Anesthetic care underwent significant changes over time due to scientific growth and development of new techniques for perioperative monitoring.
Central venous oxygen saturation (SvcO2) is a variable of great interest since it co elements hemodynamic information by evaluating oxygen supply, consumption, and global use.
General anesthesia diminishes the body’s metabolic rate and oxygen requirements, therefore an increase in SvcO2 is expected. This phenomenon has not yet been reported in the medical literature, and precise values of SvcO2 under general anesthesia remain unknown. Furthermore, contradictory reports describe SvcO2 changes in different clinical settings.
Because of the existing controversies, it is essential to analyze SvcO2 in patients undergoing surgery with general anesthesia to establish the normal reference values for this parameter. Such values can be suggested for specific clinical situations, optimizing perioperative clinical care.
We included patients without randomization aged 18 years or older undergoing major elective surgeries under general anesthesia and requiring the placement of a central line for routine monitoring. We excluded patients with cardiac disease and low ejection fraction, patients with chronic lung disease requiring supplementary oxygen therapy, pregnant women, patients with systemic inflammatory response syndrome by any cause, and those who were hemodynamically unstable at the time of second blood sample collection. All patients signed written informed consent before initiating study-related procedures. The study was conducted during a period of 6 months from August to December 2010 at Fundación Santa Fe de Bogotá University Hospital.
The sample size of 111 patients was calculated to reach a power of 80% for the study. The alpha error was set at <0.05 (5%) for the mean differences in the SvcO2 values before induction of anesthesia and at 30 minutes after induction. The calculated sample size accounted for a 10% drop-out rate.
The subjects were monitored with pulse oximetry, EKG, temperature, and non-invasive blood pressure; midazolam 3 mg IV and propofol 30-50 mg IV were used to sedate the patients. A central line was placed, the first blood sample was obtained before anesthesia induction and the second blood sample was obtained at the initiation of mechanical ventilation, 30 minutes after induction (FiO2 = 0.6). Both samples were analyzed with the I-STAT© device. We recorded patient demographic information, type of surgery, SvcO2 from both blood samples, and vital signs during the surgical procedure. Data was exported and stored in a Microsoft Excel© database for further analysis. The following anesthesia regimen was used per standard operating procedures: propofol 1 mg/kg, rocuronium 0.6 mg/kg, remifentanil 0.1-0.15 mg/kg/ min or fentanyl 1-5 mcg/kg/h, sevoflurane 0.5 MAC, and BIS between 40-60 (during maintenance).
STATA 10© software was used for data analysis, considering the variable measurement scale. Nominal and ordinal variables were described with absolute and relative frequencies. Continuous variables were described using means and standard deviations. To evaluate differences in the means, the Student’s t-test for independent samples was applied, as well as a descriptive statistic of the involved variables. The surgical procedure characteristics were identified using central tendency and dispersion measures. For continuous variables: means, standard deviations, minimum and maximum values were used; for discrete variables: absolute frequencies and percentages were used. Frequencies and percentages were also used for nominal variables. SvcO2 was stratified by age group.
This study did not compromise patient safety or quality of care. The protocol was designed based on Declaration of Helsinki international ethics recommendations and Colombian normativity regarding research involving human subjects (Resolution 8430 of 1993 by the Social Protection Ministry). Approval by the Institutional Ethics in Research Committee was obtained before execution of the study. Subject participation required a written informed consent.