pulmonary function test (PFT), a procedure for determining the capacity of the lungs to exchange oxygen and carbon dioxide efficiently. There are two general kinds of respiratory function tests. One measures ventilation, or the ability of the bellows action of the chest and lungs to move gas in and out of alveoli; the other kind measures the diffusion of gas across the alveolar capillary membrane and the perfusion of the lungs by blood. Efficient gas exchange in the lungs requires a balanced ventilation-perfusion ratio, with areas receiving ventilation well perfused and areas receiving blood flow capable of ventilation. Basic ventilation studies are performed with a spirometer and recording device as the patient breathes through a mouthpiece and connecting tube; a nose clip prevents nasal breathing. Measurements or calculations are made of the tidal volume (TV), or gas inspired and expired in a normal breath; the inspiratory reserve volume (IRV), or the maximal volume that can be inspired after a normal respiration; the expiratory reserve volume (ERV), or the maximal volume that can be expired forcefully after a normal expiration; the residual volume (RV), or the gas remaining in the lungs after maximal expiration; and the minute volume, or the gas inspired and expired in 1 minute of normal breathing. The vital capacity of the lungs is equal to TV + IRV + ERV, and the total lung capacity to TV + IRV + ERV + RV. Bronchospirometric measurements of the ventilation and oxygen consumption of each lung separately are performed by using a specially constructed double-lumen catheter with two balloons. One balloon is inflated to seal off the contralateral lung when the other lung is tested. Arterial blood gas studies, including determinations of the acidity, partial pressure of carbon dioxide and of oxygen, and oxyhemoglobin saturation, provide information on the diffusion of gas across the alveolar capillary membrane and the adequacy of oxygenation of tissues. See also blood gas determination, forced expiratory volume, maximum breathing capacity.
