ABSTRACT
We have developed a cylindrically symmetric model with which to study what physiologic variables might contribute to the impedance cardiogram signal. We find the major contributions in this model to be due to dilation of the aorta and carotid arteries, changes in conductivity of blood in these same vessels due to red cell reorientation during flow, changes in the conductivity of the lungs, and changes in heart volume. The calculations suggest that the popular equation used to determine stroke volume from thoracic impedance data is not accurate under all conditions of the circulatory system.
Subject(s)
Cardiography, Impedance , Models, Cardiovascular , Plethysmography, Impedance , Adult , Biomedical Engineering , Blood Pressure , Cardiac Output , Heart/anatomy & histology , Humans , Models, Anatomic , Stroke VolumeSubject(s)
Autoanalysis , Clinical Laboratory Techniques , Computers , Diagnosis, Computer-Assisted , HumansABSTRACT
Scholander and his coworkers have described in These Proceedings several experiments in which the osmotic pressure exerted by colloidal particles appears to be altered when the particles are subject to an external force; they interpret the experiments as showing that the traditional explanation of osmotic pressure is wrong (Proc. Nat. Acad. Sci. USA 68, 1093-1094; 68, 1569-1571; 70, 124-128). (In their view the osmotic pressure is determined by the concentration of the colloidal particles at the free surface rather than at the semipermeable membrane.) This note shows that one must also consider the way in which the external force changes the total hydrostatic pressure in the liquid. When that is done, the experiments of Scholander et al. are quantitatively consistent with the traditional interpretation of osmotic pressure.
