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Pulmonary Function Tests (PFTs)

MD Anaesthesia PG Examination - 30 Marks

1. INTRODUCTION

Pulmonary function tests (PFTs) are a group of non-invasive investigations that objectively assess the mechanical properties of the lungs, airways, and respiratory muscles. In anaesthesia practice, PFTs serve three key purposes:

Preoperative risk stratification - identifying patients at risk of postoperative pulmonary complications (PPCs)
Staging of chronic lung disease - particularly COPD and interstitial lung diseases
Guiding surgical decisions - especially in lung resection candidates

A thorough history and physical examination remain the cornerstone of preoperative pulmonary assessment. PFTs complement this evaluation in patients with known lung disease, risk factors, or planned thoracic surgery.

Miller's Anesthesia, 10e, p. 1302
Barash's Clinical Anesthesia, 9e, p. 3144

2. CLASSIFICATION OF PFTs

Category	Tests
Spirometry	FVC, FEV1, FEV1/FVC, FEF25-75%, PEFR, MVV
Lung Volumes	TLC, FRC, RV, VC, IC, ERV (via body plethysmography, N2 washout, He dilution)
Diffusing Capacity	DLCO (single breath CO transfer)
Airway Resistance	Raw, specific airway conductance (SGaw)
Respiratory Muscle Strength	PImax (MIP), PEmax (MEP)
Exercise Testing	Cardiopulmonary exercise test (CPET), 6-minute walk test

3. LUNG VOLUMES AND CAPACITIES

Understanding lung volumes is fundamental to interpreting PFTs.

Lung volumes diagram showing TLC, VC, FRC, RV, IC, tidal volume

Fig. 1 - Lung volumes and capacities (Miller's Anesthesia, 10e)

Static Lung Volumes (Normal Adult Values)

Volume/Capacity	Definition	Normal Value
Tidal Volume (VT)	Air inhaled/exhaled per breath	~500 mL
Inspiratory Reserve Volume (IRV)	Max air inhaled above VT	~3000 mL
Expiratory Reserve Volume (ERV)	Max air exhaled below FRC	~1000 mL
Residual Volume (RV)	Air remaining after maximal exhalation	~1200 mL
Total Lung Capacity (TLC)	Total lung volume at maximal inspiration	~6000 mL
Vital Capacity (VC)	IRV + VT + ERV	~4500 mL
Functional Residual Capacity (FRC)	ERV + RV; end-expiratory volume	~2200 mL
Inspiratory Capacity (IC)	VT + IRV	~3500 mL

Key point: RV, FRC, and TLC cannot be measured by spirometry alone - they require nitrogen washout, helium dilution, or body plethysmography.

Murray & Nadel's Textbook of Respiratory Medicine
Miller's Anesthesia, 10e, p. 1303

4. SPIROMETRY

4.1 Principle

Spirometry measures the volume of air inhaled or exhaled using a pneumotachometer that measures flow and integrates it to obtain volume. The test consists of:

Inhalation to TLC
A forceful, maximal exhalation to RV
Optionally, maximal inhalation back to TLC (to generate flow-volume loop)

The initial 25-30% of the maximal expiratory maneuver is effort-dependent; the remainder is largely effort-independent. This requires good patient cooperation and a trained technician.

Spirometry trace showing FEV1, FVC, and FEF25-75%

Fig. 2 - Spirometry trace: FEV1, FVC, peak flow, and FEF25-75% (Miller's Anesthesia, 10e)

4.2 Key Spirometric Parameters

Parameter	Definition	Normal Value
FVC (Forced Vital Capacity)	Total air forcibly exhaled from TLC to RV	>80% predicted
FEV1	Volume exhaled in the first second	>80% predicted
FEV1/FVC ratio	Tiffeneau index	>0.70 (>70%)
FEF25-75%	Mean flow during middle half of FVC; small airway index	>60-70% predicted
PEFR (Peak Expiratory Flow Rate)	Maximal flow in first 10 ms of exhalation	Effort-dependent
MVV (Maximum Voluntary Ventilation)	Maximum volume breathed in 12 seconds x 5	>80% predicted

4.3 Interpretation of FEV1/FVC

FEV1/FVC < 0.70 = Obstructive defect
FEV1/FVC ≥ 0.70 with reduced FVC = Restrictive defect
Both FEV1 and FVC reduced with FEV1/FVC ≥ 0.70 = Restriction
Miller's Anesthesia, 10e, p. 1302
Barash's Clinical Anesthesia, 9e, p. 3144-3145

5. MEASUREMENT OF LUNG VOLUMES

Three techniques are used to measure volumes that include RV (and hence FRC and TLC):

5.1 Body Plethysmography (Gold Standard)

Patient sits in an airtight, constant-volume chamber (body box)
Makes inspiratory efforts against a closed shutter with a pressure transducer
Applies Boyle's Law: P1V1 = P2V2
As diaphragm contracts, thoracic volume increases and box pressure rises proportionately
Measures FRC (or RV), from which TLC is derived
Advantage: Measures all gas in the thorax, including trapped gas - most accurate in severe obstruction

5.2 Nitrogen Washout (Open-Circuit Method)

Patient inhales 100% oxygen; all exhaled gas collected
N2 concentration measured in exhaled gas by microprocessors
When exhaled N2 < 1.5% for 3 breaths, test ends
N2 washed out = 80% of initial lung gas → FRC calculated
Modern ventilators use a modification of this technique for FRC estimation at the bedside

5.3 Helium Dilution (Closed-Circuit Method)

Patient rebreathes a known volume and concentration of He in a closed circuit
He equilibrates until concentration stabilizes
C1V1 = C2(V1 + FRC) → FRC calculated
Limitation: Underestimates FRC in severe obstruction (poorly ventilated regions don't equilibrate)
Miller's Anesthesia, 10e, p. 1303
Murray & Nadel's Respiratory Medicine

6. FLOW-VOLUME LOOPS

The flow-volume loop plots flow (y-axis) against lung volume (x-axis) during a maximal expiration followed by a maximal inspiration. It displays the same spirometric information more conveniently and allows identification of airway obstruction patterns.

6.1 Normal Flow-Volume Loop

Expiratory limb: rapid rise to peak flow (effort-dependent), followed by a linear decline (effort-independent)
Inspiratory limb: symmetric, semicircular shape
Flow at 75%, 50%, 25% of VC (V75, V50, V25) can be measured
FEF25-75% reflects small airway resistance (effort-independent portion)

6.2 Patterns in Disease

Flow-volume loops in obstructive disease, restrictive defect, tracheal stenosis, and normal

Fig. 3 - Flow-volume loops: Normal, Obstructive lung disease, Tracheal stenosis, Restrictive defect (Barash's Clinical Anesthesia, 9e)

Pattern	FEV1/FVC	TLC	Shape of Loop
Obstructive (COPD, asthma)	↓ (<0.70)	↑	Concave (scooped) expiratory limb, shifted left
Restrictive (fibrosis, scoliosis)	Normal	↓↓	Normal shape but reduced size (shifted right)
Fixed obstruction (tracheal stenosis)	↓	Normal	Flattening of BOTH inspiratory and expiratory limbs

6.3 Variable Airway Obstruction

Variable extrathoracic, intrathoracic, and fixed obstruction flow-volume loops

Fig. 4 - Variable extrathoracic, variable intrathoracic, and fixed obstruction patterns (Miller's Anesthesia, 10e)

Type	Flattening	Mechanism	Example
Variable extrathoracic obstruction	Inspiratory limb flattened	During inspiration, atmospheric pressure > lesion lumen pressure → collapse	Vocal cord palsy, subglottic stenosis
Variable intrathoracic obstruction	Expiratory limb flattened	During forced expiration, pleural pressure > lesion lumen pressure → collapse	Tracheomalacia, intrathoracic tracheal tumor
Fixed obstruction	Both limbs flattened equally	Obstruction is rigid; not affected by pressure changes	Tracheal ring, bilateral vocal cord fixation

Miller's Anesthesia, 10e, p. 1302
Barash's Clinical Anesthesia, 9e, p. 3145-3146

7. PATTERNS OF PFT ABNORMALITY

7.1 Obstructive Pattern

Parameter	Change
FEV1	↓↓
FVC	Normal or mildly ↓
FEV1/FVC	↓ (<0.70)
TLC	↑ (air trapping)
RV, FRC	↑
FEF25-75%	↓ (early small airway disease)

Causes: COPD (chronic bronchitis, emphysema), asthma, bronchiectasis, cystic fibrosis, small airway disease

The hallmark is a reduction in FEV1/FVC%. In emphysema, loss of elastic recoil leads to dynamic airway compression. In chronic bronchitis, airway narrowing is due to fibrosis and secretions. RV, FRC, and TLC are typically all elevated (air trapping).

7.2 Restrictive Pattern

Parameter	Change
FEV1	↓
FVC	↓↓
FEV1/FVC	Normal (≥0.70)
TLC	↓↓
RV	↓ or normal
FEF25-75%	Normal

Causes:

Pulmonary: Interstitial lung disease, pulmonary fibrosis, sarcoidosis, pneumonia
Extrapulmonary: Obesity, scoliosis, pleural effusion, neuromuscular disease (myasthenia gravis, GBS), diaphragm palsy

7.3 Mixed Pattern

Both FEV1/FVC < 0.70 AND TLC reduced
Seen in: advanced COPD with co-existing fibrosis, bronchiectasis with fibrosis

GOLD Classification of COPD (Spirometry-Based)

Stage	Criteria (Post-bronchodilator)
I - Mild	FEV1 ≥ 80% predicted
II - Moderate	50% ≤ FEV1 < 80%
III - Severe	30% ≤ FEV1 < 50%
IV - Very Severe	FEV1 < 30%

All stages require FEV1/FVC < 0.70.

Fishman's Pulmonary Diseases and Disorders
Barash's Clinical Anesthesia, 9e, p. 3145-3146

8. DIFFUSING CAPACITY FOR CARBON MONOXIDE (DLCO)

Principle

DLCO evaluates the functional capillary surface area available for gas exchange. CO is used because its high affinity for hemoglobin keeps alveolar capillary pCO near zero - so transfer is limited only by membrane thickness, not perfusion.

Fick's Law: DLCO is determined by:

Surface area of alveolar-capillary membrane
Pressure gradient (alveolar pCO - capillary pCO)
Membrane thickness
Molecular weight and solubility of gas

Method (Single Breath Technique)

Patient inhales a gas mixture containing 0.3% CO + 10% He + 21% O2 (balance N2)
Holds breath for 10 seconds
Exhales; sample analyzed for CO and He
He dilution gives alveolar volume; CO uptake gives DLCO

Clinical Significance

DLCO	Interpretation
>80% predicted	Normal
60-80%	Mild reduction
40-60%	Moderate reduction
<40%	Severe reduction

Causes of reduced DLCO:

Emphysema (loss of alveolar surface area) - often with normal spirometry in early disease
Interstitial lung disease (increased membrane thickness)
Pulmonary hypertension, pulmonary emboli (reduced pulmonary capillary blood volume)
Anemia (less hemoglobin to bind CO)
Lobectomy/pneumonectomy

Causes of elevated DLCO:

Polycythemia, left-to-right shunts, mild left heart failure (increased pulmonary blood volume)

Anaesthetic relevance: DLCO < 60% predicted = increased risk of PPCs; indication for further preoperative evaluation including CPET. - Miller's Anesthesia, 10e, p. 1303-1304

9. RESPIRATORY MUSCLE STRENGTH TESTS

Test	Description	Significance
PImax / MIP (Maximum Inspiratory Pressure)	Maximal inspiratory effort against occluded airway at RV	Reflects diaphragm and accessory inspiratory muscle strength; PImax < -25 cmH2O suggests need for ventilatory support
PEmax / MEP (Maximum Expiratory Pressure)	Maximal expiratory effort against occluded airway at TLC	Reflects expiratory/abdominal muscle strength; PEmax < +40 cmH2O → impaired cough

Both PImax and PEmax are reduced in neuromuscular disease, poor effort, and fatigue. Increased lung volumes reduce PImax (flatter diaphragm); decreased lung volumes reduce PEmax. - Fishman's Pulmonary Diseases

10. CARDIOPULMONARY EXERCISE TESTING (CPET)

CPET is a dynamic, integrated test evaluating functional capacity and the contribution of the respiratory, cardiovascular, and musculoskeletal systems.

Key Measured Parameters

Parameter	Threshold	Clinical Meaning
VO2max (max oxygen consumption)	<15 mL/kg/min = high risk	Broad indicator of cardiorespiratory reserve
Anaerobic Threshold (AT)	<10 mL/kg/min = high risk	Sustainable exercise capacity; point where anaerobic metabolism begins
VE/VCO2 slope	>35 = high risk	Efficiency of ventilation; reflects V/Q mismatch and dead space

Monitoring during CPET: ECG, SpO2, respiratory rate, exhaled gas analysis (VO2, VCO2), airflow, and volume on a cycle ergometer or treadmill with incremental workload.

Anaesthetic significance: CPET is the most comprehensive preoperative cardiorespiratory assessment, particularly for lung resection, major abdominal, and cardiac surgery candidates.

Miller's Anesthesia, 10e, p. 1307

11. PREOPERATIVE PFT ASSESSMENT FOR LUNG RESECTION

This is a core anaesthetic application of PFTs. Three goals:

Identify patients at risk of increased postoperative morbidity/mortality
Identify patients needing short- or long-term postoperative ventilatory support
Evaluate reversibility of airway obstruction with bronchodilators

Predicted Postoperative FEV1 (ppoFEV1)

ppoFEV1 = Preoperative FEV1 × % lung tissue remaining after resection

ppoFEV1	Risk
>40%	Reduced risk
30-40%	Moderate risk
<30%	High risk - likely needs postoperative ventilation

Spirometric Thresholds for Lung Resection Risk

Parameter	High Risk Threshold
FEV1	<60% predicted → increased PPCs; <800 mL historically = absolute contraindication (now less used with VATS)
FVC	Abnormal VC in 30-40% of postoperative deaths
MVV	<50% predicted
RV/TLC ratio	>50%
FEV1/FVC	Low ratio = obstructive disease present

Note: With thoracoscopic surgery (VATS) and improved pain management, patients with lower lung volumes can now be safely operated on. It is preferable to express FEV1 as % predicted rather than absolute value (accounts for age, sex, height). - Barash's Clinical Anesthesia, 9e, p. 3144-3145

12. EFFECTS OF ANAESTHESIA AND SURGERY ON PFT PARAMETERS

Understanding how anaesthesia and surgery alter lung function is equally important:

Parameter	Change After Surgery
TLC	Decreases after abdominal (not limb) surgery
VC (Vital Capacity)	Decreases by 25-50% within 1-2 days; returns to normal in 1-2 weeks
RV (Residual Volume)	Increases by ~13%
ERV	Decreases by 25% after lower abdominal, 60% after upper abdominal/thoracic surgery
VT (Tidal Volume)	Decreases by 20% within 24 hours
FRC	Decreases ~33% with reduced compliance; small airway closure occurs

Mechanism of FRC reduction under anaesthesia:

Loss of muscle tone alters balance between chest wall recoil (outward) and lung elastic recoil (inward)
Moving supine: FRC reduces ~1 L
Induction of anaesthesia: further reduces FRC by ~400-500 mL
Results in: increased elastic behavior (reduced compliance), increased airway resistance, atelectasis, V/Q mismatch, and impaired oxygenation
50% of anaesthesia-related deaths involve hypoxemia
Barash's Clinical Anesthesia, 9e, p. 3144
Miller's Anesthesia, 10e, p. 1307-1308

13. BRONCHODILATOR REVERSIBILITY TESTING

PFTs are routinely performed before and after bronchodilator administration (typically salbutamol 400 mcg by MDI or nebulization, wait 15 minutes; or ipratropium, wait 30 minutes).

Positive response: ≥15% improvement (or ≥200 mL absolute improvement) in FEV1 or FVC
Indicates reversible airway obstruction (typical of asthma)
Bronchodilator therapy should be initiated/optimized before elective surgery
Anaesthetic significance: Prognosis in COPD correlates better with post-bronchodilator spirometry than baseline values
Barash's Clinical Anesthesia, 9e, p. 3147

14. SUMMARY TABLE: PFT PATTERNS AT A GLANCE

Parameter	Normal	Obstructive	Restrictive	Mixed
FEV1	>80%	↓↓	↓	↓↓
FVC	>80%	Normal or ↓	↓↓	↓↓
FEV1/FVC	>0.70	↓ (<0.70)	Normal	↓
TLC	Normal	↑	↓↓	Variable
RV	Normal	↑↑	↓	↑
FRC	Normal	↑	↓	Variable
DLCO	Normal	↓ (emphysema)	↓ (fibrosis)	↓
FEF25-75%	Normal	↓↓	Normal	↓
Flow-volume loop	Normal	Scooped, shifted left	Small but normal shape	Mixed

15. INDICATIONS FOR PFTs IN ANAESTHESIA PRACTICE

Preoperative - known COPD, asthma, interstitial lung disease, unexplained dyspnoea, thoracic surgery, morbid obesity, neuromuscular disease
Lung resection - mandatory to calculate ppoFEV1 and guide surgical decision
Monitoring disease progression - COPD, ILD, sarcoidosis
ICU - weaning readiness (RSBI = RR/VT < 105 = ready to wean), NIF/PImax assessment
Assessment of bronchodilator response - asthma vs COPD
Occupational lung disease - silicosis, asbestosis

CONCLUSION

PFTs provide the anaesthesiologist with objective, quantifiable data about lung mechanics, gas exchange efficiency, and respiratory reserve. Spirometry (FEV1, FVC, FEV1/FVC) forms the foundation of PFT interpretation. Lung volume measurement (particularly body plethysmography) adds information about air trapping and restriction. DLCO identifies impaired gas exchange. Flow-volume loops characterize the site and nature of airway obstruction. CPET offers the most comprehensive preoperative cardiorespiratory risk assessment. Together, these tests guide patient selection for surgery, predict postoperative complications, direct perioperative optimization, and inform ventilatory management in the ICU.

Key References (Sources):

Miller's Anesthesia, 10e - Chapters 12 and 19
Barash, Cullen & Stoelting's Clinical Anesthesia, 9e - Chapter 38
Murray & Nadel's Textbook of Respiratory Medicine - Chapter 31
Fishman's Pulmonary Diseases and Disorders, 2-Volume Set - Chapter 31

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