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

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


Interstitial fibrosis

Multiple pulmonary emboli

Pulmonary oedema


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.