Richard McFee

Resistivity of Body Tissues at Low Frequencies

The resistivity of tissues of the thorax of dogs has been measured in situ under nearly normal conditions. Additional data have been obtained from humans. Approximate values of tissue resistivity found are 160 ohm-cm for blood, 2,000 ohm-cm for lung, 2,500 ohm-cm for fat, 700 ohm-cm for liver, 250 and 550 ohm-cm (anisotropic) for heart muscle and 150 and 2,500 ohm-cm (anisotropic) for skeletal muscle. Reasons for the differences between these and previously reported values have been found, and in some cases, verified experimentally. Predictions of whole trunk resistivity based on anatomical data and these measurements are within 8% of actual trunk measurements.

An orthogonal lead system for clinical electrocardiography

E inthoven, in his early investigations of the electrocardiogram, was concerned with the influence of the orientation of the heart on the form of the complexes. His interest in this relationship led to the introduction of the concepts of the manifest vector and the Einthoven triangle. These concepts are two dimensional in character, and apply to the frontal plane. Despite this limitation, they have proved to be so useful that they are routinely employed in electrocardiography today. The value of Einthoven’s approach has been so evident that the possibility of extending it to include all three components of the manifest vector has been under consideration for many years. There are several reasons why the early research with this objective has not culminated in clinical application. The Einthoven triangle itself had, for many years, no clearly demonstrable scientific basis. This basis has since been provided by the concept of the lead vector introduced by Burger and van Milaan. Another problem has been that of obtaining the sagittal component of the heart vector from potentials induced in electrodes on the chest, close to the heart. The recently developed “lead field” concept has shown that the proximity effects can be suppressed through the use of several electrodes whose potentials are averaged. Still another obstacle has been the difficulty of determining the electrical axis in three dimensions without burdening the clinician with calculations of inordinate mathematical complexity. An answer to this problem is the use of “resolvers,” electronic instruments introduced by Schmitt’ and studied by several other investigators.2-5 It has been shown9 that the approximate mean electrical axis of the “QRS” and “T” complexes can easily be determined with these devices. These instruments can be built as compact and reliable units well adapted to the clinic.6 Because of these relatively recent developments, it is now possible to determine the electrical axis of the heart in the clinic in three rather than two dimensions. All that is required is a lead system which provides the three components of the heart vector. A great number of such orthogonal lead systems have been proposed, each with merits and deficiencies of its own. It is beyond the scope of this article to review each of them. In our opinion, even the best systems, such as Schmitt’s’ and Frank’s8 do not strike the optimum balance between accuracy and simplicity. In this report an orthogonal system is described which is intended to satisfy clinical requirements. It is designed specifi-

Theory of Magnetic Detection of the Heart's Electrical Activity

The currents set up in the chest by the electromotive forces of the heart produce magnetic fields at the chest surface which have a peak intensity of about one microgauss. To detect these fields, an optimized coil assembly has been constructed which yields an rms noise level of about 10−8 G in a 1‐ to 40‐cycle band. The design of the coil assembly and the interpretation of its output are based on an analysis employing an unusual form of the reciprocity theorem. The theory shows that magnetic detection is fundamentally different from its electric counterpart and may reveal new clinical information. It also shows how immunity to magnetic interference can be achieved without resorting to bulky and expensive magnetic shields.

Electrodeless Measurements of the Effective Resistivity of the Human Torso and Head by Magnetic Induction

A magnetically coupled impedance measuring instrument has been developed for determining an effective electrical resistivity of human subjects without electrodes. The instrument operates at 100 kHz. The value obtained represents an average of the resistivity of tissue near the coils.

Optimum Input Leads for Cryogenic Apparatus

Electrical leads carrying currents into cryogenic apparatus also introduce heat. Even with an ideal Carnot cycle, the mechanical power needed to remove this heat can be one hundred or more times the heat flow itself. If the currents and hence the input leads are heavy, a very sizeable refrigerator may be required. In this article the configuration of the leads which minimizes the influx of heat is derived theoretically, taking variations in thermal and electrical conductivity into account. Graphs are given for the minimum heat flow and optimum cross section of a copper input lead carrying arbitrary current I. The optimum is found to be fairly sharp. If the diameter of the lead differs by a factor of two from the optimum, the influx of heat is increased by over 100%.

The magnetic heart vector

Abstract Records of the magnetic field due to the heart can be obtained in a hospital environment without the use of a magnetically shielded room. It is possible to build a magnetocardiograph sensitive primarily to the tangential components of the hearts EMFs. This is in contrast to ECGs where the radial component is emphasized and usually masks any tangential components. Tangential EMF components lying in frontal planes cause current to circulate around the front-to-back (z) axis, and can be assigned a vector direction along this axis. Similarly, tangential EMFs in sagittal and horizontal planes can be associated with x and y directed vectors. The vector sum of these three components is the spatial “magnetic heart vector.” The magnetic heart vector is conceptually similar to, and can be displayed using the same techniques as, the “heart vector” of electrovectorcardiography but is of radically different interpretation. Ideal magnetocardiographic leads, analogous to ideal electrovectorcardiographic leads, are defined in terms of the lead fields produced. Our present magnetocardiograph, while not specifically designed to implement the ideas of this paper, does give evidence that there are some tangentially oriented EMF components in persons without heart disease, and often much larger tangential EMFs in persons with heart disease.

Research in electrocardiography and magnetocardiography

The mathematical, physical, and engineering aspects of electrocardiography and magnetocardiography are reviewed. A brief summary of relevant physiological and clinical information is also given. The aim of the article is to provide a general perspective for engineers new to the area who want to do research. A detailed discussion of difficulties encountered in determining the heart vector and the nondipolar properties of the hearts field is included. Stress is placed on the need for development of practical ECG and MCG systems for use in the clinic. A number of research problems of current interest are pointed out.

Qualitative effects of thoracic resistivity variations on the interpretation of electrocardiograms: The low resistance surface layer

A lthough the torso is often assumed in electrocardiographic studies to be homogeneous, it is not. In a previous paper’ we have considered the resistivity changes which produce the most drastic modification of surface electrocardiograms (ECG’s). These are those variations which occur in and around the heart itself, i.e., the differences in conductivity between heart muscle, blood, and lung, as well as the lower resistivity of heart muscle in the direction of its fibers compared to that in a transverse direction. In this article, we discuss what we consider to be the second most important resistivity change, namely, that occurring in the vicinity of the surface electrodes. This is due to the relatively low resistance surface layer formed by the muscles girdling the thorax. As in our first paper, we use elementary models, and neglect the perturbing effects of other resistivity changes, e.g., the ribs, spine, sternum, liver, blood, blood vessels, pleural membranes, etc.

Applications of superconductivity to the generation and distribution of electric power

Potential applications for superconductors exist in transformers, transmission lines, generators, motors, circuit breakers, and rectifiers. Superconducting electromagnets also promise savings in power and weight when used with controlled thermonuclear fusion devices and magnetohydrodynamic generators. Discussion, however, is limited to recent developments in power apparatus

On the normalization of the electrical orientation of the heart and the representation of electrical axis by means of an axis map

R outine clinical electrocardiography currently includes the interpretation of 12 individual leads by trained readers. A variety of other methods of recording the electrical activity of the heart have been suggested. In general, each method has some desirable and some undesirable characteristics for cardiac diagnosis. For example, the vectorcardiogram reflects the time phase of voltages in various electrocardiographic leads. This advantage is obtained, however, at the expense of an adequate representation of other temporal relations. In a similar fashion, other recording methods each have characteristics which make some information more accessible and some less accessible than does the routine electrocardiogram. The purpose of this report is to describe a method of electrocardiographic recording and analysis which provides information that is not easily accessible in other records of cardiac electrical activity. Limited clinical experience with the method will also be reported. The method employs a lead system which provides the three mutually perpendicular components of the heart vector. These voltages are passed through a device (“resolver”) which alters the lead system so that the relative orientation of the heart with respect to the lead axes is changed. This process can be considered as an effective rotation of the heart within the body. The resolver is adjusted to “rotate” the heart, for both the QRS and T complexes, so that in each case the mean axis of the heart vector points toward the left side, and the plane of its motion is perpendicular to the long axis of the body. The original mean axis of the vector is then determined from the settings of the resolver knobs, and plotted on a map, along with the direction of motion of the vector. The voltages in the three new “normalized” leads are also recorded. The information inherent in the three original electrocardiograms is thus separated into (1) a map which shows the mean axes of the QRS and T heart vectors, as