The venous flow per respiratory cycle of right external iliac vein flow by ultrasound in the different position for predicting post-spinal hypotension during cesarean delivery

Abstract

Background

Post-spinal anesthesia hypotension during cesarean delivery results from reduced systemic vascular resistance due to the blockade of autonomic nerves, which is further worsened by inferior vena cava (IVC) compression by the gravid uterus. The study aimed to determine whether venous flow per respiratory cycle of right external iliac vein flow (REIVF) in the inguinal region, as measured on ultrasound in the deferent positions, could reflect the degree of IVC compression and further identify parturients at risk of post-spinal hypotension.

Methods

One hundred and twenty parturients scheduled for elective cesarean delivery under spinal anesthesia were enrolled in this study. Venous flow (VF) per respiratory cycle of the REIVF was measured using ultrasound before anesthesia in the supine position and a left tilt of 15°(A 15° slope pad is placed under the waist of the pregnant woman)Respond 1. All patients were divided into a hypotension group (n = 64) and a non-hypotension group (n = 53) Respond 11. based on whether SHS occurred. The primary outcomes included preoperative ultrasound parameters of REIVF obtained in different positions, the correlation between these parameters and SHS, and the magnitude of changes in REIVF induced by postural alteration (ΔREIVF). Hypotension was defined as a more than 20% reduction in systolic blood pressure relative to baseline. Respond 4. Multivariate logistic regression analysis was performed to explore the relationship between ΔREIVF and post-spinal hypotension. Receiver operating characteristic (ROC) curves were constructed to evaluate the predictive performance of relevant parameters for post-spinal hypotension. The corresponding area under the curve (AUC) and optimal cut-off values were subsequently calculated.

Results

At a 15° left tilt, REIVF was significantly higher in the non-hypotensive group than in the hypotensive group. In both groups, REIVF per respiratory cycle at a 15° left tilt was significantly higher than that in the supine position (P < 0.001).The difference in right external iliac vein flow between the supine position and 15° left tilt (ΔREIVF) was independently associated with post-spinal hypotension (SHS) following spinal anesthesia for cesarean delivery (OR = 1.071, 95% CI: 1.038–1.104, P < 0.001).Although systolic blood pressure (SBP) declined in both subgroups (ΔREIVF > 125.85 mL/min vs. ΔREIVF ≤ 125.85 mL/min), the reduction in SBP was markedly greater in the ΔREIVF > 125.85 mL/min group at 4 and 5 min after spinal anesthesia (P = 0.001 and P < 0.001, respectively). In addition, heart rate (HR) increased significantly in the ΔREIVF ≤ 125.85 mL/min group at 4 min after spinal anesthesia (P = 0.011).

Conclusion

Variations in REIVF among different positions prior to anesthesia are predictive of subsequent post-spinal hypotension (SHS). Respond 3.

Trial registration

This study was registered at http://www.chictr.org.cn, No. ChiCTR2100046925, 2021-05-31.

Background

The incidence of post-spinal anesthesia hypotension (PSAH) among parturients undergoing cesarean delivery is approximately 70% [1, 2]. PSAH may ead to adverse maternal and fetal outcomes, such as maternal nausea, vomiting, dyspnea, neonatal depressed Apgar scores and fetal acidosis [3–8]. Therefore, effective prediction of maternal hypotension have immense clinical importance.

After spinal anesthesia, relaxation of the uterine supporting ligaments reduces uterine support, exacerbating mechanical compression of the inferior vena cava (IVC) and simultaneously decreasing peripheral vascular resistance 6. MRI studies have shown that when pregnant women assume a left lateral tilt of more than 15°, IVC compression is reduced, significantly improving cardiac blood volume and effectively preventing supine hypotensive syndrome (SHS) [9, 10]. The IVC collapse index can serve as a predictive tool for hypotension after anesthesia induction. Specifically, variations in the maximum and minimum diameters of the IVC during the respiratory cycle can be quantified via bedside ultrasound before anesthesia, a method validated in studies involving both non-pregnant and pregnant women [11, 12].

However, due to its deep location, direct measurement of the IVC is challenging under increased uterine pressure. Consequently, some studies have focused on assessing the transverse diameter and peak flow velocity of the common femoral vein using ultrasound before spinal anesthesia to predict hypotension during pregnancy and maternal anesthesia. Studies suggest that an optimal transverse diameter exceeding 12.2 mm may effectively forecast hypotensive events [13]. Nevertheless, various factors influence the diameter and peak flow velocity of superficial veins, leading to ongoing debates regarding error avoidance strategies.

As the largest branch of the IVC, the external iliac vein is superficial and allows for easier monitoring of flow changes by ultrasound compared to the IVC. Additionally, it avoids measuring factors that may influence lateral diameter and peak flow velocity of peripheral veins [14, 15]. Studies have demonstrated that when a puerpera transitions from supine to left lateral position with an angle greater than 15°, there is reduced uterine compression on the IVC. This facilitates return of blood stasis in lower limbs and abdominal cavity to systemic circulation, resulting in a relative increase in external iliac venous flow per unit time [16, 17]. Therefore, this study aimed to measure flow changes and assess reactivity of indirect response capacity under different positions prior to anesthesia as a means of predicting post-anesthesia supine hypotensive syndrome occurrence.

Methods

Population and study design

This study (Approval No. : 2021-12) was approved by the Ethical Committee of the Henan Provincial People’s Hospital (Chairman Prof Fengmin Shao) on 25 January 2021 and this prospective observational study was registered at http://www.chictr.org.cn (ChiCTR2100046925). Written informed consent was obtained from all patients.

In accordance with the Declaration of Helsinki, a total of 120 parturients were enrolled. All participants were aged 18–40 years, with full-term singleton pregnancy (> 37-week gestation), height of 156–170 cm, and American Society of Anesthesiology (ASA) physical status I–II, and underwent elective cesarean delivery under combined spinal–epidural (CSE) anesthesia between January 2022 and June 2023. All study participants provided signed informed consent. The exclusion criteria were: an ASA score of III–IV; contraindications to spinal anesthesia; prolonged pregnancy (> 42 weeks); preexisting or pregnancy-induced hypertension or preeclampsia; placenta previa; placental abruption; multiple pregnancies; morbid obesity (body mass index [BMI] ≥ 36); fetal distress or fetal abnormalities; emergent cesarean delivery; or parturient refusal.

Parturients scheduled for cesarean delivery had fasted for a minimum of six hours prior to the procedure. Ultrasonographic assessments were conducted 15 min before anesthesia administration, with participants positioned on a transfer bed in the post-anesthesia care unit. A Philips EPIQ7 ultrasound system equipped with an L12–5 linear array probe (5–12 MHz) was utilized to evaluate right external iliac venous flow (REIVF) throughout one respiratory cycle. Measurements were obtained at the level of the inguinal ligament, between the anterior superior iliac spine and the pubic tubercle. Using pulsed-wave Doppler mode (Fig. 1a), the Doppler sample volume was positioned at the vessel center, and the calibration line was oriented parallel to blood flow at a 50°–60° angle (Figs. 1b and 2). All scans were performed by a certified ultrasound specialist, with both anesthesiologists and parturients blinded to the findings.

Fig. 1.

a Pulsed-wave Doppler mode. b Doppler sample volume placed at the center of the vessel and the calibration line aligned parallel to the blood flow at an angle of 50°–60°Respond 7

Fig. 2.

B-mode ultrasound image of the external iliac vein in the inguinal region

The parturient was then transferred to the operating room. An intravenous line was established with an 18-G catheter in the dorsum or wrist vein of the right hand. Standard monitoring, including electrocardiography, non-invasive blood pressure, and pulse oximetry, was continuously performed. The first two resting systolic pressure (SBP) and heart rate (HR) measurements (Mindray BeneVision N22) with the parturient in the supine position were recorded and averaged as baseline values. Parturients with baseline SBP exceeding 140 mmHg were excluded due to suspected hypertension. Combined spinal-epidural anesthesia was administered at the L3–4 level with the parturient in the right lateral decubitus position. After detecting cerebrospinal fluid, 12 mg of 0.75% isobaric ropivacaine was intrathecally injected via a 27-G Whitacre needle, and an epidural catheter was inserted 3 cm into the epidural space using an 18-G Tuohy needle. The parturient was immediately placed in a supine position with left uterine displacement using a wedge under the right hip until delivery. Lactated Ringer’s solution was administered at a co-loading rate of 1 mL/kg/min until delivery. Parturients with sensory block levels below T6 at the start of surgery were excluded. Epidural boluses of 5 mL lidocaine were administered intermittently until satisfactory anesthesia was achieved [13].

Hypotension was defined as a decrease in SBP greater than 20% from baseline before delivery. If hypotension occurred, a rescue bolus of 50 µg of phenylephrine was administered and repeated as needed. Bradycardia was defined as HR < 50 beats per minute (bpm), treated with 0.5 mg atropine. Apgar scores at 1 and 5 min and neonatal body weight were recorded. Additionally, 1 mL of umbilical artery (UA) blood was collected immediately after delivery for blood gas analysis using an ISTAT CG4 + test cartridge [13].

The primary outcome was the change in REIVF over one respiratory cycle measured by ultrasound in different pre-anesthesia positions and its association with post-spinal hypotension during cesarean delivery. Maternal and obstetric characteristics, including age, weight, height, BMI, gravidity, parity, gestational weeks, induction-to-delivery interval, upper sensory block level, total IV fluid administered before delivery, total doses of phenylephrine and atropine, neonatal body weight, Apgar scores at 1 and 5 min, and UA blood pH, were also recorded.

Statistical analysis and sample size calculation

The sample size was calculated to detect positional differences in REIVF, while the predictive analysis using logistic regression/ROC should be interpreted in an exploratory manner. The sample size was estimated using G-power software (v.3.1.9.2), with a two-sided test with alpha of 0.05 and test efficacy (1-β) of 90%, requiring at least 100 women. The sample size was designed to detect the medium effect size (0.48) with 90% actual efficacy using bilateral paired t-tests. Considering a 20% dropout rate, a total of 120 cases were enrolled in this study.

The patient and obstetric characteristics are presented as the mean ± SD or median (interquartile range), as appropriate and were analyzed using an unpaired Student’s t-test, Fisher’s exact probability test or Pearson’s chi-squared test, as appropriate.

The parameters of the REIVF, measured by ultrasound in the supine position, and the supine position with a left tilt of 15°, were analyzed by multiple logistic regression analysis to determine if they were independently associated with the incidence of post-spinal hypotension. Collinearity analysis was performed for each factor. The VIF values of biparietal diameter, head circumference, abdominal circumference, fetal weight and amniotic fluid index were 6.245, 5.679, 3.200, 2.283 and 1.004 respectively. Thereafter, the area under the receiver operating characteristic (ROC) curves were used to test the ability of the identified parameters to predict post-spinal hypotension, and the area under the curve (AUC) was calculated. The AUC is a measure of the accuracy of a particular parameter (AUC ≤ 0.5 indicates no predictive ability and AUC = 1.0 indicates the best possible prediction). The maximal value of Youden’s index was used as the criterion for selecting the optimum cut-off values of the predictive parameters; Youden’s index was calculated as follows:

The perioperative hemodynamic parameters were assessed by repeated-measures ANOVA(F = 8.341,P = 0.005 < 0.05). All statistical analyses were performed using SPSS for Windows, version 24.0 (SPSS Inc., Chicago, IL, USA). Statistical significance was set at a P-value < 0.05.

Results

A total of 120 patients were included in this study, of which 62 had a temperature sensory block plane below T6 and 1 patient had a temperature sensory plane above T4, so 3 patients were excluded. Finally, 117 cases were analyzed.

Hypotension occurred in 64 (54.7%) parturients. Patient characteristics in the hypotension and non-hypotension groups are presented in Table 1. There was no significant difference with respect to age, weight, height, arity, gestational age, baseline SBP and HR (Table 1).

Table 1.

Comparison of general condition and fetal information between the two groups

Group	n	Age (years)	Height (cm)	Weight (kg)	Gestational age (weeks)	Baseline SBP (mmHg)	Baseline HR (bpm)
Group hypotension	64	32.7 ± 4.1	162 ± 0.1	73.4 ± 9.9	38.2 ± 0.9	113.5 ± 10.8	87.6 ± 11.5
Group non-hypotension	53	32.3 ± 7.2	163 ± 0.1	73.7 ± 9.4	37.9 ± 2.0	117.4 ± 11.5	84.4 ± 14.1

Data are presented as mean ± SD or number of patients

Abbreviations bpm beats per minute, HR heart rate, SBP systolic pressure

Table 2 shows the venous flow per respiratory cycle of REIVF at different positions in the hypotension and non-hypotension groups. At left tilt of 15°, the REIVF in the non-hypotensive group was significantly higher than that in the hypotensive group. At left inclination 15°, the REIVF per respiratory cycle in both groups was significantly higher than in the supine position (P < 0.001).

Table 2.

The parameters of vessels probed by ultrasound between two groups

Index	Group	n	Supine position	Left inclination 15°
REIVF (mL/min)	Group hypotension	64	108.9 ± 47.9	266.8 ± 93.6ab
	Group non-Hypotension	53	125.2 ± 82.5	167.1 ± 96.3b

Data are presented as mean ± SD

a, There were statistical differences in group comparisons; b, There were statistical differences in intra-group comparisons Respond 8

Table 3 shows that the difference in external iliac vein flow measured by ultrasound in the supine position and at 15° (ΔREIVF) to the left was associated with the occurrence of SHS after spinal anesthesia during cesarean section (OR = 1.071, 95% CI: 1.038–1.104, P < 0.001). Other parameters measured by ultrasound were not associated with the occurrence of SHS. All parameters model-fit assessment was performed.

Table 3.

Results of multiple logistic regression analysis predicting the incidence of postspinal hypotension after elective cesarean section

	OR	95% CI		P-value
		Lower limit	Upper limit
ΔREIVF	1.071	1.038	1.104	< 0.001
Biparietal diameter	1.144	0.773	1.694	0.501
Head circumference	1.057	0.923	1.211	0.420
Abdominal circumference	0.982	0.922	1.047	0.579
Fetal weight	0.999	0.996	1.002	0.506
Amniotic fluid index	0.994	0.972	1.016	0.562

Abbreviations REIVF right external iliac vein flow

ROC curve analysis showed that ΔREIVF was most suitable for predicting maternal hypotension risk (area under the curve = 0.869, 95% confidence interval: 0.8011–0.9377, P < 0.001)(Fig. 3). The optimal cut-off value for the maximum Youden index was 125.85 mL/min (sensitivity 70.5%; specificity 92.5%).

Fig. 3.

ROC curve of the venous flow per respiratory cycle of ΔREIVF before the spinal anesthesia for cesarean delivery. The optimal cut-off value for predicting the incidence of hypotension in ΔREIVF was 125.85mL/min, area under the ROC curve, with 95% CIs showed in the figure. Abbreviations: ROC, receiver operating characteristics; REIVF, right external iliac vein flow; ΔREIVF = REIVF (15° tilt) – REIVF (supine position)

Table 4 shows the obstetric characteristics grouped by the venous flow per respiratory cycle of REIVF. The total dose of phenylephrine administered was significantly higher in the group with ΔREIVF > 125.85 mL/min than in the group with ΔREIVF ≤ 125.85 mL/min (P < 0.05). There was no significant difference in the total dose of atropine administered, surgery time, neonatal body weight, 1 min and 5 min Apga scores and the pH of UA blood between the two groups.

Table 4.

Obstetric characteristics by the venous flow per respiratory cycle of REIVF

Groups	Number of cases	Dosage of phenylephrine (ug)	Atropine dosage	1 min Apgar	5 min Apgar	Umbilical artery PH
ΔREIVF > 125.85 ml/min group	48	25.3 ± 12.0a	0.0 ± 0.0	9.6 ± 0.5	10.0 ± 0.0	7.29 ± 0.06
ΔREIVF ≤ 125.85 ml/min group	69	9.9 ± 10.9	0.0 ± 0.0	9.7 ± 0.4	10.0 ± 0.0	7.29 ± 0.05

Data are presented as mean ± SD

A, compared with the ΔREIVF ≤ 125.85 ml/min group, p < 0.05

Although SBP decreased in both the groups (ΔREIVF > 125.85 mL/min and ΔREIVF ≤ 125.85 mL/min), SBP decreased markedly in the group with ΔREIVF > 125.85 mL/min at the time points of 4 min and 5 min after spinal anesthesia (P = 0.001 and < 0.001, respectively; Fig. 4a). Moreover, HR increased significantly in the group with ΔREIVF ≤ 125.85 mL/min at the time point of 4 min after spinal anesthesia (P = 0.011; Fig. 4b).

Fig. 4.

a: Serial changes in systolic blood pressure (SBP) at different time points after spinal anesthesia. b: Serial changes in heart rate (HR) at different time points after spinal anesthesia. *A statistically significant difference between the two groups was assessed by repeated-measures ANOVA. Abbreviations: SBP, systolic pressure; HR, heart rate; bpm, beats per minute

Discussion

The objectives of the present study were to evaluate the variations in REIVF throughout a respiratory cycle under different postures of puerperae prior to spinal anesthesia and to predict the likelihood of supine hypotension following spinal anesthesia. It was determined that an alteration exceeding 125.85 mL/min in external iliac vein flow under various postures before spinal anesthesia could serve as an indicator for predicting the subsequent occurrence of supine hypotension. A total of 120 patients were enrolled, with a hypotension incidence rate of 54.7%, which closely corresponded to previous reports on maternal hypotension after spinal anesthesia. To ensure the safety of both mother and child, based on relevant literature and clinical experience, observations for hypotension were terminated at 4 min after spinal anesthesia while simultaneously appropriate treatment for hypotension was initiated [13, 17]. Although there were differences in the oxytocin dosage between the two groups, no statistically significant disparities were observed in fetal weight after birth or Apgar scores at 1 min and 5 min. Furthermore, no statistically significant differences were found in umbilical artery blood pH or any complications.

Several studies have demonstrated the utility of MRI in assessing uterine compression of the vena cava during late pregnancy, with the IVC diameter serving as a reliable indicator [9–16]. However, several studies have suggested that changes in the inferior vena cava collapse index (IVC-CI) and diameter may not effectively predict severe hypotension after spinal anesthesia during lower limb surgery for patients who breathe spontaneously [18, 19]. Conversely, research has shown that the ratio of maximum IVC diameter to IVC-CI at exhalation before surgery can better predict the occurrence of hypotension following spinal anesthesia in elderly patients undergoing lower limb surgery [20]. Despite attempts to elucidate the relationship between IVC compression and the incidence of hypotension during cesarean section under spinal anesthesia, no study has yet clarified this association. Given the risk posed by the supine position to pregnant women, clinicians must improve diagnostic accuracy and treatment efficacy. This study hypothesizes that measuring the external iliac vein flow using a linear array probe can serve as an effective parameter for evaluating IVC compression in high-risk parturients experiencing post-cesarean section hypotension. In order to explore the high-risk factors for supine hypotension, our research team analyzed the gravity and buoyancy of the fetus within the amniotic cavity of the uterus based on Archimedes’ principle. We propose that amniotic fluid volume determines fetal buoyancy, whereas fetal weight, abdominal circumference and head circumference are key determinants of fetal gravitational load. Therefore, in this study, we identified ΔREIVF (changes in placental blood flow index), fetal biparietal diameter, head circumference, abdominal circumference, fetal weight and the amniotic fluid index as potential high-risk factors for supine hypotension, and analyzed them using ROC curves. The final results showed that among all these high-risk factors, ΔREIVF had the largest AUC value and the strongest correlation with supine hypotension. The venous flow per respiratory cycle of the REIVF in the hypotension group was significantly greater than that in the non-hypotension group in the supine position with a left tilt of 15°. The more severe compression of the IVC in the hypotension group may be attributed to the closer proximity of the uterus to this vessel and its greater impact on blood flow, exacerbated by muscle relaxation after spinal anesthesia, which weakens uterine support from ligaments, pelvic floor muscles and fascia. This increased compression is likely to result in hypotension following anesthesia. Previous studies have demonstrated a significant association between preoperative baseline vascular tone, systemic blood volume and the incidence of hypotension following spinal anesthesia in cesarean section. Consequently, the toe perfusion index (PI) can serve as a reliable predictor for the occurrence of hypotension. Xu et al. confirmed that PI values of both left and right toes could accurately forecast the likelihood of hypotension after spinal anesthesia in cesarean section [8]. While these parameters are valuable predictors of the development of hypotension post-spinal anesthesia, their detection often necessitates additional medical devices. However, ultrasound is an indispensable tool readily accessible to anesthesiologists akin to an anesthesia machine. In recent years, imaging modalities such as ultrasound and MRI have been utilized for identifying aortic and IVC compression by the gravid uterus. Fujita et al. reported that a left lateral decubitus position of 15° consistently reduced IVC compression compared to the supine position [9]. The right lateral decubitus position at any angle did not result in an increase in average volume of the IVC when compared to the left lateral decubitus position of 15°. Yao et al. found that an increased transverse diameter of RCFV was correlated with the occurrence of hypotension after spinal anesthesia, and could be used as a predictive measure for parturients at risk before anesthesia administration [13]. Duclos et al. employed a noninvasive measurement device Clearsight combined with a digital cuff to calculate stroke volume and analyzed its performance in predicting hypotension [18]. They concluded that this digital noninvasive monitoring device could effectively predict the occurrence of hypotension after spinal anesthesia through stroke volume variation analysis. These indicators indirectly reflect the degree of compression on both the aorta and IVC located at their distal end points where compression occurs. The present study aims to further explore and predict the risk of post‑spinal anesthesia hypotension in parturients.

The ultrasound measurements in this study had two key highlights. First, the flow of the external iliac vein during one respiratory cycle was assessed using pulsed-wave (PW) mode ultrasound, and the difference ΔREIVF was obtained by measuring the flow at different positions. While B-mode ultrasound is widely used for vessel diameter measurements, PW-mode ultrasound enables visualization of the flow in the external iliac vein during a single respiratory cycle. To minimize systematic errors, we measured the flow of the external iliac vein as an observational index. Second, our focus was on comprehensively evaluating the impact of IVC compression on lower extremity venous system, including both the femoral and great saphenous veins. We measured the flow in the external iliac vein starting from below the right inguinal ligament at the position of the common femoral vein and scanned upwards.

Limitations of this study: 1. All parturients received fluid replacement before spinal anesthesia, and crystalloid solution was used. Whether the use of colloid solution in advance can reduce the occurrence of SHS is still worth further investigation;2. All parturients in this study were lying on their sides at 15°. Whether other angles of lying on their sides (such as 30° and 45°) can more accurately predict the occurrence of SHS still needs further investigation. The sample size was calculated to detect differences in REIVF between positions; therefore, the predictive findings derived from logistic regression and ROC analysis should be interpreted as exploratory and hypothesis-generating. Respond 11.

Conclusion

The changes of REIVF vein flow in different positions before anesthesia can predict the occurrence of SHS after spinal anesthesia.

Authors’ contributions

Xiaoguo Ruan: Drafting the work, acquisition, analysis, and interpretation of the data for the work.Pengfei Niu: Formal analysis, writing review and editing. Mingyang Sun, Ningtao Li, Yongfeng Zhu, Jiaqiang Zhang, Enqing Chang: Conception of the work, Supervision, critical revision for important intellectual content.All authors approved the final version of the manuscript to be published.

Funding

Not applicable.

Data availability

The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

The study was approved by the Ethics Committee of the Henan Provincial People’s Hospital (Ethics approval Number: 2021-12) and had been approved by the China Clinical Trials Registry (ChiCTR2100046925).

Written informed consent was obtained from all patients before enrolment.

Consent for publication

Not Applicable.

Competing interests

The authors declare no competing interests.