Sonographic assessment of the inferior vena cava/aorta index measured with the transducer placed in the anterior median line and right anterior axillary line - a comparison.

UNLABELLED: The aim of the study was to compare the values of the sonographic inferior vena cava/ aorta index obtained with the transducer placed in the median line and right anterior axillary line.
MATERIAL AND METHODS: The study enrolled 45 volunteers, including 33 women, aged 22.5 ± 1.26 with a negative history of circulatory and renal diseases. The study consisted in a sonographic assessment and measurement of the inferior vena cava and aorta by placing the transducer in the anterior median line and right anterior axillary line. The value of the inferior vena cava/aorta index was obtained by calculating the ratio of the diameters of the inferior vena cava and aorta. The diameter measurements were taken by placing the ultrasound transducer in the anterior median line and right anterior axillary line. Two examiners performed the measurement three times and used convex probes of 3.5-5 MHz. Additionally, the subjects' weight, height and arterial blood pressure were taken.
RESULTS: Following a statistical analysis with the use of STATISTICA software, the following values of the inferior vena cava/aorta index were obtained: in the anterior median line - 1.43 ± 0.21, and in the right anterior axillary line - 1.285 ± 0.19. There was no statistically significant difference between the measurements obtained by the two examiners (p = 0.17). A strong correlation was noted between the inferior vena cava/ aorta indices calculated in both sites of transducer placement, which was irrespective of the examiners (the correlation coefficient: r = 0.61 and r = 0.71). The study indicate that the inferior vena cava/aorta index measured in the right anterior axillary line is a simple and reproducible method for determining the body fluid status; the examinations conducted in both sites may be used interchangeably. Further studies are needed to determine reference values for the inferior vena cava/aorta index measured in the right anterior axillary line.

Introduction

An accurate assessment of the body fluid status, particularly in a pediatric patient, is a challenge(. To do so, various methods are needed. The basic one is a physical examination which includes an assessment of the skin elasticity, mucous membranes, weight, blood pressure and heart rate(. Laboratory tests, which are commonly performed to determine the fluid status, are characterized by unsatisfactory levels of sensitivity and specificity(. Moreover, central venous pressure (CVP) is a basic method to assess the hydration status of patients in intensive care units, there is also bioimpedance and it is becoming more and more common. However, these methods are burdened with a low accuracy or are invasive. Moreover, they are also time-consuming(. Frequently, they do not allow for monitoring the fluid status changes. A modality which may offer the possibility of even more accurate assessment of the fluid status and enable monitoring of patients is sonography(. The ultrasound assessment of the diameter of the inferior vena cava and aorta (IVC/Ao index) with the ultrasound transducer placed in the anterior median line is a recognized method( used in the emergency departments and intensive care units. The limitations of this method include: trauma in the substernal region, wound dressing or substantial amount of gas in the intestines. In such situations the assessment of the IVC/Ao index performed in other sites may prove useful.

The aim of the study was to compare the values of the sonographic IVC/Ao index measured with the transducer placed in the median line and right anterior axillary line.

Material and methods

The study enrolled 45 volunteers – students aged 18–26. This group consisted of 33 women and 12 men. The mean age was 22.5 ± 1.26. None of the subjects had a history of circulatory or renal diseases. In each subject, the arterial blood pressure was measured in the brachial artery, weight and height were taken and the BMI index was calculated. The study consisted in a sonographic assessment and measurement of the inferior vena cava and aorta by placing the transducer in the anterior median line and right anterior axillary line. For this purpose, a convex probe with the frequency of 3.5–5 MHz and the GE Logiq 7 ultrasound system (GE PDI, Tempe, AZ, USA, produced in 2005) were used. The examinations were conducted with the patients in the supine position. The transducer was placed in the median line, inferior to the xiphoid process. Following a slight change of the transducer placement, the IVC and Ao were visualized from the same site. The inferior vena cava was measured in its intrahepatic fragment beneath the confluence with the hepatic veins (2 cm below the diaphragm)(, where its walls were parallel (fig. 1). The diameter of the aorta was taken 1 cm above the celiac trunk(( (fig. 2). Additionally, the transducer was placed in the right anterior axillary line (fig. 3). Using the liver as an acoustic window during the patient's regular breathing cycle, both vessels were visualized and their maximum diameters were recorded at the site where the vessel's walls were parallel. The measurements were taken in the median aspect of the intrahepatic IVC, and the aorta was measured at the same level (fig. 4).

Fig. 1

Measurement of the inferior vena cava in the anterior median line

Fig. 2

Measurement of the aorta in the anterior median line

Fig. 3

Transducer placed in the right anterior axillary line

Fig. 4

Inferior vena cava and aorta in the right anterior axillary line

The examinations were performed by two examiners (M.L. and M.Ł.) – the members of the Ultrasound Student Association at the Laboratory of Diagnostic Ultrasound and Biopsy, Depar tment of Pediatrics, Oncolog y, Hematology and Endocrinology at the Medical University of Gdańsk, Poland. The measurements were consulted with experienced physicians working in the Laboratory. Each examiner took the measurements three times. The outcomes were analyzed in STATISTICA 10 software in the Polish language version (StatSoft, 2011).

Results

The following diameters of the inferior vena cava and aorta were obtained in the studied group: IVC 2.03 ± 0.28 cm, Ao 1.42 ± 0.16 cm. The IVC/Ao index equaled 1.43 ± 0.21 and was slightly higher than reported in the literature – 1.2 ± 2 SD (SD = 0.17). The measurements taken in the right anterior axillary line resulted in the following values: IVC 1.80 ± 0.28 cm, A o 1.40 ± 0.20 cm; I VC/Ao i ndex was 1.285 ± 0.19. No statistically significant differences in the results obtained by both examiners were observed (p = 0.96 for the median line and p = 0.17 for the right anterior axillary line). This attests to the reproducibility of the measurements performed in the two analyzed sites. A strong correlation was observed between the values of the IVC/Ao indices measured in both sites. The correlation coefficient equaled r = 0.61 for the first examiner (fig. 5) and r = 0.71 for the other examiner (fig. 6). This suggests that both examination sites may be used interchangeably in establishing the fluid status of patients.

Fig. 5

Correlation for the first examiner

Fig. 6

Correlation for the second examiner

Discussion

The advancement that has occurred in medicine in recent years, particularly the growing popularity of the evidence-based medicine (EBM), forces physicians to establish accurate diagnoses and qualify patients in a precise way. One of the most important parameters in the assessment of the patient's general condition is the fluid status and estimation of the dehydration/overhydration level(. The insufficient accuracy of the methods currently employed to assess the hydration status prompts the search for the new methods that are accurate, reproducible, fast and non-invasive(. Apart from the fact that after a hematocrit assessment the results arrive after a long time and are difficult to accept particularly in the emergency department, this method may also lead to an erroneous diagnosis(. The measurement of CVP, in turn, is an invasive procedure and its accessibility is limited(. Therefore, the sonographic assessment of the IVC/Ao index may constitute an alternative method to assess the hydration status. Such an examination is fast, non-invasive and reproducible(. It may accelerate implementation of an adequate therapy for dehydrated patients and those at risk of hypovolemic shock(. This is of particular significance for elderly and pediatric patients as well as for those with impaired thirst mechanism, unconscious and in a severe general condition(.

When ultrasound equipment is available, it is also possible to measure the diameter and collapsibility of the inferior vena cava(. This, however, is associated with certain limitations, namely the necessity to refer the obtained values to the body surface area of the patient, particularly the child, which requires the usage of special centile charts(. This is inconvenient and time-consuming. Furthermore, the reference values for the diameter of the inferior vena cava in the pediatric population and in the elderly are not precise(, and the result is affected by interindividual variability in the size of the inferior vena cava(.

The aforementioned limitations were overcome by the introduction of the IVC/Ao index in which the diameter of the inferior vena cava is compared to the diameter of the aorta. Thanks to this, knowledge of the patient's body surface is not required(, which facilitates and shortens the examination. There are also publications whose authors suggest that the FAST examination in patients with blunt abdominal trauma should be supplemented with the inferior vena cava/aorta index(. The available papers indicate that this index has considerable value both in the initial assessment and in further follow-up examinations(. The examination in the anterior median line is impossible in patients with abdominal injuries, burns or with wound dressings in the substernal region. In such situations, the assessment of the IVC/Ao index measured with the transducer placed longitudinally in the right anterior axillary line should be considered. In order to assess the diameters, the image of both vessels is obtained by using the liver as an acoustic window(. This method is also effective when the examination is hindered by the presence of chyme in the stomach and gas in the intestines. The results obtained by placing the transducer in both lines show a strong correlation. Mastering the technique of measuring the IVC/Ao index is not time-consuming.

Conclusions

The IVC/Ao index determined on the basis of measurements taken in the right anterior axillary line is an easy and reproducible method of the fluid status assessment.

The examinations performed in both sites may be used interchangeably.