The Effect Of Positive End-expiratory Pressure On Oxygenation And Hemodynamics During One-lung Ventilation

Objective: Thoracic surgery require anesthetists to provide the conditions for one-lung ventilation. With a wide range of video-assisted thoracoscopic surgery carried out, the request of one-lung ventilation is getting higher and higher. One-lung ventilation (OLV), not only can prevent blood and secretions to spill into the Kin-lung, keeping lung isolation, but the operative side lung can completely collapse and surgical field remained relatively static. During one-lung ventilation, however, about 9% -27% of patients will be significant hypoxemia [1], even in the pre-operative patients with normal lung function. Reason may be for contralateral one-lung ventilation, the operated lung no longer have ventilation, but there is blood flow distribution and blood flow for the absorption of oxygen. So arterial partial pressure of oxygen (PaO2) decreased. At this point, intrapulmonary shunt increase up to 21% -39% [2]. Hypoxic pulmonary vasoconstriction (HPV) in the prevention of hypoxemia during one-lung ventilation plays an important role. Side of non-ventilation lung blood flow cardiac output is usually 20% -25%. If there is no HPV, while blood flow can be increased up to 40% -50%. However, a variety of intraoperative factors such as posture, narcotic drugs, vasoactive drugs, anesthesia, etc. can inhibit HPV. Application of positive end-expiratory pressure (PEEP) to the dependent lung is often used for preventing hypoxemia. However, several studies on the effects of PEEP during OLV produced conflicting evidence,. some think PEEP improve oxygenation and decrease ventilation perfusion mismatch(V/Q) because PEEP increase end-expiratory alveoli volume, functional residual capacity(FRC) and prevents atelectasis and others show no benefit or worsening of PaO2 because it result in overtension of thoracic cavity, increase pulmonary vascular resistance(PVR), which in turn causes diversion of blood flow from ventilation lung to nonventilation lung and decrease hypoxic pulmonary vasoconstriction (HPV). In our study, we applies different level of PEEP to dependent lung during OLV in thoracic surgery. At the same time we record HR, MAP and Ppeak, arterial blood analysis and calculate Qs/Qt in order to get optimization of PEEP in OLV.Method: twenty patients who were scheduled for thoracic surgery were studied. All patients were premedicated with atropine sulfate 0.5mg intramuscularly (IM) 30min before admission to the operating room (OR). On admission to the OR, patients were placed on operating table and intraarterial and venous cannulas were inserted with local anesthesia. Additional monitoring included ECG, arterial oxygen as measured by pulse oximetry (SpO2), direct blood pressure(DBP). Anesthesia was induced with midazolam 0.1mg/kg, 3-5ug/kg fentanyl, 0.08-0.1 mg/kg vecuronium and 0.3mg/kg Etomidate was given to facilitate tracheal intubation. A left-sided double-lumen endobronchial tube was placed and positioned initially by auscultation, and confirmed by fiberoptic bronchoscopy just before initiation of OLV. The lungs of all patients were ventilated with 100% oxygen throughout this study. Two-lung volume -controlled ventilation was performed with a tidal volume of 10ml/kg and one lung volume-controlled ventilation was performed with a tidal volume of 6 ml/kg, an inspiration: expiration ratio of 1:2. Respiratory rate was adjusted to obtain arterial PaCO2 between 35 and 45 mmHg. All patients were given an infusion of propofol 1-2 mg/kg/min and remifentail 0.8-1.0ug/kg/min during anesthesia, supplemented vecuronium for muscle relax. Our experimental sequence consisted of four steps. In step one (T1), measurements were made on after 30 minutes of two lung ventilation. In step two (T2), measurements were obtained after 30 minutes of the start of OLV. In step 3 (T3), 5cmH2O PEEP was applied to the dependent lung. Measurements were made 30 minutes after step 3. In step 4 (T4), 10cmH2O PEEP was applied to the dependent lung. Measurements were made 30 minutes after step 4. On T1, T2, T3, T4, Arterial blood was analyzed by an automated arterial blood gas analyzer and the following variables were measured: mean arterial blood pressure (MAP), heart rate (HR), arterial partial pressure of oxygen (PaO2), arterial oxygen saturation (SaO2), airway peak pressure, Shunt fraction (Qs/Qt) was also calculated. The following formulas was used to calculate Qs/Qt. PAO2=FiO2×(PB-PH2O)- PACO2/R, Qs/Qt =PA-aDO2×0.0031×(PA-aDO2×0.0031+5) All data are means±SD. Changes in hemodynamic data, arterial blood sampling data, Qs/Qt and airway peak pressure were compared using T-test. Changes were considered significant at the 5% level (p < 0.05).Result: A total of 20 patients were enrolled in this study. The values for PaO2, PaCO2, Qs/Qt, PH ,SaO2 and Ppeak are shown in Table 4.1. There was no significant change in PaCO2 throughout the entire four-step experimental sequence. After OLV, PaO2 decreased significantly from 449.45±144.29 mmHg to 200.2±145.25 mmHg and Qs/Qt increased significantly from 49.84±18.86 % to 72.91±8.18 %. After applying PEEP5cmH2O to the dependent lung, PaO2 increased to 299.55±138.83 and Qs/Qt decreased to 66.8±10.88%. But after applying PEEP10cmH2O to the dependent lung, PaO2 decreased to 237.3±135.57mmHg and Qs/Qt increased to 71.01±8.61%. The values for HR, MAP are showed on tabled 4.2. Throughout the study, there are no significant difference in HR and MAP. But MAP decreased over 30% shown on two patient.Conclusion: this study demonstrates that applying PEEP5cmH2O to the dependent lung can improve PaO2 and decrease Qs/Qt. PEEP of 5cmH2O does not influence on the hemodynamics markedly during OLV and have less lung damage.