A study of oxygenation during thoracotomy.

UP TO THE BEGINNING Of the present century, operations within the thoracic cavity were greatly hampered by the "pneumothorax problem." When the chest was opened, the lung collapsed and the patient made vigorous efforts to breathe. The mediastinal structures moved violently and surgery was very diflficult. Hypoxia rapidly occurred and unless the chest was soon dosed, death occurred due to asphyxia. In an effort to prevent the lungs from collapsing, certain steps were taken: (1) The patient's body and the surgeon were placed in a negative pressure chamber, with the patient's head outside the chamber. 1 (2) Positive pressure was applied to the patient's airway, while breathing continued spontaneously. (3) Gases were insufflated through a catheter inserted to the carina. 2 Waters (1933) 8 introduced controlled respiration using cyclopropane, but it was not until 1941 that it was widely adopted for thoracic surgery# It was then recognized that in this method lay the means of preventing the paradoxical respiration and mediastinal flap which occurred with spontaneous breathing. During normal anaesthesia with the chest closed, and with intermittent positive pressure ventilation, there is rarely any problem with oxygenation in patients with normal cardiopulmonary systems, provided that at least 33 per cent oxygen is inspired and hyperventilation is employed. ~ Nunn, Bergrnan, and Coleman 6 state that there are four factors tending to lower arterial oxygen tension during anaesthesia: 1. Underventilation, associated with a high arterial carbon dioxide tension (Paco2). 2. Increase in physiological dead space, which is known to be a regular feature of anaesthesia. 7 This increase appears to be slowly progressive, and a mean increase of 33-52 per cent in physiological dead space may occur, depending on the anaesthetic agent used. The alveolar dead space shows a marked rise, while anatomical dead space is reported relatively unchanged. This increase in physiological dead space is normally compensated for under anaesthesia by by-perventilating the patient. 3. Inequalities in the ventilation/perfusion ratios. 4. Admixture of shunted venous blood with oxygenated blood leaving the pulmonary capillaries. This is probably the most important factor. Physiological shunting comprises a number of components including: (a) Pulmonary arteriovenous anastomoses. (b) Drainage of venous blood into the left heart and