Model for the physics and physiology of fluid administration

This article describes a model designed to provide an understanding of fluid flow in intravenous systems and human subjects. Experiments were developed which demonstrate that the model can represent common clinical situations. The model depicts physical devices as ideal resistors, pressure sources, and flow sources. The patient's venous system is depicted as a combination of ordinary and Starling resistors. For flows between 0 and 300 ml/hr, both physical devices and patients are adequately represented by a straight line representing the pressure-flow relationship (PFR): pressure = opening pressure + flow × resistance, where the slope is the resistance to fluid flow and the intercept is the opening pressure. The PFR for a normal vein is characterized by a flat slope (vein resistance =22±20 mm Hg/L/hr, mean ± SD) and a low intercept (opening pressure =15±8 mm Hg). The PFR for a partially obstructed vein has a resistance equal to that of an unobstructed vein and an opening pressure elevated approximately equal to the pressure obstructing the vein. For perivascular tissue, the PFR has a steep slope (tissue resistance =1,125±1,376 mm Hg/L/hr), while tissue opening pressure depends on the amount of fluid infused. At the onset of fluid extravasation (infiltration), tissue pressure usually is lower than venous pressure (8±8 versus 15±8 mm Hg), until fluid fills the distensible tissue compartment. In clinical practice, when infiltration or obstruction occurs, flow decreases and the clinician adjusts the roller clamp until correct flow resumes; no problem is obvious. The combined model for the intravenous tubing and venous systems explains the behavior of current clinical infusion devices.Presented in part at the Sixth Medical Monitoring Technology Conference, Vail, CO, March 1986; at the Annual Meeting of the American Society of Anesthesiologists, Las Vegas, NV, October 1986; at the Seventh Medical Monitoring Technology Conference, Vail, CO, March 1987; at a meeting on Computers in Critical Care and Pulmonary Medicine, San Diego, CA, June 1987; at the Annual Meeting of the American Society of Anesthesiologists, Atlanta, GA, October 1987; at the Regional Meeting of the Association for the Advancement of Medical Instrumentation, Cincinnati, OH, October 1987; at the Institute of Electrical and Electronics Engineers Ninth Annual Conference of the Engineering in Medicine and Biology Society, Boston, MA, November 1987; and at a meeting of the American Society of Hospital Pharmacists, Atlanta, GA, December 1987.Supported in part by grants from IVAC Corp.The author thanks the following individuals for important intellectual and/or technical assistance: Peter Basser, PhD, Avital Cnaan, PhD, Adriane Concus, MD, John Fox, MD, David Gissen, MD, David Joseph, MD, Anne Kamara, David Leith, MD, Leonard Lind, MD, Richard Morris, MB, BS, Barbara Orlowitz, MEE, Mary Anne Palleiko, RN, Beverly Philip, MD, Daniel Raemer, PhD, David Scott, MB, BS, John Stelling, MPH, and Marie vanRensberg, MB. At IVAC Corp: Walter Bochenko, BSEE, MBA, Robert Butterfield, BSEE, Douglas Christian, RPh, MBA, Alan Davison, BS, David Doan, PhD, Alan Somerville, BSEE, MS, Robin Wernick, BSEE, MS.