Dr.Jeena Aslam BHMS,MD(Hom)
The patient and the doctor are equally curious to get at the exact diagnosis. When the doctor asks for some tests to be done, the patient is in hope that when all the tests are over, the diagnosis will be clear. But in medicine, most of the diagnosis comes from a carefully taken history, a little of it from physical examination, and even less from investigation and still a substantial part of the diagnosis remains a secret for ever unless the patient has surgery or God forbids, an autopsy. The poor diagnostic value of investigation is probably best illustrated by pulmonary function test. It cannot give an accurate diagnosis because same degree of defect in ventilation, perfusion and diffusion may occur in wide variety of diseases. As in the case of other diseases, in lung disease also, most of the diagnosis comes from history and physical examination. Among the investigation, the most useful is X-ray chest which is not a PFT, but only a window to the structure of the lung.
It helps in giving functional nature of disease and its severity
1. It gives an idea of the progress of disease
2. Helps in assessing the efficacy of treatment by telling whether the patient is better or worse than before and to what extent
3. Helps the anesthetist whether the degree of lung function impairment is compatible with safe anaesthesia in drug surgeory.
4. In deciding the amount of compensation in cases where impairment of lung function might have been produced by an occupational hazard e.g. working in a coal mine.
Thus PFT have several uses, but only a limited value in diagnosis.
Pulmonary volumes and capacities:-
The goal of respiration is to provide O2 to the tissues and to remove CO2. To achieve this goal, respiration can be divided into four major functional events.
1. Pulmonary ventilation which means the inflow and outflow of air between the atmosphere and the lung alveoli
2. Diffusion of O2 and CO2 between the alveoli and the blood.
3. Transport of O and CO in the blood and body fluids to and from the cells
4. Regulation of ventilation and other facets of respiration.
The process of studying pulmonary ventilation by recording volume movement of air into and out of the lung is called spirometry. Volumes are basic entities, while capacities are derived from volumes. Each capacity is a sum of 2 or more volumes. If you try to make a mental picture, it would be easier for you to understand.
It consists of a drum inverted over a chamber of water, with a drum counterbalanced by a weight. In the drum is a breathing gas, usually air or O2. A tube connects the mouth with the gas chamber. When one breathes in and out of the chamber the drum raises and falls and an appropriate recording is made on a moving sheet of paper.
Mainly there are four pulmonary volumes which when added together equals the maximum volume to which the lungs can be expanded.
1. Tidal volumes: – It is the volume of air inspired or expired in one breath. The volume expired slightly less than that of inspired. In a strict sense tidal volume is the volume of air expired. There is normally considerable variation in the tidal volume. Therefore a more reliable estimate of tidal volume is obtained from the average of few breaths. It amounts to 500 ml.
2. The inspiratory reserve volume: – It is the extra volume of air that can be inspired over and above normal tidal volume. Inspiratory muscles have to be used to their maximum capacity to inhale IRV. It is usually equals to about 3000ml.
3. Expiratory reserve volume: – It is the extra amount of air that can be expired by forceful expiration after the end of a normal tidal expiration. Expiratory muscles have to be used to their maximum capacity to expel the ERV. This normally amounts to about 1100 milliliters.
4. Residual volume: – It is the volume of air remaining in the lungs at the end of a maximum expiration which you can easily imagine. This averages about 1200 ml.
Pulmonary capacities are
1. Inspiratory capacity: – It is the maximum volume of air that can be inspired following a normal expiration. It is about 3500ml. IC= TV+ IRV
2. Functional residual capacity: – It is the volume of air remains in the lung after normal end expiration.
FRC= ERV+RV. It amounts to 2300ml.
3. Vital capacity:- It is the volume of air forcefully expired after a deep inspiration.
VC=IRV+TV+ERV It amounts to 4600ml.
4. Total lung capacity: – It is the maximum volume to which lung can be expanded with the greatest possible inspiratory effort. TLC=IRV+TV+ERV+RV
All pulmonary volumes and capacities are about 20%-25% less in woman than men.
Respiratory minute volume: –
It is the amount of air inspired or expired by the lung in one minute.
MV=TV× Respiratory rate/ minute.
At rest a normal male adult inspired and expired about 12 times/mt. The amount of air inspired= 500×12=6litre. This is called respiratory minute volume or pulmonary ventilation. In exercise it may go upto 200 liters.
Maximum Voluntary Ventilation (MVV):-
It is the maximum amount of air that can be expired by the lung in one minute. This is a combined indicator of compliance and airway resistance. To determine MVV the subject is asked to breathe as fast and as deeply as possible for 15 sec. The total volume expired is 15sec ×4 to get MVV.
MVV-MV= Breathing reserve.
Peak expiratory flow rate (PEFR):-
It is the maximum rate that can be sustained during first 10millisecond of a sudden forced expiration after a full inspiration. PEFR depends on the height and surface area of the individual. It is measured by using Wright’s peak flow meter. This is an easy and convenient method to assess airway obstruction.
Maximum mid- expiratory flow rate (MMFR):-
This is the velocity of air expressed as liters per second during the middle third of the total expired volume. It is also denoted as forced expiratory flow. Average value lies between 1.5 and 5.5 liters/second in men. Determination of MMFR helps to detect borderline cases of airway obstruction.
The elastic property of lung is expressed in terms of pulmonary compliance. It is the distensibility of lung per unit charge in intrapleural pressure. Normal pulmonary compliance is 0.2litre per cm of water.
Liquids or gas flowing through tubular structures have to overcome resistance due to friction between the molecules of the substance flowing and between substance flowing and wall of the tube.
Resistance to air flow offered by air passages depends upon several factors like caliber of the passage, driving pressure, rate of flow, type of flow, density of gas, and its viscosity. Its value is increased in obstructive pulmonary disease
Pulmonary function test:-
1. Test of Ventilation:-
Include spirometry which gives some idea of the ventilatory function and a few specific measurements
- I. Timed vital capacity
- II. Maximum expiratory flow rate
- III. Maximum Voluntary ventilation
- IV. Flow-volume curve
- V. Measuring the inequality of ventilation
- VI. Functional residual capacity
- VII. Determination of dead space
Aim of the test is to distinguish between restrictive and obstructive diseases. Restrictive diseases are characterized by reduced compliance which may be because of reduced distensibility of lungs or a mechanical obstacle to expansion of lung. Obstructive diseases are characterized by increased airway resistance which may be due to bronchospasm, secretions or a tumor within the airways or pressing them from outside
1. Timed vital capacity (Forced expiratory volume):-
It is the volume of air expelled in the first one second of a forcible expiration following a full inspiration is called FEV1. FEV1 is normally more than 80%of the VC. Airway obstruction is indicated by FEV1 below 70% of normal.
2. Maximum mid expiratory flow rate
3. Maximum voluntary ventilation
4 .Flow volume curve
The rate of flow during breathing can be monitored using a pneumatograph. This flow rate is plotted against lung volume to get flow- volume curve. The curve is altered in obstructive and restrictive lung disease. Due to technical problems these curves are not routinely plotted in clinical laboratories.
Measuring the inequality of ventilation:-
To assess whether the ventilation in different parts of lung is uniform.
1. Radioactive xenon method:-
Following inhalation of radioactive xenon radioactivity is measured all over the chest. A uniformly graded distribution of radioactivity is a rough indicator of uniform ventilation. Ventilation is normally greater at the bases than at the apices of the lung.
2 .Breath nitrogen test- single breath technique:-
The patient is asked to take a single maximal inspiration or pure O2 and then to breath out maximally at a slow and steady pace. N2 concentration is measured continuously in the expired air. Normally the N2 concentration is initially zero till most of the dead space air is expired. Then N2 concentration rises steeply. This phase corresponds to the expulsion of a mixture of dead space air and alveolar air. Finally the N2 concentration reaches a plateau which corresponds to the expulsion of alveolar air. Uneven ventilation may be seen in both restrictive and obstructive diseases of the lung.
Functional residual capacity:-
It cannot be measured by spirometry. It may be measured by either Helium dilution method or the N2 washout method.
1. Helium dilution method: –
A known amount of He is added to a bag or a spirometer, the amount added being such as to achieve a He concentration of 10%. The subject rebreathes in the closed system till the concentration of He in the lung and the bag becomes equal. The concentration is measured at the end of a normal expiration. Since no He was present in the lungs to start with the same amount of He which was added to the bag get distributed between the bag and the lung. This amount and new concentration being known, the volume of the bag and lungs combined can be calculated. If we subtract the volume of bag from combined volume we get the volume of lungs at the end of expiration which is FRC
2. Nitrogen washout method:-
This method is based on the knowledge that alveolar N concentration is normally about 80%.Starting with normally end expiratory position the subject breathes 100% O for a few minutes to washout the N In a normal adult about 2minute of breathing N free air is enough to wash out the entire N. The expired air is collected in a big spirometer. The volume of air collected in the spirometer is noted and its N concentration.
The region of respiratory tract which do not participate in gas exchange, are normally constituted by the airways, anatomical dead space and in some diseases poorly perfused alveoli , the physiological dead space.
The most popular method of determination of anatomical dead space involves N analysis in the expired air following a single deep breath of 100% O2. The subject is asked to take a deep breath and then breathe out slowly, at a steady rate as much as he can. The expired air is connected to a flow meter and N2 analysis so that the flow rate and continuous N2 analysis are both available. Physiological dead space may be determined from measurement of Tidal volume and CO2 concentration in alveolar air and mixed expired air.
Tests of diffusion:-
The diffusion is the net movement of fluid from one compartment to another due to concentration difference. The rate of diffusion is directly proportional to pressure gradient and also area available for diagnosis and solubility of gas concerned (s). It is inversely proportional to diffusion distance (d) and square root of molecular weight of the gas. In this the characteristics of gas which affect rate of diffusion are S and Mw. S/Mw is called diffusion coefficient. If we consider diffusion coefficient of O2 to be 1, the relative diffusion coefficient of other gases gives an idea of how much slower or faster than O2 those gas will diffuse.
Tests of end result respiration:-
The ultimate purpose of respiration is to supply O2 to the tissues and to get rid of the CO2 produced in the tissues.
1 .Arterial Po2:-
Arterial Po2 may be measured with an O2 electrode. The O2 electrode works on the principle that the currant flowing through the platinum electrode immersed in a buffer solution is proportional to the Po2.The normal arterial Po2 is about 100mm of Hg. A low arterial Po2 is physiological in high altitude. Pathological causes of low arterial Po2 include (1) Hypoventilation (2) Diffusion defect (3) arteriovenous admixture.
2. Arterial Pco2:-
Arterial Pco2 may be measured with a co2 electrode. Co2 alters the PH of a buffer surrounding a glass electrode. The electrode is sensitive to changes in the PH which are translated in terms of Pco2 by appropriate calibration. The normal arterial Pco2 is about 40mm of Hg. Arterial Pco2 is raised in hypoventilation, but in diffusion defect and A-V Pco2 is normal.
3. Arterial PH:-
Arterial PH is measured with a PH meter using glass electrode. Rise in Pco2 tends to lower PH of the blood.
Tests during exercise:-
Lungs have enormous physiological reserve. Hence many function tested to be quiet normal at rest in lung diseases. But during exercise the reserve is encroached upon, thereby unmasking abnormality. The exercise is performed on a treadmill. It should be performed for ventilatory function, diffusion and overall respiratory function. The tests during exercise may be combined with the exercise stress test for coronary insufficiency. The combined testing is not only more convenient and economical but also gives a more comprehensive evaluation of the patient.
PFT has only a very limited value in diagnosis. It is better to restrict the tests to the minimum. The most valuable tests are timed vital capacity, arterial Po2, and co2 tension