Secondary Logo

Journal Logo

Obstetric Anesthesiology: Original Clinical Research Report

Differential Roles of the Right and Left Toe Perfusion Index in Predicting the Incidence of Postspinal Hypotension During Cesarean Delivery

Xu, Zifeng MD, PhD*; Xu, Tao MD*; Zhao, Puwen MD*; Ma, Rui MD*; Zhang, Mazhong MD, PhD; Zheng, Jijian MD, PhD

Author Information
doi: 10.1213/ANE.0000000000002393
  • Free

Spinal anesthesia is the standard anesthetic for elective cesarean delivery.1,2 Approximately 70% of parturients develop hypotension after spinal anesthesia, as compared to only about 33% of nonpregnant patients, and the degree of hypotension is often more severe and abrupt.3 Aortocaval compression by the gravid uterus, low baseline vasomotor tone, and spinal anesthesia–related sympathetic blockade likely contribute to this abrupt onset and profound hypotension in parturients under spinal anesthesia.4–6 Without proper management, severe postspinal hypotension can increase the risks of maternal and fetal complications. Therefore, it is essential for anesthesiologists to predict the incidence of postspinal hypotension in parturients and identify the possible mechanisms.

A number of noninvasive methods based on indirect evaluation of autonomic nervous system activity and vasomotor tone has been explored to predict spinal hypotension in parturients, but none reflect aortocaval compression by the gravid uterus.4,7–9 These predictive methods include the measurement of thoracic electrical bioimpedance, heart rate (HR) variability, and perfusion index (PI) parameters. The PI provides a noninvasive and continuous method for assessment of peripheral tissue perfusion by calculating the ratio between the pulsatile (arterial compartment) and nonpulsatile components (venous and capillary blood and other tissues) of the light reaching the detector of the pulse oximeter at peripheral tissues.10 Peripheral PI changes correspond to changes of local blood volume pulsations and intravascular pulse pressure; both are affected by the distensibility of the vascular wall or vascular tone. Low PI usually reflects peripheral vasoconstriction with or without severe hypovolemia and high PI usually reflects dilation of peripheral blood vessels.10–12 A previous study showed that a finger PI >3.5 measured before spinal anesthesia for cesarean delivery identifies women at increased risk for spinal hypotension, and has been proposed to reflect low baseline vascular tone.4

In contrast to the observation that high finger PI values (>3.5) before spinal anesthesia predict spinal hypotension in parturients,4 our pilot observation suggested that supine parturients with low toe PI before spinal anesthesia were more likely to develop spinal hypotension. The most likely reason underlying the difference of finger PI and toe PI in supine parturients before spinal anesthesia would be the effects of direct aortocaval compression by the gravid uterus. Therefore, we speculated that measurement of baseline toe PI and changes of toe PI after spinal anesthesia would reflect aortocaval compression by the gravid uterus and would be associated with the incidence of postspinal hypotension in parturients. In addition, the predictive accuracy might be different between left toe PI and right toe PI because right iliac vessels are more often compressed by gravid uterus.13,14

METHODS

Patients

The study protocol was approved by the Institutional Ethics Committee and verbal informed consent was obtained from all included parturients. One hundred consecutive uncomplicated singleton full-term parturients (American Society of Anesthesiologists physical status I, II) scheduled for elective cesarean delivery under combined spinal–epidural anesthesia were included in this study between December 2014 and August 2015. The exclusion criteria were as follows: parturient age ≤18 years or ≥40 years, gestational age <36 or ≥41 weeks, emergency cases, placenta previa, pre-eclampsia, cardiovascular or cerebrovascular disease, diabetes, autonomic neuropathy, known anxiety disorder, any known medicine use, known fetal abnormality, morbid obesity (body mass index [BMI] ≥40), and any contraindications to spinal anesthesia.

Anesthesia and Monitor Methods

No premedication or prophylactic antiemetics were given to the parturients. All the parturients were fasted at least 6 hours for solid food and 2 hours for clear liquid before spinal anesthesia. Room temperature lactated Ringer’s solution was administered about 10 mL/kg/h just before the initiation of spinal anesthesia until the end of surgery. All the parturients were directly transported to the operating room from the ward without prewarming, and the operating room temperature was maintained at 22°C. All the parturients were monitored by automatic noninvasive arterial pressure on the right arm, finger pulse oximetry on the left index finger, and electrocardiogram (IntelliVue MP30, PHILIPS Healthcare, Boblingen, Germany). In addition, 2 Masimo Radical 7 pulse oximeter probes (Masimo Corp, Irvine, California) were put on the left and right second toes of the parturients for continuous monitoring of the toe PI and pleth variability index (PVI) until the fetus was delivered. After spinal anesthesia, the blood pressure measurement interval was changed from every 3 minutes to every 1 minute until the fetus was delivered. The toe PI and PVI values before spinal anesthesia and 3 minutes after spinal anesthesia were used for further analysis. The anesthesiologists in this study were blinded to the values of the PI and PVI. Baseline values of systolic blood pressure (SBP), diastolic blood pressure (DBP), HR, peripheral oxygen saturation, PI, and PVI were recorded in the supine position without left uterine displacement.

Participants turned to the right lateral decubitus position for standard combined spinal–epidural anesthesia, performed at the L3–4 or L2–3 vertebral interspace. After a 16-gauge Tuohy needle was inserted in the epidural space, a 25-gauge Whitacre spinal needle was punctured into the subarachnoid space through the Tuohy needle. Two milliliters of 0.75% ropivacaine were diluted with cerebrospinal fluid to 3.0 mL, after which approximately 2.4 mL was injected into the subarachnoid space over 15–30 seconds. After the epidural catheter was secured, the parturient returned to the supine position without left uterine displacement. Oxygen at 5 L/min was administered by nasal cannula. The sensory blockade level was checked 5 minutes after spinal injection using alcohol swabs. If a T6 sensory block level was not achieved, 2% lidocaine was administered through the epidural catheter in 4–10 mL increments until T6 sensory block level was achieved, and these parturients were excluded from further analysis.

Criteria and Interventions to Treat Postspinal Hypotension in Parturients

Postspinal hypotension in parturients was defined as SBP <80 mm Hg5,15 or complaints of symptoms consistent with hypotension (eg, faintness, dizziness, breathlessness, nausea, or vomiting) even without SBP <80 mm Hg.9 Once postspinal hypotension occurred, a bolus of 50 µg phenylephrine or 15 mg ephedrine was administered as a rescue medication depending on the patient’s HR. If HR was lower than 60 beats/min, ephedrine was administered; if HR was higher than 60 beats/min, phenylephrine was used. A bolus of 0.5 mg atropine was administered if the HR dropped below 55 beats/min without the occurrence of postspinal hypotension. The incidence of postspinal hypotension was recorded from the onset of spinal anesthesia to the time when the fetus was removed from the uterus.

The primary hypothesis in this study was that toe PIs would predict the incidence of postspinal hypotension in parturients. The secondary hypothesis was that left toe PI would be more useful than right. Other exploratory hypotheses included that baseline toe PVI, maternal body weight, BMI, and the difference in toe PI measured 3 minutes before and 3 minutes after spinal anesthesia would be associated with the incidence of supine hypotensive syndrome.

Statistical Analysis

Medcalc 12.5 (MedCalc Software, Ostend, Belgium) and Origin 8.0 (OriginLab Corporation, Northampton, MA) software were used to analyze the collected data. The patient data are presented as the mean (standard deviation) or median (interquartile range [IQR]). Data normality was assessed by Shapiro-Wilk test. General characteristics of the parturient and fetus, surgery time, fluid volume, preanesthetic hemodynamic parameters, baseline toe PI and PVI between parturients who developed postspinal hypotension and those who did not were compared by unpaired t test or Mann-Whitney test, as appropriate. The anesthetic hemodynamic parameters, toe PI and PVI before and after anesthesia were compared by paired t test for normally-distributed data and Wilcoxon signed rank test for nonnormally-distributed data. The relationships of PI, PVI, and the patient characteristic data with the incidence of postspinal hypotension were first analyzed by individual logistic regression analysis to find variables univariably associated with the incidence of postspinal hypotension. Then multivariable forward selection logistic regression analysis was used to find variables independently associated with outcome at P < .05. The areas under the receiver operating characteristic curves (AUROC) were used to measure our primary hypothesis. An AUROC value of 0.5 shows no predictive ability for toe PIs, AUROC = 0.7–0.79 corresponds to acceptable prediction ability, AUROC = 0.8–0.89 corresponds to excellent prediction ability, and AUC = 0.9–1.0 corresponds to outstanding prediction ability16; Z statistics/overlap of 95% confidence interval (CI) was used to test our secondary hypothesis through comparing the difference of AUROCs between left toe PI and right toe PI. The optimal threshold value was determined by Youden index J. Bootstrap method with 1000 resampling times was used to calculate a BCa bootstrapped 95% CI for the Youden index and its associated criterion value. All hypothesis tests were 2 tailed, and P < .05 was considered statistically significant. AUROC was used to estimate sample size by Medcalc software, the AUROC of 0.7 for toe PI test was considered to be a clinically important difference from the null hypothesis value (0.5), and the expected incidence rate of postspinal hypotension was more than 33%, α level at .05, β level at .10.

RESULTS

General Characteristics of Parturient, Neonate, Anesthesia, and Surgery

Table 1.
Table 1.:
General Characteristics of Parturient, Fetus, Anesthesia, and Surgery

Two hundred fifty-two parturients were screened and 100 parturients met inclusion criteria and agreed to participate this study. Six parturients were excluded from further analysis because of an inadequate blockade level of spinal anesthesia. The patient and obstetric characteristics are shown in Table 1. Forty-two of 94 parturients (44.7%) developed postspinal hypotension; 13 of these 42 parturients had an SBP >80 mm Hg but complained of faintness, dizziness, breathless, nausea, vomiting, or other symptoms. No significant differences were found in the age, height, gestational age, neonatal weight, administered fluid volume before spinal anesthesia and the total volume during surgery, or surgical duration between the postspinal hypotension and non–postspinal hypotension parturients, whereas body weight and BMI were significantly higher in the postspinal hypotension parturients.

Hemodynamic Changes Associated With Spinal Anesthesia

Figure 1.
Figure 1.:
Hemodynamic changes induced by spinal anesthesia during cesarean delivery. A, Changes of SBP; (B) changes of DBP; (C) changes of HR. Black bar, NPSH parturients, before spinal anesthesia. Red bar, NPSH parturients, after spinal anesthesia. Cyan bar, PSH parturients, before spinal anesthesia. Magenta bar, PSH parturients, after spinal anesthesia. **P < .001, compared to preanesthetic hemodynamic parameters in PSH parturients; # P < .05, compared to preanesthetic hemodynamic parameters in NPSH parturients. DBP indicates diastolic blood pressure; HR, heart rate; NPSH, non–postspinal hypotension; PSH, postspinal hypotension; SBP, systolic blood pressure.

There were no significant differences in the baseline SBP, DBP, and HR between the postspinal hypotension and non–postspinal hypotension parturients. In the postspinal hypotension parturients, the median of SBP and DBP changes before and after spinal anesthesia were 51.5 mm Hg (IQR, 35–62; P < .0001) and 33 mm Hg (IQR, 26–43; P < .0001), respectively. In the non–postspinal hypotension gravidae, the median of SBP and DBP changes before and after spinal anesthesia were 5.5 mm Hg (IQR, 1.0–17; P < .0001) and 5.5 mm Hg (IQR, 1.0–12.5; P = .0001), respectively. Compared to the nonpost-spinal hypotension parturients, there were significantly larger changes of SBP and DBP values before and after spinal anesthesia in the postspinal hypotension parturients, P < .001 (Figure 1).

Toe PI 3 Minutes Before Spinal Anesthesia

Figure 2.
Figure 2.:
The receiving operator characteristic curves for baseline toe PIs. Red dotted line, left baseline toe PI. Blue dotted line, right baseline toe PI. PI indicates perfusion index.

The baseline median values of the left and right toe PIs were significantly lower in the postspinal hypotension parturients (1.2 [IQR, 0.79–1.9] and 1.1 [IQR, 0.66–1.8]) than those in the non–postspinal hypotension parturients (2.8 [IQR, 1.8–3.4] and 2.2 [IQR, 1.4–3.0]), P < .001 for both comparisons. The left toe area under the ROC curve was 0.81 (95% CI, 0.71–0.88). The optimal cutoff point of the preanesthetic PI to predict the occurrence of postspinal hypotension was 2.2 (95% CI, 1.4–2.2), with a sensitivity of 92.9% (95% CI, 80.5%–98.5%) and specificity of 61.5% (95% CI, 47.0%–74.7%). The right toe area under the ROC curve was 0.76 (95% CI, 0.66–0.84). The optimal cutoff point was 1.3 (95% CI, 0.99–2), with a sensitivity of 61.9% (95% CI, 45.6%–76.4%) and specificity of 84.6% (95% CI, 71.9%–93.1%). Based on z statistics, there was no significant difference of predictive accuracy between the left and right baseline toe PIs, with a 0.046 difference in areas under the ROC curves (95% CI, −0.06 to 0.15; P = .3721; Figure 2).

The Difference in Toe PI Measured 3 Minutes Before and 3 Minutes After Spinal Anesthesia

Figure 3.
Figure 3.:
Changes of toe PI and PVI induced by spinal anesthesia during cesarean delivery. A, Changes of left toe PI; (B) changes of left toe PVI; (C) changes of right toe PI; (D) changes of right toe PVI. Black box, NPSH parturients, before spinal anesthesia. Red box, NPSH group, after spinal anesthesia. Cyan box, PSH parturients, before spinal anesthesia. Magenta box, PSH parturients, after spinal anesthesia. ## P < .001, compared to preanesthetic PI in NPSH parturients; **P < .001, compared to preanesthetic PI in NPSH parturients. The horizontal line in the box is the statistical median; the maximum and minimum values are displayed with vertical lines (“whiskers”) connecting the points to the center box; the horizontal line to the top of the box is the upper quartile values; the horizontal line to the bottom of the box is the lower quartile values. NPSH indicates non–postspinal hypotension; PI, perfusion index; PSH, postspinal hypotension; PVI, pleth variability index.
Figure 4.
Figure 4.:
The receiving operator characteristic curves for the toe PI changes at 3 min after spinal anesthesia. Red dotted line, left toe PI changes. Blue dotted line, right toe PI changes. PI indicates perfusion index.

In the non–postspinal hypotension parturients, the median values of toe PIs increased after spinal anesthesia by 2.7 (IQR, 0.7–4.0; P < .0001) and 2.3 (IQR, 0.55–3.8; P < .0001), respectively. In the postspinal hypotension parturients, the median values of the either left or right toe PI did not increase after spinal anesthesia, −0.11 (IQR, −0.7 to 1.34; P = .67) and −0.28 (IQR, −0.94 to 0.51; P = .33), respectively (P = .33; Figure 3A, C). The AUROC was 0.67 (95% CI, 0.56–0.76) for toe PI changes measured on the left toe, and 0.76 (95% CI, 0.66–0.84) for changes measured on the right (Figure 4). The difference of areas under the ROC curves between the left and right toe PI changes at 3 minutes after spinal anesthesia was 0.087 (95% CI, −0.029 to 0.202; P = .1401).

Exploratory Logistic Regression Analysis of Covariates

Table 2.
Table 2.:
Results of Individual Variable Logistic Regression Analysis to Predict the Incidence of Postspinal Hypotension During Elective Cesarean Delivery

Individual logistic regression analysis demonstrated that maternal body weight, BMI, and baseline toe PIs were associated with the incidence of postspinal hypotension (Table 2). Variance inflation factor analysis showed that BMI and weight were collinear; therefore, only maternal BMI and baseline toe PI were then included in multivariable forward logistic regression analysis and the result showed that baseline toe PIs (left and right) and BMI had a significant association with the incidence of postspinal hypotension. Further ROC curves analysis showed that BMI was not a good predictor of postspinal hypotension since the AUROC of BMI was <0.7 (0.66, 95% CI, 0.55–0.75).

DISCUSSION

In the present study, we demonstrated that baseline toe PI in the postspinal hypotension parturients was smaller than that in the non–postspinal hypotension parturients. Baseline toe PI might be used to predict the incidence of hypotension of pregnancy after spinal anesthesia. Although the maternal body weight and BMI in parturients with postspinal hypotension were higher compared to those without, these measures of obesity were not associated with the incidence of postspinal hypotension in parturients. Conversely, the preanesthetic HR and toe PVI were not predictive factors. In contrast, Yokose et al17 identified preanesthetic HR, but not other parameters derived from pulse oximetry or HR variability, as a prognostic factor for hypotension associated with spinal anesthesia.

Our results showed that the areas under the ROC curves were 0.81 and 0.76 for left and right toe PI, respectively, before spinal anesthesia. Although there was no significant difference between left and right toe PIs in predicting the incidence of postspinal hypotension in parturients, the lower limit of 95% AUROC CI of left toe PI was 0.71, which also was above the cutoff of 0.7, while the right was only 0.66.

Although our data suggest that baseline toe PI can be used to predict the incidence of postspinal hypotension in parturients, the normal PI value varies dramatically, ranging from 0.02% to 20%, and is affected by environmental temperature, stress, and circulating blood volume, among other factors.18,19 Therefore, we also analyzed the change in PI after spinal anesthesia, because spinal anesthesia eliminates the sympathetic-related PI reduction. In contrast to the increase of toe PI observed after neuraxial anesthesia in nonobstetric patients20,21 and in the non–postspinal hypotension parturients herein, the toe PI in the postspinal hypotension parturients did not increase significantly. These findings are consistent with the theory that spinal-induced sympathectomy causes the toe PI to increase, while direct compression of the artery and vein results in PI reduction, and that spinal anesthesia tends to worsen the aortocaval compression from the gravid uterus, which is in line with previous reports that spinal anesthesia significantly increased the incidence of severe hypotension during cesarean delivery.3,22 Although radiological angiography, radionuclide venography, ultrasound, and magnetic resonance imaging, among other techniques, can be used to measure the aortocaval compression by the pregnant uterus,23–26 these techniques cannot be used in the operating room to reflect the dynamic aortocaval compression by the pregnant uterus in real-time. In contrast, toe PI monitoring may provide a noninvasive, continuous measure of aortocaval compression by the gravid uterus during cesarean delivery under spinal anesthesia.

Similar to previous reports,17,27 we did not demonstrate a predictive effect of baseline toe PVI. No significant differences in the baseline median PVI values were found in either the left or right toe between postspinal hypotension parturients and non–postspinal hypotension parturients. The accuracy of PVI has been reported to depend on mechanical ventilation28; in the present study, all patients breathed spontaneously.

There are some limitations in our study. First, we did not simultaneously measure the changes of PI and PVI at the fingers and toes, owing to insufficient equipment. We also did not confirm the aortocaval compression by imaging techniques. Intervention based on toe PI prediction of postspinal hypotension was not simultaneously conducted in our study. Due to the dramatic decrease of blood pressure in the postspinal hypotension parturients and the urgent need for intervention, we only performed logistic regression analysis to establish the relationships between the baseline parameters and the incidence of supine hypotension instead of assessing the degree of the decrease in SBP from the baseline.

In summary, our study demonstrated that baseline measures of toe PI predict subsequent spinal hypotension in parturients. In addition, exploratory findings suggest that dynamic changes in the toe PI after induction of spinal anesthesia might reflect the direct compression of the abdominal inferior vena cava and aorta by the gravid uterus.

DISCLOSURES

Name: Zifeng Xu, MD, PhD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Name: Tao Xu, MD.

Contribution: This author helped conduct the study and analyze the data.

Name: Puwen Zhao, MD.

Contribution: This author helped conduct the study and analyze the data.

Name: Rui Ma, MD.

Contribution: This author helped conduct the study and analyze the data.

Name: Mazhong Zhang, MD, PhD.

Contribution: This author helped design the study and write the manuscript.

Name: Jijian Zheng, MD, PhD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

This manuscript was handled by: Jill M. Mhyre, MD.

REFERENCES

1. Cyna AM, Andrew M, Emmett RS, Middleton P, Simmons SW. Techniques for preventing hypotension during spinal anaesthesia for caesarean section. Cochrane Database Syst Rev. 2006:CD002251.
2. Heesen M, Stewart A, Fernando R. Vasopressors for the treatment of maternal hypotension following spinal anaesthesia for elective caesarean section: past, present and future. Anaesthesia. 2015;70:252–257.
3. Klöhr S, Roth R, Hofmann T, Rossaint R, Heesen M. Definitions of hypotension after spinal anaesthesia for caesarean section: literature search and application to parturients. Acta Anaesthesiol Scand. 2010;54:909–921.
4. Toyama S, Kakumoto M, Morioka M, et al. Perfusion index derived from a pulse oximeter can predict the incidence of hypotension during spinal anaesthesia for Caesarean delivery. Br J Anaesth. 2013;111:235–241.
5. Hanss R, Bein B, Ledowski T, et al. Heart rate variability predicts severe hypotension after spinal anesthesia for elective cesarean delivery. Anesthesiology. 2005;102:1086–1093.
6. Holmes F. Spinal analgesia and caesarean section; maternal mortality. J Obstet Gynaecol Br Emp. 1957;64:229–232.
7. Ouzounian JG, Masaki DI, Abboud TK, Greenspoon JS. Systemic vascular resistance index determined by thoracic electrical bioimpedance predicts the risk for maternal hypotension during regional anesthesia for cesarean delivery. Am J Obstet Gynecol. 1996;174:1019–1025.
8. Hanss R, Bein B, Weseloh H, et al. Heart rate variability predicts severe hypotension after spinal anesthesia. Anesthesiology. 2006;104:537–545.
9. Berlac PA, Rasmussen YH. Per-operative cerebral near-infrared spectroscopy (NIRS) predicts maternal hypotension during elective caesarean delivery in spinal anaesthesia. Int J Obstet Anesth. 2005;14:26–31.
10. Goldman JM, Petterson MT, Kopotic RJ, Barker SJ. Masimo signal extraction pulse oximetry. J Clin Monit Comput. 2000;16:475–483.
11. Mowafi HA, Ismail SA, Shafi MA, Al-Ghamdi AA. The efficacy of perfusion index as an indicator for intravascular injection of epinephrine-containing epidural test dose in propofol-anesthetized adults. Anesth Analg. 2009;108:549–553.
12. Dorlas JC, Nijboer JA. Photo-electric plethysmography as a monitoring device in anaesthesia. Application and interpretation. Br J Anaesth. 1985;57:524–530.
13. Bieniarz J, Yoshida T, Romero-Salinas G, Curuchet E, Caldeyro-Barcia R, Crottogini JJ. Aortocaval compression by the uterus in late human pregnancy. IV. Circulatory homeostasis by preferential perfusion of the placenta. Am J Obstet Gynecol. 1969;103:19–31.
14. Eckstein KL, Marx GF. Aortocaval compression and uterine displacement. Anesthesiology. 1974;40:92–96.
15. Kato J, Tanaka T. [Shock due to supine hypotensive syndrome]. Sanfujinka No Jissai. 1967;16:118–123.
16. Jamal Talabani A, Endreseth BH, Lydersen S, Edna TH. Clinical diagnostic accuracy of acute colonic diverticulitis in patients admitted with acute abdominal pain, a receiver operating characteristic curve analysis. Int J Colorectal Dis. 2017;32:41–47.
17. Yokose M, Mihara T, Sugawara Y, Goto T. The predictive ability of non-invasive haemodynamic parameters for hypotension during caesarean section: a prospective observational study. Anaesthesia. 2015;70:555–562.
18. Granelli Ad, Ostman-Smith I. Noninvasive peripheral perfusion index as a possible tool for screening for critical left heart obstruction. Acta Paediatr. 2007;96:1455–1459.
19. Lima A, Bakker J. Noninvasive monitoring of peripheral perfusion. Intensive Care Med. 2005;31:1316–1326.
20. Ginosar Y, Weiniger CF, Meroz Y, et al. Pulse oximeter perfusion index as an early indicator of sympathectomy after epidural anesthesia. Acta Anaesthesiol Scand. 2009;53:1018–1026.
21. Xu Z, Zhang J, Shen H, Zheng J. Assessment of pulse oximeter perfusion index in pediatric caudal block under basal ketamine anesthesia. ScientificWorldJournal. 2013;2013:183493.
22. Marx GF. Supine hypotension syndrome during cesarean section. JAMA. 1969;207:1903–1905.
23. Higuchi H, Takagi S, Zhang K, Furui I, Ozaki M. Effect of lateral tilt angle on the volume of the abdominal aorta and inferior vena cava in pregnant and nonpregnant women determined by magnetic resonance imaging. Anesthesiology. 2015;122:286–293.
24. Dhekne RD, Barron BJ, Koch ER. Radionuclide venography in pregnancy. J Nucl Med. 1987;28:1290–1293.
25. Lee SW, Khaw KS, Ngan Kee WD, Leung TY, Critchley LA. Haemodynamic effects from aortocaval compression at different angles of lateral tilt in non-labouring term pregnant women. Br J Anaesth. 2012;109:950–956.
26. Kerr MG, Scott DB, Samuel E. Studies of the inferior vena cava in late pregnancy. Br Med J. 1964;1:522.4–52533.
27. Sun S, Huang SQ. Role of pleth variability index for predicting hypotension after spinal anesthesia for cesarean section. Int J Obstet Anesth. 2014;23:324–329.
28. Cannesson M, Desebbe O, Rosamel P, et al. Pleth variability index to monitor the respiratory variations in the pulse oximeter plethysmographic waveform amplitude and predict fluid responsiveness in the operating theatre. Br J Anaesth. 2008;101:200–206.
Copyright © 2017 International Anesthesia Research Society