Lung function Tests
Assessment of lung function
values obtained from spirometry
Many spirometry measurements e.g. FEV 1, FVC are stated as a percentage of predicted values that are determined from many normal individuals grouped on the basis of sex, age, and height. The range of normal is 80%-120% of the predicted value.1 . Tidal volume (V T ) - volume of air in one breath during normal quiet breathing. The portion of the V T that participates in gas exchange is the alveolar volume (V A ); the remainder, about 30% of the V T, is "wasted" or "dead space."
2. VC vital capacity - maximal volume of air that can be expelled from the lungs following a maximal inspiration. Decreases progressively with restrictive lung disease.
3. Forced vital capacity (FVC) is the same as VC, except that the exhalation is performed as rapidly and forcefully as possible, which causes the airways to narrow, slowing the rate of expiration.
4. Forced expiratory volume in 1 second (FEV 1 ) - volume of air forcefully expired during the first second after a deep breath; ie the portion of the FVC exhaled in 1 second. Mainly reflects the status of large airways.
5. FEV 1 /FVC % is the ratio of the FEV 1 to FVC, expressed as a percentage (normal > 75%). The FEV 1 /FVC ratio is effort-dependent; ie it increases with increasing expiratory effort. The FEV 1 /FVC ratio is particularly useful in evaluating obstructive disorders, but is also helpful in the evaluation of restrictive disorders. If only the FEV 1 is low (FEV 1 /FVC ratio < 70%), it suggests obstruction; if both the FEVl 1 and FVC are low, (but therefore giving a normal ratio
(FEV 1 /FVC ratio > 75%)), it suggests restriction.
Values calculated indirectly
Obtaining the following values requires the use of spirometry and helium dilution techniques (Which measure the volume of gas in the lungs by dilution of helium, an inert gas. The volume obtained depends at which point the patient is connected into the system i.e. RV, FRC or TLC)1. Total lung capacity (TLC) - volume of air in the lungs after a maximal inspiratory effort.
2. Functional residual capacity (FRC) is the volume of air remaining in the lungs at the end of a normal expiration. The FRC reflects the resting position of the lungs and chest wall; it is the lung volume at which the inward recoil of the lungs is balanced by the outward recoil of the chest wall. The FRC has two components:
a. The expiratory reserve volume (ERV) is the amount of the FRC that can be expelled by a maximal expiratory effort.
b. The residual volume (RV) is the volume of air remaining in the lungs after a maximal expiratory effort.
3. Lung volume relationships
TLC = VC + RV
RV = FRC - ERV
Other Tests
1. Gas Transfer Factor. Total lung transfer of carbon monoxide (TL CO ). ( Diffusing capacity)The TL CO indicates the adequacy of the alveolar-capillary membrane. It is determined by measuring the amount of carbon monoxide (CO) transferred from the alveolar gas to the pulmonary capillary blood after the patient inhales a known amount of CO (0.3 %); it is expressed in ml/min/mm Hg.
The TL CO has a number of uses. It helps in distinguishing between asthma, chronic bronchitis, and emphysema and can indicate the severity of emphysema. It is useful in monitoring in sarcoidosis and in interstitial lung disease, showing progression of disease. A low TL CO indicates alveolar damage.
| Disorders that decrease TL CO Emphysema Interstitial fibrosis Multiple pulmonary emboli Pulmonary oedema Sarcoidosis Pulmonary resection Anaemia (due to reduced binding of CO by haemoglobin) |
Disorders that increase TL CO Pulmonary haemorrhage (due to uptake by intra alveolar RBCs) Intracardiac left-to-right shunt Vascular congestion, but only prior to oedema Polycythaemia rubra vera |
2. Arterial Blood Gases
3. Arterial Saturation (SaO 2 )
The % of oxyhaemoglobin in relation to total Hb in arterial blood is normally between 95 and 98%. Measured by pulse oximeter. (finger probe) Excellent non invasive indicator of hypoxaemia. Sensitive test of alveolar block if exercise saturation falls.
4. End-expired CO 2 ( PECO2 )
Approximation of alveolar and therefore arterial CO 2. (Ins < 0. 5% Exp 5%) Measured by capnograph. Non invasive indicator of hypoventilation.
5. Compliance (Lung Elasticity)
Relevant in emphysema and restrictive disease
6. Ventialation-Perfusion Tests
Physiological dead space
Alveolar-arterial PO 2 gradient
Intrapulmonary shunt
By measuring PO 2 and PCO 2 in the arterial and mixed venous blood and expired gases the above V-Q indices can be derived
7. RA Factor
Rheumatoid factors - antibodies directed against IgG
8. RAST Test (radioallergosorbent test)
Measurable allergen-specific antibodies. Used to study certain respiratory and food allergies.
Lung Disease
Obstructive and restrictive
Obstructive Lung Disease
1. Asthma2. Chronic Bronchitis
3. Emphysema
or 4. a combination of these
COPD Chronic obstructive pulmonary disease is a general term which may refer to pure emphysema, pure bronchitis, or a mixture of the two (excludes asthma)
1. Asthma: Characterised by reversible airway constriction resulting from a combination of airway hyperactivity, mucous secretion and airway oedema. Known precipitants of hyperactivity include exercise, cold air, infection, drugs, and occupational exposure. Car exhaust pollution may be another precipitant.
2. Chronic Bronchitis: Characterised by excessive mucous with narrowing of airways.
3. Emphysema: - involves the destruction of alveoli and support structures, leading to loss of normal elastic recoil of lungs and subsequent premature airway closure (collapse) at higher than normal lung volumes during exhalation.
Pattern of Impairment
a. Flow rates. A reduced FEV 1 /FVC ratio (less than 70%) is the indicator of obstructive airway disease. However, the FEV 1 /FVC may be normal even with considerable peripheral airway obstruction.
a. Flow rates. A reduced FEV 1 /FVC ratio (less than 70%) is the indicator of obstructive airway disease. However, the FEV 1 /FVC may be normal even with considerable peripheral airway obstruction.
b. Lung volumes. Changes may be seen in moderate to severe obstructive airway disease.
(1 ) Lung volume measurements are useful in identifying hyperinflation due to premature airway closure.
(i) During a forced expiration, if the terminal airways close before all the air is expelled, hyperinflation results, causing an increase in the FRC, RV, and RV/TLC.
(ii) In small airway disorders, because of air trapping, the RV may increase while the FRC and FEV 1 remain normal.
(2) In emphysema, the alveolar wall destruction and loss of lung elastic recoil cause an increase in the TLC.
c. Compliance is increased in emphysema, because lung elastic recoil is reduced. (1 ) Lung volume measurements are useful in identifying hyperinflation due to premature airway closure.
(i) During a forced expiration, if the terminal airways close before all the air is expelled, hyperinflation results, causing an increase in the FRC, RV, and RV/TLC.
(ii) In small airway disorders, because of air trapping, the RV may increase while the FRC and FEV 1 remain normal.
(2) In emphysema, the alveolar wall destruction and loss of lung elastic recoil cause an increase in the TLC.
Restrictive Disease
examples include
A. Intrinsic (within lung) - low T L co
1. Pulmonary interstitial disease ( e.g. fibrosis asbestosis silicosis)
2. Pulmonary oedema
3. Aspiration pneumonitis
4. Acute respiratory distress syndrome (ARDS)
B. Extrinsic (outside lung) - Normal T L co ( gas exchange normal) A. Intrinsic (within lung) - low T L co
1. Pulmonary interstitial disease ( e.g. fibrosis asbestosis silicosis)
2. Pulmonary oedema
3. Aspiration pneumonitis
4. Acute respiratory distress syndrome (ARDS)
1. Chest wall deformity
2. Pleural fluid
3. Diaphragmatic compression by obesity, ascites, pregnancy
4. Respiratory muscle weakness e.g. myasthenia gravis, polio
Pattern of Impairment
a. Flow rates FEV 1 /FVC may be normal or may be increased due to increased traction on the intrathoracic airway walls.
a. Flow rates FEV 1 /FVC may be normal or may be increased due to increased traction on the intrathoracic airway walls.
b. Lung volumes
(1) A reduction in VC and TLC is the most useful indicator of a restrictive ventilatory defect.
(2) Lung stiffness in restrictive diseases increases the lung elastic recoil and lowers the FRC.
(3) Chest wall stiffness lowers lung volumes because it restricts lung expansion.
(1) A reduction in VC and TLC is the most useful indicator of a restrictive ventilatory defect.
(2) Lung stiffness in restrictive diseases increases the lung elastic recoil and lowers the FRC.
(3) Chest wall stiffness lowers lung volumes because it restricts lung expansion.
c. Compliance is reduced because lung elastic recoil is increased.
d. Airway resistance is decreased because the elastic forces maintain wider airways at any given lung volume. Respiratory failure
Where Pao2 < 8kPa
Type 1 and Type 2 Respiratory Failure - category depends on PaCO2 (normal range PaO 2 is 12 - 13kPa - but depends on age).
Type 1 respiratory failure
PaCO 2 <6.5kPa (alkalosis – “pink puffer”)
Alveolar hyperventilation to compensate for low PaO 2 – CO2 is lost as it diffuses better. Treat with 60% O2.
Alveolar hyperventilation to compensate for low PaO 2 – CO2 is lost as it diffuses better. Treat with 60% O2.
type 2 respiratory failure
PaCO 2 >6.5kPa (acidosis – “blue bloater”)
Alveolar hypoventilation as exhaustion occurs and the patient is no longer able to hyperventilate. Treat with 24% O2 (respiratory centre is insensitive to CO2 and respiration is driven by hypoxia – hence high percentage oxygen could remove respiratory drive completely). If treatment fails, intubate and ventilate.
Alveolar hypoventilation as exhaustion occurs and the patient is no longer able to hyperventilate. Treat with 24% O2 (respiratory centre is insensitive to CO2 and respiration is driven by hypoxia – hence high percentage oxygen could remove respiratory drive completely). If treatment fails, intubate and ventilate.
Summary
Type 1
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Type 2
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PaO 2 <8kPa
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PaO 2 <8kPa
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PaCO 2 <6.5kPa
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PaCO 2 >6.5kPa
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