Lung Function tests Flashcards
(8 cards)
What is FRC?
Functional Residual Capacity (FRC) refers to the amount of air remaining in the lungs after a normal, passive exhalation. It’s the volume at the end of a normal breath and represents the point where the lungs’ elastic recoil and chest wall expansion are balanced. In healthy adults, FRC is typically around 3 liters.
The volume of the FRC must be made by indirect measurement since it is not
possible to exhale all the air from the lungs
ADVANTAGES / DISADVANTAGES OF
DIFFERENT METHODS USED TO MEASURE
FRC
Helium Dilution
Advantages
Relatively inexpensive.
Small by comparison with body plethysmograph.
Negligible resistance during the test.
Disadvantages
Requires a 10 litre spirometer therefore quite a bulky piece of equipment.
May under estimate the FRC in chronic obstructive airways disease.
Nitrogen washout
Advantages
Relatively inexpensive.
Smallest and most portable as it does not incorporate a 10 litre spirometer.
Leaks during the FRC measurement are easily and quickly identified.
Nitrogen analysis may be used for other tests such as closing volume and
measurements of intrapulmonary gas mixing.
Disadvantages
Higher breathing resistance than He dilution (although still within BTS
guidelines).
May under estimate the FRC in chronic obstructive airways disease.
Plethysmography
Advantages
Measure the FRC with a greater degree of accuracy in chronic obstructive
airways disease.
Measurements obtained within a shorter time than N2 washout and He
dilution in patients with obstructive lung disease.
In uncooperative patients, useful measurements such as airways resistance
and conductance may be made using the plethysmograph.
Disadvantages
Often expensive.
Not portable.
Some patients refuse to be shut inside the plethysmograph.
May be affected by external pressure changes eg slamming doors.
DISTURBANCES OF THE ALVEOLAR-CAPILLARY GAS TRANSPORT
During exercise the effective surface area of the alveolar-capillary membrane
increases, shown as a rise in the transfer-coefficient. An increase in pulmonary
blood flow as the result of abnormality in the circulation (eg Fallot’s tetralogy) is
also associated with a rise in transfer coefficient.
Where pathology occurs in the alveolar-capillary membrane (inflammation,
fibrosis, oedema, embolism) the gas transfer properties are reduced and
distributed unequally. The transfer coefficient is then abnormally low.
Where there is a local loss of function of lung tissue (atelactasis tumors,
inflammation, resection) the effective alveolar surface area is abnormally small,
whereas transfer in the remaining normal alveoli may not be disturbed. The latter
compensate for the loss of function of the diseased alveoli. The transfer
coefficient is usually normal.
Where there is an obstruction in the pulmonary circulation, eg through stenosis of
the mitral valve, the blood volume per alveolus increases; filling of the capillaries
is greater and so closed capillaries open. This is associated with a decrease in
pulmonary blood flow and thus an abnormally low transfer factor and transfer
coefficient. Where there is long standing obstruction the gas transfer disturbance
is exacerbated by parenchymal abnormalities of the membrane
Why perform bronchial provocation challenges?
The functional consequences of an alteration in bronchial tone depends on the
site at which change takes place. Bronchoconstriction of the large airways is
usually the result of neural stimulation and mainly causes changes in flow
resistance in the airways. Bronchoconstriction of the small airways, down to
alveolar level, is due to humoral factors (eg histamine, prostaglandins, oxygen,
carbon dioxide; the predominant changes here are in lung volume, and
compliance.)
The clinical signs of asthma are wheeze, abnormal breathlessness and possibly
cough. The symptoms are associated with changes in lung function. The
symptoms may occur first at night on account of the circadian variation in airway
diameter. Atopic asthma often occurs in subjects who are allergic to one or more
allergens such as house dust mite, cat, dog, pollen etc. Asthma caused by
allergen is usually referred to as extrinsic asthma. Asthma occurring in non-
atopic subjects and for which no allergen can be identified is called intrinsic or
non-atopic asthma.
If lung function and skin tests detect no abnormality at the time of examination
then a provocation test for bronchial reactivity may assist the diagnosis of
asthma. This may take the form of an exercise test to induce bronchoconstriction,
or by inhaling a bronchoconstrictor substance such as histamine in increasing
doses.
METHACHOLINE CHALLENGE
The subject inhales aerosols of methacholine in increasing doses in order to
provoke a fall in FEV1. The doses are determined by the apparatus, method and
duration of exposure of the subject. Various methods may be used such as
single inhalation of each dose via a dosimeter or by continuous breathing from a
wrights nebuliser. The solutions contain histamine acid phosphate in the
following concentrations: 0.03, 0.06, 0.125, 0.25, 0.5, 1.0, 2.0, 4.0, and 8.0
mg/ml.
Following a baseline measurement of FEV1 the subject inhales nebulised
phosphate buffered saline (PBS). This is the solution in which the histamine
powder is diluted. It is important to check that the subject does not show a drop
in FEV1 caused by PBS alone.
The subject then inhales the starting dose of histamine and repeats the FEV1. If
there is no change then the subject continues by inhaling the next dose of
histamine
followed by a repeat FEV1 . This procedure continues until the FEV1 drops by
20%
or more from the baseline Phosphate Buffered Saline FEV1, or the highest
concentration of histamine (8.0 mg/ml) is reached. Depending on the concentration at
which the subject reacts the histamine reactivity may be graded as mild, moderate, or
severe.
BRONCHIAL CHALLENGE
A wide range of substances used in industry have been found to cause airways
obstruction such as Di-isocyanates, fluxes, resins, constituents of printers ink, grain
spores, vinyl chloride vapor, some wood dusts etc. Patients often experience cough,
chest tightness, breathlessness and a reduction in ventilation of the obstructive type
occurring progressively throughout the working day.
If a patient is thought to have become sensitised to the material that they work with then
an industrial bronchial challenge should be performed. The patient is admitted to
hospital for a minimum of three days.
Day 1) The patient performs baseline routine lung function measurements at
9.00 am. Serial measurements of Vital Capacity, FEV1, and Transfer
Factor are made throughout the day to allow for diurnal variation in
measurements.
Day 2) The patient is exposed to a placebo visibly similar to that which may
cause the respiratory impairment in a challenge chamber. Serial
measurements of Vital Capacity, FEV1, and Transfer Factor are made at
regular intervals during the first hour following exposure and at hourly
intervals throughout the rest of the day. No response should be detected
to placebo.
Day 3) The patient is exposed to the substance that is thought to cause
respiratory impairment. Serial measurements of Vital Capacity, FEV1, and
Transfer Factor are made at regular intervals during the first hour
following exposure and at hourly intervals throughout the rest of the
day. If the patient is sensitised then a drop of at least 10 - 15% will be
observed in lung function.
Day 4) If day 3 produces little or no change in lung function then it may be
possible to increase the dose or exposure time in an attempt to
demonstrate respiratory impairment.
Immediate reaction to an allergen is known as atopy. Subjects who suffer from it are
described as atopic. Atopy is defined as the tendancy to make an immunoglobulin E
(IgE) type immune response to common environmental antigens
What is happening in the lungs when a subject is atopic? The first time the allergen is
inhaled it enters the body through the surface layer of the airways. There it is
encountered by cells which recognise it as foreign. Specific antibodies are formed. They
are a type specifically produced by the atopic subject and called IgE (immunoglobulin,
class E). Very little of this antibody is released in to the circulation. Most of the antibody
clings to the surface of rather specialised cells known as mast cells. These cells are
amongst those that migrate around the body. When they settle down they tend to
choose the lining tissues of the bronchi and skin.
The next time the subject inhales the allergen it meets the mast cells coated by specific
IgE antibody. Antibody and allergen combine together. The union is disruptive to the
mast cell. Its cell wall breaks down and from the interior of the cell, granules are
released into the surrounding tissue. These granules contain a complex array of
chemical substances, which have a wide variety of effects. Enough can be understood
about the development of asthma under these circumstances by considering just one:
histamine. This substance has at least two actions relevant to the production of asthma.
It causes fluid to leak out of small blood vessels into the loose connective tissues lying
beneath the surface of the bronchi. The tissue swells and so the bronchi are narrowed.
The histamine also stimulates the bronchial smooth muscle causing it to contract which
also narrows the airway.
Exposure to an allergen will produce wheezing within ten minutes. Once the allergen is
removed the wheezing will normally disappear within an hour
What is skin testing
Skin Testing
Atopy is often reflected in a special sensitivity of the skin. Atopic skin sensitivity is
demonstrable to certain allergens commonly found in the environment such as animal
hair, pollen, house dust moulds etc. Extracts or solutions can be prepared from these
materials. A drop of the extract is placed on the skin of an atopic person. The skin is
then pricked to allow a minute amount of fluid into the skin. If the subject is sensitive to
an allergen it will be displayed within 15 minutes. The skin will become itchy and red.
Finally a small weal, usually half a centimeter in diameter, will appear. A true positive
skin prick test reaction (a weal 3 mm greater in diameter than the negative control)
indicates that specific IgE is fixed to mast cells in the skin and has led to a vasoactive
response due to release of histamine.
Contraindicated in widespread eczema as can cause false positives and exacerbate eczema
