Nomogram for Prediction of Alveolo-arterial Gradient During One-lung Ventilation

The shunt that occurs during one lung ventilation varies for each patient and depends on many parameters. In this study, our aim was to predict the level of shunting during anaesthesia using readily available preoperative data. It is important to predict the hypoxia that will develop due to the shunt and to plan the methods that can be applied to reduce the shunt in terms of patient safety. One of the most reliable data reflecting the shunt level is the gradient of alvelo- arterial oxygen concentration. The calculation of alveolo-arterial oxygen gradient (A-a O2 gradient) is based on easily accessible parameters (pO2, pCO2, FiO2, age). The secondary aim of the study is to follow how the A-a O2 gradient (oxygen difference between alveolar and arterial blood) changes over time during surgery.

Demographic data, planned operation, comorbidities, side of surgery, ASA score, preoperative haemoglobin level, FEV1 (forced expiratory volume in 1 minute), FEV1/FVC (forced expiratory volume in 1 minute/forced vital capacity) ratio and EF (ejection fraction) will be recorded before surgery.FEV1, FEV1/FVC ratio values will be measured by spirometry test. Spirometry is a preoperative test routinely performed in all patients undergoing one lung ventilation. EF will be measured by preoperative echocadiography, which is also performed during the preoperative preparation of all patients scheduled for lung surgery. Patients undergoing VATS (video-assisted thoracoscopy) surgery with planned lung resection, lobectomy, pulmonectomy, segmentectomy etc. will be included in the study and all will be operated in the lateral decubitus position.

Once the patient is on the operating table, routine haemodynamic monitoring (heart rate, non-invasive blood pressure and blood oxygen saturation) will be performed.

The patient will then have an arterial cannula inserted for invasive arterial monitoring. It is a routine practice in major surgeries for blood gas monitoring, haemorrhage monitoring and monitoring of sudden blood pressure fluctuations. After insertion of the arterial cannula, blood gas will be taken in room air and arterial pO2 (partial oxygen pressure), arterial pCO2 (partial carbon dioxide pressure) and arterial SpO2 (oxygen saturation) will be recorded. At the same time, a venous blood sample (from any vein in the upper limb) will be taken in room air and vSpO2 (venous blood oxygen saturation) will be recorded. The difference between SpO2 of arterial and venous blood is determined as Delta-SpO2 and is a specific parameter of this study.

All patients will undergo general anaesthesia and orotracheal intubation with a double lumen tube.

For anaesthesia induction, 1 mcg/kg fentanyl, 2 mg/kg propofol and 0.6 mg/kg rocuronium will be administered .After adequate mask ventilation and complete muscle relaxation, a double lumen intubation tube will be placed into the trachea by direct laryngoscopy. After the position of the tube and whether it is in the correct bronchus is supported by bronchoscopy, the patient will be connected to the mechanical ventilator and lateral decubitus (provided that the side to be operated on remains on top) will be given. Ventilator parameters will be selected the same for all patients. During two-lung ventilation, tidal volume 8 ml/kg, respiratory rate 12, peep 5, and FiO2 (fraction of inspired oxygen) = 60%. During one-lung ventilation, tidal volume will be set to 6 ml/min, respiratory rate 15, peep 5, and FiO2 = 70%. All patients will be ventilated in volume control mode.

At this stage, EtCO2 (end-tidal carbon dioxide) will be measured separately in both lungs. For this, first the airway from the lower lung will be clamped and the EtCO2 value will be recorded (dEtCO2 - dependent lung EtCO2). Then, vice versa, the airway from the lower lung will be clamped and the EtCO2 value obtained (ndEtCO2 - non-dependent lung EtCO2) will be recorded. The difference between these two values, D-EtCO2 (delta - end-tidal carbon dioxide) will be added to the data.

After these steps, one-lung ventilation will be started and the first A-a oxygen gradient will be recorded 30 minutes after initiation of OLV. The consequent calculations of OLV will be made every 30 minutes until the end of the surgery. The consequent measurements are necessary for estimation the A-a gradient change over time.