Nondependent Lung Ventilation and Fluid Responsiveness

In all patients, standard monitors, and state and response entropy (SE and RE, respectively) based-depth of anesthesia will be applied. Normothermia will be maintained by using forced-air warming blankets. Anesthetic technique will be standardized in all studied patients. Anesthesiologists who give the anesthetic will be not involved in the collection of outcome data. General anesthesia will be induced with propofol (2-3 mg kg-1) and fentanyl (2-3 µg kg-1) to achieve a SE value less than 50 and the difference between RE and SE less than 10.

Cisatracurium (0.2mg kg-1) will be administered to facilitate the placement of a left-sided double-lumen tube, and the correct position of its tip will be confirmed with a fiberoptic bronchoscope.

Anesthesia will be maintained with 0.7 to 1.5 minimum alveolar concentration of sevoflurane and 0.5 µg kg-1 increments of fentanyl to maintain the SE values less than 50 and the difference between RE and SE less than 10. Suppression of the second twitch in train-of-four stimulation of the ulnar nerve will be maintained with 0.03mg kg-1 increments of cisatracurium.

The radial artery will be catheterized. Cardiac index (CI) and SVV will be measured by using a Vigileo-FloTrac system (v1.14, Edwards Lifescience, Irvine, CA).

Patients' two lungs (TLV) will be mechanically ventilated with a pressure-controlled ventilation mode, a fraction of inspired oxygen (FiO2) of 0.4 in air, TV of 8 mL kg-1 (predicted body weight (PBW)), inspiratory to expiratory (I: E) ratio of 1:2.5 and PEEP of 5 cm H2O, fresh gas flow (FGF) of 1.5-1.7 L min-1, and RR adjusted to achieve a PaCO2 of 35-45 mm Hg.

During OLV,TV, FiO2, I: E ratio, PEEP, FGF, and RR, will be maintained as during TLV and the lumen of the nondependent lung will be left open to air.

After thoracostomy, patients will be randomly allocated to one of two by drawing sequentially numbered sealed opaque envelopes containing a computer-generated randomization code.

The patient's nondependent lung during OLV will be ventilated with a CPAP of 2 cm H2O or HFPPV as randomized.

All patients will receive lactated Ringer's solution at 2mL kg-1 h-1 during surgery. The capability of SVV to predict fluid responsiveness during CPAP or HFPPV will be assessed at 30 min after randomization.

Hemodynamic control will be standardized according to the authors' protocol. If MAP dropped down to 60 mmHg, 250 mL of plasma protein fraction 5% will be administered, and, if this will not enough, repeated doses of intravenous of ephedrine 5 mg or phenylephrine 100 µg, will be administered to maintain urine output to be equal or greater than 0.5 mL kg-1 hour-1. A hemoglobin concentration of 8 g dL-1 or greater will be compensated with red blood cell concentrates.

An independent investigator blinded to the study groups who will not be involved in the patients' management collected the data.

As in previous studies,14-15 the sample size is determined by considering that an area under the ROC curve ≥ 0.8 is clinically reliable to predict fluid responsiveness. To detect a 0.3 difference from the null hypothesis of 0.5, 28 patients will be required in each group with a type-I error of 0.05 and a power of 80% under the ROC curve, assuming equal number of responders and non-responders. To compensate for a dropout rate of 10%, 31 patients will be included in each group.

Normal distributions of data will be assessed using Kolmogorov-Smirnov test. Student's paired t-test or the Wilcoxon signed-rank test will be used to compare hemodynamic variables obtained at the 2 time points (T0, T1) before and after volume expansion. Hemodynamic variables between responders and non-responders within the group at each time point will be compared using Student independent t-test or Mann-Whitney U-test where appropriate. The x2 test will be used when indicated. The Pearson rank method tests linear correlations between SVV before volume loading (T0) and absolute changes in SVV (∆SVV) and percentage change in SVI (∆SVI) after volume. The responders are defined as those patients who demonstrates a ≥ 15% increase in CI after volume expansion between T0 and T1.16 A ROC curve for each variable will be generated and an area under the ROC curve will be calculated. Using this analysis, the optimal threshold value, sensitivity and specificity of SVV during each study intervention could be determined. The ROC curves will be compared using the DeLong test. Data will be expressed as mean ± SD, medians [IQR], or number (%). A value of p< 0.05 is considered to be statistically significant.