Body plethysmography: Which parameters can be measured?
Welcome to this Chalk Talk episode on body plethysmography.
In the last episode in this course, we saw how spirometry is used to measure mobilizable
lung volumes.
Body plethysmography can additionally measure non-mobilizable volumes.
These include the functional residual capacity, residual volume, total lung capacity, and
airway resistance.
It is mostly used in specialized departments.
So, let’s start with two examples of when body plethysmography can be useful.
Obstructive lung diseases can be determined by spirometry: Here, the FEV1 value and forced
vital capacity are measured and their ratio determined, resulting in the FEV1:FVC ratio.
In obstructive diseases, the FEV1 value and, therefore, the FEV1:FVC ratio is reduced.
However, both the FEV1 and the forced vital capacity are dynamic volumes, that is, they’re
measured under forced expiration.
Patients need to correctly perform the breathing maneuver required for measurement.
In contrast, with body plethysmography, an obstruction can be determined by airway resistance,
a parameter independent of patient cooperation.
It’s measured during normal breathing, so that there’s no need for a special breathing
maneuver.
Therefore, body plethysmography is a good alternative for patients with poor compliance
and it can objectify spirometry findings well.
Another reason to perform body plethysmography could be the differentiation between restriction
and emphysema, that is, chronic lung hyperinflation.
Spirometry provides only a clue, namely a reduced vital capacity.
Though, differentiation requires knowing the residual volume of the lung and, therefore,
the total lung capacity: Compared to a normal finding, the vital capacity
is reduced in restriction because the total lung volume is decreased.
In contrast, in lung hyperinflation, the vital capacity is reduced as the residual volume
is increased at the expense of the vital capacity.
The total lung capacity isn't reduced.
The term body plethysmography is derived from the Greek word plethysmós meaning "enlargement"
and gráphein for "to write".
It literally means describing the enlargement of a body.
Through this procedure, the total volume in the lungs is determined, including the volume
that can’t be exhaled.
The idea behind body plethysmography is to measure lung volume non-invasively and outside
the thorax.
For this, thoracic movements during breathing are recorded as pressure changes in the surrounding
air, which are converted subsequently into changes in the lung volume.
This type of measurement only works in a closed set-up.
Accordingly, patients sit in an airtight cabin, breathing air in and out from outside the
cabin via a mouthpiece.
Inside the cabin, there’s a pressure sensor that registers the pressure change within
the cabin.
A second pressure sensor is in the mouthpiece that can simultaneously measure pressure in
the mouth and airflow.
Let's first look at what the pressure sensor in the cabin measures.
Since the cabin is closed and doesn’t change its shape, the volume and pressure in the
cabin always change simultaneously and in opposite directions.
This means if one value increases, the other value decreases automatically and conversely.
So, how can this relationship be described mathematically?
The product of pressure and volume in the cabin is constant.
This relationship is also known as Boyle’s law.
It applies to our measurement set-up, namely when breathing in the cabin.
During inspiration, the volume of the lungs and, therefore, the thorax increases.
The body, shown here as lungs for simplicity's sake, occupies more space in the chamber.
As a result, the volume of surrounding air in the chamber decreases by the same amount
through which the volume of the thorax increases.
The surrounding air is compressed and cabin pressure increases.
During expiration, the lungs and, therefore, the thorax constrict again.
The space available to the surrounding air enlarges.
The air expands into this additional space and its volume increases by the same amount
through which the volume of the thorax decreases.
The pressure in the cabin sinks.
Through this interplay, the change in thoracic volume can be derived from the change in cabin
pressure: The product of volume and pressure before inspiration must remain constant and,
therefore, equal to the product of volume and pressure after inspiration.
The pressures of the surrounding air, which are used in this formula, are measured.
The volume of surrounding air before inspiration can be calculated as the volume of the cabin
minus the volume of the body.
The latter is estimated for the patient using their height and weight.
The only unknown value is the volume after inhalation, which can now be calculated.
The change in volume of the surrounding air, therefore, corresponds to the change in volume
of the thorax.
It’s termed displaced volume.
To avoid any confusion at this stage: The displaced volume isn’t identical to the
tidal volume but smaller.
It corresponds to the lung motion required to alter the lung volume so that a negative
pressure develops in the alveoli, leading to inspiration.
The displaced volume of air and the airflow can be depicted graphically during measurement.
As a reminder, airflow is recorded with the mouthpiece that occurs simultaneously with
cabin pressure measurement.
Airflow is plotted on the y-axis of the diagram, and the displaced volume on the x-axis.
Plotting is performed clockwise.
During inspiration, the displaced volume of air, as well as airflow, initially rise.
However, the more air flows into the lungs, the lower the negative pressure in the alveoli
generated for inspiration.
As a result, airflow decreases again during inspiration, finally dropping to zero.
Expiration occurs passively, because the inspiratory muscles relax and lung tissue starts to contract
again.
Through this reduction in volume, the pressure in the alveoli increases and air flows out
of the lungs.
The excess pressure in the alveoli decreases until it’s depleted.
In turn, airflow decreases again towards the end of expiration, finally dropping to zero.
Because of its shape, the diagram is also referred to as a loop.
It visualizes the work of breathing, as it indicates which change in volume produces
a specific airflow.
In healthy individuals, the loop is an almost straight line.
The gradient of the curve corresponds to the specific airway resistance, i.e., the bronchial
resistance that needs to be overcome by airflow during breathing.
The flatter the loop, the greater the airway resistance, in other words, the greater the
displaced volume needed for generating a specific airflow.
In clinical practice, a flat curve can indicate obstruction, namely narrowing of the bronchi.
However, the specific resistance depends on lung volume.
A lung that is twice as large also requires twice as much displaced volume to generate
the same airflow as a lung that’s half its size.
The specific resistance must, therefore, be related to lung volume, i.e. the total lung
capacity, to accurately determine an obstruction.
This results in a numerical value, the airway resistance, also termed resistance.
But how do we determine the lung volume required?
To calculate the lung volume, we need to know the displaced volume, as well as the relative
change in volume in the lung.
The latter can be determined using the pressure sensor in the mouthpiece: After the patient
has breathed normally for a while, occlusion pressure is measured.
Here, a valve in the mouthpiece closes the moment the patient inspires.
Now, the thorax expands but no air flows into the lungs.
The negative pressure in the alveoli responsible for inspiration remains and spreads to the
mouth.
Therefore, we can assume that the pressure measured in the mouthpiece corresponds to
the pressure in the alveoli.
Perhaps you’ve already noticed that when the valve is closed, the lungs are transformed
into a closed system.
So Boyle's law applies here, just like in the cabin: The product of volume and pressure
in the lungs remains constant during breathing when the valve is closed.
For the product of the two variables to remain constant, lung movement and alveolar pressure
need to change conversely.
For example, if the pressure in the lungs decreases by 1% through inspiration, lung
volume increases by 1%.
Occlusion pressure can, therefore, be measured to determine the relative change in volume
in the lungs during breathing.
Let’s assume that the displaced volume required for inspiration is 1% of the total volume.
By measuring the change in cabin pressure, we know how large the displaced volume is
in milliliters.
From this, we can derive which volume in milliliters corresponds to 100%, that is, the total lung
volume.
We can calculate its value using Boyle’s law, if we transform it a little: Lung volume
corresponds to the volume of surrounding air in the cabin multiplied by the quotient of
the pressure change in the cabin and the relative pressure change in the lungs measured at the
mouthpiece.
Using the determined lung volume, airway resistance can now be calculated from the ratio of the
displaced volume to airflow.
The ratio between the displaced volume and pressure in the mouthpiece yields the total
lung capacity.
At the end of body plethysmography, spirometry usually follows, which can be performed using
the same measuring instrument.
The patient is requested to maximally exhale and then maximally inhale, followed by forced
exhalation.
Spirometry provides the inspiratory vital capacity, forced vital capacity, and the FEV1.
Also, the PEF and FEFs are determined at different timepoints.
Ok, so that wraps it up on the body plethysmography procedure and values that can be measured.
With the quiz on the next slide, you can check if you’ve understood the underlying principle
of measurement.