ACADEMIC EMERGENCY MEDICINE 2011; 18:98–101 © 2011 by the Society for Academic Emergency Medicine
Objectives: Inferior vena cava ultrasound (IVC-US) is a noninvasive bedside tool to assess intravascular volume status. This study set out to investigate the interrater reliability of IVC-US by bedside clinician sonographers and determine whether alternative methods of IVC-US such as B-mode and visual estimation are equally reliable to traditional M-mode.
Methods: A convenience sample of adult emergency department (ED) patients was prospectively enrolled. Each patient underwent IVC-US by two different emergency physicians (EPs), each of whom first performed visual estimation of IVC percent collapse and of volume status, followed by caliper measurements in M-mode and B-mode. EPs were blinded to patient data and to the other sonographer’s results. For each technique, interrater reliability was determined between the two EPs’ assessments using intraclass correlation coefficients (ICC) for continuous data and Cohen’s weighted kappa for categorical data. In addition, analysis was performed on M-mode diameter measurements to determine the relationship between sonographer and patient characteristics on interrater reliability.
Results: Five EPs performed 92 US exams on 46 patients. Using M-mode, the ICC for maximum IVC diameter was 0.81 (95% confidence interval [CI] = 0.67 to 0.89), and for minimum diameter was 0.77 (95% CI = 0.62 to 0.87). There were no statistically significant differences between the caliper methods used for IVC measurements (M-mode diameter, B-mode diameter, or B-mode area). Agreement for visually estimated IVC collapse (0.60, 95% CI = 0.36 to 0.76) was similar to agreement for calculated M-mode IVC collapse index (0.52, 95% CI = 0.27 to 0.71). Cohen’s weighted kappa for volume status based on visual estimation of IVC filling (size, shape, and collapse) was 0.64 (95% CI = 0.53 to 0.73). ICC values for M-mode diameter measurements were significantly higher in studies involving patients who were noneuvolemic and studies in which sonographers had each performed at least five prior IVC-US.
Conclusions: Emergency physicians’ US measurements of IVC diameter have a high degree of interrater reliability. IVC percent collapse by visual estimation or based on caliper measurements have lower, but still moderate to good reliability. The use of the visual estimation technique should be considered by clinicians who have learned to obtain measured parameters of IVC filling because it is equally reliable to traditional M-mode and can be performed more rapidly.