Copyright © American Society of Artificial Internal Organs. Unauthorized reproduction of this article is prohibited. 463 ASAIO Journal 2016 Clinical Critical Care Assessment of volume status in critically ill patients poses a challenge to clinicians. Measuring changes in the inferior vena cava (IVC) diameter using ultrasound is becoming a standard tool to assess volume status. Ultrasound requires physicians with significant training and specialized expen- sive equipment. It would be of significant value to be able to obtain this measurement continuously without physician presence. We hypothesize that dynamic changes in limb’s bio- impedance in response to respiration could be used to predict changes in IVC. Forty-six subjects were tested a hemodialy- sis session. Impedance was measured via electrodes placed on the arm. Simultaneously, the IVC diameter was assessed by ultrasound. Subjects were asked to breathe spontaneously and perform respiratory maneuvers using a respiratory train- ing device. Impedance (dz) was determined and compared with change in IVC diameter (dIVC; r = 0.76, p < 0.0001). There was significant relationship between dz and dIVC (p < 0.0001). Receiver-operator curves for dz at thresholds of dIVC (20% to70%) demonstrated high predictive power with areas under the curves (0.87–0.99, p < 0.0001). This evalua- tion suggests that real-time dynamic changes in limb imped- ance are capable of tracking a wide range of dynamic dIVC. This technique might be a suitable surrogate for monitoring real-time changes in dIVC to assess intravascular volume sta- tus. ASAIO Journal 2016; 62:463–469. Keywords: bioelectrical impedance analysis, hemodialysis, ultrasound Monitoring and assessment of a patient’s volume status and fluid responsiveness is an integral part of the management plan of critically ill patients as well as others for whom volume management has significant outcome implications. Rapid and accurate volume determinations are important for preventing under or over resuscitation of critically ill and injured patients that can result in higher rates of secondary injury and mortality. These same considerations also extend to patients with chronic conditions such as heart failure and chronic kidney disease. 1–6 Development of noninvasive and easy to use tools to make these assessments would be a welcome addition to the man- agement of these patients. The dynamic relationship between venous return, the function of the right ventricle, and its interaction with lung mechanics are key determinants of estimating intravascular volume status and more importantly, the patient’s functional response to addition or removal of volume. 7,8 The use of real-time ultrasound to view the collapse or distention of the inferior vena cava (IVC) during respiration is a useful tool to estimate volume responsiveness and guide intravenous fluid management. 9–18 Inferior vena cava ultrasound requires phy- sicians with significant training and specialized expensive equipment. Inferior vena cava measurements are difficult to obtain in obese patients and patients with significant amounts of bowel gas. Inferior vena cava measurements require a physi- cian to be present at the bedside for each measurement making frequent measures to follow treatment logistically challenging. It would be of significant value to be able to obtain this mea- surement on a continuous basis without physician presence for each measurement. Bioimpedance—a measure of a tis- sue’s resistance to an induced current or voltage 19,20 —has been used to monitor fluid status, nutritional status, and lung water. When applied to either the whole body or a portion thereof, bioimpedance becomes the cumulative effect of the individual impedances of components under examination. These com- ponents in the body consist of muscle tissue, fat, intracellular and extracellular fluid, and blood. Blood, as a good conductor of electricity, has a distinct effect on limb impedance as the respiratory cycles significantly shift blood volume in the limb. That is, the impedance of a limb increases with decreased blood volume in the limb—as occurs during spontaneous inspiration as blood return is increased to the thorax through the mechanism of enhanced intrathoracic negative pressure— and decreases with increased blood volume in the limb—as is the case during spontaneous expiration. These effects are exaggerated with respiratory maneuvers like deep inspiration. Furthermore, because venous compliance is up to 30 times greater than its arterial counterpart, 21,22 volume change occur- ring within a limb during respiration will largely be a function of venous blood return. Aim We tested a new noninvasive technique to assess vol- ume status. Our approach was to utilize single-frequency Dynamic Limb Bioimpedance and Inferior Vena Cava Ultrasound in Patients Undergoing Hemodialysis MOHAMAD H. TIBA,*¶ BARRY BELMONT,†¶ MICHAEL HEUNG,‡¶ NIK THEYYUNNI,*¶ ROBERT D. HUANG,*¶ CHRISTOPHER M. FUNG,*¶ AMANDA J. PENNINGTON,*¶ BRANDON C. CUMMINGS,¶ GERARD T. DRAUCKER,*¶ ALBERT J. SHIH,§¶ AND KEVIN R. WARD*¶ Copyright © 2016 by the American Society for Artificial Internal Organs DOI: 10.1097/MAT.0000000000000355 From the *Department of Emergency Medicine, †Department of Bio- medical Engineering, ‡Division of Nephrology, Department of Internal Medicine, §Department of Mechanical Engineering, and ¶Michigan Center for Integrative Research in Critical Care (MCIRCC), University of Michigan, Ann Arbor, Michigan. Submitted for consideration November 2015; accepted for publica- tion in revised form February 2016. MHT, KRW, BB, MH, NT, RDH, and AJP have received grant funding support from Baxter Healthcare Corporation for this study. Disclosures: MHT and KRW have submitted a patent application (61/859,615). Filed on July 29, 2014 and published on January 29, 2015. AJS, CMF, GTD, and BCC declare no conflict of interest. Correspondence: Kevin R. Ward, 2800 Plymouth Rd., 10-A103, Ann Arbor, MI 48109. Email: keward@umich.edu.