L. A. Geddes

The Specific Resistance of Canine Blood at Body Temperature

The specific resistance (?) of canine blood, having a hematocrit (H) range extending from 0 to 70 percent, was measured at 37°C and 25 kHz using a variable-length conductivity cell connected to a constant-current impedance bridge. A least-squares exponential fit for the experimentally determined dataprovided the relationship ?= 56.8e 0.025H with a correlation coefficient of 0.989. The data obtained were also fitted to the Maxwell-Fricke equation; an excellent fit was obtained in the range of 0-50 percent hematocrit using a form factor of 2.5.

Threshold 60-Hz Current Required for Ventricular Fibrillation in Subjects of Various Body Weights

This paper reports the threshold 60-Hz alternating-current values required to induce ventricular fibrillation when the current is applied to electrodes at different sites on the surface of the bodies of rabbits, puppies, one monkey, dogs, goats, and ponies. It is shown that for a given body weight, the duration of exposure to current influences the fibrillation threshold; exposure times shorter than 1 s require more current. For a given duration of current flow, the threshold current for fibrillation is a function of body weight and electrode location. The lowest current for fibrillation was required with lead III (left forelimb-left hindlimb) and lead I (right-left forelimbs) required the highest current. For a 5-s exposure, the threshold current for fibrillation varies almost as the square root of body weight (W), the general expression being I = KW ? , where ? is nearly 0.5. Values for K and ? are presented for leads I, II, and III.

Observations on cardiac energetics: I. Theoretical introduction

Equations are developed to describe the energy expenditure of the human heart. As well as the external potential and kinetic energy terms, general consideration is given to other possible avenues of energy consumption. Emphasis is placed upon using mathematical variables which are readily available for experimental verification. The errors involved in assuming that mean values for the physiological parameters give reasonable estimations for the external mechanical performance are examined, and a theoretical estimation for the discrepancy in the kinetic component is presented. Logical extension of the mathematical derivation leads to a determination of cardiac external mechanical efficiency and clearly demonstrates the significance of the ventricular pressure-volume loop in this context. Finally, experimental procedures are suggested to clarify further some of the conclusions reached through the theoretical analysis.

Response to Passage of Sinusoidal Current Through the Body

It is becoming increasingly popular to introduce current to human and animal subjects either to measure a physiological event or to stimulate (or inhibit) irritable tissue. The type and intensity of current and the location of the electrodes is dependent on the result desired. The published literature abounds with descriptions of the various techniques using intentionally injected electrical currents for the measurement of physiological events by impedance, stimulation of nerve and muscle, and the production of an anesthesia-like state. Although all of these techniques are in popular use, when injecting current into animals and man it is necessary to employ the safest possible procedure. Except in the emergency life-saving situation, for example when high current is employed in trans-chest ventricular defibrillation, it is highly desireable to avoid stimulation of cutaneous receptors under the electrodes and to exclude passage of current through the cardiac ventricles because of the high risk of producing ventricular fibrillation which if not arrested immediately results in irreversible damage to the central nervous system. This paper describes the response to passage of current through various parts of the body and, in particular, presents data on the threshold values for sensation and the total thoracic current for ventricular fibrillation. Leakage of low-intensity current from catheters in the ventricles can produce fibrillation; accordingly, the threshold current for ventricular fibrillation is presented for this current path.