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During surgical pneumothorax operations, measurements were made of the oxygen saturation, pH, and pCO2 of the arterial blood in 32 patients, and the circulatory effect of collapse of the operated lung was observed by determining these 3 quantities. The values obtained indicated that, in patients with minimal or moderate unilateral disease, the total collapse of the operated lung was followed by diversion of the blood flow to the unoperated side. In patients with extensive disease of the operated lung, the absence of change in the 3 variables indicated a preexisting diversion of the blood from the diseased lung. In patients with severe bilateral disease, the data indicated a minimal diversion of the blood. DURING the past quarter of a century the dis¬ turbances of ventilatory functions produced by surgical pneumothorax have been the subject of numerous laboratory and clinical studies. As a result, there is available a vast mass of data, but unfortunately many of these are conflicting, prob¬ ably because of the great variety of animal species and preparations and investigative techniques used. Consequently, despite the surfeit of data many aspects of the subject remain incomplete. One of the most important of these, at least from a clinical viewpoint, is the relation of the degree of collapse and the magnitude of dysfunctions to the various factors which determine this relationship. In the past it was generally assumed that the greater the degree of pneumothorax, the more serious the inter¬ ference with ventilation and circulation. This con¬ cept has caused many surgeons to compromise operating conditions in their desire to minimize the degree of dysfunction. However, with the advent of operations requiring collapse of major portions of lungs and the use of endobronchial anesthesia and of bronchial blockers, evidence began to accumulate that cast doubt on this concept. In order to help resolve this and other problems, we have embarked on a long-term study. In the initial report ' data on 19 patients were presented which indicated that the magnitude of arterial oxygen desaturation and hypercarbia was not as great as expected. The study has been extended in an attempt to delineate the role played by various factors and to elucidate on the homeostatic mecha¬ nisms which are brought into play during surgical pneumothorax. The purpose of this communication is to present data pertinent to the surgical and anesthesiologic factors which determine the degree of ventilatory impairment. MethodsThe method of study has been detailed in the first report.' Briefly, it entailed measurements of arterial oxygen saturation, carbon dioxide tension, and pH during various phases of surgical pneumo¬ thorax in 32 patients undergoing intrathoracic op¬ erations for pulmonary tuberculosis (14), neoplastic From the

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