Regional Tissue Oxygen Saturation as a Predictor of Post-Spinal Anesthesia Hypotension for Cesarean Delivery

Since the discovery of neuraxial blockade, spinal anesthesia has been widely used for cesarean section due to its effectiveness and rapid onset. However, one of the most common complications associated with spinal anesthesia is hypotension, which occurs in 70% to 80% of parturient women. This condition is primarily caused by sympathetic vasomotor blockade, leading to reduced systemic vascular resistance, venous pooling in capacitance vessels, and decreased cardiac output. Severe hypotension can result in adverse maternal and fetal outcomes, including reduced uterine blood flow, decreased placental perfusion, fetal hypoxia, and acute fetal acidosis. While preoperative stress tests have been used to predict the risk of post-spinal anesthesia hypotension, their sensitivity is only 69%. Therefore, there is a critical need for non-invasive, reliable, and readily available techniques to predict and manage spinal anesthesia-induced hypotension during cesarean delivery.

Near-infrared spectroscopy (NIRS) is a non-invasive technique that allows continuous assessment of regional tissue oxygen saturation (rSO2). NIRS measures blood oxygenation in the microvasculature by utilizing near-infrared light at wavelengths absorbed by hemoglobin, providing real-time perfusion data. Peripheral nerve blocks have been shown to induce local vasodilation and increase subcutaneous tissue oxygen saturation through sympathetic blockade. Recent studies have suggested that increased subcutaneous tissue oxygen saturation in the middle anterior thigh can predict the success of femoral nerve blocks. However, the potential role of NIRS in predicting hypotension after spinal anesthesia has not been thoroughly investigated. This study aimed to determine the changes in rSO2 measured by NIRS after spinal anesthesia and to evaluate whether rSO2 could predict post-spinal hypotension in parturients.

This prospective study was conducted at Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, from January 2019 to November 2019. Ethical approval was obtained from the local ethics committee, and written informed consent was provided by all participants. The study enrolled 97 patients aged 20 to 40 years, with American Society of Anesthesiologists (ASA) physical status I–II, and a gestational age of 36–41 weeks, who were scheduled for elective cesarean delivery under spinal anesthesia. Exclusion criteria included placenta previa, pre-eclampsia, pregnancies complicated with cardiovascular or cerebrovascular diseases, gestational diabetes, body mass index (BMI) ≥ 40, and contraindications to spinal anesthesia. Two sensors connected to a four-channel NIRS device (INVOS 4100; Covidien, Mansfield, MA, USA) were applied to the middle anterior thigh of both the left and right limbs. Baseline rSO2 data were obtained before spinal anesthesia.

Upon arrival in the operating room, standard monitoring with non-invasive blood pressure, heart rate, and oxygen saturation was established. Baseline systolic and diastolic pressures were recorded in the supine position. All parturient women received oxygen via nasal cannula at 2 L/min. Patients were then turned to the right lateral decubitus position for combined spinal and epidural anesthesia using a midline approach. The epidural needle was inserted through the ligamentum flavum into the epidural space between the L3 and L4 spinous processes. The spinal needle was advanced through the epidural needle into the subarachnoid space until free-flowing clear cerebrospinal fluid was observed. Two milliliters of 0.6% ropivacaine were intrathecally injected over 30 to 60 seconds. After removal of the spinal needle, the epidural catheter was placed at a depth of 4 to 5 cm into the epidural space. The patient was immediately shifted into the supine position, and cold sensory tests were performed to assess the sensory block level. If the extent of sensory block did not reach the Th6 level, an epidural dose of 2% lidocaine was injected, and these patients were excluded from the study. Non-invasive blood pressure, heart rate, and rSO2 values were recorded at 1-minute intervals from the completion of spinal anesthesia up to the delivery of the fetus, and then at 3-minute intervals after childbirth.

Post-spinal anesthesia hypotension was defined as a systolic pressure of 25% of the baseline value. A standardized protocol was followed in cases of hypotension: intravenous ephedrine was administered if the heart rate was 60 beats/min. Episodes of hypotension were recorded from the completion of spinal anesthesia up to childbirth.

Statistical analyses were performed using SPSS for Windows (Version 22.0; SPSS, IBM Corp., NY, USA). Line graphics were constructed using GraphPad Prism version 6.01 for Windows (GraphPad Software, San Diego, CA, USA). Normally distributed variables were expressed as mean ± standard deviation, and intergroup comparisons were performed using an independent sample t-test. Non-normally distributed data were presented as median (range) and analyzed using the Mann-Whitney U test. Categorical data were reported as numbers (percentages) and analyzed using the Chi-square test. A P-value of <0.050 was considered statistically significant. Receiver operating characteristic (ROC) curves were generated to evaluate the diagnostic power of rSO2 for predicting post-spinal anesthesia hypotension. The optimal cut-off value was determined by the Youden index, which provided equally weighted sums of the true positive rate and true negative rate.

Five parturients were excluded due to inadequate spread of sensory block. Out of the remaining 92 patients, 45 (48.9%) developed post-spinal anesthesia hypotension. There were no significant differences in baseline characteristics (age, height, weight, gestational age, and duration of surgery) between the hypotension and non-hypotension groups. The sensory block level was Th4 (Th2–Th6) in the hypotension group and Th5 (Th3–Th6) in the non-hypotension group, which was not significantly different. Neonatal body weight was similar in both groups. The baseline rSO2 values of the left side in post-spinal hypotension and non-hypotension patients were 82.5 ± 4.8% and 85.9 ± 4.4%, respectively, which were significantly different. The baseline rSO2 value of the right side was also significantly lower in the hypotension group (81.6 ± 5.8% vs. 83.8 ± 4.0%). At 3-minute intervals after spinal anesthesia, parturients in the hypotension group had significantly lower rSO2 values of the left side compared to those in the non-hypotension group (82.8 ± 5.0% vs. 88.2 ± 3.3%). Similarly, the average rSO2 values of the right side after 3 minutes post-spinal anesthesia were significantly lower in the hypotension group (81.8 ± 5.7% vs. 85.2 ± 3.6%).

The ROC curve analysis showed that the baseline rSO2 value in the left side could predict post-spinal anesthesia hypotension with an area under the curve (AUC) of 0.723. The sensitivity and specificity were 81% and 73%, respectively, with an optimal cut-off value of 84.50. The AUC for the baseline rSO2 value in the right side was 0.628, with a sensitivity of 72% and specificity of 64%, and an optimal cut-off value of 82.50. The diagnostic power of rSO2 value at 3 minutes after spinal anesthesia for predicting post-spinal hypotension was also evaluated. The rSO2 value in the left side at 3-minute intervals after spinal anesthesia had an AUC of 0.823, with a sensitivity of 96% and specificity of 71%, and an optimal cut-off value of 84.50. The rSO2 value in the right side at 3-minute intervals after spinal anesthesia had an AUC of 0.703, with a sensitivity of 74% and specificity of 71%, and an optimal cut-off value of 84.50.

This study demonstrated that NIRS measurement of rSO2 in the lower limb is a useful diagnostic tool for predicting post-spinal anesthesia hypotension in patients undergoing cesarean delivery. The rSO2 value in the left side at 3-minute intervals after spinal anesthesia showed excellent predictive ability, with an AUC of 0.823. Baseline rSO2 values also showed acceptable predictive ability, with an AUC of 0.723 in the left side and 0.628 in the right side. The optimal threshold cut-off value of rSO2 in the left side was the same initially and 3 minutes after spinal anesthesia, suggesting that baseline rSO2 may have immediate clinical value for anesthesiologists in preventing hypotension after spinal anesthesia.

NIRS is a clinically available and easily transportable technique with a user-friendly interface and efficient data acquisition. It provides continuous and real-time perfusion data, reflecting blood oxygenation and perfusion distribution in the microvasculature. The sensors are patient-friendly, making monitoring of ischemic threats to the brain and body tissues safe and easy, thus guiding clinicians’ assessment and intervention. However, the potential disadvantages of routine NIRS monitoring include the high costs of single-use optodes and limited spatial resolution. While NIRS has been widely used to evaluate cerebral blood oxygenation changes, its use in assessing regional oxygen saturation of the anterior thigh is less common. This study highlighted the practical advantages of NIRS in predicting post-spinal hypotension and its potential role in improving maternal and fetal outcomes during cesarean delivery.

In maternal pregnancy, blood flow to the uterus from the pelvis is increased, and aorto-caval compression due to the pregnant uterus contributes to hemodynamic changes associated with spinal anesthesia. Dextro-rotation of the uterus in pregnancy is common, and left uterine displacement in the pre-delivery phase may help minimize hemodynamic changes after subarachnoid block. Future studies are warranted to evaluate the correlation between rSO2 in pregnancies with left uterine displacement and the incidence of post-spinal hypotension during cesarean section. This study had some limitations, including the arbitrary assessment of rSO2 value and blood pressure at 3-minute intervals after fetal childbirth instead of continuous monitoring.

In conclusion, NIRS measurement of rSO2 in the lower limb represents a valuable diagnostic tool for predicting post-spinal anesthesia hypotension in patients undergoing cesarean delivery. A value for rSO2 >84.50 is a good predictor of post-spinal hypotension after subarachnoid block in parturients. This technique can assist in the early administration of vasopressors, thereby preventing and managing maternal hypotension post-spinal anesthesia during cesarean section, ultimately leading to better maternal and fetal outcomes.

doi.org/10.1097/CM9.0000000000001647

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