Inferior Vena Cava Dispensability Index versus Central Venous Pressure in Volume Status Assessment in Shocked Patients

Article information Background: The central venous pressure [CVP] remains the most frequently used variable to guide fluid resuscitation in shocked patients and is considered a direct measurement of the blood pressure in the right atrium and vena cava. The ultrasound-guided measurement of the inferior vena cava [IVC] and its changes with respiration has recently been utilized to calculate a patient's fluid status. The Aim of the work: This work aimed to compare ultrasound guided inferior vena cava dispensability index to central venous pressure in shocked patients and to assess the volume status in shocked patients after measurement of CVP and IVC dispensability index [IVC DI]. Patients and Methods: The study was conducted on 60 shocked patients, who were selected from the ICU, Al-Azhar University Hospitals. Results: At cutoff point 7.5 cmH 2 O, CVP has a sensitivity of 97% and specificity of 96.3% for prediction of hypovolemia. Also, at cutoff point 15.5, IVC-DI has sensitivity of 93.9% and specificity of 100% to predict hypovolemia. In addition, CVP at cutoff point 8.5 cmH 2 O has sensitivity of 50% and specificity of 55% for predicting fluid responsiveness. Also, at cutoff point 14.5 IVC DI has sensitivity of 100% and specificity of 91.9% for predicting fluid responsiveness. Conclusion: Ultrasound guided CVP and IVC DI were reliable markers in predicting fluid responsiveness and hypovolemia among shocked patients. IVC DI was superior than central CVP.


INTRODUCTION
Shock is a life-threatening condition that leads to global tissue hypo perfusion and circulatory collapse.Bedside ultrasound is a useful tool for diagnosing some types of shock [1] .
The central venous pressure can be measured using a central venous catheter advanced via the internal jugular vein and placed in the superior vena cava near the right atrium.A normal central venous pressure reading is between 8 to 12 mmHg.This value is altered by volume status and/or venous compliance [2] .
The ultrasound-guided measurement of the IVC diameter and its changes with respiration has recently been utilized to calculate a patient's fluid status.It is often used to evaluate patients' volume status as an alternative to central venous catheterization.It is a dynamic measure of intravascular volume status because it reflects the volume changes that happen with respiration.The IVC adapts to the body's volume condition by modifying its diameter based on total body fluid volume.The IVC distends with insufflation as enhanced intra thoracic pressure results in higher RV afterload and a transient boost in pulmonary artery pressure with a whole net reduction in venous return [3] .
Inferior vena cava [IVC] diameters are assessed during a normal respiratory cycle.While patients are supine, the IVC diameters are measured 2 to 3 cm from the right atrial border in a long-axis subxiphoid or subcostal view with a 4-2 MHz curvilinear probe based on the best available view [4] .
The IVC collapsibility index is calculated by the following formula: IVC collapsibility index = [maximum diameter on expiration -[minimum diameter on inspiration/maximum diameter on expiration]].In mechanically ventilated patients, the IVC distensibility index is calculated using the formula: IVC distensibility index = [[maximum diameter on inspirationminimum diameter on expiration]/minimum diameter on expiration] [5] .
This study was aimed primarily to compare between ultrasound guided inferior vena cava dispensability index and central venous pressure in shocked patients regarding their accuracy.And aimed secondarily to assess volume status in shocked patients after measuring of central venous pressure [CVP] and inferior vena cava dispensability index [IVC DI].

Ultrasound measurement of IVC diameters:
The IVC collapsibility [in spontaneously breathing patients] and distensibility [in mechanically ventilated patients] indices were measured with bedside ultrasound.A Mindray ultrasound device [North American Distributor for Mindray, Foundation in China] with a 3.5-5 MHz convex probe was used for ultrasound training in our department and was used for the measurements.All ultrasound measurements were carried out by the same PICU fellow, who was experienced in bedside ultrasound.Measurements were performed while the patient was in the supine position.IVC images were acquired in the sagittal section.Images of the IVC draining into the right atrium were obtained while the probe was in the sub xiphoid area, using the liver as the acoustic window.The minimum IVC diameter on inspiration and the maximum IVC diameter on expiration were recorded using the M-mode just beyond the point, where the hepatic veins drain into IVC.The maximum IVC diameter on inspiration and the minimum IVC diameter on expiration were measured using the same ultrasound scanning method.In mechanically ventilated patients, the IVC distensibility index was calculated using the following formula: IVC distensibility index ¼ [[the maximum diameter on inspiration -the minimum diameter on expiration]/the minimum diameter on expiration] Statistical analysis: Statistical package for social science [SPSS] software, version 20 [IBM® Armonk, USA] was used to perform the statistical analyses of patient data.The continuous variables were expressed by the means, standard deviation [SD], minimum and maximum.However, the qualitative variables were expressed by the relative frequency and percentages.Values of CVP and IVC DI were correlated with other variables.Pearson's correlation efficient was calculated to assess these correlations.P value >0.05 was considered significant.

RESULTS
The According to volume status, patients were divided into those who had hypovolemia [n =27] and those with no hypovolemia [n =33].Table [3] reflected that, the hypovolemic patients had significantly lower CVP and IVC DI.  4].
Table [5] reflected sensitivity and specificity of CVP and IVC DI in predication of hypovolemia and fluid-responsiveness. Results showed that, ICV DI was more specific in predication of hypovolemia and more sensitive for prediction of fluid-responsiveness than CVP.

DISCUSSION
Clinical determination of the intravascular volume in critically ill and injured patients can be extremely difficult.This is problematic because fluid loading is considered the first step in hemodynamically unstable patients' resuscitation.Yet, multiple studies have shown that only approximately 50% of hemodynamically unstable patients in the intensive care unit [ICU] and operating theatre respond to a fluid challenge [6] .
The present study showed that there was no significant association between age or Body mass index [BMI] with CVP or IVC distensibility index.This was consistent with Ibrahim et al. [7] who enrolled 67 critically ill patients with mean age of 42.3 ± 14.51 years and male predominance [65%].The study revealed that there was no significant association between age, sex or BMI with CVP or IVC distensibility index.
The present study showed statistically significant positive correlation between CVP, ICV DI the blood pressure and significant negative correlation between CVP and ICV distensibility index of the study population and CRT.These results are supported by the systematic review carried out by Hutchinson and Shaw [8] who revealed that CVP play a significant role in the hemodynamic monitoring of the critically ill patients.In addition, Ibrahim et al. [7] showed that there was a statistically significance increase in SBP, DBP and MABP among cases that had CVP >10 than who had CVP ≤10.Also, there was a negative correlation between ICV with SBP.Our results are in line with Çelik et al. [9] who revealed that there was a significant association between ICV distensibility index and SBP.However, Jassim et al. [10] reported that there was no significant association was found between CVP and MAP or HR.This could be attributed to different inclusion and exclusion criteria and sample size.
In our work, there was a statistically significant, positive correlation between CVP, ICV DI and hemoglobin levels.Similar association had been reported by Çelik et al. [9] .
As regard association between CVP, and ICV DI with pH and PaCO2, our results agree with Rahim-Taleghani et al. [11] who reported a significant correlation between CVP with pH and HCO3 in patients with septic shock.The same results were reported by Baratloo et al. [12] .
The most important finding of the current study is the significantly positive correlation between CVP and ICV distensibility index.This agrees with Ibrahim et al. [7] who reported a statistically significant positive correlation between CVP and ICV DI.
Regarding the relation between CVP and IVC distensibility index and volume status, it was revealed that CVP and IVC DI were significantly lower in hypovolemic subjects than non-hypovolemic subjects.In a comparative study to gauge the relationship between stroke volume variation [SVV] and inferior vena cava distensibility index [IVC-DI] as a measure of fluid responsiveness in mechanically ventilated hypotensive ICU patients, Kaur et al. [13] showed a direct link between IVC-DI and SVV usage during a clinical setting of low blood pressure suspicious of being attributable to hypovolemia.Furthermore Dalimunthe et al. [14] revealed that a significant correlation positive was found between CVP and volume status [r= 0,940, p= <0,001] but it had negative correlation with IVC-DI [r= -0,573, p=0,008].
The association between CVP, and IVC DI with fluid response showed significant reduction in fluid responsive subjects than non-fluid responsive subjects.The results were supported by Atallah et al. [15] who revealed that there was significant association between CVP and fluid responsiveness.In addition, Saber et al. [16] showed a significant association between IVC-DI and fluid responsiveness in mechanically ventilated patients with septic shock.Comparable results were reported by Ali Khalil et al. [17] In contrast to the current study, El-Gazzar et al. [18] revealed that there was no significant difference between fluid responsive subjects than non-fluid responsive subjects as regard CVP and IVC-DI in mechanically ventilated patients after CABG, the disagreement may be due to the difference in inclusion criteria.
Our results indicated that IVC-DI have higher accuracy in predicting hypovolemia and fluid responsiveness.These results are in line with Saber et al. [16] who showed that IVC-DI have higher accuracy than CVP in predicting fluid responsiveness.At cutoff point 7.5 CVP has sensitivity of 84.2% and specificity of 95.9% for predicting fluid responsiveness.Also, at cutoff point 14.5 IVC DI has sensitivity of 85% and specificity of 90% for predicting fluid responsiveness.
According to Huang et al. [19] , IVC-DI had an AUC of 0.82 [95% CI: 0.79-0.85],a specificity of 80%, and a sensitivity of 69% in mechanically ventilated shocked individuals.The IVC-DI has been proven to be a valid predictor of fluid responsiveness in critically ill patients who are mechanically ventilated, with a cut-off value of 18%, differentiating responders from non-responders in the research group.
In addition, Ali Khalil et al. [17] showed that IVC distensibility index [IVC DI] can be used in prediction of fluid responsiveness.IVC-DI index cutoff value was > 12.6% with sensitivity 80% and specificity 80%.However, Ismail et al. [20] showed that CVP, despite having a good sensitivity of 88.6%, high specificity of 100%, and a significant p-value, is not a reliable detector of fluid responsiveness due to its small AUC value and low 95% CI.
In conclusion, the current study showed that ultrasound guided central venous pressure and inferior vena cava distensibility index we reliable markers in predicting fluid responsiveness and hypovolemia among shocked patients.Inferior vena cava distensibility index was found to be superior with higher accuracy than central venous pressure in predicting fluid responsiveness and hypovolemia among shocked patients.However, the current study was limited by small sample size, the lack of control group, being a single center study and relatively short follow up period.Thus, further studies with a larger sample size and longer follow-up are needed to confirm our results and to identify risk factors of adverse events.

Financial and non-financial relations and activities of interest: None
According to fluid response, patients were assigned to one of each two groups, the fluidresponse [n=22] and non-responsive [n=38].Patients with fluid-response had significantly lower values of CVP and IVC DI, than those who had no fluid response [Table This prospective comparative study was conducted on 60 shocked patients.They were selected from the intensive care unit [ICU], Al-Azhar university Hospital [Damietta].This study began in October 2022 and continued until the end of October 2023.

Table [ 2
]: Correlation between each CVP and IVC DI with clinical data of the study population Table[3]: Relation between volume status and each of CVP and IVC DI Table[4]: Relation between fluid response and each of CVP and IVC DI

Table [ 5
]: Sensitivity and specificity of CVP and IVC DI in prediction of hypovolemia and fluidresponsiveness