European Journal of Cardiovascular Medicine

Hypotension following spinal anesthesia for cesarean section (CS) is a common problem that can lead to maternal and fetal complications.[1,2] The incidence of post-spinal hypotension during non-elective CS has been estimated to be between 39%–75%.[3-6] Routinely, most elective CS cases receive volume replacement and vasopressor as a precaution against potential hypotension during spinal anesthesia. However, due to time constraints, most patients undergoing non-elective CS are unable to receive adequate volume replacement, which puts them at higher risk of experiencing the hypotensive effects of spinal anesthesia. This is especially concerning for patients who already have underlying fluid depletion. Additionally, abrupt changes in hemodynamics after spinal anesthesia for non-elective CS further exaggerate hypotension.[7]

Despite attempts to prevent post-spinal hypotension with measures such as co-loading and vasopressor use, these interventions are not completely capable of eliminating the incidence of hypotension. Therefore, it is important to identify patients at risk for post-spinal hypotension in non-elective CS, but currently, there is limited research available in this field.

According to a systematic review, the use of baseline heart rate (HR) and systolic blood pressure (SBP) to predict the occurrence of post-spinal hypotension in CS has yielded inconsistent results.[8] It has been observed that early hypovolemia does not usually result in significant changes in HR or SBP.[9] As an alternative, the shock index (SI), calculated as the ratio of heart rate to systolic blood pressure, may be a better indicator of acute hypovolemia.[10] Studies have shown that SI might be a better measure of the degree of hemodynamic stability than HR or SBP alone.[10,11]

In obstetric population, the threshold of SI of ≥0.9 has been associated with increased risk of maternal adverse outcomes.[12-14] Normal SI values typically range from 0.5 to 0.9, with values above 0.9 indicating worsening hemodynamics. Based on this cut-off value, we hypothesized that an SI value of ≥0.9 may be associated with an increased risk of hypotension following spinal anesthesia for non-elective CS. This study investigates how calculating SI preoperatively affects the prediction of hypotension during spinal anesthesia, with the aim to provide insights that can inform clinical practice and guide future research in this area.

Study Design and Setting This prospective, single-center, cross-sectional observational study was conducted at R.L. Jalappa Hospital, Tamaka, Kolar from January 2024 to June 2024. Ethical approval was obtained from the Institutional Ethics Committee of Sri Devaraj Urs Academy of Higher Education and Research. Written informed consent was obtained from participants either in the labor room or obstetric emergency ward prior to the study procedure. This study followed the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) guidelines. Participants The study included term pregnant laboring women (gestational age ≥37 weeks) with American Society of Anesthesiology physical status (ASA PS) I or II who underwent emergency CS under spinal anesthesia, with no immediate threat to life of pregnant women and fetus. Inclusion Criteria: • ASA physical status I and II • Term pregnancy (≥37 weeks gestation) • Non-elective cesarean section • No immediate threat to maternal or fetal life Exclusion Criteria: • Pregnancy-induced hypertension or gestational hypertension • Known fetal abnormalities • Contraindication to spinal anesthesia • Multiple gestation • Baseline systolic blood pressure (SBP) <100 mmHg • Stillbirth • Height <150 cm • Cardiovascular, cerebrovascular, or endocrine disease • Patients requiring conversion to general anesthesia (GA) • Patients unwilling to participate Sample Size Calculation Sample size was estimated using the incidence of post-spinal hypotension at 34% from the study by Silwal et al.[15,16] using the single proportion formula: Sample size (N) = Z²₁₋α/₂ × p(100-p) / d² Where: • Z₁₋α/₂ = 1.96 (at 5% type 1 error, P<0.05) • p = Expected proportion = 34% • d = Absolute error/precision = 10% Using the above values at 95% confidence level, a sample size of 87 was calculated. Considering 10% non-response rate, a final sample size of 96 subjects was included in the study. Anesthetic Protocol During the pre-anesthetic visit, patients' data including age, gestational age, height, weight, body mass index (BMI), and indication for CS were recorded. Before the patient was shifted to the operating room (OR), ranitidine 50 mg and metoclopramide 10 mg were administered intravenously via 18 G cannula. Once in the OR, patients were positioned supine with a 15° left lateral tilt, and standard monitors were attached to record their 5-lead ECG, pulse oximetry, and noninvasive blood pressure. The patient's systolic blood pressure (SBP) and heart rate (HR) were measured three times, with each measurement taken one minute apart. The averages of these measurements were recorded as the baseline SBP and HR. Additionally, shock index (HR/SBP) was calculated before administering spinal anesthesia. Vein patency was maintained with a minimal rate infusion of Ringer's lactate (RL) solution. In sitting position and under all aseptic precautions, 2.0 ml of 0.5% hyperbaric bupivacaine was administered in L3-L4 interspace with 25 G spinal needle, after ensuring free flow of cerebrospinal fluid. The patient was then placed in a supine position with a right hip wedge, and immediately after the spinal injection, 1 L of RL was rapidly co-loaded over 10–15 min. Surgery commenced once the sensory level reached T6, which was determined by checking for loss of cold sensation with alcohol-soaked cotton swabs. Hemodynamic parameters were measured at baseline, immediately after spinal anesthesia, every 3 minutes for the first 15 min after spinal injection, and every 5 min until the end of surgery. Mnagement of Complications Hypotension (SBP <80% of baseline or SBP <100 mmHg) was treated with phenylephrine 50 µg bolus and rapid infusion of RL 200 ml. If the hypotension was associated with bradycardia (HR <55 beats/min), an IV injection of atropine 0.5 mg was given. After the baby was delivered, 3 IU of oxytocin was administered IV over at least 30 s followed by an infusion of 10 IU/h. The amount of IV fluids given during surgery, estimated blood loss, and use of uterotonic agents were all recorded. Outcome Measures The primary outcome measure was the occurrence of post-spinal hypotension, defined as SBP <80% of the baseline reading or SBP <100 mmHg from the administration of spinal anesthesia until the delivery of the baby. The secondary outcome measure was post-delivery hypotension, defined as SBP <80% of baseline reading or SBP <100 mmHg observed from the initiation of oxytocin until the end of surgery. Statistical Analysis Data normality was assessed by Shapiro-Wilk test and by visually inspecting histograms. Patient data were presented as mean (standard deviation) or median (interquartile range) or frequency (percentage). Demographic and surgical profiles of the parturients, baseline and intraoperative hemodynamic parameters were compared using unpaired t-test or Mann-Whitney test, as appropriate. Chi-square tests were used for categorical variables or Fisher exact tests if expected cell counts were less than five. We conducted univariable and multivariable logistic regression analysis to explore the association between baseline shock index (categorized as <0.9 and ≥0.9) and hypotension after spinal anesthesia. Variables that were univariably associated (P<0.15) with hypotension, along with clinically relevant variables, were included in the multivariable model building. It was reported as odds ratios (OR) with 95% confidence intervals (CI). We assessed the degree of multicollinearity using the variance inflation factor (VIF) and dropped variables with VIF value >10. The diagnostic ability of the baseline SI to predict post-spinal and post-delivery hypotension was analyzed using receiver operating characteristic (ROC) curves. We used Hosmer-Lemeshow test to assess the goodness of fit of the logistic regression for multivariable analysis and characterized the multivariate model discrimination using a ROC curve C statistic. A P-value <0.05 was considered statistically significant. SPSS version 22.0 (IBM SPSS Statistics, Somers NY, USA) was used for all analyses.

Patient Demographics

A total of 96 parturients were enrolled in the study and all were included in the final analysis. Out of the 96 parturients, 80 parturients (83.33%) had baseline SI <0.9 (considered as the low SI group), while 16 (16.67%) had a baseline SI ≥0.9 (the high SI group). Table 1 summarizes patient demographics and the perioperative variables for both low SI and high SI groups.

Incidence of Hypotension

In 45 (46.88%) parturients, hypotension was observed before delivery, whereas hypotension after delivery was observed in 33 (34.38%) parturients. Among parturients with SI <0.9, 35 (43.75%) had hypotension before delivery compared to 10 (62.50%) parturients with SI ≥0.9 (p-value=0.168). Similarly, 24 (30.00%) parturients with SI <0.9 and 9 (56.25%) with SI ≥0.9 had hypotension after delivery (p-value=0.045) (Table 1).

Table 1: Comparison of demographic characteristics and perioperative profiles between low and high shock index (SI) groups

Risk Factors for Post-Spinal Hypotension

Univariable analysis revealed that sensory block height >T4 was significantly associated with hypotension before delivery (OR 2.28, 95% CI 1.01–5.16, p=0.047) (Table 2). In multivariable analysis, both baseline SI ≥0.9 (AOR 2.84, 95% CI 1.08–7.46, p=0.034) and block height >T4 (AOR 2.41, 95% CI 1.02–5.68, p=0.045) were identified as independent risk factors for hypotension before delivery, i.e., post-spinal hypotension (Table 2).

Risk Factors for Post-Delivery Hypotension

For hypotension after delivery, univariable analysis showed that maternal height (OR 0.89, 95% CI 0.81–0.98, p=0.015), gestational age (OR 1.22, 95% CI 1.01–1.48, p=0.042), and SI (OR 2.96, 95% CI 1.06–8.27, p=0.039) were all associated factors (Table 3). In multivariable analysis, parturient height (OR 0.90, 95% CI 0.82–0.99, p=0.029) and baseline SI (AOR 4.52, 95% CI 1.58–12.91, p=0.005) were significant independent risk factors for hypotension after delivery (Table 3).

Predictive Performance of Shock Index

The independent variables in the regression model had a VIF of less than 3, indicating that no significant multicollinearity was observed. The ROC curve for SI alone in predicting hypotension before delivery was 0.54 (95% CI 0.43–0.65), while for predicting hypotension after delivery, it was 0.57 (95% CI 0.46–0.68). Combination of SI with block height >T4 produced a ROC curve of 0.59 (95% CI 0.48–0.70) for hypotension before delivery.

Table 2: Univariable and multivariable analysis of risk factors associated with hypotension before delivery

Table 3: Univariable and multivariable analysis of factors associated with hypotension after delivery

The model performance as reflected by ROC curve for the multivariable logistic regression analysis was 0.638 (95% CI 0.52–0.75) for post-spinal hypotension (Figure 1), and 0.692 (95% CI 0.58–0.80) for post-delivery hypotension (Figure 2), respectively. The Hosmer-Lemeshow test was not significant for both multivariable analyses (p=0.18 for post-spinal hypotension and p=0.14 for post-delivery hypotension, respectively).

Figure 1: The receiver operating characteristic (ROC) curve of the multivariable logistic regression model to predict post-spinal hypotension for non-elective cesarean delivery. The variables included in the model were maternal age, BMI, gestational age, shock index ≥0.9, baseline heart rate, baseline systolic blood pressure, and sensory block height >T4. Area under ROC curve = 0.638 (95% CI 0.52–0.75).

Figure 2: The receiver operating characteristic (ROC) curve of the multivariable logistic regression model to predict post-delivery hypotension for non-elective cesarean delivery. The variables included in the model were maternal age, height, BMI, gestational age, shock index ≥0.9, baseline heart rate, baseline systolic blood pressure, and sensory block height >T4. Area under ROC curve = 0.692 (95% CI 0.58–0.80).

In this prospective single-center cross-sectional study, we found that patients with a high baseline SI (≥0.9), compared to those with low SI (<0.9), were more likely to experience hypotension after spinal anesthesia for non-elective CS. Additionally, the study found that sensory block height above 4th thoracic dermatomes was significantly associated with an increased odds of developing post-spinal hypotension, while patient height was significantly associated with hypotension after delivery. However, we did not observe substantial diagnostic capability of SI alone for predicting post-spinal and post-delivery hypotension.

Numerous studies have shown that SI can serve as an early warning sign for circulatory shock in cases of non-specific shock, trauma, and sepsis.[17-21] Compared to traditional indicators such as heart rate or blood pressure, SI is believed to be more effective in detecting concealed hypovolemia that may not be apparent through those traditional indicators.[9] In obstetric populations, a normal SI value ranges from 0.7 to 0.9, with an SI of ≥0.9 being associated with adverse maternal outcomes.[12-14] Drawing from the studies mentioned earlier, we opted to utilize an SI cutoff of ≥0.9 and found it to be a significant independent risk factor for post-spinal and post-delivery hypotension in non-elective CS. These findings contribute to the growing body of evidence suggesting that an SI ≥0.9 is a strong indicator of adverse maternal outcomes.

Surprisingly, our study found that SI alone had a weak discriminating ability for predicting post-spinal and post-delivery hypotension, as indicated by a low area under ROC curve. However, when other risk variables were included along with shock index in the model, there was an improvement in predicting ability of the model with ROC 0.638 for post-spinal hypotension and 0.692 for post-delivery hypotension. Bishop and colleagues found that a composite PRAM score based on pulse rate >90 beat/min, age >25 years, and mean arterial pressure <90 mmHg showed good discrimination for predicting post-spinal hypotension, with a c-statistic of 0.626.[22] These findings support the need for composite parameters to develop a better prediction model, as there is a complex interaction between cardiovascular, autonomic nervous system, and endocrine systems in maintaining hemodynamic stability.[23]

Unlike the significant association found with SI, no notable relationships were found between baseline HR or baseline SBP and post-spinal or post-delivery hypotension in our study. Our findings are further strengthened by the recent systematic review that revealed inconsistent results regarding the predictive value of HR and SBP for post-spinal or post-delivery hypotension.[8] Dynamic parameters have been suggested as potentially better predictors[8], but they require specialized equipment and personnel, and are time-consuming. In contrast, static parameters such as SI are easy to record, inexpensive, non-invasive tool, and are feasible to use in resource-constrained settings. Moreover, an added advantage of SI is that it is more reliable than HR or SBP in reflecting the adequacy of volume status before spinal anesthesia for emergency CS.

While previous studies have thoroughly investigated the risk factors for hypotension induced by spinal anesthesia, those associated with post-delivery hypotension remain underexplored. Apart from baseline patient characteristics and hemodynamic factors, other contributors to post-delivery hypotension include factors like blood loss following delivery and the amount of oxytocin administered intraoperatively. Interestingly, we found that a baseline high SI was associated with post-delivery hypotension, even though there were no significant differences related to total blood loss or oxytocin administered between the high and low SI groups. Notably, baseline SI has demonstrated its predictive value for postpartum bleeding following vaginal delivery.[24] Hence, larger-scale future studies are necessary to investigate the correlation between baseline SI and post-delivery bleeding after CS under spinal anesthesia.

Limitations

There are some limitations in our study. We could not determine whether high baseline SI was related to hypovolemia. Perhaps ultrasound or echocardiography would be an appropriate tool to find out the correlation between high baseline SI and hypovolemia. Thus, our results are suggestive of an association and do not necessarily imply a causal relationship. Moreover, it would be interesting to observe the changes in SI from baseline after preloading prior to administration of spinal anesthesia. In addition, there are many factors that can influence post-spinal hypotension, and despite controlling for key confounding variables in our analysis, there may still be residual confounding from unmeasured factors. Besides, SI was dichotomized based on previous studies, so further research is needed to determine the optimal cutoffs for the relationship between SI and post-spinal hypotension following emergency CS. Finally, although we tested the discriminating ability of our prediction model, it lacks robustness because we did not use bootstrapping techniques to validate our model internally. Since our study was not intended to develop a risk prediction tool, future studies are required to validate (both internally and externally) the prediction model incorporating SI.

In this study, we demonstrated that baseline SI of ≥0.9 was associated with post-spinal and post-delivery hypotension in parturients undergoing emergency CS. Our findings provide novel evidence that SI may be a useful tool in identifying parturients who are more likely to develop hypotension, and as a result, aggressive preventive strategies can be undertaken at the early stage. We suggest that the predicting ability of the SI on incidence of post-spinal hypotension should be assessed in further trials.

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