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Lecture 11

BIOC34H3 Lecture Notes - Lecture 11: Pulmonary Function Testing, Restrictive Lung Disease, Functional Residual Capacity


Department
Biological Sciences
Course Code
BIOC34H3
Professor
Stephen Reid
Lecture
11

Page:
of 4
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Lecture 11: Pulmonary Function Tests and Alveolar Ventilation
1. Spirometry and Lung Volumes
A spirometer is a device used to measure lung volumes. The old-fashioned models consisted of a
drum of air upside down in water. A tube allows the subject to breathe in and out of the air drum.
As the person inspires, air is removed from the drum causing it to move downward thereby pulling
the pen upward and creating a positive deflection on the chart paper. When expiration occurs air is
blown into the drum causing it to move upward and the pen to move downward, giving a negative
deflection.
2. Lung Volumes and Capacities
The volume of a breath is referred to as tidal volume. The average value for tidal volume is
approximately 500 ml. At a normal breathing frequency of 12 breaths per minute, this translates
into 6 000 ml of air taken into the lungs every minute. However, not all of this air enters the alveoli
and is involved in gas exchange. A portion of the inspired air remains in the anatomical dead space
of the conducting zone, where no gas transfer occurs (see below).
The maximum amount of air exhaled following a maximal inspiration is called vital capacity.
However, no matter how hard we expire, there will always be approximately 1.2 L of air left in the
lungs that cannot be exhaled (residual volume). Standard spirometry techniques which can
measure breath volume can't be used to measure this residual volume of air - but there is another
technique, the inert gas technique, that can measure this. The residual volume, plus our expiratory
reserve volume, is termed our functional residual capacity. The entire air capacity of the
respiratory system, including the reserve volumes and residual volume, is referred to as total lung
capacity.
3. Pulmonary Function Tests
Lung volumes are measured by pulmonary function tests which can also be used to diagnose
various pathological conditions or lung disorders. Generally, these disorders can be classified as
either restrictive or obstructive. A restrictive pulmonary disease interferes with lung expansion,
and thus inspiration, and may be caused by damage to the chest wall, the lungs or the pleura.
Examples of restrictive pulmonary diseases include pulmonary edema, where the expansion of the
alveoli is inhibited by fluid in the lungs and pulmonary fibrosis.
The other type of lung diseases assessed using these pulmonary function tests are classified as
obstructive. These conditions hinder expiration. Since air does not leave the lungs efficiently, the
lungs over-inflate. Diseases that damage the alveoli, such as asthma or emphysema, are classic
examples of obstructive disorders.
When we perform a lung function test, we are often trying to measure forced vital capacity, by
breathing in as hard as one can, and then exhaling as hard as one can. This gives us then values for
both our maximal inspiration and our maximal exhalation. Forced vital capacity is determined by
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several factors, including the strength of the chest and abdominal muscles, airway resistance
(which can be altered by bronchitis or asthma), lung size (which can be altered by the size of the
person, or by diseases such as tuberculosis), and the elasticity of the lung tissues also plays a factor
(this can be altered by diseases, or simply by age).
A second variable that is measured by lung function tests is forced expiratory volume, which is
the proportion of forced vital capacity that can be exhaled in a given time, for example in one
second (what is termed FEV1). Generally within five or six seconds a person can breathe out their
entire vital capacity, so forced expiratory volume over five or six seconds (FEV5 or FEV6) would
approximate FVC.
However, the vast majority of air (about 80%) is breathed out within the first second - in a healthy
person. Under disease conditions, this proportion changes (lower FEV1/FVC ratio with obstructive
lung diseases and slightly higher ratio with restrictive) and this is why both forced vital capacity
and forced expiratory volume are important in these tests.
We can make these measurements using various spirometry techniques, from the old-fashioned
air-filled drum in water, or more modern calibrated electronic spirometers. The one thing that can't
be measured with these spirometry techniques is residual volume. However, this too can be
measured via a different process called the inert gas technique.
4. The Inert Gas Technique
When using the inert gas technique, the subject is connected to a spirometer containing 10%
helium in air. We know the volume of the spirometer (V1), and the concentration of the helium in
the air (C1). Helium is insoluble, and so it is not taken up in the blood - thus it will stay in the lungs,
and not diffuse throughout the body during the test. There is a valve in between the spirometer and
the subject, and the lung volume we will be measuring is the lung volume at the time the valve is
opened. When the valve is opened, the helium previously contained in the spirometer will be taken
into the lungs, and over time the concentration of helium in the spirometer and in the lungs will be
equal (since no helium is leaving the lungs and entering the blood). Thus, when we measure the
concentration of helium in the spirometer following equilibration it will be the same as the
concentration of He in the lungs (called C2). V2 is the volume of the lungs. Since we know the
volume of the spirometer, by using the equation:
C1V1 = C2 (V1 + V2)
Or by rearranging it as:
C2 = V1 (C1 - C2)/C2
We can find out what lung volume is - including the normally hidden residual volume, if we open
the valve immediately after maximal exhalation.
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5. Minute Ventilation and Alveolar Ventilation
Minute ventilation is the amount of air taken into the lungs in one minute. It can be calculated by
multiplying breathing frequency (that is, breaths per minute) by tidal volume (the amount of air
breathed in per breath, measured in ml or mL/kg). Typical values for breathing frequency are about
12 breaths per minute, and a typical tidal volume is about 500 ml - so the value for standard minute
ventilation is 6000 ml/minute.
However, not all of the air that we take in (tidal volume) is available for gas exchange as some of
the inspired air remains within the anatomical dead space that is the conducting zone. The
amount of air that actually reaches the alveoli, per unit of time, is termed the alveolar ventilation.
It is calculated by subtracting dead space ventilation from minute ventilation, or more technically,
VA = (VT X fR) - (DSV X fR)
Where VA is alveolar ventilation, VT is tidal volume, fR is breathing frequency, and DSV is the
volume of the dead space - approximately about 150 ml. Later we'll look at a way to calculate
alveolar ventilation by measuring CO2 levels in expired gas and the arterial blood. First, however,
we'll look at what changes in tidal volume and breathing frequency do to alveolar ventilation.
Remember how, in the cardiovascular system, increasing heart rate (in the absence of sympathetic
stimulation) would only increase cardiac output by so much, before the decrease in stroke volume
would cause cardiac output to decrease at very high heart rates. A similar situation occurs in the
respiratory system: one can only increase breathing frequency by so much before the decrease in
tidal volume causes a decrease in alveolar ventilation. It is much more efficient to increase tidal
volume than to increase breathing frequency, because it means a smaller proportion of each breath
is taken up in the anatomical dead space.
It is not easy to measure dead space volume directly, and this would seem to make it impossible to
measure alveolar ventilation. Since we know there is no gas exchange occurring in the dead space
we can use a number of assumptions and substitutions to calculated alveolar ventilation indirectly.
This approach involves measuring the levels of CO2 in the arterial blood and exhaled air.
First, we shall assume that there is no CO2 in the ambient air - in fact there is, about 0.03%, but it is
so close to nothing we will, for the sake of this calculation, assume it is zero - and then we may
assume that at the end of inspiration, when the conducting zone is full of outside air, that there is no
CO2 in the dead space. This means that all expired CO2 must come from the alveolar gas. We will
use this equation, which can tell us the volume of CO2 exhaled per unit of time:
VCO2 = VA X FCO2
FCO2 is the fractional concentration of CO2 in the alveolar gas, equivalent to %CO2/100: for
example, if 10% of the expired gas is CO2, then the value for FCO2 will be 0.1. This equation can be
rearranged as:
VA = VCO2 / FCO2