

P₁ × V = P₂ (V - ΔV)

• P₁ and P₂ = Pressure changes
• V = Functional residual capacity
• ΔV = Change in volume

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(D) N₂ WASHOUT METHOD

• Concentration of N₂ in air is 80%. So if total quantity of nitrogen in lungs is measured, the volume of air present in lungs can be calculated.
• Subject is asked to breathe normally after normal expiration. The subject inspires pure O₂ through valve.
  • Expire into a Douglas bag
  • Repeated for 6 to 7 minutes
  • Until nitrogen in lungs is displaced by O₂
  • Nitrogen comes to Douglas bag
  • After that, Functional Residual Capacity is calculated

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RESTRICTIVE & OBSTRUCTIVE DISORDERS

• On basis of PFT, lung disorders can be classified as restrictive or obstructive.
• In restrictive (e.g. pulmonary fibrosis), the vital capacity is reduced.
• In obstructive, the vital capacity is normal because lung tissue is not damaged.
• In asthma for example, the VC is normal, but expiration is more difficult and takes a longer time because bronchoconstriction increases resistance to air flow.
• Obstructive disorders are therefore diagnosed by the rate of expiration.

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PEAK EXPIRATORY FLOW RATE (PEFR)

• Maximum rate at which air can be expired after a deep inspiration.
• Normal value: In normal person it is 400 L/minute
• Measured by Wright peak flow meter or mini peak flow meter.
• Significance: Is reduced in all types of respiratory disease. Reduction is more significant in obstructive disease than in restrictive disease.
  • Restrictive disease: PEFR is 200 L/min
  • Obstructive disease: 100 L/min

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VARIATION IN LUNG VOLUMES

1. Age (volume ↓ with age)
2. Gender (males have larger volumes than females)
3. Height (tall individuals have larger volumes)
4. Weight
5. Race (American Indians, Blacks, Asians have as much as 12%-14% lower volumes than whites)

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TABLE: TYPICAL EFFECT OF OBSTRUCTIVE & RESTRICTIVE DISEASE ON SPIROMETRIC AND AIRFLOW VOLUME MEASUREMENTS

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🧠 NOW — SIMPLE LANGUAGE EXPLANATIONS

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What is PFT?

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Static vs Dynamic LFT - Simple Meaning

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Spirometry - Simple Meaning

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Waterseal Spirometer vs Computerised Spirometer

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Indications - Why is Spirometry Done?

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Contraindications - When NOT to Do It?

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Key Spirometry Values - Simple Meaning

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Obstructive vs Restrictive - The MOST IMPORTANT Concept

🔴 Obstructive (e.g. Asthma, COPD, Emphysema)

• The hose is kinked or blocked - water (air) comes out slowly
• The total amount of water in the hose (lung size) is okay
• So: FVC normal, but FEV₁ is LOW → FEV₁/FVC ratio FALLS
• TLC can be normal or even increased (air gets trapped)

🔵 Restrictive (e.g. Pulmonary Fibrosis)

• The hose is smaller/stiffer - holds less water (air) overall
• Air comes out at normal speed but there is just less of it
• So: FVC LOW, FEV₁ LOW, but FEV₁/FVC ratio stays NORMAL or goes UP
• TLC is decreased

Quick memory trick:

• Obstruction = Problem with FLOW (pipe is narrow)
• Restriction = Problem with VOLUME (lung is small/stiff)

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FEV₁/FVC Ratio - Grading Obstruction

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Helium Dilution Method - Simple Meaning

1. Fill a spirometer with a known amount of helium (say, 15% concentration)
2. Patient breathes in and out of it until helium spreads equally into both the lungs AND the spirometer (reaches equilibrium)
3. Helium does not enter the blood (insoluble), so all helium stays in the gas space
4. Now the concentration drops (say, to 10%) because helium spread into a bigger space (lungs + spirometer)
5. Using the formula C₁V₁ = C₂(V₁+V₂), we calculate the unknown lung volume (V₂ = FRC)

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Total Body Plethysmography - Simple Meaning

• Patient sits inside a sealed airtight glass box (like a phone booth)
• When patient tries to breathe in (but shutter is closed), their lungs expand → lung volume increases → box volume decreases → box pressure increases
• Based on Boyle's Law: when volume goes up, pressure goes down (and vice versa)
• By measuring pressure changes inside the box, we calculate FRC and other volumes

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N₂ Washout Method - Simple Meaning

• Normal air is 80% nitrogen (N₂)
• If you make the patient breathe pure oxygen for 6-7 minutes, all the nitrogen gets washed out of their lungs
• The nitrogen that comes out is collected in a Douglas bag
• Since we know air is 80% N₂, we can back-calculate: If this much N₂ came out, there must have been this much air (FRC) in the lungs

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PEFR - Simple Meaning

• This is simply the maximum speed at which you can blow air out
• Measured with a simple handheld device called a Peak Flow Meter (Wright or Mini)
• In obstructive diseases like asthma, PEFR drops a lot because the airways are narrowed (restricting speed)
• In restrictive diseases, PEFR drops slightly but not as dramatically
• Asthmatics often measure their own PEFR at home to monitor their condition

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Summary Table - Who Has What?

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1. Static LFT = how much, Dynamic LFT = how fast
2. Obstructive = can't blow fast (FEV₁/FVC ↓), Restrictive = can't blow much (TLC ↓)
3. Spirometry measures airflow; plethysmography/helium/nitrogen washout measure volumes like FRC, RV, TLC
4. FEV₁/FVC ratio is the single most important number for classifying lung disease