Subcostal versus right lateral ultrasound measurements of inferior vena cava: Measurements obtained from these two views are not equivalent in non-ICU patients

Abstract

Objective:

The objective of this study is to assess concordance between the subcostal and right lateral view for ultrasonographic inferior vena cava measurements including the end-inspiratory diameter, end-expiratory diameter and respiratory variation represented by the caval index in spontaneously breathing healthy adults.

Methods:

We recruited a convenience sample of 33 healthy adults. A phased array ultrasound probe was used to obtain inferior vena cava measurements from a subcostal view in the sagittal plane and from a right lateral view in the coronal plane with B-mode ultrasound. End-inspiratory diameter, end-expiratory diameter and caval index were obtained for each view. A two-tailed t-test was performed to compare the caval indices obtained by the two views. Bland–Altman analysis was used to obtain the limits of agreement for the inferior vena cava diameter and caval index across the two views.

Results:

Subcostal and right lateral caval indices across all participants were significantly different according to a paired t-test (p < 0.0001). The Bland–Altman analysis showed wide limits of agreement in end-inspiratory diameter (−0.97 and 0.50 cm) and in end-expiratory diameter (−0.94 and 0.90 cm). The right lateral view underestimated the inferior vena cava caval index relative to the subcostal view.

Conclusions:

The subcostal and right lateral views are not equivalent in obtaining inferior vena cava measurements in spontaneously breathing healthy adults. Current cut-off values for measurement-based applications of inferior vena cava ultrasound, including fluid responsiveness using caval indices, may not be accurate when values are obtained from the right lateral view in the coronal plane of the inferior vena cava in patients.

Keywords

Inferior vena cava fluid responsiveness caval index respiratory variation

Introduction

Numerous studies suggest that ultrasound measurement of respiratory changes in the inferior vena cava (IVC) diameter and caval index (CI) can provide valuable information for the management of patients in emergency, critical care and other inpatient settings. These measurements have been supported as indicators of right atrial pressure,¹ volume status,² fluid responsiveness^3,4 and congestive heart failure.⁵ IVC ultrasound has been integrated into multiple aspects of medicine for multiple clinical applications based on such studies.

The clinical utility of the CI arises from the physiologic changes in the IVC that occur during spontaneous unsupported ventilation. Specifically, the diameter of the IVC varies with changes in intrathoracic pressure. During spontaneous inspiration, negative intrathoracic pressures increase venous return to the right atrium, which reduces the intraluminal volume and pressure in the IVC leading to decreased diameter. During spontaneous expiration, the increased intrathoracic pressures decrease venous return to the right atrium which results in less emptying and increased pressure within the IVC, increasing its diameter. This respiratory variation in the diameter of the IVC can be quantified as a percent change called the CI. The IVC diameter and CI are readily obtained using sonography.

Because of the clinical value of the respiratory variation in the IVC diameter and resulting CI, it is unfortunate that different studies obtained these measurements using different scanning windows of the IVC. Wallace et al. reviewed several of these studies and the different windows they used to obtain measurements.⁶ Despite the use of different IVC windows to obtain measurements, it appears that most studies used a subcostal approach to visualise the IVC in the sagittal plane. This is important because the IVC can be visualised with a right lateral view in the coronal plane as well. This approach can be easier to obtain in certain individuals, such as patients with obesity and postprandial patients. However, few studies have been conducted to verify the reliability of CI or other IVC measurements using this approach. Since the majority of studies validating the CI and IVC diameter cut-off values used a subcostal view, IVC values obtained with a right lateral view may not be as reliable for guiding clinical decision-making.

To our knowledge, only four studies to date have attempted to compare the IVC measurements taken from subcostal and right lateral views in adults.^7–10 Zhang et al. used a long axis approach in the subcostal and right lateral view to obtain end-inspiratory, end-expiratory and variability measurements of the IVC. Their study enrolled spontaneously breathing and mechanically ventilated patients in the ICU and found that measurements obtained from the two views were significantly different.⁷ Kulkarni et al. studied mechanically ventilated patients in shock and found discordance among the two views in the minimum and maximum diameter measurements. Interestingly, they found that the percent variability (CI) was not significantly different and therefore concluded that the two views are comparable in mechanically ventilated patients.⁸ Shah et al. studied the IVC measurements in spontaneously breathing patients in the ICU. They found that the two views provided significantly different measurements and that the right lateral CI consistently underestimated the CI obtained from the subcostal view.⁹ Most recently, Valette et al. studied ICU patients who were on either mechanical ventilation, controlled ventilation or spontaneous breathing. They found limits of agreement that were not clinically acceptable in comparing IVC diameters from the two views. Although the results were better when analysing the respiratory variation in the IVC, they still reported doubtful clinical utility due to wide limits of agreement.¹⁰

Still there has not been sufficient comparison of these IVC views in patient populations outside of the ICU. Current study results may not be applicable to patient populations with different physiologic parameters than ICU patients. ICU patients tend to be supine for long periods of time and may or may not be mechanically ventilated. It is unclear if these results can be generalised to patients who have recently been upright, standing or sitting, and are not critically ill. By examining the questions of IVC view concordance in healthy populations, we can examine the opposite end of the spectrum. If results are consistent in healthy subjects with results from ICU populations-based studies, it would be reasonable to extrapolate the results to other patient populations outside of the ICU as well.

In our study, we compared measurements of the IVC obtained from the subcostal and right lateral views in healthy subjects. Measurements consisted of the diameter at end inspiration and end expiration in the long axis of the IVC, which were used to calculate the CI. Concordance between the two views of the IVC would allow values obtained from either view to be easily applied to clinical applications, such as fluid management, that have been primarily studied using the subcostal view. This would be valuable clinically for patients who have difficult subcostal IVC windows but sufficient right lateral IVC windows. Conversely, discordance among these views would imply that the two views cannot be used interchangeably and that separate values for guiding medical decision-making should be established for each view of the IVC.

Materials and methods

Sampling and demographic information

This was a single-centre cross-sectional study that was approved by the Institutional Review Board. A convenience sample of 33 healthy volunteers above the age of 18 years was recruited from a medical student population.

Data collection

After providing written informed consent, volunteers provided their age, sex, height and weight. Ultrasound imaging was done in grayscale (B-Mode) using a Mindray ultrasound machine (Mindray Medical International Limited, Shenzhen, China) with a phased array probe (2–4 MHz). Volunteers were imaged while lying supine by one of the two medical students who had completed IVC ultrasound training with an ultrasound fellowship-trained physician. This training consisted of a system-based, 2-year long, biweekly curriculum and several dedicated sessions on IVC imaging. Volunteers were asked to breathe normally (spontaneous unsupported ventilation) while the IVC was imaged in the long axis via the subcostal view. This provided a sagittal plane image which allowed for the measurement of the anteroposterior diameter of the IVC. The smallest and largest diameter in this view was recorded 3 cm from the atrio-caval junction (Figure 1a and b). The IVC was then imaged in the long axis from the right lateral view at the level of the midaxillary line. This provided a coronal plane image which allowed for the measurement of the medial–lateral diameter of the IVC. The smallest and largest diameter in this view was recorded 3 cm from where the IVC passes the diaphragm, which was used as an approximation for the atrio-caval junction in this view (Figure 1c and d).

Figure 1.

Representative image showing IVC measurements: (a) subcostal view at end-expiratory diameter, (b) subcostal view at end-inspiratory diameter, (c) right lateral view at end-expiratory diameter, and (d) right lateral view at end-inspiratory diameter. These images do not belong to any of our study participants. IVC: inferior vena cava.

Data analysis

The CI was calculated by subtracting the minimal IVC diameter (during inspiration) from the maximal IVC diameter (during expiration), dividing by the maximal IVC diameter, and multiplying by 100. The CI was calculated for subcostal and right lateral views. Analyses were performed using Prism 9.0 (GraphPad, La Jolla, CA) and RStudio (RStudio, Boston, MA). To determine concordance or discordance between the subcostal and right lateral CIs, a two-tailed t-test assuming a gaussian distribution was performed. Demographic variables were compared to CIs using a multiple regression model. Bland–Altman analysis was used to obtain the limits of agreement for the end-inspiratory and end-expiratory diameters and the CI across the two views.

Results

Subcostal and right lateral CIs across all participants were significantly different according to a paired t-test (t = 5.21, df = 32, p < 0.0001). The mean difference between CIs (subcostal and right lateral) was 14.78 (95% confidence interval = 9.01–20.55). The t-test pairing was found to be significantly effective (R = 0.050, p < 0.01).

In the Bland–Altman analysis of the CI, the mean bias between subcostal and right lateral CIs was 14.78%, whereas the limits of agreement were −17.14 and 46.69 (Figure 2a). For the end-expiratory Bland–Altman analysis, a mean bias of −0.02 cm was seen with limits of agreement of −0.94 and 0.90 cm (Figure 2b). The Bland–Altman analysis of end-inspiratory diameter showed a mean bias of −0.24 cm with limits of agreement of −0.97 and 0.50 cm (Figure 2c). Pearson correlation between subcostal and right lateral CIs showed significant, moderately positive correlation (Figure 3). Pearson correlation coefficients for CI, end-expiratory diameter and end-inspiratory diameter can be viewed in Table 1.

Figure 2.

Bland–Altman analysis between subcostal and right lateral views: (a) Bland–Altman plot for CI, (b) end-expiratory IVC diameter, and (c) end-inspiratory IVC diameter. Solid line shows average bias, and dashed lines show limits of agreement. IVC: inferior vena cava.

Figure 3.

Relationship between subcostal and right lateral CIs in percent. Solid line represents linear regression with dashed lines representing 95% confidence intervals. CI: caval index.

Table 1.

Correlation of IVC parameters between subcostal and right lateral view.

	Correlation	p	95% Confidence interval
Caval index	0.50	<0.01	0.19–0.72
End-expiratory diameter	0.36	0.04	0.016–0.62
End-inspiratory diameter	0.58	<0.001	0.29–0.77

IVC: inferior vena cava.

IVC parameters that were measured included end-expiratory diameter and end-inspiratory diameter for each of the two views. CIs were calculated as continuous variables for right lateral and subcostal views based on IVC parameters as described in the methodology. The calculated IVC parameters can be found in Table 2.

Table 2.

IVC and CI values for all participants.

	Parameter	M	Median	Range (low, high)	SD
Right lateral view	End-inspiratory diameter (cm)	1.11	1.07	(0.49, 1.88)	0.41
	End-expiratory diameter (cm)	1.53	1.56	(0.70, 2.17)	0.44
	Caval index (%)	28.41	27.93	(8.00, 54.48)	12.58
Subcostal view	End-inspiratory diameter (cm)	0.87	0.87	(0.22, 1.68)	0.40
	End-expiratory diameter (cm)	1.51	1.52	(0.77, 2.20)	0.39
	Caval index (%)	43.19	37.97	(7.84, 82.18)	18.37

IVC: inferior vena cava.

Our study population (N = 33) included 13 (39%) females and 20 (61%) males. In addition to sex, other demographics collected include age, weight, height and BMI (Table 3). A multiple regression model was run in RStudio (RStudio, Boston, MA) to determine whether these demographic variables were related to the CI obtained from either view. In our first model, the right lateral CI was regressed onto age, weight, height, BMI and sex. Demographic variables including age (B = −0.45, β = −0.088, p = 0.28), weight (B = −2.48, β = −2.53, p = 0.24), height (B = 2.22, β = 1.68, p = 0.24), BMI (B = 7.55, β = 1.72, p = 0.23) and sex (male relative to female, B = −0.18, β = −0.01, p = 0.98) were not significantly related to right lateral CI. The predictors did not explain significant variance in right lateral CI (R² = 0.06, p = 0.86).

Table 3.

Participant demographics.

	Female (N = 13)	Male (N = 20)
Age (years)	24.85	26.10
Weight (kg)	63.77	79.68
Height (cm)	164.22	179.71
BMI (kg/m²)	23.66	24.60

BMI: body mass index.

In our second model, subcostal CI was regressed onto age, weight, height, BMI and sex. Demographic variables including age (B = 0.07, β = 0.01, p = 0.96), weight (B = −1.42, β = −0.99, p = 0.64), height (B = 0.71, β = 0.37, p = 0.80), BMI (B = 4.80, β = 0.75, p = 0.60) and sex (male relative to female, B = 13.36, β = 0.36, p = 0.27) were not significantly related to subcostal CI. The predictors did not explain significant variance in subcostal CI (R² = 0.07, p = 0.85).

Discussion

Our results demonstrate the discordance between the subcostal and right lateral views when obtaining measurements of the IVC in the long axis. Based on this, the two windows are not equivalent for obtaining measurements of the IVC diameters or calculating CIs. Furthermore, the CI obtained from the right lateral view underestimates the CI relative to values obtained from the subcostal view.

This has important clinical implications for which IVC views are valid for use in the clinical setting. Since many studies assessing applications of IVC ultrasound measurements were conducted using the subcostal view,^4,11,12 those study results cannot be readily applied to IVC measurements obtained using the right lateral view. For example, several studies examine the sensitivity and specificity of different CI values to assess potential fluid responsiveness. This is a common clinical application for IVC ultrasound, which could potentially misidentify patients as fluid non-responders using values obtained from the right lateral view. The smaller CIs obtained, relative to the CIs obtained with a subcostal view, would lead to underestimation of IVC collapsibility. This has the potential to do harm by miscategorising patients as fluid non-responders and supporting inappropriate treatment plans.^13,14 However, it is worth noting that miscategorising patients as fluid responders may also have the potential to do harm, which speaks to the importance of correctly identifying those patients who would benefit from fluid administration.^15,16 The differences in measurements between right lateral and subcostal view affect the clinical use of static measurements of IVC diameter as well. Measurements of IVC diameter, such as end-expiratory diameters, recommended by the guidelines from the American Society for Echocardiography for right atrial pressure estimates,¹⁷ would not provide accurate clinical information if obtained using a right lateral view while applying commonly used cut-off values derived from subcostal view-based data. In addition, measurements obtained by the right lateral view cannot be easily compared with those obtained from a subcostal view when assessing response to treatment or changes in clinical status over time. This means one cannot compare measurements or observations of IVC physiology over time using these views interchangeably. Instead, a single IVC view should be used for comparison of measurements or observed physiology over time. Given so many studies of IVC ultrasound use the subcostal view, one could argue that this should be the primary IVC window used to guide medical decision-making and that the right lateral view should only be used as a rescue view when the subcostal view is unobtainable. If the right lateral view is the only obtainable view, the smaller CI that will be obtained and lack of applicability of studied values for fluid responsiveness and other clinical uses should be taken into consideration.

One potential explanation for the discordance between the two windows is based on IVC geometry. Collapsibility and diameters of the IVC would be expected to be similar in sagittal and coronal sections if the IVC was a circular tube collapsing uniformly. Naruse et al. found in 2007 that the IVC is an ellipse that collapses either along the sagittal plane, which they called the horizontal ellipse morphology, or along the coronal plane, which they called the vertical ellipse morphology.¹⁸ The horizontal ellipse morphology has been shown to be the predominant form.^9,18 This was an intrinsic property of the IVC in every participant. The elliptical shape of the IVC may explain why its collapsibility and diameters are discordant across the sagittal and coronal planes. Based on these previous findings, we suspect that compression of the IVC by abdominal viscera in the supine position may lead to the elliptical shape of the IVC and its increased collapsibility in the sagittal plane. It is also possible that the subcostal probe placement of the anterior transabdominal window applies additional pressure to the IVC that does not occur in the right lateral window that is protected from this potential effect by ribs underlying the probe position. Regardless of the cause, there exists a difference in measured collapsibility and diameters of the IVC in different planes which should be taken into consideration when visualising the IVC using ultrasound.

Our study results build on the work of previous studies that compared these two windows for ultrasound imaging of the IVC and allow for the potential extrapolation of the collective data to a wider spectrum of patients now. Previous studies examined the difference in measurements obtained from these two views in ICU patient populations. There has been little examination of patient populations outside of the ICU comparing the values from these different IVC views. While there are conflicting results regarding the interchangeability of the subcostal and right lateral IVC windows in mechanically ventilated patients,^7,8 it appears that there is a consistent difference in IVC measurements for spontaneously breathing ICU patients.^7,9

However, the generalisability of these different windows and their measures to patients outside of the ICU is unclear. ICU patients are likely to be supine for long periods of time and the variations observed in IVC measurements and collapsibility might not exist in patients who have recently been in other positions, such as standing or sitting up. Our results improve generalised knowledge for these IVC windows to patients outside of the ICU by examining the opposite end of the patient spectrum with healthy subjects. Our study demonstrates that a difference exists in IVC measurements and CI values obtained using the subcostal and right lateral views in healthy adults. Given that these differences in measurements and physiology have been observed in both critically ill and healthy spontaneously breathing patient populations, it is now reasonable to extrapolate that these IVC views are not interchangeable in most spontaneously breathing patients. In addition, the demonstration of a consistently smaller CI from the right lateral view supports using measurements from this view as ‘rule in’ assessment only for fluid responsiveness for most spontaneously breathing patients. The right lateral view’s relatively smaller CI lowers its sensitivity for fluid responsiveness because the subcostal view was used to determine cut-off values for fluid responsiveness in many studies. Thresholds in these studies have ranged from 18%¹¹ to 42%.⁴ Although CIs for most of our participants would fall above the lower threshold of 18%, the discordance between the two views would be more meaningful if a higher threshold like 40%¹² or 42% is used. For the right lateral view to be used more precisely for clinical assessment, new studies of the IVC with measurements obtained from this view would need to be conducted. Given that the differences in measurements hold up across the extremes of spontaneously breathing patient populations, it is reasonable to conclude that the subcostal and right lateral IVC views cannot be used interchangeably and that measurements obtained in the right lateral view are not as sensitive for the assessment of volume responsiveness.

Despite the improved generalisability that our study adds to knowledge of the discordance of the subcostal and right lateral IVC views, there remain some limitations. The first limitation is the relatively small sample size. In addition, the study population is healthy young adults, which may have different physiology than hospitalised, non-ICU patient populations.¹⁹ Despite the possible limitation that these results may not generalise to acutely ill patients, these data add context and validation to the field of IVC ultrasound. Given that recent studies have highlighted technical challenges and confounding factors in determining fluid responsiveness using IVC ultrasound,²⁰ further studies are needed to explore other potentially unique patient populations and conditions under which IVC measurements are valid for this purpose. Furthermore, these studies must determine the populations in which measurements from the subcostal and right lateral views may be concordant or discordant.

Our study is the first study to demonstrate the discordance between measurements of the IVC obtained from the subcostal and right lateral views in adult non-ICU populations. The two views are not interchangeable, so one cannot compare measurements obtained between the two views. The right lateral view underestimates collapsibility and the subsequently derived CI. Using measurements obtained from the right lateral view of the IVC would lead to underestimating patients’ volume responsiveness when using cut-off values for responsiveness derived from the many studies that use subcostal views to generate their results. In addition to the discordance betwenn CIs, we find a similar discordance in the measured diameters of the IVC. IVC diameter-based assessments of right atrial pressure should not be performed using the right lateral view. Based on our study results, the subcostal view of the IVC should be the standard window for viewing the IVC. The right lateral view should only be used as a rescue window for viewing the IVC when the subcostal view is unobtainable and should be interpreted with consideration of the limitations of this view.

Footnotes

Acknowledgements

Not applicable

Contributors

FH, MR, and JC conceived and designed the study; FH and MR performed experiments; FH, CSS, and JC analysed the data; FH, CSS, and JC interpreted the results; CSS prepared figures; FH and CSS drafted the manuscript; FH, MR, CSS, and JC edited and revised the manuscript; FH, MR, CSS, and JC approved the final version of the manuscript.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Ethics approval

This study was approved by the Institutional Review Board at the University of Utah.

Application ID: IRB_00130825, Approved 6/29/2020

Guarantor

FH is the guarantor of this article.

Permission from patient(s) or subject(s) obtained in writing for publishing their case report

YES

Permission obtained in writing from patient or any person whose photo is included for publishing their photographs and images

YES

Confirm that you are aware that permission from a previous publisher for reproducing any previously published material will be required should your article be accepted for publication and that you will be responsible for obtaining that permission

YES

ORCID iD

Fadi Haroun

References

Kircher

Himelman

Schiller

. Noninvasive estimation of right atrial pressure from the inspiratory collapse of the inferior vena cava. Am J Cardiol 1990; 66: 493–496. DOI: 10.1016/0002-9149(90)90711-9.

Brennan

Ronan

Goonewardena

, et al. Handcarried ultrasound measurement of the inferior vena cava for assessment of intravascular volume status in the outpatient hemodialysis clinic. Clin J Am Soc Nephrol 2006; 1: 749–753. DOI: 10.2215/CJN.00310106.

Feissel

Michard

Faller

, et al. The respiratory variation in inferior vena cava diameter as a guide to fluid therapy. Intensive Care Med 2004; 30: 1834–1837. DOI: 10.1007/s00134-004-2233-5.

Airapetian

Maizel

Alyamani

, et al. Does inferior vena cava respiratory variability predict fluid responsiveness in spontaneously breathing patients? Crit Care 2015; 19: 1–8. DOI: 10.1186/s13054-015-1100-9.

Blehar

Dickman

Gaspari

. Identification of congestive heart failure via respiratory variation of inferior vena cava diameter. Am J Emerg Med 2009; 27: 71–75. DOI: 10.1016/j.ajem.2008.01.002.

Wallace

Allison

Stone

. Inferior vena cava percentage collapse during respiration is affected by the sampling location: an ultrasound study in healthy volunteers. Acad Emerg Med 2010; 17: 96–99. DOI: 10.1111/j.1553-2712.2009.00627.x.

Zhang

Liu

Wang

, et al. [Inferior vena cava diameter and variability on longitudinal plane measured through ultrasonography from different sites: a comparison study]. Zhonghua nei ke za zhi 2014; 53: 880–883.

Kulkarni

Janarthanan

Harish

, et al. Agreement between inferior vena cava diameter measurements by subxiphoid versus transhepatic views. Indian J Crit Care Med 2015; 19: 719–722. DOI: 10.4103/0972-5229.171390.

Shah

Spiegel

, et al. Relationship between the subcostal and right lateral ultrasound views of inferior vena cava collapse: implications for clinical use of ultrasonography. Chest 2018; 153: 939–945. DOI: 10.1016/j.chest.2017.10.002.

10.

Valette

Ribstein

Ramakers

, et al. Subcostal versus transhepatic view to assess the inferior vena cava in critically ill patients. Echocardiography 2020; 37: 1171–1176. DOI: 10.1111/echo.14802.

11.

Barbier

Loubières

Schmit

, et al. Respiratory changes in inferior vena cava diameter are helpful in predicting fluid responsiveness in ventilated septic patients. Intensive Care Med 2004; 30: 1740–1746. DOI: 10.1007/s00134-004-2259-8.

12.

Muller

Bobbia

Toumi

, et al. Respiratory variations of inferior vena cava diameter to predict fluid responsiveness in spontaneously breathing patients with acute circulatory failure: need for a cautious use. Crit Care 2012; 16: R188. DOI: 10.1186/cc11672.

13.

Rivers

Nguyen

Havstad

, et al. Early goal-directed therapy in the treatment of severe sepsis and septic shock. N Engl J Med 2001; 345: 1368–1377. DOI: 10.1056/NEJMoa010307.

14.

Rhodes

Evans

Alhazzani

, et al. Surviving sepsis campaign: international guidelines for management of sepsis and septic shock: 2016. Vol. 43. Berlin: Springer, 2017. DOI: 10.1007/s00134-017-4683-6.

15.

Boyd

Forbes

Nakada

, et al. Fluid resuscitation in septic shock: a positive fluid balance and elevated central venous pressure are associated with increased mortality. Crit Care Med 2011; 39: 259–265. DOI: 10.1097/CCM.0b013e3181feeb15.

16.

Alsous

Khamiees

DeGirolamo

, et al. Negative fluid balance predicts survival in patients with septic shock: a retrospective pilot study. Chest 2000; 117: 1749–1754. DOI: 10.1378/chest.117.6.1749.

17.

Lang

Badano

Victor

, et al. Recommendations for cardiac chamber quantification by echocardiography in adults: an update from the American Society of Echocardiography and the European Association of Cardiovascular Imaging. J Am Soc Echocardiogr 2015; 28: 1–39. DOI: 10.1016/j.echo.2014.10.003.

18.

Naruse

Sakaguchi

Nakayama

, et al. A novel method for dry weight assessment in hemodialysis patients: utilization of inferior vena cava flat ratio to correct for individual variations in vessel diameter. Ther Apher Dial 2007; 11: 42–48. DOI: 10.1111/j.1744-9987.2007.00454.x.

19.

Via

Tavazzi

Price

. Ten situations where inferior vena cava ultrasound may fail to accurately predict fluid responsiveness: a physiologically based point of view. Intensive Care Med 2016; 42: 1164–1167. DOI: 10.1007/s00134-016-4357-9.

20.

Millington

. Ultrasound assessment of the inferior vena cava for fluid responsiveness: easy, fun, but unlikely to be helpful. Can J Anesth 2019; 66: 633–638. DOI: 10.1007/s12630-019-01357-0.