Comparison of Oxygen Saturation During Different Intraoperative Positions

Overview

One-lung ventilation (OLV) during thoracic surgery and its effect on oxygen saturation has been discussed in multiple studies. Literature shows different ventilation strategies and possible hypoxemia has been attributed to several issues. Port-access cardiac surgical procedures ("key hole surgery" including mitral valve repair or prosthesis, tricuspid valve repair or prosthesis, atrial septal defect closure or a combination of these) require the use of OLV as well. Its effect on oxygen saturation has not been studied previously. The aim of this study is to investigate the effect of extracorporeal circulation (ECC) and patient positioning on oxygen saturation and on the progress of hypoxemia throughout OLV during port-access in comparison to oxygen saturation throughout OLV during lateral decubitus thoracotomies.

OLV is a technique that allows isolation of the individual lung under anesthesia. It is required for a number of thoracic surgical procedures as well as port-access procedures. The use of a double lumen tube allows easy switching from two-lung to single lung ventilation. For many thoracic operations the patient will be in the lateral decubitus position. The primary physiologic change that takes place on initiation of OLV is the presence of trans-pulmonary shunting, causing impaired oxygenation and occasionally hypoxemia. The degree of shunt is reduced due to the effect of gravity which increases pulmonary blood flow to the dependent lung and it will be further decreased due to a phenomenon termed hypoxic pulmonary vasoconstriction (HPV). Hypoxemia may occur during OLV in lateral decubitus position due to the restriction of the dependent lung expansion caused by mediastinal weight therefore leading to atelectasis. Increased ventilation pressure and lung volume of the dependent lung impedes perfusion and contributes to hypoxemia. Presentation of hypoxemia during OLV is multifactorial. It depends on the operation side (OLV better in left thoracotomies), on preoperative lung function \[best indicator = Partial Pressure of Oxygen in Arterial Blood (PaO2) divided by Fraction of Inspired Oxygen (FIO2)\] and on the distribution of perfusion. The PaO2/FIO2 is the best parameter to describe the full spectrum of ventilation-perfusion abnormalities, independent of the inspired oxygen concentration level. The patients are divided into 2 groups according to the surgical procedure as follows: Port-access group (n=25), and lateral decubitus lung surgery group (n=25). Standard monitoring will be applied intraoperatively \[electrocardiography (ECG), pulse-oxymetry (SpO2), invasive and non-invasive blood pressure\]. After induction of anesthesia, an appropriately sized double lumen tube will be inserted and its position will be checked using fibroscopy. All patients will be placed on a volume controlled mode of ventilation with the following settings: tidal volume between 5 and 10 ml/kg and FIO2 at 50% and 4 cm H2O of positive end-expiratory pressure (PEEP). Respiratory frequency and tidal volume will be adjusted to maintain end-tidal carbon dioxide (ETCO2) between 30 and 40 mmHg. ETCO2 levels and SpO2 will be monitored continuously. If SpO2 declines below 90% the ventilator settings will be adapted according to the following protocol: First the FIO2 is increased to 80%. If this is insufficient FIO2 is increased to 100%. If this remains insufficient, continuous positive airway pressure (CPAP) \[5 cm water (H2O) pressure\] is applied to the non-ventilated lung. If this is still insufficient the surgeon will be asked to allow for temporary recruitment of the non-ventilated lung.