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Pulmonary Function Test (PFT)

  1. Lung Volumes
    1. Total lung capacity – Volume of gas contained in the lungs after a maximal inspiration (TLC = VC + RV).
    2. Residual volume – Volume of gas remaining in the lungs after a maximal expiration.
    3. Vital capacity – Volume of gas that is exhaled from the lungs in going from TLC to RV.(IRV+TV+ ERV)
      Total lung capacity (TLC) and residual volume (RV) are useful in distinguishing obstructive and restrictive diseases. TLC and RV are increased Q in obstructive airways disease and reduced Q in restrictive lung diseases and musculoskeletal abnormalities.
  2. Peak expiratory flow: (PEF) is measured by a maximal forced expiration through peak flow meter. It correlates well with the forced expiratory volume in 1 Sec (FEV1) and is used as an estimate of airway caliber. Peak flow rates should be measured regularly in asthmatics to monitor response to therapy and disease control.
  3. SPIROMETRY – Recording of exhaled volume versus time.
    1. FEV1 (Forced expired volume in one second)–The maximum amount of air that can be exhaled in the first second of expiration. Normally it is 70-80% of the FVC.
    2. FVC (Forced vital capacity) – The maximum volume of air that can be forcibly expired after inhaling as deep as possible.
    3. FEV1 /FVC(as percentage)- The proportion of the total volume of air that can be expired in the first second of expiration.
    4. PEFR(Peak expiratory flow rate)-The maximal expiratory flow rate achieved.   
    5. FEF (Forced expiratory flow) :  the rate of airflow recorded in measurements of forced vital capacity.
    6. FEF25 – 75%  (Maximal Mid Expiratory Flow Rate). (MMEFR)- 
      1. Average expiratory flow rate during middle 50% of the FVC. i.e. the average rate of airflow measured between exhaled volumes of 25 and 75 per cent of the vital capacity during a forced exhalation
      2. It is regarded as a more sensitive measure of small airway narrowing than FEV1.   
      3. In spirometry -. It is effort independent.
      4. With obstructive disease only in the small airways, the only abnormality noted on routine testing of pulmonary function may be a depression in FEF 25-75 and an abnormal terminal portion of forced expiratory flow volume curve.
      5. FEV1 /FVC may be normal in very early stage of  obstructive disease.

Important Point about FEV 25% - 75%

  1. Values between 80 – 120% of predicted values are normal.
  2. Mild obstructive (small airway) diseases have decrease FEF25 – 75% with normal FEV1/FVC.
  3. Maximum Voluntary ventilation (MVV)
    This measures the amount of air one can breathe in and out during one minute.
  4. Ventilatory function can be assessed in two ways.
    1. Under static conditions → By measuring lung volumes e.g. total lung capacity, vital capacity and inspiratory capacity.
    2. Under dynamic conditions →  By determining forced expiratory flow rates e.g. forced expiratory volume, forced vital capacity.    


Fig: Spirogram demonstrating a slow vital capacity maneuver and various lung volumes. Harrison-18th fig 252.2 Pg- 2088)



Important Points:


TLC, RV & ERC are measured using:-

  1. Helium dilution method
  2. Body plethysmography (AIPG 2009)
    1. RV, FRC, and TLC- cannot be measured by spirometry because they include the volume of gas present in the lungs
    2. Two techniques of commonly used to measure these volumes:  helium dilution and plethysmography.
    3. The helium dilution method may underestimate the volume of gas in the lungs if they are slowly communicating airspaces, such as bullae.
    4. In this situation, lung volumes can be measured more accurately with a body plethysmograph, a sealed box in which the patient sits while panting against a closed mouth piece.

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