Inferior vena cava/aorta diameter index in the assessment of the body fluid status - a comparative study of measurements performed by experienced and inexperienced examiners in a group of young adults.

The assessment of the body fluid status is one the most challenging tasks in clinical practice. Although there are many methods to assess the body fluid status of patients, none of them is fully satisfactory in contemporary medical sciences. In the article below, we compare the results of measurements performed by experienced and inexperienced examiners based on the inferior vena cava/aorta diameter index in a sonographic hydration assessment. The study enrolled 50 young students at the age of 19-26 (the median age was 22.95) including 27 women and 23 men. The volunteers were examined in the supine position with GE Logiq 7 system and a convex transducer with the frequency of 2-5 MHz. The measurements were performed in the longitudinal and transverse planes by two inexperienced examiners - the authors of this paper, following a four-hour training conducted by an experienced sonographer. The longitudinal values of the inferior vena cava/aorta diameter index obtained in this study were similar to those found in the literature. The reference value for the inferior vena cava/aorta index determined by Kosiak et al., which constituted 1.2 ± 2 SD, for SD = 0.17, was similar to the values obtained by the authors of this paper which equaled 1.2286 ± 2 SD, for SD = 0.2. The article presented below proves that measuring the inferior vena cava/aorta diameter index is not a complex examination and it may be performed by physicians with no sonographic experience. Furthermore, the paper demonstrates that the inferior vena cava/aorta diameter index measured in the transverse plane is similar to the inferior vena cava/aorta diameter index determined in the longitudinal plane. Thus, both measurements may be used interchangeably to assess the hydration status of patients.

Introduction

An accurate assessment of the body fluid status is a challenging task in clinical practice( and inappropriate assessment of hydration or overhydration may result in severe complications, even in the patient's death(. Due to their limitations, the methods currently used for the hydration status assessment do not meet the expectations of clinicians to a satisfactory level(. A commonly used physical examination is burdened with the considerable risk of making an erroneous assessment(. The advancement that has occurred in the field of ultrasound imaging allows for the application of this method directly at the patient's bed as an extension of the basic physical examination, not only when imaging examinations are necessary, but also for the purposes of clinical assessment of the patient's condition.

Numerous authors assess new methods in terms of their diagnostic value, possibility of easy and fast application, dependency of the outcomes on the examiner's experience as well as time needed for mastering the technique(.

The presented paper constitutes an attempt to assess the value of the selected diagnostic index used for the hydration status assessment by providing answers to questions connected with the technique of examination and by comparing ultrasound-based measurements of the inferior vena cava/aorta diameter (IVA/Ao) index taken by experienced and inexperienced examiners.

Questions connected with examination technique

Is the measurement of IVC/Ao index an easy and fast examination? How much time is needed for mastering this method? Is sonographic experience vital to perform the examination or can it be conducted by persons without such experience?

Material and methods

The study enrolled 50 healthy volunteers (students) at the age of 22.5 ± 3.5 including 27 women (54%) and 23 men. The subjects were examined in the supine position with GE Logiq 7 system and a convex transducer with the frequency of 2–5 MHz. The measurements were taken in longitudinal and transverse views by placing the transducer below the xiphoid process. The diameters of IVC and aorta in a B-mode examination were measured during a regular breathing cycle. In addition, each subject had their arterial pressure taken, as recommended in the guidelines of the Polish Society of Hypertension, with the use of Omron 3 – an automatic blood pressure monitor. Moreover, for BMI index calculation, the weight and height of each subject were taken. The examinations were preceded by an interview in terms of chronic diseases and taken medicines. The measurements were performed by two inexperienced examiners – the authors of this paper, following a four-hour training conducted by an experienced sonographer. The examinations were performed under continuous observation of the supervisors of this study. In order to minimize errors in measurements, all values were calculated three times and the analysis included the median value. The analysis of the outcomes was performed by means of the Student's t-test. The p value lower than 0.05 was regarded as statistically significant. The outcomes were discussed with the reference to the literature. The study was approved by the Independent Ethics Committee for Academic Research at the Medical University of Gdańsk (Poland).

Results

In sonographic measurements performed in the transverse view, the diameter of aorta (Ao) constituted 15.7 mm, the longer dimension of the inferior vena cava (IVC1) equaled 28.4 mm and the shorter one (IVC2) – 15.5 mm. In the longitudinal view, Ao equaled 15.3 mm and IVC – 18.8 mm (tab. 1). The mean body weight in the study group constituted 62 kg and the mean height – 171 cm; BMI index was normal (22.52 ± 2.78). The measurements conducted in the longitudinal view resulted in obtaining IVC/Ao index values similar to those quoted in the literature(1). The mean IVC/Ao index obtained by the authors of this paper equaled 1.2286 ± 2 SD, for SD = 0.2 (tab. 1) and was similar to the value determined by Kosiak et al. which constituted 1.2 ± 2 SD, for SD = 0.17. Moreover, a significant correlation was observed between the IVC/Ao indices calculated in the longitudinal and transverse views: r = 0.508, r2 = 0.457 (the value in the distribution of the Student's t-test: t = 3.776, t = 3.286 for p = 0.002, p = 0.001).

Tab. 1

Median IVC and aorta diameters in the study group

	Longitudinal IVC [mm]	Longitudinal Ao [mm]	IVC/Ao	BMI [kg/m2]
Examiner 1	19	15,7	1,21	22,22
SD	2,95	1,6	0,23	2,78
Examiner 2	18,5	14,9	1,24	22,22
SD	2,7	1,78	0,22	2,78
Median (a!! examinations)	18,8	15,3	1,23	22,22
SD	2,5	1,6	0,2	2,78

IVC - inferior vena cava; Ao - aorta; IVC/Ao - inferior vena cava/aorta co!!apsibi!ity index; BMI - body mass index; SD - standard deviation.

Thus, it was demonstrated that these measurements may be used interchangeably (figs. 1 and 2).

Fig. 1

Correlation between the measurements of IVC1/Ao and IVC/Ao

Fig. 2

Correlation between the measurements of IVC2/Ao and IVC/Ao

However, no correlation was observed between the value of the IVC/Ao index measured in the longitudinal view and the ratio of the shorter and longer dimensions of the IVC measured in the transverse view.

Discussion

The assessment of the body fluid status is a significant element of a physical examination(1). A fast diagnosis of considerable dehydration and implementation of adequate treatment (admission to hospital, oral/intravenous rehydration) are of vital importance. On the other hand, a failure to identify dehydration or erroneous assessment of its degree may lead to groundless hospitalization, which is associated with unnecessary costs, and puts the patient at risk of hospital-acquired infections(. In daily medical practice, the following methods are used to diagnose dehydration: subjective means, such as physical examination (dry mouth, skin tension, blood pressure, venous return or, in extreme cases, even enophthalmos), laboratory tests (hemoglobin level, hematocrit as well as sodium and total protein plasma levels) and information obtained during the interview (24-hour diuresis, vomiting, diarrhea, edema, excessive sweating, thirst). Although this information is helpful, it does not enable accurate assessment of the hydration status(. A more precise assessment is possible by means of invasive procedures, such as taking central venous pressure (CVP). However, when this method of monitoring is selected, one should bear in mind the risk of complications, such as: bleeding, stroke, air embolism, formation of an aneurysm during accidental puncturing of an artery and even death(. A different and not very common method of a quantitative assessment of individual components in the organism is bioimpedance, the usage of which is constrained by considerable cost and limited access to specialized equipment.

The usefulness of the sonographic hydration status assessment has been previously confirmed both in pediatric patients and in adults in several independent studies(. In 1979, Natori et al. for the first time proved the correlation between the alterations in IVC diameter and right atrial blood pressure(. At present, the measurement of the inferior vena cava (IVC) and inferior vena cava collapsibility (IVCCI) are effectively used to determine the body fluid status of dialysis patients(. A branch of medicine in which IVC assessment is more and more commonly used is emergency medicine, particularly in the case of patients with multiple organ trauma(. Moreover, it was shown that ultrasound examination of the fluid status may serve as an early sign of hypovolemia in healthy blood donor candidates and may help determine the degree of inadequate blood supply in patients with hemorrhages after traumas(. In spite of numerous positive reports connected with IVC measurement, this method is burdened with certain limitations, such as: the absence of clear reference values for the diameter of the inferior vena cava and insufficient availability of adequately trained medical staff(.

The IVC/Ao index is a relatively new element of sonographic assessment of the hydration status. The selection of the aorta as a reference for the diameter of the inferior vena cava has a particular explanation. In the fetus, the aorta usually develops simultaneously with the IVC. Its diameter correlates with age, sex and body surface but does not correlate with the fluid status. Therefore, when comparing the diameter of the IVC with the diameter of the abdominal aorta, the necessity to calculate it per the body surface may be avoided(. The conducted studies demonstrated a significant correlation between weight and diameter of the aorta both in transverse and longitudinal views (correlation coefficient >0.7). The literature suggests that the measurements of the index may be used interchangeably with CVP which is currently considered the most accurate method in assessing the fluid status of patients(. The non-invasive character of sonography suggests its greater usefulness in daily clinical practice. Moreover, a linear correlation of 0.9696 (Pearson's linear correlation coefficient) between CVP and IVC/Ao index was determined(.

The longitudinal IVC/Ao index measurements performed in a group of young, healthy adults aged 20–25 and conducted by the authors of this paper after a four-hour training are similar to the outcomes of the previous studies which for the first time were presented during the eighteenth European Ultrasound Congress – Euroson 2006 in Bologna and published in the “American Journal of Emergency Medicine”(. The similarity of the obtained results suggests that measuring the diameters of the IVC and aorta as well as calculation of the IVC/Ao index is an easy and fast examination that can be mastered during a short training and may be performed by inexperienced examiners in a manner comparable to the experienced ones. The application of this method by physicians who are not radiologists would enable fast, emergency sonographic assessment of the hydration status in daily practice. The fact that the examination is easy to perform is also reflected in the reproducibility of results obtained by both examiners in consecutive measurements. Moreover, the lack of experience in sonography did not affect the outcomes in a significant way. The results of the study confirm the hypothesis that the value of the index determined by Kosiak's team may be treated as a commonly accepted reference.

The study also included the transverse measurements of the IVC/aorta index. The obtained results revealed correlations between the indices measured in longitudinal and transverse views (r = 0.508, r = 0.457). This suggests a possibility of interchangeable usage of these measurements, particularly when in one of the planes, the assessment of the abdominal vessels is difficult or prevented.

In the hands of an experienced examiner, sonography, as a tool for hydration assessment, may also be used for searching for a direct cause of hydration disorders. During one examination of the IVC and aorta, it is also possible to assess abdominal (kidneys, spleen, liver) and thoracic organs (heart, lungs)(. Detection of a pathology in the aforementioned area may help to identify the cause of disorders and allows for fast commencement of targeted therapeutic actions.

Conclusions

After a four-hour training, persons without experience in sonography are capable of measuring the diameters of the IVC and aorta with the accuracy comparable to the precision of experienced examiners.

From technical point of view, the sonographic assessment of the IVC/Ao index is an easy examination and may be effectively conducted by persons without prior experience in the field of sonography.

The IVC/Ao index obtained in the transverse view is similar to the IVC/Ao index obtained in the longitudinal view. Thus, for the purposes of the hydration status assessment, the measurements taken in both planes may be used interchangeably.