Pulmonary Function Testing

Objectives:
• Conduct spirometry tests and make calculations from raw test results.
• Interpret results in light of published norms.
• Basic spirometry and terminology
– Tidal volume (TV): volume of air inhaled or exhaled in one normal breath.
– Inspiratory reserve volume (IRV): maximal amount of air that can be inhaled following a normal inhalation.
– Expiratory reserve volume (ERV): maximal volume of air that can be exhaled following a normal exhalation.
– Inspiratory Capacity (IC): maximal amount of air a subject can inhale following a normal exhalation.
– Vital capacity (VC): maximal amount of air that a subject can exhale after a maximal inhalation.
• Volumes not easily measured with spirometer
– Residual volume (RV): volume of air remaining in lungs after maximal inhalation.
– Functional residual capacity (FRC): volume of air left in lungs after a normal exhalation.
– Total lung capacity (TLC): total volume of air the lungs can hold.

• Calculated forced volumes and flows
– Forced vital capacity (FVC): total volume of air expired after a maximal inhalation when the subject is attempting to exhale as rapidly and forcefully as possible. In healthy subject, FVC = SVC.
– Forced expiratory volume - one second (FEV1.0): the amount of air exhaled in the first one second of FVC maneuver.
– Forced expiratory flow from 25-75% (FEF25-75) or Maximal Mid-Expiratory Flow (MMEF): the flow rate during the middle 50% of the FVC maneuver (from 25% to 75% of expired volume).
– Maximal voluntary ventilation (MVV): the maximal amount of air that a person can breathe in or out in a short period of time - usually 10, 12, or 15 seconds.

Use of Data:
• Deviations from normal indicators of pulmonary disease
– Asthma - constriction - restricts flow
– Emphysema - destruction of alveoli and trapping of air - inability to rapidly exhale and increase in residual volume.
– Smoking and air pollution effects on lungs
• Residual volume important in body composition measurements.
• FEV, FEV1.0, best predictors of disease.
– FEV/FEV1.0 also used to detect disease
• MVV sometimes used to evaluate respiratory muscle weakness.

Pulmonary Function Issues Related to Measured Values:
• Overinflation of Lungs
– Emphysema - COPD - Permanently
– Asthma - Acutely
• ? RV + Ratio RV/TLC
• FEV1/ FVC ratio falls below 80% - Also flow rates fall
– With age (lungs less compliant)
– Falls with obstructive diseases; e.g. asthma/bronchitis
• Asthma - obstructive disease ? increased collapsing force of large airways
– obstruction to expiratory flow ? lung volume
– bronchodilators may return flow to normal
• Early COPD - characterized by irreversible ? in small airway resistance that reduces expiratory flow
– not very responsive to dilators
• Severe COPD - ? small & large airway resist
– severe flow limitations – bronchodilators ? little help
– chronic bronchitis and emphysema
• Emphysema - loss of elastic recoil ? ? small airway collapse during expiration, thereby ? resistance
– Max Expiratory Flow ?
– Bronchodilators have no effect
– ? FRC + TLC

Training:
• In general, lung volumes and capacities ? little with training. VC may ? slightly. TLC doesn’t change much, slightly ? possible
• MVV may ? considerably
– Due to ? TV and ? rate of respiration

Procedure Notes:
• Conversion to Body Temperature Pressure Saturated (BTPS)
– Body Temp ? 37o C
– Saturation with water vapor = 100%
– All pulmonary function values reported in BTPS, but measurements taken at Ambient Temperature Pressure Saturated (ATPS)
– Conversion (Ref: CCJ, p. 50)

VBTPS = VATPS * BTPSCF

Where:
BTPSCF = TB(?C) + 273 X PB – PH2O at room temp
TR(?C) +273 PB – PH2O at body temp
TB = body temp in degrees Celsius (? 37? C)
TR = room (or spirometer) temp in degrees Celsius
273 = factor to convert Celsius to Kelvin
PB = barometric pressure
PH2O = water vapor pressure at room and body temp (CCJ,p. 50)

• FRC by Nitrogen Washout - Breathing Pure Oxygen (Source: West, Respiratory Physiology, pp. 146-147)
– General Formula
V1 * C1 = (V1 * C3) + (V2 * C2)

Where: V1 = Lung Volume
V2 = Volume of gas exhaled over washout procedure
C1 = [N2] in lungs before washout (atmospheric ? 80%)
C2 = [N2] of exhaled gas over washout ([N2] in V2)
C3 = [N2] left in lungs after washout measured at
end-expiration

• Solve for unknown V1:
(V1 * C1) - (V1 * C3) = V2 * C2
V1 * (C1 - C3) = V2 * C2
V1 = V2 * C2)
C1 – C3

Remember general constants given for atmospheric air:
• Pb at sea level = 760 mm Hg
• FIO2 = 0.2093 (or 20.93%)
• FICO2 = 0.0004 (or 0.04%)
• FIN2 = 0.7903 (or 79.03%)
• ? PIO2 = 0.2093 x 760 = 159 mm Hg
• PICO2 = 0.0004 x 760 = 0.3 mm Hg
• PIN2 = 0.7903 x 760 = 600 mm Hg

Assumption: since atmosphere is composed almost entirely of N2, O2, and CO2, then:
N2% = 100 - O2% - CO2%
Calculate residual volume as: (Ref. Wilmore, MSSE, 1980):
RV (L) = VO2bag (L) * (b-a) ÷ (c-d)
Where: RV = residual volume in liters
VO2bag = volume of oxygen in liters added to bag
(usually 3-5 L)
a = % nitrogen impurity in original oxygen (assume
to be 0.0 for practical purposes)
b = % nitrogen in rebreathing bag after subject completes breathing maneuver
c = % nitrogen in alveolar air at beginning of test (assume 80.0%)
d = % nitrogen in alveolar air during last maximal breath (assume 0.2% nitrogen higher than equilibrium %, I.e., b + 0.2% nitrogen)

Simplified (CCJ, p. 49):
RV = VO2bag * b or VO2bag * b
8.0.0 – (b + 0.2) 79.8 - b

Where:
VO2bag = volume of O2 in bag at start
b = percent of N2 in bag after rebreathing
N2 = 100% - %O2 - % CO2
NOTE: If nitrogen analyzer is available, bag nitrogen concentration can be measured directly from the bag.

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Dr. D.S. Merchant is a Gold Medalist in (Anatomy & Histology), Nephrology Fellow in AKUH, Pakistan. For more information on Nephrology or visit www.ehealthguide.info is a popular website that offers information on Tuberculosis and Pulmonary Sarcoidosis Vs Tuberculosis

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