pulmonary function tests

pulmonary function tests

1. spirometry

spirometry is used to measure how much air can be taken in and out as wellas how fast air can move also in and out.
the spirometry is used for the following 3 tests:

• flow volume loops

flow volume loops provide a graphical illustration of a patient`spirometric efforts. flow is plotted against volume to display a contiuous loop from inspiration to expiration.
the loops show FVC and FEV1.
FEV1/ FVC is normally 80%.

COPD: decrease in expiratory flow rate at any giving lung volume, invrease in residual volume.
restrictive disease; decrease in all lung volumes.
fixed large airway ostruction: plateau in both ispiratory and expiratory cycles.
variable extrathoracic obstruction: plateau in the inpiratory limb.

• The severity of the abnormality might be graded as follows:
• May be a physiologic variant: Predicted FEV₁ ≥100%
• Mild: Predicted FEV₁ <100% and ≥70%
• Moderate: Predicted FEV₁ <70% and ≥60%
• Moderately severe: Predicted FEV₁ <60% and ≥50%
• Severe: Predicted FEV₁ <50% and ≥34%

• Maximum volunatry ventilation

it is the maximum volume of air that can be exhaled by volunatry  effort in a 15 second interval. this volume is multiplied by 4 and expressed as liters/ min.

• postbronchodilator testing

reversibility of airway obstruction can be assessed with the use of bronchodilators.
after spirometry is completed, the patient is given an inhaled bronchodilator and the test is repeated.
in general > 12 % increase in FEV1( or absolute improvement of 200 ml) or FVC after inhaling a B agonist is considered a significant response.


            2.lung capacity tests
the lung capacity tests determine the total amount of air in the lung.

The following terms describe the various lung (respiratory) volumes:

• The tidal volume (TV), about 500 ml, is the amount of air inspired during normal, relaxed breathing.
• The inspiratory reserve volume (IRV), about  is the additional air that can be forcibly inhaled after the inspiration of a normal tidal volume.
• The expiratory reserve volume (ERV),  is the additional air that can be forcibly exhaled after the expiration of a normal tidal volume.
• Residual volume (RV),  is the volume of air still remaining in the lungs after the expiratory reserve volume is exhaled.
  Summing specific lung volumes produces the following lung capacities:
  • The total lung capacity (TLC),  is the maximum amount of air that can fill the lungs (TLC = TV + IRV + ERV + RV).
  • The vital capacity (VC),  is the total amount or air that can be expired after fully inhaling (VC = TV + IRV + ERV = approximately 80% TLC).
  • The inspiratory capacity (IC), is the maximum amount of air that can be inspired (IC = TV + IRV).
  • The functional residual capacity (FRC),  is the amount of air remaining in the lungs after a normal expiration (FRC = RV + ERV).

Average lung volumes in healthy adults

Volume	Value (litres)
	In men	In women
Inspiratory reserve volume	3.3	1.9
Tidal volume	0.5	0.5
Expiratory reserve volume	1.0	0.7
Residual volume	1.2	1.1

Lung capacities in healthy adults^[3]

Volume	Average value (litres)		Derivation
	In men	In women
Vital capacity	4.8	3.1	IRV plus TV plus ERV
Inspiratory capacity	3.8	2.4	IRV plus TV
Functional residual capacity	2.2	1.8	ERV plus RV
Total lung capacity	6.0	4.2	IRV plus TV plus ERV plus RV

the following 3 tests measure lung volumes:

• body blethysmography

it is an alternative method of measuring lung volumes thattake advantages of boyle`s law.

 a patient is placed in sitting position in a closed body box with a known volume. from FRC, the patient pants with an open glottis against a closed shutter to produce changes in the box pressure proportionate to the volume of air in the chest.

• helium dilutional method

During helium dilution measurement of lung volumes, patients breathe from a known volume and concentration of helium gas for a period of typically 4 to 7 minutes. The oxygen concentration in the starting mixture is set at 30% to ensure patients with COPD can remain comfortable during the test. A carbon dioxide absorber is situated in line with expired breath to keep the closed-circuit CO2 level below 0.5% and avoid discomfort and hyperventilation. Oxygen is added to the system to maintain the starting volume in the spirometer. Since the type of thermal conductivity helium analyzer employed in the SpiroAir can be affected by changes in concentrations of CO2, O2 and Water Vapor, a chemical absorber removes the interference of CO2 and Water Vapor. A simple algorithm corrects the helium signal for changes in oxygen background.

Once connected to the closed-circuit, equilibration between the starting and final helium concentrations should occur within 7 minutes. A state of equilibrium is defined as helium concentration changes of less than 0.02% over a 30 second interval.

The functional residual capacity (FRC) is calculated from the helium concentrations as follows:

FRC = (% helium initial - % helium final) / % helium final x system volume

•  Nitrogen-washout technique 

In the nitrogen-washout technique, the patient breathes 100% oxygen, and all the nitrogen in the lungs is washed out. The exhaled volume and the nitrogen concentration in that volume are measured. The difference in nitrogen volume at the initial concentration and at the final exhaled concentration allows a calculation of intrathoracic volume, usually FRC. 

          3.diffusion capacity/DLCO2

The volume of CO transferred across the alveoli per minute per unit alveolar partial pressure.

CO is rapidlly taken up by Hb therefore it is transferred mainly by diffusion.

a single breath of 0.3% CO and 10 % helium is held for 20 sec, expired partial pressure of CO is then measured.

normal value 17-25 ml/min/mmhg

Processes Associated with Alterations in DLCO
Obstructive Lung Diseases
Cystic fibrosis Emphysema
Parenchymal Lung Diseases
Drug reactions (e.g., amiodarone, bleomycin) Idiopathic Interstitial lung disease Lung disease caused by fibrogenic dusts (e.g., asbestosis) Lung disease caused by biologic dusts (e.g., allergic alveolitis) Sarcoidosis
Pulmonary Involvement in Systemic Diseases
Dermatomyositis-polymyositis Inflammatory bowel disease Mixed connective tissue disease Progressive systemic sclerosis Rheumatoid arthritis Systemic lupus erythematosus Wegener's granulomatosis
Cardiovascular Diseases
Acute and recurrent pulmonary thromboembolism Acute myocardial infarction Fat embolization Mitral stenosis Primary pulmonary hypertension Pulmonary edema
Other
Acute and chronic ethanol ingestion Bronchiolitis obliterans with organizing pneumonia (BOOP) Chronic hemodialysis Chronic renal failure Ciagarette smoking Cocaine freebasing Diseases associated with anemia Marijuana smoking
Increases In DLCO
Diseases associated with increased pulmonary blood flow (e.g., left-to-right intracardiac shunts) Diseases associated with polycythemia Exercise Pulmonary hemorrhage DLCO, diffusing capacity of carbon monoxide.

© 2003 The Cleveland Clinic Foundation.

Exhaled Nitric Oxide

The measurement of exhaled nitric oxide as a reflection of airway inflammation is gaining rapid acceptance as a pulmonary function test. Normal values have been shown to depend on the exhaled flow rate during the measurement. The test is repeated until three reproducible results are obtained. The mean value is reported. Patients are asked to inspire to total lung capacity and then exhale into an analyzer using a steady, controlled exhaled flow rate. The test is rapid and safe and can be performed by most patients. The normal values shown in Table 1 are for a measurement flow rate of 50 mL/sec.¹⁴

Table 1: Exhaled Oral Nitric Oxide

	Volume (ppb)
Category	Adult	Child	Interpretation
Normal	5-20	5-15	Normal
High normal or increased	20-35	15-25	Moderately raised exhaled nitric oxide can indicate underlying airway inflammation
			Colds and influenza can transiently raise exhaled nitric oxide, and some patients have higher baseline exhaled nitric oxide levels than others
Elevated	>35	>25	Indicates ongoing eosinophilic inflammation Symptomatic patients are likely to respond to steroids Possible causes (if already on steroids) include poor compliance, recent allergen exposure, inadequate steroid dose, and poor steroid response Not all patients with high exhaled nitric oxide levels experience symptoms