Lung Volume Studies

Objective — Perform a lung volume study and use lung volume measurements to identify restrictive disease, air trapping, hyperinflation, and mixed defects.

---

Overview

Lung volume studies are pulmonary function tests that measure functional residual capacity (FRC).

FRC is important because it cannot be directly exhaled from the lungs. Once FRC is measured, other lung volumes can be calculated.

Three methods are used to measure FRC:

VTG is also called:

• Thoracic gas volume
• Body plethysmography
• BodyBox

VTG is used to compute the patient's total lung capacity (TLC).

---

Indications, Contraindications & Hazards

Indications

Lung volume studies are used to:

• Diagnose restrictive lung disease patterns
• Differentiate obstructive and restrictive patterns when vital capacity is reduced
• Assess response to therapeutic interventions
• Perform pre-operative assessment

Contraindications

Contraindications are the same as spirometry, plus inability to access the body box:

• Hemoptysis of unknown origin
• Pneumothorax
• Unstable cardiovascular status
• Recent MI or PE
• Thoracic, abdominal, or central aneurysms
• Inability to access the body box

Hazards

---

Equipment

Equipment may include:

• Spirometer
• Helium analyzer
• Nitrogen analyzer
• Plethysmograph

---

Nitrogen Washout

Open-Circuit Technique

Nitrogen in the lungs is presumed to be equal with the atmosphere, about 75-80%.

During the test:

1. The patient breathes 100% O2.
2. The O2 washes nitrogen out of the lungs.
3. A nitrogen analyzer and spirometer provide breath-to-breath analysis of expired N2.
4. The test lasts up to 7 minutes or until exhaled N2 concentration is less than 1%.

Nitrogen Corrections

Corrections must be made for:

• N2 washed out of the blood and tissue
• Small amounts of N2 in the pure O2 source

For each minute of breathing O2, approximately 30-40 mL of N2 is removed from blood and tissue.

0.04 x time = N2 tissue correction

The tissue correction is subtracted from the volume of exhaled N2 when computing FRC.

---

Helium Dilution

Helium is metabolically inert. It can be distributed throughout the lungs without significant absorption.

During the test:

1. The spirometer is filled with a known volume, usually 600-800 mL.
2. Helium is added until the helium concentration is 10%.
3. The patient rebreathes the gas in the spirometer.
4. The helium concentration falls until it reaches a stable level.
5. In healthy patients, equilibration usually takes 3-4 minutes.

The system must include:

• A CO2 absorber, such as soda lime
• Added oxygen to meet the patient's oxygen demand

Helium Correction

About 100 mL is subtracted from FRC for helium that dissolves in the blood.

If this volume is not subtracted, FRC will be overestimated.

---

Thoracic Gas Volume (VTG)

Why VTG Is Different

VTG is the quickest and most accurate of the three methods for determining FRC.

It is based on Boyle's law:

P1V1 = P2V2

Boyle's law describes the relationship between volume and pressure when temperature is constant.

VTG is the volume of gas contained in the thorax, whether the gas communicates with open airways or is trapped in any thoracic compartment.

VTG measurement begins at end tidal expiration, which is FRC.

Because plethysmography includes trapped gas, VTG measurements are often larger than FRC values measured by gas dilution methods, especially in emphysema with air trapping and uneven distribution of ventilation.

Body Box Procedure

1. Seat the patient in the box with the door shut for at least 30 seconds so temperature can equalize.
2. Place the patient on the mouthpiece and ask them to breathe normally.
3. Record at least three tidal breaths that vary by less than 100 mL.
4. Have the patient perform an SVC maneuver.
5. Confirm SVC is greater than or equal to FVC.
6. When the patient returns to baseline, prepare them for panting.

During panting:

• The patient places hands on cheeks.
• Panting frequency is typically 30-60 breaths/min, depending on the manufacturer.
• The airway is occluded and the patient pants against a closed shutter.
• With no airflow, mouth pressure equals alveolar pressure.
• Airway pressure and box pressure are measured simultaneously.

As the patient inspires against the closed shutter:

• Thoracic volume increases.
• Pressure in the box decreases.
• The pressure and volume changes are used to compute VTG using Boyle's law.

---

Core Lung Volumes

Tidal Volume (VT or TV)

Tidal volume is the volume of air moved in and out of the lungs during normal breathing.

It is measured while the patient breathes quietly until three consecutive breaths vary by less than 100 mL.

Increasing tidal volume during measurement can alter end-inspiratory or end-expiratory levels and cause inaccurate values.

Inspiratory Reserve Volume (IRV)

IRV is the largest volume of gas that can be inspired from the resting end-tidal inspiration level.

Expiratory Reserve Volume (ERV)

ERV is the largest volume of gas that can be expired from the resting end-expiratory level.

High-yield points:

• ERV normally comprises about 25% of VC.
• ERV changes usually parallel increases or decreases in VC.
• Reduced ERV is consistent with a restrictive defect.
• Erroneous ERV estimates cause miscalculation of RV because ERV is subtracted from FRC to calculate RV.

Residual Volume (RV)

RV is the volume of gas remaining in the lungs at end maximum expiration.

RV = FRC - ERV

Increased RV means the lungs still contain a large amount of gas despite maximal expiratory effort.

Increased RV is characteristic of:

• Emphysema
• Bronchial obstruction
• Chronic air trapping

RV and FRC usually increase together.

RV is usually decreased in restrictive disorders.

Inspiratory Capacity (IC)

IC is the largest volume of gas that can be inspired from the resting expiratory level.

High-yield points:

• IC is a subdivision of VC.
• IC comprises approximately 75% of VC.
• IC changes usually parallel VC.
• Reduced IC is consistent with restrictive defects.

Functional Residual Capacity (FRC)

FRC is the volume of gas remaining in the lungs at the end tidal expiratory level.

FRC can only be measured indirectly because this gas cannot be exhaled from the lungs.

FRC comprises about 40% of TLC.

Vital Capacity (VC or SVC)

Vital capacity is the volume of gas measured on a slow, complete expiration after maximal inspiration, without forced or rapid effort.

It is measured by having the patient:

1. Inspire maximally.
2. Exhale completely with no time limit.

It can also be measured from maximum expiration to maximum inspiration.

Decreased VC can be caused by loss of distensible lung tissue.

Total Lung Capacity (TLC)

TLC is the volume of gas in the lungs at end maximal inspiration.

TLC = FRC + IC
TLC = VC + RV

TLC can also be determined by radiologic methods using A-P and lateral chest films.

TLC may be normal or increased in obstructive disease.

If TLC is less than LLN, a restrictive process should be suspected.

---

Interpretation

Restrictive Abnormality

Restrictive disease interferes with expansion and contraction of the chest or lungs.

Restriction is identified by a decreased TLC.

Obstructive Abnormality

Combined Abnormality

A combined abnormality requires both:

• Obstruction confirmed by spirometry: FEV1 less than LLN
• Restriction confirmed by lung volumes: TLC less than LLN

---

Airway Resistance and Conductance

Body plethysmography can also measure values associated with airway resistance and conductance.

Airway Resistance (Raw)

Raw is opposition to flow caused by friction.

Raw = (Pmouth - PA) / flow

Raw is a measure of the difficulty with which air flows through the lungs.

It is measured in:

cmH2O/L/sec

Normal Raw:

0.6-2.4 cmH2O/L/sec

Airway Conductance (Gaw)

Gaw is flow generated per unit of pressure drop in the airway.

Gaw is the reciprocal of Raw and measures the ease of gas movement through the airways.

It is measured in:

L/sec/cmH2O

Normal Gaw:

0.42-1.67 L/sec/cmH2O

Specific Resistance and Specific Conductance

Airway diameter changes with lung volume. Airway caliber increases as the lungs expand.

Raw and Gaw are affected during normal breathing and abnormal physiology, especially obstructive disease. Specific values help normalize this effect by evaluating resistance and conductance at the lung volume where they were measured, usually VTG at FRC.

sGaw = Gaw / VTG
sRaw = Raw x VTG

Normal sGaw:

greater than 0.15 L/sec/cmH2O

There are no reliable reference sets for sRaw.

Obstructive Pattern

Obstruction is suggested by:

• Raw greater than 2.4 cmH2O/L/sec
• sGaw less than 0.15 L/sec/cmH2O

Restrictive disease will show normal airway resistance and conductance values.

Where Normal Airway Resistance Occurs

Resistance in the normal airway is distributed as:

Disease Effects

All obstructive diseases can increase Raw and decrease Gaw.

Examples:

• Large airway obstruction from tumor, trauma, or foreign body can cause a significant increase in Raw.
• Acute asthma attack can increase Raw up to three times normal.
• Emphysema can increase Raw due to airway narrowing and collapse, especially in the bronchioles.
• Bronchitis may increase Raw relative to disease severity.

---

Quick Review

Three Ways to Measure FRC

• Helium dilution
• Nitrogen washout
• Plethysmography / VTG

Must-Know Equations

RV = FRC - ERV
TLC = FRC + IC
TLC = VC + RV
sGaw = Gaw / VTG
sRaw = Raw x VTG

Must-Know Pattern Clues