Sonographic evaluation of intravascular volume status in the surgical intensive care unit: a prospective comparison of subclavian vein and inferior vena cava collapsibility index.

BACKGROUND: Traditional methods for intravascular volume status assessment are invasive and are associated significant complications. While focused bedside sonography of the inferior vena cava (IVC) has been shown to be useful in estimating intravascular volume status, it may be technically difficult and limited by patient factors such as obesity, bowel gas, or postoperative surgical dressings. The goal of this investigation is to determine the feasibility of subclavian vein (SCV) collapsibility as an adjunct to IVC collapsibility in intravascular volume status assessment.

METHODS: A prospective study was conducted on a convenience sample of surgical intensive care unit patients to evaluate interchangeability of IVC collapsibility index (IVC-CI) and SCV-CI. After demographic and acuity of illness information was collected, all patients underwent serial, paired assessments of IVC-CI and SCV-CI using portable ultrasound device (M-Turbo; Sonosite, Bothell, WA). Vein collapsibility was calculated using the formula [collapsibility (%) = (max diameter - min diameter)/max diameter × 100%]. Paired measurements from each method were compared using correlation coefficient and Bland-Altman measurement bias analysis.

RESULTS: Thirty-four patients (mean age 56 y, 38% female) underwent a total of 94 paired SCV-CI and IVC-CI sonographic measurements. Mean acute physiology and chronic health evaluation II score was 12. Paired SCV- and IVC-CI showed acceptable correlation (R(2) = 0.61, P < 0.01) with acceptable overall measurement bias [Bland-Altman mean collapsibility difference (IVC-CI minus SCV-CI) of -3.2%]. In addition, time needed to acquire and measure venous diameters was shorter for the SCV-CI (70 s) when compared to IVC-CI (99 s, P < 0.02).

CONCLUSIONS: SCV collapsibility assessment appears to be a reasonable adjunct to IVC-CI in the surgical intensive care unit patient population. The correlation between the two techniques is acceptable and the overall measurement bias is low. In addition, SCV-CI measurements took less time to acquire than IVC-CI measurements, although the clinical relevance of the measured time difference is unclear.