Understanding and identifying the lung volume components is essential in pulmonary function testing.
Measurements of lung volumes are important to confirm or clarify the nature of lung disorders. The flow volume loop may indicate an obstructive or restrictive or obstructive/restrictive pattern, but a further test of lung volume is often necessary for clarification.
In an obstructive lung disease, airway obstruction causes an increase in resistance. During normal breathing, the pressure volume relationship is no different from a normal lung. However, when breathing rapidly, greater pressure is needed to overcome the resistance to flow, and the volume of each breath gets smaller. The increase in the effort to breathe can cause an overdistention of the lungs.
The flow volume loop may show lower than normal FEV1 and FEF25-75, but it is not until a lung volume has been determined that an increase in TLC, FRC and RV can be confirmed.
Common obstructive disease include asthma, bronchitis and emphysema.
In a restrictive lung disease, the compliance of the lung is reduced which increases the stiffness of the lung and limits expansion. In these cases, a greater pressure than normal is required to give the same increase in volume.
The flow volume loop may show lower than normal FVC, but the FEV1 and FEF25-75 may only be mildly effected. The lung volume measurement will clearly show a reduction in TLC, FRC and RV.
Common causes of decreased lung compliance are pulmonary fibrosis, pneumonia and pulmonary edema. Patients whose respiratory muscles are unable to perform normally because of a neuromuscular disease or paralysis can show a restrictive pattern.
The total volume contained in the lung at the end of a maximal inspiration is subdivided into volumes and also into capacities.
Inspiratory Reserve Volume (IRV). Maximum amount of additional air that can be inspired from the end of a normal inspiration.
Expiratory Reserve Volume (ERV). The maximum volume of additional air that can be expired from the end of a normal expiration.
Residual Volume (RV). The volume of air remaining in the lung after a maximal expiration. This is the only lung volume which cannot be measured with a spirometer.
Lung capacities are subdivisions of total volume that include two or more of the 4 basic lung volumes.
Vital Capacity (VC). The maximum volume of air that can be forcefully expelled from the lungs following a maximal inspiration. Called a capacity because it is the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume. VC = IRV+TV+ERV=TLC-RV
Functional Residual Capacity (FRC). The volume of air remaining in the lung at the end of a normal expiration. Called a capacity because it equals residual volume plus expiratory reserve volume. FRC = RV+ERV
Inspiratory Capacity (IC). Maximum volume of air that can be inspired from end expiratory position. Called a capacity because it is the sum of tidal volume and inspiratory reserve volume. This capacity is of less clinical significance than the other three. IC = TV+IRV
If you have a closed container where volume can be adjusted using a reciprocating pump (typically 30ml) then the pressure in the container increases in amount proportional to the fractional decrease in container volume (i.e. PV=k).
In body plethysmography, the patient sits inside an airtight box, inhales or exhales to a particular volume (usually FRC), and then a shutter drops across their breathing valve. The subject makes respiratory efforts against the closed shutter causing their chest volume to expand and decompressing the air in their lungs. The increase in their chest volume slightly reduces the box volume and thus increases the pressure in the box. This method of measuring FRC actually measures all the conducting pathways including abdominal gas; the actual measurement made is VTG (Volume of Thoracic gas).
To compute the volume of air in the lungs, we first compute the
change in volume of the chest. Using Boyle's Law (P1 V1 = P2 V2 at
constant temperature), we set the initial pressure in the box times the
initial volume of the box (both of which we know), equal to the pressure
times volume of the box at the end of a chest expansion (of which we
only know the pressure).
The volume of the box during respiratory effort is solved. The difference between this volume and the initial volume of the box is the change in volume of the box, which is the same as the change in the volume of the chest.
During helium dilution measurement of lung volumes, patients
breathe from a known volume and concentration of helium gas for a period
of typically 4 to 7 minutes. The oxygen concentration in the starting
mixture is set at 30% to ensure patients with COPD can remain
comfortable during the test. A carbon dioxide absorber is situated in
line with expired breath to keep the closed-circuit CO2 level below 0.5%
and avoid discomfort and hyperventilation. Oxygen is added to the
system to maintain the starting volume in the spirometer. Since the type
of thermal conductivity helium analyzer employed in the SpiroAir
can be affected by changes in concentrations of CO2, O2 and Water
Vapor, a chemical absorber removes the interference of CO2 and Water
Vapor. A simple algorithm corrects the helium signal for changes in
oxygen background.
Once connected to the closed-circuit, equilibration between the starting and final helium concentrations should occur within 7 minutes. A state of equilibrium is defined as helium concentration changes of less than 0.02% over a 30 second interval.
The functional residual capacity (FRC) is calculated from the helium concentrations as follows:
FRC = (% helium initial - % helium final) / % helium final x system volume
The dead space of the system (patient valve, filter and mouthpiece) is subtracted from this value.
The FRC can be underestimated with the helium dilution technique in conditions such as bullous emphysema or severe airways obstruction. Trapped lung gas does not communicate with the inhaled helium mixture. Plethysmographic measurement of lung volumes is preferred in these cases.
The FRC can be overestimated or unmeasurable when leaks are present. Leaks may develop in the equipment valves or circuitry, or, more commonly, at the mouthpiece. A system leak is likely to be the cause if the graph of the delivered helium concentration does not flatten, ie, when equilibrium is not reached, within 7 minutes. The ComPAS Freedom™ software will warn of possible leak conditions.
Measurements of lung volumes are important to confirm or clarify the nature of lung disorders. The flow volume loop may indicate an obstructive or restrictive or obstructive/restrictive pattern, but a further test of lung volume is often necessary for clarification.
In an obstructive lung disease, airway obstruction causes an increase in resistance. During normal breathing, the pressure volume relationship is no different from a normal lung. However, when breathing rapidly, greater pressure is needed to overcome the resistance to flow, and the volume of each breath gets smaller. The increase in the effort to breathe can cause an overdistention of the lungs.
The flow volume loop may show lower than normal FEV1 and FEF25-75, but it is not until a lung volume has been determined that an increase in TLC, FRC and RV can be confirmed.
Common obstructive disease include asthma, bronchitis and emphysema.
In a restrictive lung disease, the compliance of the lung is reduced which increases the stiffness of the lung and limits expansion. In these cases, a greater pressure than normal is required to give the same increase in volume.
The flow volume loop may show lower than normal FVC, but the FEV1 and FEF25-75 may only be mildly effected. The lung volume measurement will clearly show a reduction in TLC, FRC and RV.
Common causes of decreased lung compliance are pulmonary fibrosis, pneumonia and pulmonary edema. Patients whose respiratory muscles are unable to perform normally because of a neuromuscular disease or paralysis can show a restrictive pattern.
The total volume contained in the lung at the end of a maximal inspiration is subdivided into volumes and also into capacities.
There are four lung volume subdivisions which:
a) do not overlap.
b) can not be further divided.
c) when added together equal total lung capacity (TLC).
Identifying The Lung Volumes
Tidal Volume (TV). The amount of gas inspired or expired with each breath.Inspiratory Reserve Volume (IRV). Maximum amount of additional air that can be inspired from the end of a normal inspiration.
Expiratory Reserve Volume (ERV). The maximum volume of additional air that can be expired from the end of a normal expiration.
Residual Volume (RV). The volume of air remaining in the lung after a maximal expiration. This is the only lung volume which cannot be measured with a spirometer.
Lung capacities are subdivisions of total volume that include two or more of the 4 basic lung volumes.
Identifying The Lung Capacities
Total Lung Capacity (TLC). The volume of air contained in the lungs at the end of a maximal inspiration. Called a capacity because it is the sum of the 4 basic lung volumes. TLC = RV+IRV+TV+ERVVital Capacity (VC). The maximum volume of air that can be forcefully expelled from the lungs following a maximal inspiration. Called a capacity because it is the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume. VC = IRV+TV+ERV=TLC-RV
Functional Residual Capacity (FRC). The volume of air remaining in the lung at the end of a normal expiration. Called a capacity because it equals residual volume plus expiratory reserve volume. FRC = RV+ERV
Inspiratory Capacity (IC). Maximum volume of air that can be inspired from end expiratory position. Called a capacity because it is the sum of tidal volume and inspiratory reserve volume. This capacity is of less clinical significance than the other three. IC = TV+IRV
Measuring Lung Volumes
Body Plethysmography
The penultimate way to measure lung volumes is body plethysmography. With this instrument, the volumes of the lung are evaluated by pressure change. Body plethysmography is the most accurate means available at this time to assess lung volumes because it is not limited by air trapping.If you have a closed container where volume can be adjusted using a reciprocating pump (typically 30ml) then the pressure in the container increases in amount proportional to the fractional decrease in container volume (i.e. PV=k).
- Boyle's Law states that:
- V1 P1 T1 = V2 P2 T2
- For the plethysmograph, the temperature is kept constant so:
- P1 V1 = P2 V2
- Where:
P1 and V1 are initial pressure and volume.
P2 and V2 are final pressure and volume.
Note: Both measurements are made at a constant temperature.
- Where:
- V1 P1 T1 = V2 P2 T2
In body plethysmography, the patient sits inside an airtight box, inhales or exhales to a particular volume (usually FRC), and then a shutter drops across their breathing valve. The subject makes respiratory efforts against the closed shutter causing their chest volume to expand and decompressing the air in their lungs. The increase in their chest volume slightly reduces the box volume and thus increases the pressure in the box. This method of measuring FRC actually measures all the conducting pathways including abdominal gas; the actual measurement made is VTG (Volume of Thoracic gas).
The volume of the box during respiratory effort is solved. The difference between this volume and the initial volume of the box is the change in volume of the box, which is the same as the change in the volume of the chest.
Helium Dilution
Helium dilution is a classic method used to measure the lung volume and capacities. The origins of the technique are reputed to date back to Sir Humphrey Davy's book, "Researches Chemical and Philosophical", published in 1799, describes the measurement of his own lung volumes, including the first recorded measurement of the residual volume. He also measured his own rates of oxygen consumption and carbon dioxide production. He is famous for his investigations into nitrous oxide, but he also investigated the effects of breathing nitric oxide and carbon monoxide. He made these observations with a gasometer and analysis of his expired air, and his work anticipated the invention of blood gas analysis.
Why use helium?
Helium is an inert (lighter than air), colorless, odorless,
tasteless gas and is not toxic. Furthermore, it cannot transfer across
the avleolar-capillary membrane and is thus contained when in the lungs.
Closed circuit helium dilution studies have been a standard
method of measuring the 'hidden' residual volume in the lung for many
years.
The method employs the simple principle of gas dilution using helium, an insoluble inert gas that mixes easily in the lungs.
Once connected to the closed-circuit, equilibration between the starting and final helium concentrations should occur within 7 minutes. A state of equilibrium is defined as helium concentration changes of less than 0.02% over a 30 second interval.
The functional residual capacity (FRC) is calculated from the helium concentrations as follows:
FRC = (% helium initial - % helium final) / % helium final x system volume
The dead space of the system (patient valve, filter and mouthpiece) is subtracted from this value.
The FRC can be underestimated with the helium dilution technique in conditions such as bullous emphysema or severe airways obstruction. Trapped lung gas does not communicate with the inhaled helium mixture. Plethysmographic measurement of lung volumes is preferred in these cases.
The FRC can be overestimated or unmeasurable when leaks are present. Leaks may develop in the equipment valves or circuitry, or, more commonly, at the mouthpiece. A system leak is likely to be the cause if the graph of the delivered helium concentration does not flatten, ie, when equilibrium is not reached, within 7 minutes. The ComPAS Freedom™ software will warn of possible leak conditions.