Perioperative management
·
All
patients will receive standard intra-operative monitoring, including
heart rate (HR), peripheral oxygen saturation
(SpO2), continuous
electrocardiography (ECG) and noninvasive blood pressure monitoring (NIBP).
·
After pre-oxygenation, general anaesthesia will
be induced
with
propofol, fentanyl and rocuronium (0.8
to 1 mg kg—1), and maintained with propofol (100 - 150 mcg kg—1
min—1) or sevoflurane (1to 2%) along with fentanyl (1 to 2 mcg kg—1
hr—1).
·
Anesthetic
administration was targeted to maintain a bispectral index (BIS) of 40 to 60
throughout the surgical procedure.22
·
All
patients will receive Sodium chloride(0.9%) solution at 4 ml/kg/hour as a maintenance
fluid infusion during surgery and will be ventilated at baseline in
volume-control mode with the following settings: VT of 6 ml/ kg
PBW and positive end-expiratory pressure set 5 cmH2O to
achieve and maintain a peripheral oxygen saturation of 96% and an end-tidal
carbon dioxide (EtCo2) concentration between 30 and 35 mmHg.
·
The PBW
(kg) will be calculated as follows: X 0.91[height (cm) - 152.4]; (X 50 for men
and 45.5 for women).
·
After
induction of general anesthesia, invasive blood pressure monitoring will be
obtained by inserting a 20-G cannula into the radial artery. A square-wave test will be used in all
patients to exclude under or overdamping of the pressure signal.23
Hemodynamic monitoring
Radial artery catheter will be connected to the FlowTrac
system (version 4.0; Edwards
Lifesciences, Irvine, CA, USA). We will continuously monitor stroke volume
(SV), stroke volume index (SVI), cardiac output (CO), cardiac index (CI) and
SVV through FlowTrac /EV1000 system. 43,44 PPV will be continuously
recorded on a Nihon -Kohden monitoring system (Nihon Kohden Corporation,
Japan).
Study protocol
Measurements will be started during a
period of hemodynamic stability (defined
as changes in mean arterial
pressure less than 10% over 5 min7,26) after positioning the patient in lateral. All readings will be
taken before the skin incision.
The study protocol will start with recording an initial set of measurements (Baseline). TVC will be applied by increasing the TV up to 8 ml/ kg PBW and PPV and SVV
changes were recorded
(∆ PPV and ∆ SVV, respectively, calculated as percentage of variations
between the values of PPV and SVV recorded at bassline -1 and 1 min after the VT
increase).
Following this, the TV will be reduced back to 6 ml/ kg PBW and
after 1 min, a set of measurements will be recorded (Baseline 2) and the fluid
challenge (FC) of 250 ml of sodium chloride ( 0.9%) solution will be infused
over 10 min and SV,
SVI will be noted. Only
the haemodynamic data, obtained from the first fluid challenge administered to
each enrolled patient will be used for the analysis. Positive end-expiratory
pressure will be kept constant during the study period. For the safety of the patient, the interruption of the protocol
will be at discretion of the attending anesthetist.
Sample
size calculation
·
The sample size of the study was calculated by means of the
comparison of the areas under the ROC curves test (AUC). For this purpose, we predicted an AUC of at least 0.75,
which is the threshold for considering a diagnostic test as accurate,29 and compared
it with the null hypothesis (AUC 0.50; no discriminating
power). Accordingly, a sample size of 38 patients was calculated (type I error of 5% and type II error of 20%).
With the expectation of a 10% dropout rate, 42 patients will be enrolled in the
study.
Statistical analysis
A receiver operating characteristic (ROC) curve approach
will be used to evaluate the reliability of TVC test in predicting fluid responsiveness. ROC curves will
be constructed for PPV and SVV values
before fluid challenge administration; for the percentage change
in PPV and SVV between 6-
and 8-ml/ kg PBW ventilation after 1
min of TVC application.
A patient will be
considered fluid responsive if the SVI increased at least 10% after fluid
challenge administration.7,8 Cut-off values will be chosen with the highest
Youden index and, finally, statistically significant ROC curves (P < 0.05) were compared using the De
Long test.27 Normal
distribution will be evaluated by means of the D’Agostino-Pearson test and,
accordingly, data are expressed as median with interquartile [IQR] range or
mean (SD).
Changes
in continuous variables after TVC will be compared using a paired t test or Wilcoxon signed rank sum test,
while an independent t test or
Mann–Whitney U test was used for
subgroup comparisons, as appropriate. For dichotomous
or categorical variables, a Chi-square test for comparison of proportions will
be applied. Statistical analyses will be conducted using STATA and or R
software.
For
all comparisons, we will consider significant P values less than 0.05. |