INTRODUCTION:
It is unusual for a specific lung function test to diagnose a disease. At
Jbest, a series of tests may place a lung disorder into one of several
categories and when other features such as history, physical
examination, radiology and pathology are added to the equation, a
possible diagnosis is considered. Most requests for tests will include
£ provisional diagnosis. When the most appropriate tests have been
Selected, further hurdles must be overcome before any reliable
results can be obtained. The patient must be able to co-operate fully
Ind there is no substitute for an experienced, sympathetic but firm
technician to ensure that maximum performance has been achieved.
I rushed estimate of total lung capacity (TLC) in a claustrophobic
|nd panicking, body-box-bound patient is no use to anyone. The
Technician must have the right to question the advisability of a test
|n a particular day as the patient may have become too ill to cooperate
or be recovering from fractured ribs or even recent surgery,
finally, it is essential that the equipment used is serviced regularly
id reliably calibrated.
^The main uses of lung function testing are:
j) To help define more clearly the type of functional disorder.
To measure serially natural progression (or regression with
therapy) of the disorder.
* To decide on the feasibility of thoracic surgery.
: To assess the degree of respiratory failure.
|If the brain is functioning normally, a breath begins with
gntraction of inspiratory muscles enlarging the thorax, lowering
ga-thoracic and pleural pressures, enlarging the alveoli and
expanding alveolar gas so reducing its pressure below
nospheric. Air at atmospheric pressure must flow into the thoraxwhere it is conducted to, and diffuses out into, the alveoli.
At any one moment, approximately iooml of desaturated blood, with a strong affinity for oxygen, is spread over an area of 70 square metres (area of pulmonary capillary bed) separated from air by a membrane 0.2.x thick. Oxygen from alveolar air diffuses rapidly across the alveolar-capillary membrane and is finally chemically combined with haemoglobin molecules within the circulating red blood cells. Carbon dioxide diffuses in the opposite direction and is eliminated in expired gas.
Appreciation of these steps in respiration is important in helping to understand how a specific lung function test may pinpoint a particular malfunction in the respiratory system.
Three broad categories of testing will be considered:
1. Tests of ventilatory function.
2. Tests of gas exchange.
3. Exercise testing.
TESTS OF VENTILATORY FUNCTION
These measure the ‘bellows’ action of the thoracic cage and lungs, detecting any abnormal interference in the free flow of air from atmosphere to alveoli and back. Figure shows how the total lung capacity (TLC) is "broken down into its various volumes.
Their size and relationship to each other give clues ta underlying functional disorder. How normal a
volume is will depend On what we predict it should be for that person’s height, sex and age. If the value obtained is more than 1.7 standard deviations from the predicted it is very likely to be abnormal.
During normal resting ventilation air moves gently back and forth in a tidal manner and the total volume of each breath is called the tidal volume (VT). The largest volume that can be inspired from the
endpoint of tidal expiration is called the inspiratory capacity (IC) and the largest volume that can be expired after full inspiration is called the vital capacity (VC). The lungs cannot collapse to an airless state because of the compliance of the thoracic cage and there is always some residue of gas, the residual volume (RV). At the end of a tidal expiration much more air remains in the lungs and this is called the functional residual capacity (FRC). This is the neutral position of the respiratory system, at which point the inward recoil of the lungs is exactly balanced by the outward recoil of the chest wall.
All these volumes, apart from RV and FRC can be measured by a spirometer.
We can see from Figure 3/1 that the total lung capacity
(TLC) can be derived by adding IC and FRC.
There are three methods of measuring TLC:
(1) HELIUM DILUTION METHOD
The patient breathes into a closed circuit spirometer containing a mixture of gas with a known concentration of helium. The patient is switched into the system at the end of a normal expiration and this mixture gradually equilibrates with the gas in the lungs. Oxygen is added and carbon dioxide is absorbed, maintaining a stable FRC.
When the helium concentration has stopped falling (having been diluted by the lung volume) the patient takes a full inspiration and this IC is added to the FRC (derived from the equation below) to - give the TLC.
. SV x Hex = He2 x (SV + FRC)
l Hex = helium concentration before equilibration
t He2 = helium concentration after equilibration
r SV = spirometer volume
Equilibration between spirometer and lungs is achieved within 5-10 ^minutes in normal subjects, but in severe airways obstruction mixing M much slower and it may make it more difficult to estimate the
point at which equilibration has been achieved.
(2) WHOLE BODY PLETHYSMOGRAPHY
The patient sits in an airtight box and breathes the air contained within; the flask shape represents alveoli and conducting ? airways and Vr the unknown volume. The pointers in circles \ represent pressure gauges - one measuring box pressure (which after ] calibration gives the alveolar volume) - and the other, airway J pressure (which equals alveolar pressure when there is no airflow). In (A), at end expiration, alveolar pressure equals atmospheric I pressure and Vr is unknown.
The subject then attempts to inspire against an occluded airway j (B) which results in a reduced intrathoracic pressure (AP) and a rise \ in the pressure of gas within the plethysmograph. This latter 1
pressure change is proportional to change in intrathoracic volume j (A V) which can be estimated. Knowing PI5 P2 (i.e. + AP) and AV we can derive Vr (initial thoracic gas volume or TGV) using]
Boyle’s Law (assuming a constant temperature):
Pi X V, = P2 X V2
When the shutter opens the patient inspires fully and this volume is] added to the value of VT to give the total lung capacity (TLC).
Body plethysmography will .measure all gas within the thorax which may include emphysematous bullae, hiatus hernia or pneumothorax. When there is uneven ventilation distribution with generalised airflow obstruction or a large poorly communicating bulla, the TLC estimate by helium dilution may be much less than the plethysmographic estimate; the discrepancy being greater the more severe the disorder.
(3) RADIOLOGICAL
Good postero-anterior and lateral radiographs, at full inspiration, are divided into a series of elliptical slices and the volume of each is calculated. The heart volume is subtracted and the tissue and blood
volume are taken into consideration. This is not a widely used technique.
Tests of forced expiration
One of the simplest tests of forced expiration was the Snider match test in which the patient attempted to blow out a lighted match held 15cm from the mouth and, if unable to, was considered to have serious impairment of ventilatory function. All variations on this theme have in common the same manoeuvre, that is, a deep full inspiration followed by expulsion of air from the lungs with the maximum force possible.
(a) PEAK EXPIRATORY FLOW RATE (PEFR)
The peak flow sustained over a 10msec period at the beginning of a forced expiratory manoeuvre can be measured using a Wright’s peak flow meter (or the inexpensive mini-Wright meter). From a position
of full inspiration, air is forcibly expired across a pivoted vane (Wright’s) or a lightweight piston (mini-Wright) both of which are ] spring-loaded and encased. The displacement of the vane or piston is
; proportional to maximum flow rate. 1 This is a highly effort-dependent test but in practice it is very
reproducible. It is one piece of equipment which should be available not only to the hospital doctor, nurse and physiotherapist but also to the general practitioner and, of course, the patient. It is one of the
simplest ways of measuring lung function. In spite of this, many general medical wards do not have such a meter though they would find it inexcusable to be without a sphygmomanometer.
(B) FORCED EXPIRATORY VOLUME IN ONE SECOND (FEVi)
| This can be obtained by measuring change in expired volume against started from full inspiration (i.e. TLC) flow rises rapidly to peak value and then, because of progressive airway narrowing due to| combination of (1) high pleural pressure compressing airways (2) lowering of lung volume with reduction in elastic recoil of lung there is a rapid fall off of flow rate to zero when residual volume^
reached and no more air can be expelled. In normal people the usually takes about 4 seconds and the full volume expired is forced vital capacity (FVC). The volume expired in the first second the manoeuvre is the FEVX and this is usually at least 75 per cent the FVC.
The FEV and PEFR are well correlated, but the FEVX measure average flow rate over a larger lung volume than the PEE| Both FEVj and PEFR are the most widely used and reproduce measures of forced expiration.
(c) FLOW-VOLUME LOOPS
If, instead of plotting the FVC manoeuvre as volume against we plot maximum flow rate against changing lung volume, it possible to record the maximum expiratory flow volume (ME! curve. If this is followed immediately by a full forced inspiration TLC we can record the maximum inspiratory flow volume (Mil curve, which completes the flow volume loop.
It enables us to record maximum flow rates not only at large lung volumes near TLC, but also at small volumes near RV at which point flow through smaller airways will predominate. The shape of the MIFV curve is quite different as there is no sudden flow limitation on inspiration in normal people and peak flow occurs about midway up to TLC.
Airway resistance
The major site of resistance to airflow during normal breathing is in the larger central airways. Measurement of airways resistance is more sensitive to changes in larger airways and relatively insensitive to changes in smaller airways. It is possible, therefore, for there to be extensive disease present in the peripheral airways before a significant increase in total airways resistance occurs. To measure resistance we need to know the pressure difference between mouth and alveoli (the driving pressure) and the simultaneous flow rate.
Airways resistance = driving pressure/flow rate
or
= mouth press. — alveolar press/flow rate
Flow rate and mouth pressure are measured relatively easily but an indirect estimate of alveolar pressure has to be made using a body plethysmograph. One advantage of this method is that an estimate of thoracic gas volume can be made at the same time. As airways resistance varies with lung volume (airway calibre being greater at larger lung volume) it is useful to know die lung volume at which resistance is measured. Specific airways resistance (SRAW) is the product of RAW and the lung volume at which it was measured and corrects for the effect of the latter. The reciprocal of RAW is conductance (GAW) which has a more linear relationship to lung volume and is sometimes used in preference in the form of specific airways conductance (SGAW). Measurement of airways resistance is a more sensitive test of airway calibre than FEVX but is less reproducible. It also avoids the need for a full inspiration or forced expiration which can in itself cause bronchoconstriction in asthmatics and, therefore, is preferred by some workers when assessing dose response studies using bronchodilators or bronchoconstrictors.
Compliance
Compliance is a measure of elasticity or, in the case of the lung, distensibility or stiffness. The stiffer the lungs, the greater the external force (pressure) required to produce a given increase in lung volume. Compliance is measured by plotting changes in intraoesophageal pressure (which is an approximation of intrapleural pressure) against changes in lung volume. This pressure is obtained by positioning a thin-walled balloon in the lower third of the oesophagus connected by a catheter to a pressure transducer. From the FRC position the patient inspires a measured volume at which the breath is held while transpulmonary pressure (mouth pressure oesophageal pressure) is measured. Several measurements are made between TLC and FRC and a pressure-volume curve constructed, the slope of which gives compliance. The lower the value of compliance the stiffer are the lungs. As with RAW, compliance is related to lung volume. Specific compliance is the ratio, compliance/FRC and is independent of age and sex.
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