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Muscle fibre conduction velocity is an important measurement in electrophysiology, both in the research laboratory and in clinical practice. It is usually measured by placing electrodes spaced at known distances and estimating the transit time of the action potential. The problem, common to all methods, is the estimation of this time delay. Several measurement procedures, in the time and frequency domains, have been proposed. Time-domain strategies usually require two acquisition channels, whereas some frequency-domain methods can be implemented using a single one. The method described operates in the time domain, making use of the autocorrelation function of the difference signal obtained from two needle electrodes and only one acquisition channel. Experimental results were obtained from the electromyogram of two biceps muscles (two adult male subjects, nine records each) under voluntary contraction, yielding an average of 3.58 m s−1 (SD=0.04 m s−1) and 3.37 m s−1 (SD=0.03 m s−1), respectively. Several tests showed that the proposed method works properly with electromyogram records as short as 0.3 s., Instituto de Investigaciones en Electrónica, Control y Procesamiento de Señales

An apparatus for the measurement of bacterial growth is described. The instrument applies alternate adequate sequential currents of two different frequencies through a pair of electrodes immersed in a cultured medium. It monitors, detects and quantifies the growth of micro-organisms based on the measurement of the impedance across the two electrodes and, simultaneously, it measures the variation in the medium turbidity. The medium conductivity and the interface electrode impedance changes can be extracted from the measured impedance. The variations in turbidity can be calibrated in absorbance or optical density units. Moreover, all these parameters are also proportional to bacterial proliferation. The computer-controlled apparatus processes and displays the parameters on a monitor showing bulk resistance, electrode impedance and turbidity changes as time course events. The equipment can detect aerobic or anaerobic micro-organisms and permits the operator simultaneously to assess impedance and turbidity, or it can produce each parameter as a separate event. Time growth curves of different micro-organisms are presented in the results.

Left ventricular volume calibration based on the conductance catheter depends on the correct determination of the parallel conductance (G p ). Baan's saline manoeuvre procedure leads to G p by finding the end-systolic (G es ) and end-diastolic (G ed ) conductances, for each beat of the dilution curve rising limb. After plotting such values in an xy-system, their linear regression is back-projected to intersect the identity line, so yielding an estimated Gp. The objective is to theoretically analyse all possible lines, G es =aG ed +b (Baan's line) and, based on experimental results, to establish their limitations. This was attained by calculating the regression lines using, first G ed =f1(G es ) and thereafter, Ges=f2(G ed ), which led to two values, G p2 and G p1 , for the parallel conductance. The morphology of the saline curve was also modified to assess its effect on the extrapolation. Multiple dilutions were recorded in eight experimental dogs injecting different concentrations. Each curve was classified according to the maximum change (VAR) reached by the total average conductance. Over 138 manoeuvres, 276 regressions were processed yielding correlations higher than 0.65. Of this total, 92.4% gave positive parallel conductances. The rest produced negative values and, thus, were neglected. If the two (G ed , G es ) statistical relationships were ideal, they should yield G p =G p1 =G p2 ; however, there were differences which, when G p1 was studied against G p2 , led to: G p1 =0.97 G p2 +0.055, with r=0.9476, and n=85. The remaining 53 were discarded because either some Gp values were negative, or the correlation of G es which G ed (or vice versa) was 15% and, finally, with (G es , G ed ) correlation better than 0.85. Theoretical analysis of Baan's line offers a reference frame, which contains only a limited number of practical possibilities.

By means of a selfadjusting experimental protocol based on the evaluation of the cardiac impedance, peak current thresholds were determined to defibrillate canine hearts with transventricular capacitor discharges. From 346 successful defibrillations performed on 20 mongrel dogs, an average value of 0·82 A/kg (s.d.=0·3) was found. The energy showed considerably more variability (mean of 0·22 J/kg, s.d.=0·156), suggesting that it is not the most adequate electrical parameter to describe the dose for defibrillation. From these results, a recommended dose equal to the average value plus two standard deviations (i.e. 1·42 A/kg) would appear as acceptable at an essentially 100% success level, assuming it is the first trial and that no other deterioration exists. The procedure applied in these determinations made use of a constant current 12 kHz impedance meter to estimate the load imposed on the defibrillator by the fibrillating heart. The overall average value was 25·6 Ω (s.d.=4·9) ranging from 17 to 44 Ω. The average time constant RC of the exponential discharge was 1·63 ms (s.d.=0·31) using in all cases a 63 μF capacitor. There is, of course, an error in taking the modulus of Z as equal to R and also due to the low current used in the measurements, as opposed to the high current that flows during actual defibrillation. However, it does not seem to be significant from a practical point of view. Defibrillators with a built-in Z meter and a microprocessor to automatically set the value of the discharge are, as a consequence, quite conceivable. The results given here confirm, and, in a sense, improve on those already reported in the literature.