Biomedical subjects
Publications and source records attributed to O E Clavin.
The digital impedance meter is a microprocessor-based instrument able to detect, quantify and identify micro-organisms. The equipment makes use of the bipolar technique of measuring the impedance modulus of six cells containing inoculated culture broth. It performs temperature compensation automatically. Growth curves are stored in memory as time course events and can be displayed on any suitable device.
By means of the bipolar impedance technique, we detected bacterial growth in an inoculated broth as its time course absolute impedance. From it, the impedance change relative to sterile medium was obtained, calculating also its time derivative. The repeatability of the derivative curves (they overlapped within a band better than 3.3%) permitted the identification of a double-hump pattern which, in principle, could be accepted as an indicator of the type of bacteria (Escherichia coli). After six experimental series, the growth curves appeared as sensitive to the initial concentration of bacteria and to the culture time preceding inoculation; they were also dependent on the temperature and on the average basal impedance. Temperature showed a greater effect (one order of magnitude) on the lag-phase of the growth curve than on the stationary-phase. This effect occurs because the impedance growth curves tend to get away from the reference offered by the sterile medium. The best working conditions were obtained for an average basal impedance of 510 ohms under well controlled temperature conditions (variations smaller than or equal to 0.20 degrees C) with wire stainless steel electrodes vertically immersed in the culture broth. This impedance technique appears as inexpensive and easy to automatizing for large number of samples.
Left ventricular function can be assessed with pressure-volume diagrams (DPV). Since there exists a relationship between ventricular volume changes and changes in intraventricular impedance (ZIV), we propose a diagram, DPZ, obtained from the intraventricular pressure (PIV) and the ZIV signals. This method was tested in 10 anesthetized mongrel dogs (morphine, sc, 30 mg/kg, and pentobarbital, sc, 5-7 mg/kg). PIV was measured with a Millar microtip catheter advanced under fluoroscopy via the femoral artery. ZIV was measured with a laboratory impedance meter (100 microA rms constant current, 12 kHz) in a tetrapolar configuration using another intraventricular catheter. ECG, aortic pressure and basal ZIV were also recorded to help identification of the different cardiac cycle phases on the diagram. Single-beat and multiple-beat loops were obtained in bradycardia and tachycardia to test for repeatability. These loops were always of a quadrangular-like shape with clockwise rotation. No attempt was made to calibrate the impedance axis in terms of volume. Apparent small volume changes in the isovolumic phases were probably due to ZIV catheter bending and/or movement. It is suggested that DPZ can serve in itself as a complementary evaluation tool of ventricular function by application of different maneuvers (physical, physiological of pharmacological).
UNLABELLED: By means of tetrapolar intraventricular impedancimetric measurements and by modifying the aortic impedance with a brief volume unit step afterload increase, with a balloon catheter, we obtained the geometric locus of the end-systolic pressure-admittance points (PES-YES) from the beat to beat pressure-impedance diagram (DPZ), usually three or four beats after inflation. Six experiments in five anesthetized mongrel dogs (average weight 21.6 kg, SD 2.5) were carried out under control conditions and after a slow infusion (4 micrograms/kg/min) of epinephrine to change the myocardial inotropic state. In control, the PES-YES points fell in all cases on a straight line (average correlation 0.968, SD 0.033). After epinephrine, linearity was still good in four experiments (average correlation 0.938, SD 0.059). All animals showed displacements of the regression lines after epinephrine with respect to control. However, the direction of the shifts was not always consistent: three dogs yielded an upward-leftward shift; one showed a crossing point between the two regression lines with an increase in slope after epinephrine; in the remaining two experiments (same dog in different days), the post-epinephrine regression lines of the pressure-admittance end-systolic points were displaced downwards and rightwards. Five out of the six experiments increased the slopes of the lines after infusion of epinephrine. IN CONCLUSION: the method supplied the end-systolic point locus obtained from simple cardiac catheterization and this locus was altered by inotropic intervention.
With an 8-electrode catheter, this instrument measures simultaneously five intraventricular sections of admittance and, also, total admittance, the latter obtained as the sum of five component sections. It injects through the two outermost electrodes of the catheter a constant current (828 microA rms) of high frequency (14 kHz). Actually, the instrument measures the mathematical inverse of the resistive component (conductance) of the impedance seen by the voltage sensing electrodes. The sensitivity per channel was adjusted to 25 V/mho, resulting in an overall sensitivity of 5.28 V/mho for the total intraventricular admittance. Nonlinearity in any single channel was less than or equal to 3.4% while, for the total admittance, it was less than or equal to 1.5%. The difference between any two individual channels when the same calibration resistor was connected, relative to the expected value, was less than or equal to 4.8%. The equipment demonstrated good performance in animal experiments and showed the possibility of detecting localized dysfunctions. Human applications still remain to be done.
In 260 successful transventricular simple capacitor-discharge defibrillations performed on 20 mongrel dogs under conditions of body hypothermia, an overall average peak current threshold of 69.5 mA/g of heart (SD 30.4) was found. This value, when compared by means of the unpaired t test with previous data obtained under conditions of relative normothermia (89.5 mA/g of heart, SD 32.8, 346 defibrillations, 20 dogs) yielded a highly significant difference (P less than 0.1%). When comparing the deviation of the regression equation (current vs. temperature) from the horizontal line, the Snedecor F test gave also a high level of significance (P less than 1%). These results led to the conclusion that body hypothermia significantly reduces transventricular defibrillation thresholds. After normalizing the regression equations, this reduction was found to be on the average equal to 4.1%/degrees C (SD 1.4) for current and to 5.9%/degrees C (SD 1.4) for energy over the 20 dogs. In all animals, the coefficient of variation was greater for energy than for current (about twice as much), suggesting that current is a better descriptor of what is needed for electrical defibrillation. The transventricular impedance was rather constant, yielding an overall average of 28.5 omega (SD 6.0).
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In a population of ten young normal male subjects (average height of 174.2 cm +/- 3.8, average weight of 70.2 kg +/- 6.16), which was subjectively classified as mainly ectomorph (slightly biased towards the mesomorph type) a good correlation was found between the transthoracic impedance changes due to normal respiratory movements and tidal volume (average r of 0.963 +/- 0.034) and a mean coefficient b1 of 3.64 ohms/liter +/- 1.82. For equal volume, inspiratory and expiratory movements, on the average, produced impedance changes with no significant difference (p less than 0.55). Attempts to quantitatively correlate the coefficient b1 (ohms/liter) with the anthropometric parameters did not produce any consistent result.
A high linearity instrument for biological use which permits the measurement of impedance modulus and its variations in the range of 50 to 1,000 ohms is described. It has two outputs (Z and delta Z) which can be connected to an oscilloscope or to any conventional physiological recorder. The deviation from linearity is not greater than 0,06%, with a sensitivity of up to 100 mV/ohm, and an estimated average error better than 1% in absolute value with standard deviation better than 3% and an expected range smaller than +/- 10%. The instrument can stand changes of +/- 10% in the a.c. power supply without showing significant alterations in its behavior. Besides, it has good isolating characteristics in order to prevent undesirable leak currents which are potentially dangerous for the patient. The instrument was tested satisfactorily in human beings and in experimental dogs by recording respiratory movements and cardiac activity.