Physiology of the Lateral Decubitus Position, Open Chest and One-Lung Ventilation

Early attempts at intrathoracic surgery in nonventilated patients were fraught with rapidly developing respiratory distress and a fast moving operative field. The difficulty with performing a thoracotomy in a spontaneously breathing patient, for both the patient and the surgeon, is easily explained by two phenomena: Pendel-luft and Mediastinal shift (Fig. 5.1) [1]. Both phenomena can be explained by the fact that the pleural interface has been disrupted in the open hemithorax so that no negative intrathoracic pressure is being created in response to a spontaneous inspiratory effort and chest-wall expansion. In the closed hemithorax, on the other hand, chest-wall expansion and the resulting negative intrathoracic pressure will produce gas flow into the lung via the mainstem bronchus. However, inspiratory gas flow will not only come from the trachea, but also from the operative lung, which is free to collapse due to the surgical pneumothorax. Inspiration therefore results in nonoperative lung expansion and operative lung retraction. The reverse process occurs during expiration, where bulk expiratory gas flow, from the nonoperative lung, not only escapes via the mainstem bronchus into the trachea, but also back into the re-expanding operative lung. This process results in the “pendular” motion of the lung with inspiration and expiration. Mediastinal shift occurs due to a similar process. The negative inspiratory pressure in the closed hemithorax is equally applied to the mediastinum, which is secondarily pulled away from the open thorax during inspiration. The reverse is true during expiration, where positive intrathoracic pressure pushes the mediastinum across into the open thorax. When combined, these two mechanisms explain the difficult exposure for the operating surgeon due to a fast moving operating field, and the rapidly developing respiratory distress in the patient secondary to inefficient to-and-fro ventilation with limited CO 2 elimination and fresh gas entrainment (Fig. 5.1). Interestingly, awake thoracic surgery is being re-popularized in certain high-risk individuals, but the use of video-assisted thoracoscopic surgery (VATS), which avoids opening of the hemithorax, minimizes the above stated issues [3]. Selective ventilation of one lung was first described in 1931 and quickly resulted in increasingly complex lung resection surgery [4]. While infinitely better tolerated than spontaneous respiration, hypoxia was a frequent occurrence during the early years of OLV. Extensive research over the ensuing decades has clarified the basic physiology governing pulmonary perfusion (Q) and ventilation (V), as well as the disturbances that are Introduction .................................................................................................................. 71