Imaging/brief research report
Emergency Department Bedside Ultrasonographic Measurement of the Caval Index for Noninvasive Determination of Low Central Venous Pressure

Presented as preliminary results at the Society for Academic Emergency Medicine annual meeting, June 2008, Washington, DC.
https://doi.org/10.1016/j.annemergmed.2009.04.021Get rights and content

Study objective

Among adult emergency department (ED) patients undergoing central venous catheterization, we determine whether a greater than or equal to 50% decrease in inferior vena cava diameter is associated with a central venous pressure of less than 8 mm Hg.

Methods

Adult patients undergoing central venous catheterization were enrolled in a prospective, observational study. Inferior vena cava inspiratory and expiratory diameters were measured by 2-dimensional bedside ultrasonography. The caval index was calculated as the relative decrease in inferior vena cava diameter during 1 respiratory cycle. The correlation of central venous pressure and caval index was calculated. The sensitivity, specificity, and positive and negative predictive values of a caval index greater than or equal to 50% that was associated with a central venous pressure less than 8 mm Hg were estimated.

Results

Of 73 patients, the median age was 63 years and 60% were women. Mean time and fluid administered from ultrasonographic measurement to central venous pressure determination were 6.5 minutes and 45 mL, respectively. Of the 73 participants, 32% had a central venous pressure less than 8 mm Hg. The correlation between caval index and central venous pressure was –0.74 (95% confidence interval [CI] –0.82 to –0.63). The sensitivity of caval index greater than or equal to 50% to predict a central venous pressure less than 8 mm Hg was 91% (95% CI 71% to 99%), the specificity was 94% (95% CI 84% to 99%), the positive predictive value was 87% (95% CI 66% to 97%), and the negative predictive value was 96% (95% CI 86% to 99%).

Conclusion

Bedside ultrasonographic measurement of caval index greater than or equal to 50% is strongly associated with a low central venous pressure. Bedside measurements of caval index could be a useful noninvasive tool to determine central venous pressure during the initial evaluation of the ED patient.

Introduction

Determination of intravascular volume status can sometimes be challenging in the emergency department (ED) patient. Recent research indicates that invasive hemodynamic monitoring of central venous pressure is a useful guide in directing early resuscitative efforts and assists in reducing the morbidity and mortality of ED patients with severe sepsis/septic shock.1, 2 Specifically, in patients with severe sepsis/septic shock, a central venous pressure less than 8 mm Hg is considered an indicator for aggressive intravenous fluid replacement. Unfortunately, obtaining invasive hemodynamic monitoring can lead to complications (arterial puncture, venous thrombosis, infection, etc), may be time consuming, and is typically begun after increased lactate measurements are obtained or intravenous fluid boluses fail to improve blood pressure. There are some practical limitations to using invasive methods to monitor central venous pressure in the ED, including the need for special monitoring equipment, supportive resources, and trained personnel who can devote themselves solely to conducting monitoring.3, 4 Perhaps because of these limitations and the lack of broad-based campaigns about early severe sepsis interventions, a survey in 2004 indicated that only 7% of academic EDs have initiated protocols for invasive hemodynamic monitoring.5

Using bedside ultrasonography as a noninvasive method for hemodynamic monitoring might be a useful adjunct for the ED clinician. In conjunction with other more common clinical characteristics (urine output, pulse rate, blood pressure, etc), a noninvasive determination of central venous pressure could be a useful aid. Research from cardiology, nephrology, and critical care medicine has evaluated the ability of ultrasonography to evaluate central venous pressure among selected patients in the non–acute care setting by measuring inferior vena cava diameter and inferior vena cava collapsibility, termed the caval index.6, 7, 8 These studies found strong correlations between central venous pressure and inferior vena cava diameter and with inferior vena cava and cavel index. A study by Brennan et al9 among hemodynamically stable patients undergoing right-sided heart catheterization found that an inferior vena cava and caval index of 40% was predictive of a central venous pressure of 10 mm Hg.9 As a demonstration of the potential use of ultrasonography for intravascular volume status determinations in the ED, Randazzo et al10 evaluated the ability of ED clinicians to estimate central venous pressure with ultrasonographically-measured inferior vena cava collapsibility among stable patients undergoing echocardiography. However, the criterion standard used in the study was not central venous pressure, but rather a visual estimation of inferior vena cava collapsibility, as estimated by a cardiologist. To our knowledge, there have been no published studies evaluating correlation between bedside ultrasonographic inferior vena cava measurements performed by emergency physicians and measured central venous pressure. A noninvasive method of assessing volume status among ED patients may be a useful adjunct in the care of those with suspected hypovolemia, thereby allowing the clinician to initiate rapid fluid resuscitation before other objective and invasive measurements are determined.

Bedside ultrasonographic evaluation of the inferior vena cava could be a noninvasive marker of low volume status for the emergency physician, thereby aiding the clinician in fluid management early in the course of resuscitation before more invasive measurements are undertaken.

The objective of this study was to determine whether noninvasive bedside ultrasonographic measurement of the inferior vena cava and caval index could identify a low central venous pressure among ED patients who require central venous catheterization. Specifically, we hypothesized that a caval index of greater than or equal to 50% was associated with a central venous pressure less than 8 mm Hg. We also examined the relationship between inferior vena cava and caval index and central venous pressure and investigated whether clinical factors (patient characteristics, vital signs, lactate level, time elapsed from ultrasonographic measurement to central venous pressure measurement, and amount of normal saline solution infused between measurements) influenced this relationship.

Section snippets

Study Design and Setting

This prospective, observational study was conducted at an urban, academic, adult medical center ED in New England. This ED serves more than 98,000 adult patients annually and has an overall 25% admission rate. Approximately 8.5% of all patients are evaluated in the critical care area of the ED. The hospital institutional review board approved the study.

Selection of Participants

A convenience sample of ED patients undergoing evaluation in the critical care area of the ED and who had a central venous catheter placed was

Results

Eighty-two patients were initially enrolled in the study. Inferior vena cava measurements could not be obtained in 9 (11%) patients. Table 1 provides a description of the 73 study participants and a comparison of the participants by their central venous pressure measurements (central venous pressure <8 mm Hg versus central venous pressure ≥8 mm Hg). Each emergency physician enrolled at least 12 patients. There were no differences in central venous pressure measurements by age and sex, but there

Limitations

There are several limitations to this study. The selection of study participants was not random; therefore, unintended factors in choosing the convenience sample of participants, such as when the investigators were available to conduct the study, might have influenced the results. However, the participants were enrolled without regard to category or severity of illness, hour of the day, or day of the week, so we believe this influence was small.

The ultrasonographic measurements of the inferior

Discussion

Our study examined the association of inferior vena cava measured by bedside ultrasonography with direct invasive measurement of central venous pressure. We found that an inferior vena cava and caval index greater than or equal to 50% was strongly associated with a central venous pressure less than 8 mm Hg. We believe that ED clinicians can use ultrasonography as an accurate tool to aid in determining the need for aggressive fluid replacement before initiating central venous catheterization and

References (14)

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Supervising editor: David T. Overton, MD, MBA

Author contributions: ANN conceived of the study, oversaw the data collection, and participated in the analysis and preparation of the article. RCM conducted the data analysis and prepared the article. ANN, AT-G, CAS, and MCM participated in the data collection. AT-G, CAS, and MCM participated in the review of the preparation of the article. ANN takes responsibility for the paper as a whole.

Funding and support: By Annals policy, all authors are required to disclose any and all commercial, financial, and other relationships in any way related to the subject of this article that might create any potential conflict of interest. The authors have stated that no such relationships exist. See the Manuscript Submission Agreement in this issue for examples of specific conflicts covered by this statement.

Publication date: Available online June 25, 2009.

Please see page 291 for the Editor's Capsule Summary of this article.

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