Effects of high thoracic epidural anesthesia on mixed venous oxygen saturation in coronary artery bypass grafting surgery

Background

It is quite well known that increased metabolism during and after coronary artery bypass grafting surgery (CABGS), impaired immune response and hemodynamic, respiratory and thromboembolic complications are mostly due to the unsuppressed neuroendocrin stress response [1–6]. Sympathetic activity leading to hypertension, tachycardia and increased metabolism impairs platelet activation and peripheral blood flow on the one hand and impairs peripheral oxygen saturation on the other which leads to paradox ischemia despite the high FiO2 level. Therefore, it is critical to determine the balance between oxygen delivery (DO2) and consumption (VO2) in CABS [4,7].

Mixed venous oxygen saturation is an important parameter in determining general tissue oxygen balance. It is closely related with the arterial oxygen content and cardiac output. Mixed venous oxygen saturation (SvO2) has been suggested to be more sensitive than the hemodynamic parameters to indicate early findings of sympathetic activity [8].

Respiratory complications are mainly due to patient’s inability to use the protective mechanisms such as cough, deep breath and sigh which arise from not only the pump lung but also insufficient pain control [9]. Improvement in postoperative diaphragm and respiratory functions has been reported with high thoracic epidural anesthesia (HTEA) [10,11]. Compliance (C) is a widely-used parameter in anesthetized or mechanically ventilated patients in intensive care unit (ICU) to evaluate respiratory functions [12]. It has been reported that HTEA suppresses mediator release, significantly decreases need for analgesics, minimizes respiratory complications by perfect postoperative pain control eventually enabling early extubation and discharge from intensive care unit [1,13–15].

Possible effects of HTEA in patients with coronary artery diseases have not been clarified, yet. Therefore, the present study was planned to investigate likely influences of HTEA on SvO2, hemodynamic and respiratory parameters in CABGS.

Material and Methods

STATISTICAL ANALYSIS:

The primary outcome measure was mixed venous oxygen saturation. An a priori power analysis based on the previously published data suggested that a minimum of 32 patients in each group would be required to detect a 15% increase in mixed venous oxygen saturation, with a power of 90% at the 0.05 level of significance.

A statistical software program (SPSS 16.0 for Windows, SPSS Inc., Chicago, IL, USA) was used for statistical analysis. Qualitative parameters were compared between the study groups by Chi-Square and Fisher Exact tests. Numerical variables were compared by independent samples t test. Intragroup comparisons were made by repeated measures ANOVA test and Bonferroni Post-hoc test. All tests were performed at α=0.05 significance level.

Results

The study groups were similar in terms of demographic variables (Table 1). Data about MAP, HR, CO, CI, SVR, PVR and C are presented in Table 2. Before induction, MAP was similar in the study groups, but it was significantly lower in the HTEA group before and after the opening of sternum, before the closure of sternum (p<0.05). There was no significant difference between the groups after the closure of sternum and the data was within normal ranges. There are significant differences between the recordings before and after induction (p<0.05) (Table 2). Intraoperative HR and SpO2 values were similar in the study groups. CO, CI and SVR were similar in the study groups before induction, but after induction CO and CI were significantly higher and SVR was significantly lower in the HTEA group (p<0.05) (Table 2).

Pulmonary compliance values were higher in the HTEA group after the closure of sternum, but there was no significant difference between the study groups. Intragroup comparisons in the control group indicated that the recordings decreased significantly after the closure of sternum compared to those before induction (p<0.05) (Table 2). PVR and SVR were similar in the study groups before induction, but they were lower in the HTEA group after induction (p<0.05) (Table 2). CVP, PCWP, PAP were all within normal ranges and similar in the study groups at all recording time points (Table 2).

There was no significant difference in SvO2 between values before and after induction, but it started to increase in the HTEA group at 10 min and stayed high until 60 min after CPB (p<0.05) (Table 3).

Hematocrit values before induction were 35.2±4.6% and 33.5±5.3% in the HTEA and control groups, respectively. At 5 min after CPB it was 31.4±4.6% and 28.2±4.1% in the HTEA and control groups, respectively (p<0.05). At 60-min after CPB it was 32±4.5% and 27.8±4.2% in the HTEA and control groups, respectively (p<0.05). Hematocrit values decreased significantly in the control group compared to the HTEA group (p<0.05).

Intraoperative complications such as hypotension (13–22%), disrythmia (12.5–16%), need for inotropic agent after CPB (15.6–22%) were all lower in the HTEA group, but there was no significant difference between the groups. There was significantly less need in the HTEA group than the control group for transfusion after CPB (56–84%) before the patient is transferred to ICU (p<0.01). Mean arterial blood pressure and heart rate were higher in the control group after the patients’ transfer to ICU (p<0.05). Intergroup and intragroup comparisons revealed similar and within normal range recordings of postoperative SpO2, CVP and compliance (p<0.05) (Table 4). Development of postoperative hypertension was significantly higher in the control group (p<0.001). There were no significant differences in the frequencies of postoperative disryhtmia, bradycardia, hypotension need for inotropic agents. Vasodilatator agents and additional analgesics were more frequently needed in the control group during the ICU stay (p<0.001). Transfusion requirement was significantly less in the HTEA group (p<0.05), as less than 30% of patients in this group required transfusion. Recovery scores were less in the HTEA group (p<0.001). Extubation time, stay in ICU and stay in hospital were all shorter in the HTEA group (p<0.001) (Table 5).

Intergroup comparisons with regard to VAS indicated that the scores at 30 min, 1, 2, 4 and 8 hours were all less in the HTEA group than the control group (p<0.001). The VAS in the control group approached to that of the HTEA group only at hour-12.

Discussion

Surgery stress, insufficient pain control, hypercoagulability, release of proinflammatory cytokines, and activation of neuroendocrin system are among the major factors determining postoperative morbidity [16,17]. HTEA improves the balance between oxygen presentation and usage in ischemic myocardium via selective blockage of cardiac sympathetic innervation (T1–T5) [11,17]. Thoracic epidural anesthesia decreases myocardial infarcts by 40% and protects lung functions during the postoperative 24 hours [18,19].

The risk for spinal hematoma due to the heparinisation, haemodilution and coagulation factors limit the usage of HTEA in open cardiac surgeries [20,21]. Goldstein et al. [13] gave heparin at least 1 h after placement of epidural catheter and reported no neurological complication in patients with normal coagulation test results who are not under anticoagulation therapy. Accordingly, systemic anticoagulation was applied at least 1 h after placement of epidural catheter in the present study and our findings are similar as no complication was detected.

In patients undergoing CABGS, the transient sympathetic blockage provided by HTEA has been reported to decrease perioperative myocardial ischemia, regulate coronary perfusion, and significantly improve functions of left ventricle [14,22–24]. Mendez-Tellez [25] stated that intrathoracic blood volume, PVR and hypoxic pulmonary vasoconstrictor response may be induced by the change in pulmonary vascular tonus due to the sympathetic disinnervation, and HTEA may reduce PVR through the vasodilatation in pulmonary blood vessels. Accordingly, we found similar intraoperative SpO2, CVP, PCWP, PAP and C in the study groups, whereas in the HTEA group MAP, HR, SVR and PVR measurements and intraoperative hypotension, disrhythmia development, need for inotropic and transfusion were all less and CO, CI, SvO2 and Htc measurements were higher than the control group.

Kessler et al. [14] have placed the thoracic epidural catheter at T1–T2 or T2–T3 level, while Fillinger et al. [24] placed it between the T1 and T10 levels in a cranial direction. In the present study, we placed the thoracic epidural catheter at the C6–C7 level in a caudal direction. We think this application provided stabile MAP and HR in the HTEA group through the direct blockage of cardiac sympathetic nerves. The level of CO changes inversely with the change in systemic vascular resistance when the arterial pressure is stabile. Therefore, the increase observed in the CO and CI levels in the present HTEA group may be explained by the decrease in SVR.

The amount of oxygen presentation to tissues may be indirectly evaluated by the changes in DO2 and VO2. Mixed venous oxygen saturation is a marker of the balance between whole body DO2 and VO2. SvO2 shows a positive correlation with CO, Htc and SaO2 together with a negative correlation with VO2. Therefore, continuous monitorization of SvO2 enables simultaneous monitorization of these four parameters. Normally, peripheral VO2 is independent from DO2. When CO and DO2 decrease, peripheral O2 extraction increases to keep VO2 stabile and as a result SvO2 decreases. Sepsis on the other hand, decreases peripheral O2 usage and thereby increases SvO2[12]. Lorentzen et al. [8] reported that CI, Htc, SpO2, aortic valve surgery, vasodilatation treatment and VO2 are the factors affecting SvO2; they result in low SpO2, Htc or CI, increased peripheral VO2 during recovery as well as affecting veins. The present findings revealed increased intraoperative SvO2 values in the HTEA group compared to the control group after the epidural injection. HTEA decreased usage of oxygen significantly leading to an increase in SvO2 measurements. These findings may be explained by the suppressive effect of HTEA on sympathetic neuroendocrin stress response. Moreover, hypotension development was significantly less in the HTEA group during the intraoperative period, together with significantly less disrhythmia, less need for inotropic agent and less need for transfusion during the period after the CPB before the patient’s transfer to ICU. These findings are in agreement with the previous reports and suggest that more stabile hemodynamic conditions can be provided by HTEA [1,5].

Kessler et al. [14] detected higher MAP measurements in the postoperative ICU in the general anesthesia group. In the present study, we found higher postoperative MAP values in the control group. Less hypertension seen in the HTEA group during the postoperative ICU period may be explained by the efficient pain control and sympatholitic affects of HTEA.

Early extubation has potential benefits such as decrease in intubation-related complications, fast normalization of cilia functions, early recovery of coughing reflex, less pulmonary atelectasis development, significant improvement in intrapulmonary shunt fraction and elimination of disadvantages like deterioration of venous return, decrease in cardiac flow [26]. Numerous studies have previously reported that HTEA decreases the duration of intubation [20,21,27–29]. Similarly, the present study revealed shorter intubation time in the HTEA group. Furthermore, pulmonary compliance measurement which is considered as a marker of postoperative lung functions was similar in the study groups. This finding may be due to the similarities in the duration of cardiopulmonary bypass and surgery.

Requirement for additional analgesics or methods as well as nausea and vomiting were also significantly less in the HTEA group. The segment where epidural catheter was placed and the type and volume of the local anesthetic agent used may explain these findings [14,20,21,23]. The VAS was also less in the HTEA group, whereas the control group revealed smaller VAS value only 12-hour after surgery. The requirement for additional postoperative analgesics was significantly more in the control group. The fast and efficient analgesia provided in the HTEA group explains this difference between the study groups.

Despite that HTEA has been reported to have no superiority in terms of stay in ICU [14], there are studies revealing less stay in ICU and hospital in the HTEA group than the systemic opioid group [15,27]. In the present study, mean duration of stay in ICU as well as hospital was significantly shorter in the HTEA group. These findings suggest that the quality of analgesia provided by HTEA enables early recovery of pulmonary functions without suppressing protective reflexes such as coughing and deep breath. Eventually, hospitalization becomes shorter by early mobilization of patients.

Conclusions

As a conclusion, HTEA may be suggested to improve the balance between oxygen presentation and usage by suppressing neuroendocrin stress response, provide efficient postoperative analgesia, more stabile hemodynamic and respiratory conditions, and finally lower the duration of stay in ICU and in hospital after early extubation.