A Non-invasive Method for Assessment of Intravascular Fluid Status: Inferior Vena Cava Diameters and Collapsibility Index

Objective: To evaluate the correlation between central venous pressure (CVP) and inferior vena cava (IVC) diameters measured by ultrasonography (Ultrasound) in critically ill patients. Methods: Intubated critically ill patients were enrolled. The CVP values were measured using a U-tube manometer and were compared to the IVC diameters and collapsibility index, which were measured by bedside Ultrasound. Patients younger than 18 years old, who were not intubated, who had an abdominal pressure greater than 12 mmHg, and/or who were admitted for trauma were excluded from the study. Results: Eighty three patients with a mean age of 73.6±11.2 years were enrolled. The most common diagnosis was sepsis (21 patients, 25.30%). IVC inspiration measurements were statistically significantly correlated with CVP measurements (p0.05, r: 0.1). IVC collapsibility measurements showed a negative correlation with CVP measurements (p<0.01, r: 0.68). Conclusions: There is a strong correlation between CVP and IVC diameters and the collapsibility index. This is a new formula for evaluating CVP, based on our statistical analyses.


Inferior vena cava intravascular fluid status
Trans-abdominal Ultrasound measurements of the IVC are noninvasive and thus are not associated with complications. In addition, many emergency departments have Ultrasound systems that can easily be used by emergency physicians. 8 In particular, bedside Ultrasound of the IVC diameter is a useful and easy method for assessing a patient's volume status.In the present study, we evaluated the correlation between CVP and IVC diameters as measured by Ultrasound in critically ill patients.

METHODS
This prospective observational study was conducted at a tertiary care hospital emergency department. Our emergency department serves 360,000 patients annually with a 15% admission rate. Approximately 5% of the patients are evaluated in the critical care area. Hospital ethics committee approval was received for this study (B104ISM4340029/3). An informed consent was obtained from patients because they were intubated. A data collection form was used to gather standard data. Patient age, gender, final diagnosis, arterial blood pressure, pulse rate, CVP measurement at the end of expiration, IVC diameters during expiration and inspiration, and intra-abdominal pressure values were recorded. All the measurements were made on intubated patients. Selection of Participants: Between June 1 and October 25, 2012, patients who were treated in the critical care area were included in the study. All of the patients were mechanically ventilated using the Synchronized Intermittent-Mandatory Ventilation (SIMV) Volume Control mode and had a central venous line. Patients who met the following criteria were excluded: younger than 18 years old, not intubated, having had trauma, being pregnant, and/or having known or newly diagnosed intraabdominal hypertension is defined as an intraabdominal pressure greater than or equal to 12 mmHg.

Methods of Measurement:
The CVP, intraabdominal pressure, and IVC diameters were measured in all patients. IVC diameters were measured by emergency department residents and specialists who received a standard basic Ultrasound course for two days. Ultrasound was performed using a Toshiba Aplio500 Ultrasound device and 3 mHz convex probe. The M-mode subxiphoidal window used to view the IVC, and both ends of the inspiration and expiration diameters were recorded in millimeters (Fig.1).
The CVP measurements were recorded from the fourth costal cartilage intersection point with the midaxillary line taken as a reference point. The CVP was measured using a U-tube manometer. The CVP results were recorded at the end of expiration in cmH 2 O.
The bladder was drained using a urinary catheter before measuring intra-abdominal pressure, and 50-100 mL normal saline was injected into the bladder; the distal portion was clamped. A 16 gauge needle was inserted into the output of the urinary catheter. The needle was connected to a three-way tap and water manometer. Symphysis pubis was taken as a reference point. After being filled with saline, the patient side of the manometer was opened. The intra-abdominal pressure results were measured and recorded in cmH 2 O and converted into mmHg (1mmHg= 1.36 cmH 2 O). 9

Primary Data Analysis: The NCSS Number Cruncher Statistical System 2007 and Power
Analysis and Sample Size 2008 Statistical Software (NCSS, Kaysville, UT, USA) program were used for the statistical analysis of the data. Data analysis included descriptive statistics; to assess the differences between contineous variables, a Student's t test was used. The normal distribution of variants was evaluated using Shapiro-Wilk and Kolmogorov-Smirnov analyses. Pearson and Spearman correlation analyses were used to assess correlations between CVP and IVC diameters. To assess the effects of measurements of IVC inspiration and expiration on CVP, multivariate regression analysis was used. The results were evaluated with 95% confidence intervals (CIs), and statistical significance was set at a p value less than 0.05. The data are presented as the mean ± standard deviation (SD) for continuous variables with ranges in parentheses.

RESULTS
Eighty three patients were enrolled between June 2012 and October 2012, and 48 of these patients were male. The mean age was 73.6±11.2 years. The most common diagnosis was sepsis (n=21, 25.3%) The characteristics of patients are given in Table-

DISCUSSION
We evaluated the correlation between CVP and IVC diameters as measured by Ultrasound in intubated patients, and found a strong correlation between CVP and IVC inspiration diameter and caval index.
In general, CVP measurements, pulmonary arterial catheterization, esophageal Doppler Ultrasound, and trans-esophageal echocardiography are the most widely used methods for volume assessments of critically ill patients. 10 However, none of these methods are perfect and all of them have both benefits and risks. 11 In emergency departments, measurements of CVP are most commonly used. However, this procedure can cause some early and late complications. Thus a noninvasive, costefficient diagnostic approach to assess fluid status in emergency departments is desired. 6 Ultrasound is a noninvasive method with broad applications in cardiac and hemodynamic evaluations. It can be used to assess acute respiratory failure, acute circulatory failure, and cardiac arrest. It can also be used for venous cannulation. Furthermore, IVC diameters can be measured by Ultrasound, and the European Association of Echocardiography recommends quantification as follows: an IVC diameter <2.1 cm that collapses >50% with a sniff suggests a normal RA pressure of 3 mmHg (0-5 mmHg); an IVC diameter >2.1 cm that collapses <50% with a sniff suggests a high RA pressure of 15 mmHg (10-20 mmHg); in scenarios in which the IVC diameter and collapse do not fit this paradigm, an intermediate value of 8 mmHg (5-10 mmHg) may be used. [12][13][14][15] On the other hand, technological improvements related to Ultrasound have allowed being sought new methods. Many studies have shown that bedside Ultrasound should be performed after appropriate training and should be more solution oriented. 16,17 Initially, nephrologists and cardiologists evaluated the IVC collapsibility index to assess volume status. 18,19 However, Ultrasound also has some limitations. It is results are user dependent, and the image quality is affected by anatomical obstacles such as large fat deposits and bowel gas. Thus Ultrasound can be subjective. 7 IVC is the largest vein, with a low pressure in the venous system. Dilation occurs due to changes in veous pressure. Weil 19 first demonstrated IVC dilation in patients with right ventricular heart failure. Visualization of the IVC is easier using Ultrasound, and the values can be measured in M-mode and B-mode, an advantage over the Doppler mode. 17 Previous studies have reported that IVC expiration diameter is correlated with volume loss. 20,21 However, in our study, IVC expiration diameter did not correlate with CVP. In Kusba. 20 the volume status of patients was based on the amount of fluid sampled and blood re-infusion. In Tetsuka, 21 it was evaluated in association with weight loss. Moreover, neither study assessed intubated patients. In another study, IVC diameters of 30 trauma patients with hemorrhagic shock were compared at admission and after fluid resuscitation. IVC diameters were a good indicator of volume loss. 22 A subsequent study on 31 blood donors reported the same results. 19 Lorsomradee et al. 23 found a correlation between CVP and IVC diameter during cardiac surgery in patients with a CVP equal to or less than 11 mmHg. Other studies have reported correlations between CVP and IVC collapsibility. 24,25 However, none of these studies assessed intubated patients.
Limitations: There were some limitations to our study. The selection of participants was not randomized, which may have affected the results. However, subjects were enrolled without regard to the severity of illness and with no time limitations. In addition, when measuring CVP, we used a U-tube manometer instead of a piezoelectricity transducer. However, it is not easy to obtain a piezoelectricity transducer. It is an outdated method but it has a wide range of use. Finally, another limitation was that many different residents and specialists performed the procedures. Nonetheless, they were instructed about the ultrasound procedure before starting the study, and all measurements were checked by a physician.

CONCLUSION
In intubated patients, there is strong correlation between CVP and IVC diameter during inspiration. We also found a correlation between CVP and IVC diameters and collapsibility. IVC diameter measurement can be used to assess CVP; thus, this could be an effective method for estimating the volume status of patients.

Declaration of interest:
None.