The helium dilution technique involves adding a known concentration (C1) of an inert gas, such as helium, to a box of known volume (V1). This box is then connected to a volume (V2) that is unknown (the lung volume to be measured). After allowing sufficient time for the inert gas to distribute, the new concentration (C2) of the inert gas is measured. The change in concentration is used to determine the new volume in which the inert gas has been distributed (Fig. 21.4).

In the body plethysmograph (body box), Robert Boyle's gas law is used to measure lung volumes. This law states that pressure multiplied by volume is constant at a constant temperature. The patient sits in an airtight box (Fig. 21.5) and breathes through a mouthpiece connected to a flow sensor (pneumotach). The patient then performs a panting respiratory effort against a closed mouthpiece. During the expiratory phase of the maneuver, the gas in the lung becomes compressed, causing a decrease in lung volume and a decrease in pressure inside the box. Once the volume of the box and the change in pressure at the mouth are known, the change in lung volume (ΔV) can be calculated using the equation ΔV = P1V1 / P2 - V1, where P1 and P2 are mouth pressures and V is FRC (functional residual capacity). From the measurement of FRC, inspiratory capacity can be recorded as the volume of air inspired above FRC, and expiratory reserve volume can be determined as the volume of gas exhaled from FRC. These measurements can then be used to determine other lung volumes.

In determining lung volume, the volume of air at total lung capacity (TLC) or residual volume (RV) is influenced by the properties of the lung parenchyma and the interaction between the lungs and the chest wall. The lungs and chest wall always move together as a unit in healthy individuals. The lung contains elastic fibers that stretch when stress is applied, resulting in an increase in lung volume, and recoil passively when the stress is released, resulting in a decrease in lung volume. The elastic recoil of the lung parenchyma is high, and in the absence of external forces, the lungs become almost airless (10% of TLC). Similarly, the chest wall volume can increase when the respiratory muscles are stretched and decrease when respiratory muscle length is shortened. In the theoretical absence of the lung parenchyma, the resting volume of the chest wall increases and is approximately 60% of TLC.

Lung volumes are determined by the balance between the lung's elastic properties and the properties of the muscles of the chest wall. The maximum volume of air within the lung and the chest wall (TLC) is controlled by the muscles of inspiration. As lung volume increases, the chest wall muscles lengthen progressively, resulting in a decrease in their ability to generate force. TLC is reached when the inspiratory chest wall muscles are unable to generate the additional force required to further distend the lung and chest wall. Similarly, the minimal volume of air in the lung (RV) is controlled by the muscles of expiration.
Intheheliumdilutiontechnique,aknownconcentration(C1)ofaninertgas(suchashelium)isaddedtoaboxofknownvolume(V1).Theboxisthenconnectedtoavolume(V2)thatisunknown(thelungvolumetobemeasured).Afteradequatetimefordistributionoftheinertgas,thenewconcentration(C2)oftheinertgasismeasured.Thechangeinconcentrationoftheinertgasisthenusedtodeterminethenewvolumeinwhichtheinertgashasbeendistributed(
Fig.21.4 ).Specifically,
Inbodyplethysmograph(bodybox),RobertBoyle’sgaslaw—thatpressuremultipliedbyvolumeisconstant(ataconstanttemperature)—isusedtomeasurelungvolumes.Thepatientsitsinanairtightbox(
Fig.21.5 )andbreathesthroughamouthpiecethatisconnectedtoaflowsensor(pneumotach).Thepatientthenmakesapantingrespiratoryeffortagainstaclosedmouthpiece.Duringtheexpiratoryphaseofthemaneuver,thegasinthelungbecomescompressed,lungvolumedecreases,andthepressureinsidetheboxfallsbecausethegasvolumeintheboxincreases.Oncethevolumeoftheboxandthechangeinpressureoftheboxatthemouthareknown,thechangeinvolume(ΔV)ofthelungcanbecalculated: whereP1andP2aremouthpressuresandVisFRC.FromthemeasurementofFRC,inspiratorycapacitycanberecordedasthevolumeofairinspiredaboveFRC,andexpiratoryreservevolumecanbedeterminedasthevolumeofgasexhaledfromFRC.Thesemeasurementscanthenbeusedtodeterminetheotherlungvolumes.
•Fig. 21.5 The Body Plethysmograph. Notethattheboxinwhichthepatientsitsisnotdepicted.
Determinants of Lung Volume
What determines the volume of air in the lung at TLC or at RV? The answer lies in the properties of the lung parenchyma and in the interaction between the lungs and the chest wall. The lungs and chest wall always move together as a unit in healthy individuals. The lung contains elastic fibers that (1) stretch when stress is applied, which results in an increase in lung volume, and (2) recoil passively when this stress is released, which results in a decrease in lung volume. The elastic recoil of the lung parenchyma is very high. In the absence of external forces (such as the force generated by the chest wall), the lungs become almost airless (10% of TLC). Similarly, chest wall volume can increase when the respiratory muscles are stretched and decrease when respiratory muscle length is shortened. In the theoretical absence of the lung parenchyma, the resting volume of the chest wall increases and is approximately 60% of TLC.
Lung volumes are determined by the balance between the lung’s elastic properties and the properties of the muscles of the chest wall. The maximum volume of air contained within the lung and the chest wall (i.e., TLC) is controlled by the muscles of inspiration (see ). With increasing lung volume, the chest wall muscles lengthen progressively. As these muscles lengthen, their ability to generate force decreases. TLC occurs when the inspiratory chest wall muscles are unable to generate the additional force needed to further distend the lung and chest wall. Similarly, the minimal volume of air in the –40–20 0 20 40 Pressure (cm H2O) (PL, P , or P ) •Fig. 21.6 Relaxation Pressure-Volume Curve of the Lung, Chest Wall, and Respiratory System. Thecurvefortherespiratorysystemisthesumoftheindividualcurves.Thecurveforthelungisthesameastheoneforthenormallungin