Nonbronchospirometric measurement of differential lung function.

The introduction of bronchospirometry by Jacobacus in 1932, and subsequent improvements in the technique, have led to many valuable observations on the differential and relative function of the two lungs. The method, however, can be seriously misleading to those without long experience in its use, and Gaensler and Watson (1952) and Comroe and Kraffert (1950) regard it as a form of investigation primarily suitable for the larger thoracic centres. Most of the difficulties of bronchospirometry for patient and for physician are contingent upon its principle of complete physical separation and measurement of the products of each lung's respiration. These objectives may be defeated by the presence of bronchial deformity, and the means of achieving them involves a degree of narrowing of the airway barely tolerable by the more dyspnoeic patients. The artificial separation of the airways, moreover, imposes on an under-ventilated lung the need to ventilate a larger dead space than under natural conditions, where the effect on each lung of the supracarinal dead space is in proportion to its share of the total ventilation. The original closed-circuit procedure involves high oxygen concentrations in the breathing-mixture which, as Gaensler has shown by his ambient-air method, may cause a material overestimate of the usual oxygen uptake of an underventilated lung. The use has been suggested of differences in respiratory exchange ratio (R.Q.) between simultaneous samples of expired air from each main bronchus and trachea in order to calculate the share of each lung in oxygen uptake and ventilation (Armitage and Arnott, 1951). By this principle the main difficulties of bronchospirometry could be avoided and, at little cost to the convenience of the patient, results obtained comparable with those of bronchospirometry in value to a surgeon contemplating major surgery of lung or pleura. The R.Q. was selected as the index for comparison because it is unaltered by simple dilution of an expired air sample by any volume of air, and because when two sources of expired air mix the proportion of the oxygen deficit in the final mixture for which each is responsible may be calculated from the three R.Q. differences alone. That unilateral partial loss of function would give rise to an R.Q. difference between the two lungs seemed certain, since, just as generalized pulmonary disease invariably causes change in alveolar air composition, so must unilateral disease change the gas tensions within the affected lung; and the exposure of the same …