Ultrasound Assessment of CVP: Useful Tool or Wishful Thinking?

Erin K. Broderick

Disclosures

AccessMedicine from McGraw-Hill 

Background
According to River’s Early Goal Directed Therapy, if severe sepsis or septic shock is likely, the treating physician should begin aggressive fluid resuscitation in the emergency department (ED).[1] It has been demonstrated that ultrasound can noninvasively measure the inferior vena cava (IVC) size and variation with respiration which has been correlated to right atrial pressure and hemodynamic status.[2,3] An emergency physician can use this as an adjunct early in the course of resuscitation in those suspected to be hypovolemic. A recent article published in the Annals of Emergency Medicine hypothesized that a caval index of greater than or equal to 50% was associated with a central venous pressure (CVP) less than 8 mmHg.[4]

Methods
This was a prospective, observational study at an urban, academic, adult medical center ED in New England. A convenience sample of ED patients were enrolled if central venous access and invasive monitor was indicated based on the patient’s condition. The caval index was calculated as the relative decrease in IVC diameter from expiration to inspiration. Measurements were taken two to three centimeters from the right atrial border in a long-axis during one respiratory cycle by an ultrasound credentialed study physician. CVP was recorded by nursing staff after ultrasound evaluation was completed. A low CVP was defined as less than 8 mmHg.

Figure 1 and Figure 2.

Figure 1 shows the IVC diameter at end inspiration and Figure 2 shows the IVC diameter at end expiration. The Caval Index calculation is (expire IVC diameter – inspire IVC diameter) / expire IVC diameter and the Caval Index Percentage = caval index x 100

Results
Of the 73 patients enrolled, the sensitivity of caval index greater than or equal to 50% to predict a low CVP was 91%. The specificity was 94% with a positive and negative predictive value of 87% and 96%, respectively. The study suggests the caval index was a strong predictor of low CVP and particularly strong in determining which patients did not have a low CVP. The IVC could not be visualized in 9 potential study patients who were not included in the final analysis.

Relevance to Emergency Medicine
Ultrasound is a painless, non-irradiating, non-invasive imaging tool that can be used at the bedside. Evaluation of the IVC to assess volume status can be performed rapidly and, if reliable, could avoid the complications associated with invasive monitoring, including arterial puncture, venous thrombus, and infection. In this study, physicians took approximately three minutes to complete their exam. However, even with advanced training, they were unable to image 12% of patients, likely due to large body habitus, excessive intra-abdominal bowel gas, or large amounts of intrathoracic air. This raises the question if the average emergency physician could produce the same timely results.

There are several potential flaws in the study design. The original correlation of IVC diameter and respiratory variability was described in healthy subjects during quiet respiration. The authors noted that a greater percentage of intubated patients were in the higher CVP group. IVC measurements with mechanical ventilation are likely less reliable because of the differences in intrathoracic pressures with positive pressure ventilation versus normal negative pressure ventilation, although these effects have not been well studied. It is questionable whether or not intubated patients should have been included. Also, within the ultrasound community, there is not yet standardization of the location of IVC diameter measurements or probe orientation (long versus short axis) when taking IVC measurements. However, another recent study in Academic Emergency Medicine compared the three most common sites and did find consistency in measurements between a location two centimeters caudal to the hepatic vein inlet in long axis, and at the level of the left renal vein in short axis.[5]

More importantly, while ultrasound may be useful in helping determine the CVP, recent evidence shows that the CVP alone is not necessarily predictive of volume status or fluid responsiveness, demonstrated by increased cardiac output after a fluid challenge.[6] A recent study published in Intensive Care Medicine suggested passive leg raising-induced changes in cardiac output can reliably predict fluid responsiveness regardless of ventilation mode and cardiac rhythm.[7] This further confirms that bedside ultrasound is intended to be used as an adjunct to, not a replacement for, physical exam and other means off assessing volume status and adequacy of end organ perfusion when initiating treatment in the ED.

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