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The term Organismic Sets described a puzzling and difficult area of research that apparently appears nowadays forgotten, for recent publications cannot be pinpointed. The objective here intends to find out what this subject deals with, not trying to go deeper in its intrincacies, for it would exceed by far the possibilities of the article. Instead, the article only calls the attention and perhaps stimulates the young mathematically oriented researcher.

The relationship among cardiac pacemakers is characterized by the fact that one pacemaker is usually dominant and all the others are subsidiary. The sinoatrial node acts as the dominant pacemaker, and all other potential pacemaker tissues are discharged by a conducted impulse before their respective diastolic depolarizations attain threshold. These pacemakers are called subsidiary to emphasize the fact that, under normal circumstances, they are engaged in conducting impulses, but, under abnormal circumstances, they may become actual pacemakers.

Medical science developed in tandem with the evolution of biological species and their associated diseases. Because of the close interaction between humans and other animals, even those in the wild, taking care of the former also means caring for the latter. Several scientific forerunners delved into animals' anatomical and physiological secrets in their quest to better understand animal biology and functions, thereby laying the foundation for animal medicine. Here, I briefly explore the long and complex road that led to the current state of veterinary science and provide a few examples of its present standing. (Contributions from the ancient world and eastern countries are not considered, as they represent a different area of interest.).

A biological pacemaker is one or more types of cellular components that, when implanted into certain regions of the heart, produce electrical stimuli that mimic that of the body’s natural pacemaker cells. Somatic gene transfer, cell fusion, or cell transplantation provide a way to realize it as somatic reprogramming strategies, which involve transfer of genes encoding transcription factors to transform working myocardium into a surrogate sinoatrial node, are furthest along in the possibilities. The idea, no doubt, is bright and appealing. The objective herein intends to dig into the subject trying to find out how realizable it really is.

The universe is the most fundamental wonder: we, as humans, face it every day, contemplate it in endless amazement, question it in our search for answers. And long ago, at a particular moment in a tiny piece of that great wonder, a second wonder, perhaps deeper in reach, emerged: life. Then, slowly, life evolved to contain within it a third wonder, possibly greater in some respects than the universe itself: the human mind.

The conductance catheter has gained momentum since its introduction in cardiovascular dynamics back in 1980. However, measuring errors are still blurring its clinical acceptance. The main objective here was to study the effects of the injected saline concentration and-temperature on the evaluation of the parallel conductance, G/sub p/, and thus, on the correction volume V/sub p/. That conductance, G/sub p/, and its associated volume, V/sub p/, were computed using 167 saline dilution curves obtained with boluses at different concentrations and temperatures, injected in seven anesthetized closed-chest dogs. The excursion of the total conductance relative to the steady-state value during a saline manoeuvre showed good correlation with the injected concentration at both studied temperatures. The reference parallel volume (one reference per dog) was defined as the average value obtained with three successive manoeuvres, at 6-M concentration and at body temperature; therefore, the method acted as its own reference. The variation of V/sub p/ relative to the reference value was clearly dependent on the injected concentration and on its temperature; dispersion was greater at 22/spl deg/C than at 40/spl deg/C. The variability would recognize also other causes, such as uncertainty of the extrapolation procedure and the thoracic redistribution of electrical field lines. As a conclusion, it is recommended to characterize each manoeuvre by its concentration and temperature. Body temperature and 6-M concentration appear as the most recommendable combination for the injectate in most animals. Finally, these results intend to characterize the V/sub p/ estimation procedure in order to minimize errors. The variability of V/sub p/, in different experimental conditions, demonstrated that both concentration and temperature are additional parameters that may modify the G/sub p/ estimate.

On the basis of the strict exclusion of the vis vitalis, the demand was raised by Carl Ludwig, Helmholtz, Du Bois-Reymond, and Brucke for a physiology which was causal-analytical and physically and chemically experimental. If, out of these four investigators, we pick Ludwig as the actual founder of modern physiology, the grounds for this must be justified specifically. That modern physiology is not to be contemplated without the works of the three great students of Johannes Muller is explicitly emphasized. However, Carl Ludwig occupies a special position for physiology.

Background An allometric relationship between different electrocardiogram (ECG) parameters and infarcted ventricular mass was assessed in a myocardial infarction (MI) model in New Zealand rabbits. Methods A total of fifteen animals were used, out of which ten underwent left anterior descending coronary artery ligation to induce infarction (7–35% area). Myocardial infarction (MI) evolved and stabilized during a three month-period, after which, rabbits were sacrificed and the injured area was histologically confirmed. Right before sacrifice, ECGs were obtained to correlate several of its parameters to the infarcted mass. The latter was normalized after combining data from planimetry measurements and heart weight. The following ECG parameters were studied: RR and PR intervals, P-wave duration (PD), QRS duration (QRSD) and amplitude (QRSA), Q-wave (QA), R-wave (RA) and S-wave (SA) amplitudes, T-wave peak amplitude (TA), the interval from the peak to the end of the T-wave (TPE), ST-segment deviation (STA), QT interval (QT), corrected QT and JT intervals. Corrected QT was analyzed with different correction formulae, i.e., Bazett (QTB), Framingham (QTFRA), Fridericia (QTFRI), Hodge (QTHO) and Matsunaga (QTMA) and compared thereafter. The former variables and infarcted ventricular mass were then fitted to the allometric equation in terms of deviation from normality, in turn derived after ECGs in 5 healthy rabbits. Results Six variables (JT, QTB, QA, SA, TA and STA) presented statistical differences among leads. QT showed the best allometric fit (r = 0.78), followed by TA (r = 0.77), STA (r = 0.75), QTFRA (r = 0.72), TPE (r = 0.69), QTFRI (r = 0.68) and QTMA (r = 0.68). Corrected QT’s (QTFRA, QTFRI and QTMA) performed worse than the uncorrected counterpart (QT), the former scaling allometrically with similar goodness of fits. Conclusions QT, TA, STA and TPE could possibly be used to assess infarction extent in an old MI event through the allometric model as a first approach. Moreover, the TPE also produced a good allometric scaling, leading to the potential existence of promising allometric indexes to diagnose malignant arrhythmias.

Biomedical engineering is clearly present in modern neuroendocrinology, and indeed has come to embrace it in many respects. First, we briefly review the origins of endocrinology until neuroendocrinology, after a long saga, was established in the 1950's decade with quantified results made possible by the radioimmunoassay technique (RIA), a development contributed by the physical sciences. However, instrumentation was only one face of the quantification process, for mathematical models aiding in the study of negative feedback loops, first rather shyly and now at a growing rate, became means building the edifice of mathematical neuroendocrinology while computer assisted techniques help unravel the associated genetic aspects or the nature itself of endocrine bursts by numerical deconvolution analysis. To end the note, attention is called to the pleiotropic characteristics of neuroendocrinology, which keeps branching off almost endlessly as bioengineering does too.

Background In Impedance Microbiology, the time during which the measuring equipment is connected to the bipolar cells is rather long, usually between 6 to 24 hrs for microorganisms with duplication times in the order of less than one hour and concentrations ranging from 101 to 107 [CFU/ml]. Under these conditions, the electrode-electrolyte interface impedance may show a slow drift of about 2%/hr. By and large, growth curves superimposed on such drift do not stabilize, are less reproducible, and keep on distorting all over the measurement of the temporal reactive or resistive records due to interface changes, in turn originated in bacterial activity. This problem has been found when growth curves were obtained by means of impedance analyzers or with impedance bridges using different types of operational amplifiers. Methods Suspecting that the input circuitry was the culprit of the deleterious effect, we used for that matter (a) ultra-low bias current amplifiers, (b) isolating relays for the selection of cells, and (c) a shorter connection time, so that the relays were maintained opened after the readings, to bring down such spurious drift to a negligible value. Bacterial growth curves were obtained in order to test their quality. Results It was demonstrated that the drift decreases ten fold when the circuit remained connected to the cell for a short time between measurements, so that the distortion became truly negligible. Improvement due to better-input amplifiers was not as good as by reducing the connection time. Moreover, temperature effects were insignificant with a regulation of ± 0.2 [°C]. Frequency did not influence either. Conclusion The drift originated either at the dc input bias offset current (Ios) of the integrated circuits, or in discrete transistors connected directly to the electrodes immersed in the cells, depending on the particular circuit arrangement. Reduction of the connection time was the best countermeasure.

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.

The impedancimetric method is a technique for the rapid evaluation of milk bacterial content and also of its subproducts. Several authors have made use of culture conductance changes during bacterial growth for quantitative and qualitative assessments of microbial growth. However, interface capacitance curves, Ci, have not been used. In this paper, we quantify bacteria in cow raw milk by following their growth as the above-mentioned capacitance change time course event. With it, bigger growth variations, shorter detection times and a better coefficient of correlation with the plate count method were obtained than those yielded by conductance curves. Calibration was performed by plotting initial known concentrations, IC (CFU/ml), as a function of the time detection theshold (TDT).

Background Physiological experiments have shown that the mean arterial blood pressure (MAP) can not be regulated after chemo and cardiopulmonary receptor denervation. Neuro-physiological information suggests that the nucleus tractus solitarius (NTS) is the only structure that receives information from its rostral neural nuclei and from the cardiovascular receptors and projects to nuclei that regulate the circulatory variables. Methods From a control theory perspective, to answer if the cardiovascular regulation has a set point, we should find out whether in the cardiovascular control there is something equivalent to a comparator evaluating the error signal (between the rostral projections to the NTS and the feedback inputs). The NTS would function as a comparator if: a) its lesion suppresses cardiovascular regulation; b) the negative feedback loop still responds normally to perturbations (such as mechanical or electrical) after cutting the rostral afferent fibers to the NTS; c) perturbation of rostral neural structures (RNS) to the NTS modifies the set point without changing the dynamics of the elicited response; and d) cardiovascular responses to perturbations on neural structures within the negative feedback loop compensate for much faster than perturbations on the NTS rostral structures. Results From the control theory framework, experimental evidence found currently in the literature plus experimental results from our group was put together showing that the above-mentioned conditions (to show that the NTS functions as a comparator) are satisfied. Conclusions Physiological experiments suggest that long-term blood pressure is regulated by the nervous system. The NTS functions as a comparator (evaluating the error signal) between its RNS and the cardiovascular receptor afferents and projects to nuclei that regulate the circulatory variables. The mean arterial pressure (MAP) is regulated by the feedback of chemo and cardiopulmonary receptors and the baroreflex would stabilize the short term pressure value to the prevailing carotid MAP. The discharge rates of rostral neural projections to the NTS would function as the set point of the closed and open loops of cardiovascular control. No doubt, then, the RNS play a functional role not only under steady-state conditions, but also in different behaviors and pathologies.

Transventricular peak current defibrillation thresholds using quarter (5 ms) and half (10 ms) sinusoidal pulses were determined in dogs under conditions of normothermia. For 5 ms, the overall average was 59·6 mAg−1 of heart (s.d. 21·00) in a total of ten animals and 88 successful defibrillations. For 10 ms, the average over seven dogs and 51 determinations was 52·5 mAg−1 of heart (s.d. 15·4). This difference was not statistically significant, whereas the energy delivered in the latter was about twice as much as in the former, suggesting the quarter sine wave pulse was better for defibrillation. A tighter definition of threshold is also introduced, i.e. the midrange between the maximum value which did not defibrillate and the minimum that did defibrillate. Such a definition is more realistic because it takes into account the successful and also the unsuccessfulvalues within a band of overlapping data points. The thresholds so obtained (52·3 mAg−1, s.d. 15·7, nine dogs, 5 ms; 42·9 mAg−1, s.d. 6·56, fourdogs, 10 ms) were, as expected, somewhat lower than the above mentioned ones showing, at the same time, lower coefficients of variation.

Peak current defibrillation thresholds (PCDT), i.e., values with 50% probability of success, were determined after ligation of the left anterior descending coronary artery (LADCA) while, simultaneously, body temperature was slowly decreased until central venous temperature reached an average minimum of 22.8 degrees C (s.d. 3.1). In 14 dogs with over 166 successful defibrillations, the average time elapsed between the moment of ligation and the last defibrillation was 179.1 min. (s.d. 31.9), with an average PCDT of 53.6 mA g-1 of heart (s.d. 20.5). This value was compared by means of the unpaired Student's t test with three previous values obtained, respectively, under hypothermia with no occlusion (HNO), normo-thermia with coronary occlusion (NCO), and normothermia with no occlusion (NNO), that is, HNO 69.5 (s.d. 30.4), NCO 81.1 (s.d. 29.3), and NNO 89.5 (s.d. 32.8), all expressed in mA g-1 of heart. All three differences even after Bonferroni's correction, were statistically significant (P less than 0.3%). We concluded that, (1) defibrillation thresholds were decreased by coronary occlusion and by hypothermia, (2) the decrease due to hypothermia was greater than that due to coronary occlusion, (3) both decrements appeared as additive.

Fibrillation is more likely to occur in animals showing a high degree of cellular differentiation. Lower species, with cardiac tissue histologically and electrophysiologically more uniform, very rarely, if ever, can fall into this uncoordinated activity. We describe experiments during which two specimens of the South American toad, Bufo paracnemis (body weights, 363 g and 297 g; ventricular weights, 1.8 g and 1.4 g, respectively), were repeatedly fibrillated and defibrillated almost at will. However, we failed when we tried a series of experiments in a given number of animals. Over a total of 16 fibrillation-defibrillation episodes, a fibrillation threshold of about 3.8 mA/g was estimated. The lowest defibrillation values were, respectively, 43 mA/g and 507 mA/g, using rectangular pulses (less than 7 msec pulses width). There were also a few spontaneous reversals before and after defibrillation shocks. These cases point to our still poor understanding of the mechanisms involved in the process. It is suggested that reentry or multiple ectopic foci might not be the basic mechanisms, as traditionally accepted. Rather, it might well be a new behavior of the cells or group of cells like that predicted by the theory of deterministic chaos.

Toad ventricles were externally driven by periodic pulses while monophasic action potential (MAP) signals were recorded in seven excised and seven in situ ventricles. As the frequency was slowly increased in steps, the stimulated tissue displayed several dynamic characteristics. Hierarchies of periodic behavior, like phase-locking and period-doubling sequences leading to chaos, were observed. Results showed that subharmonic bifurcations (order one and two) and chaotic-like behavior may systematically occur in the MAP signal within a definite frequency interval in the 1:1 phase locking regime. The chaotic, or more cautiously expressed, chaotic-like behavior is characterized by the power spectrum, the autocorrelation function, the Poincaré map, and the reconstructed 2-D phase portrait. It is concluded that (a) bifurcations of order one and two and the characteristic irregular behavior are evidences of local universal chaotic dynamics in cardiac tissue; (b) there are no qualitative differences in the dynamics of the in situ and excised ventricles; and (c) fibrillation seems to be related to chaotic behavior, but whether they are similar or equivalent phenomena still remains to be seen.

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).

The average intestinal-prostatic impedance in unanaesthetised bulls at ejaculation was experimentally evaluated by the application of known stimulating rectangular pulses (2 ms width, repetition rates between 25–35 pulses/s) and found, on average, to be of the order of 65·9Ω (s.d.=9) over a total of 14 ejaculations in eleven animals. The corresponding mean ejaculatory stimulating current dose was 0·507 mA/kg (s.d.=0·181). In all cases, the total current for full ejaculation was higher than 170 mA. An empirical relationship between biological impedance and applied current (124 data points) was also determined, showing, as expected, a decrease with increasing current and tending to a constant value in the order of 62Ω (s.e.e.=12) for la=288 mA. This impedance (which is faced by the stimulating electrodes inserted in the rectum) can be very easily estimated by means of the well known impedance technique at higher frequencies. In these experiments and at 12 kHz, the average over 44 measurements was 59·3 Ω (s.d.=10·7), a value very close to those given above and, therefore, useful in the design of adequate electroejaculators.

The open-loop gainG0 reference pressureP0 and basal heartrateR0 of the blood-pressure regulatory system have been estimated under four different experimental conditions by employing an aortic balloon and assuming a theoretical approach based on a 1st-order linear model. Mean values of 1·7–3·7, 75–130 mmHg, and 95–130 beats/min were obtained forG0,P0 andR0, respectively. Significant variations in these parameters were observed when the anaesthetic conditions were changed. It is suggested thatG0,P0 andR0 may have useful clinical applications both in characterising hypotensive and hypertensive syndromes and also in assessing the effect of anaesthetic agents.

A method has been developed to evaluate the heartrate sensitivity (S 0) of the baroreceptor system, which is defined as the decrease in heartrate per unit increase in mean arterial pressure. Average values, for the intact system, of the order of 2·23–3·65 beats min−1 mmHg−1 were found in the dog, and of 0·5 beats min−1 mmHg−1 in one pony. The carotid sinus baroreceptors add considerable, but not preponderant, sensitivity to the cardiovascular regulatory system for blood pressure. Decreases inS 0 of the order of 40% were found when these receptors were essentially eliminated by occlusion of the carotid arteries. The type of anaesthesia used has a marked influence on the sensitivity of the baroreceptor system, suggesting the possibility of using this sensitivity coefficient to evaluate the influence of anaesthetics on the homeostatic mechanisms for blood pressure.

Summary The electrocardiogram of the snake can be reliably recorded with two leads: one longitudinal, the other transverse. The longitudinal lead consists of a cephalad electrode located at 10% of the body length and a caudad electrode at 50% of the body length; both electrodes are on the ventral surface of the animal. The transverse electrodes are located bilaterally at the level of the heart (approximately 24% of the body length for the species reported here). The most appropriate calibration for adequately large deflections of the recording pen is 2cm=1mV or, in some cases, 3cm=1mV. A longitudinal and a transverse distribution of the cardiac potentials have been obtained for each of the three components of the ECG. They would agree qualitatively with the predictions of the dipole theory. With the two orthogonal leads described above, a reasonable estimate of the cardiac vectors can be obtained. Changes in the position of the heart, either by handling or by artificial respiration, may introduce significant changes in the magnitude and orientation of the vectors. When transverse leads are recorded, rotation around the YY-axis usually produces large variations in the amplitudes of the ECG waves. Spontaneous changes in T-amplitude are relatively common in the snake.

Summary o 1. Body surface electrocardiograms have been recorded in the anesthetized and unanesthetized snake from leads approximating the conventions of limb leads. 2. They shown evidences of a sequence of activity in this form: from a pacemaker in the sinus venous to the atria and to the ventricles, by an electrocardiogram including an SV complex preceding the P wave. 3. The ventricular T wave in the snake is generally in the same direction as the major deflection R; Q and S are poorly developed. The T wave varies greatly in amplitude and may become completely inverted without apparent change in the position of the leads relative to the heart. 4. Direct leads and myograms from the individual chambers of the snake heart confirm the designations applied to the body surface electrocardiogram. 5. T waves have been detected following the P wave as well as the SV wave in direct leads, confirming the potentially biphasic nature of all the complexes of the electrocardiogram. 6. Studies of the duration of electrical systole related to the cardiac cycle yield ratios of 0.20, 0.40, and 0.60 for the sinus venosus, atria, and ventricle in the same heart and in comparable circumstances. They suggest a growing preponderance of the contractile element of ventricular systole. 7. As the heart deteriorates and dies the relative and absolute amplitude of the SV complex recorded from direct leads increases remarkably. 8. Spontaneous premature sinus waves and sino-atrial blocks appear as clear-cut electrographic entities.

Summary The sinus venosus-atrial Wenckebach-Lucianiphenomenon was fully and directly demonstrated in snakes and turtles. Its pattern is similar to that shown by the phenomenon at the atrio-ventricular level, and presumably the mechanisms are the same. Since a continuum of arrhythmias was observed between the 1:1 ratio and the final 2:1 block, it is inferred that the synchronization theory of Roberge and Nadeau could be extended to this case postulating the existence of three coupled oscillators.

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.

Nine adult mongrel dogs were instrumented with ultrasonic microcrystals to measure left ventricular basal anteroposterior diameter and midwall myocardial segment length near the cardiac apex. Pneumatic cuff occluders were positioned around the left circumflex coronary artery near its origin and around the left anterior descending coronary artery two thirds of the way along its length. A pressure microtransducer was implanted into the left ventricle. Ten days after instrumentation the animals were anaesthetised with morphine chlorhydrate and pentobarbital sodium. An eight electrode catheter was advanced into the left ventricle to measure ventricular apical and basal regional and total electrical conductance. Minor ischaemia caused by occlusion of the left anterior descending artery was detected only by the electrode pair located near the apex, as decreased local ejection fraction. Major ischaemia caused by left circumflex artery occlusion was detected by both apical and basal electrode pairs and by total conductance, the three conductance signals indicating reduced ejection fractions compared with control values. The basal diameter signal indicated that basal regional motility changed only during major ischaemia, thus confirming the specificity of the changes in the basal conductance signals. The apical segment length signal confirmed the altered motility indicated by the apical conductance signal. These results suggest that regional wall motion abnormalities may be detected by the use of a multielectrode conductance catheter.

Transventricular impedance during fibrillation was measured with two methods: 1) by injecting a low constant sinusoidal current (obtaining a Z value), and 2) by using the time constant of an exponential defibrillatory discharge (giving an R value). One group of eight dols (Group 300) was normothermic and another group of 14 dogs (Group 400) was hypothermic. The left anterior descending coronary artery was ligated in both groups. For Group 300 (values in ohms), Z = 29.5 (SD 5.1), R = 30.9 (SD 4.7). The paired t-test gave a statistically significant difference (p 0.1 percent). For Group 400, Z = 34.5 (SD 7.3); R = 33.8 (SD 5.8), also with a significant difference, but at a lower level (p 5 percent). Nonetheless, from the practical point of view of calculating the voltage to be applied after establishing an adequate dose current, they (the differences) do not seem to have any important quantitative weight, supporting method 1 (for its easiness) to estimate transventricular impedance in this particular application.

The impedance coefficient in ohms/litre obtained by means of bipolar impedance pneumography exhibits an inverse relationship with body weight in kilogrammes; this relationship appears valid on an inter-species basis and is probably also true from an intra-species point of view. The relationship is linear on log-log paper and nearly hyperbolic on a linear scale. A statistical regression analysis produced the expression log(ΔZ/ΔV)=2·656–1·08 logW with a Standard Error of Estimate of 0·24 and a correlation coefficientr=−0·976, where ΔZ/ΔV is the impedance coefficient in ohms/litre andW the body weight in kilogrammes. The impedance coefficient per kilogramme, obtained as the derivative of the expression above, is given by (log 490–2·08 logW) and describes with accuracy the change in sensitivity with different weights. For low values of body weightW, the impedance coefficient is very high; for high values ofW, the impedance coefficient is very low and, beyond 1000 kg, negligible signals are to be expected. This observation was verified in the elephant.

Three approximate methods for the computation of cardiac output from dye-dilution curves have been analysed using the classical Stewart-Hamilton technique as a reference in a total of 89 curves obtained from 11 dogs. The gamma-function method has a tendency to overestimate on the average (+7·1%±1·6%), with an 82% probability of falling within ±20% of the Stewart-Hamilton value. The forward-triangle and compartmental methods are approximately equivalent, the first giving a small overestimation of (0·3%±1·2%), and the second a small underestimation of (−3%±1·2%); both methods have a probability of better than 92% of being within ±20% of the Stewart-Hamilton value. The analysis of human radiocardiograms by the compartmental method was encouraging. A number N of equivalent compartments greater than 2·2 for either the right-or left-heart peaks was associated with an abnormal radiocardiogram. In eight normal subjects, N yielded an average of 1·73 for the right heart and 1·69 for the left heart. The compartmental analysis of radiocardiograms, together with a study of other parameters of the curves, would seem to provide a useful assessment of the cardio-vascular function.

Summary Q-T changes have been demonstrated in the snake electrocardiogram when the heart rate HR is decreased by vagal stimulation. Two types of relationships were studied with this method: Q-T as a function of HR (or R-R interval) and Q-T/R-R as a function of HR. Second degree polynomial approximations were obtained for these data. The data were also used to determine Bazett's coefficient for the snake, which was 5.6 when Q-T=K/√HR or 0.724 when Q-T=K/√R-R, where Q-T and R-R are in sec. and HR in beats/min. The second variable Q-T/R-R was found to be a nearly linear function of heart rate in the same animal at room temperature. There was no essential difference when interindividual variations were considered, although the dispersion increased significantly and a tendency of the function to become constant appeared evident with higher heart rates. This means that the relative decrease of Q-T is smaller at higher cardiac frequencies than it is at lower frequencies, and the curve begins to deviate from a straight line with a slope given by the initial angle of the second degree polynomial fit. Also, if Q-T is assumed to be constant, a chart can be constructed and superimposed on the Q-T/R-R vs HR scattergram so that the range and most probable normal value of Q-T are easily identified.

Background The notion of the nucleus tractus solitarius (NTS) as a comparator evaluating the error signal between its rostral neural structures (RNS) and the cardiovascular receptor afferents into it has been recently presented. From this perspective, stress can cause hypertension via set point changes, so offering an answer to an old question. Even though the local blood flow to tissues is influenced by circulating vasoactive hormones and also by local factors, there is yet significant sympathetic control. It is well established that the state of maturation of sympathetic innervation of blood vessels at birth varies across animal species and it takes place mostly during the postnatal period. During ontogeny, chemoreceptors are functional; they discharge when the partial pressures of oxygen and carbon dioxide in the arterial blood are not normal. Methods The model is a simple biological plausible adaptative neural network to simulate the development of the sympathetic nervous control. It is hypothesized that during ontogeny, from the RNS afferents to the NTS, the optimal level of each sympathetic efferent discharge is learned through the chemoreceptors' feedback. Its mean discharge leads to normal oxygen and carbon dioxide levels in each tissue. Thus, the sympathetic efferent discharge sets at the optimal level if, despite maximal drift, the local blood flow is compensated for by autoregulation. Such optimal level produces minimum chemoreceptor output, which must be maintained by the nervous system. Since blood flow is controlled by arterial blood pressure, the long-term mean level is stabilized to regulate oxygen and carbon dioxide levels. After development, the cardiopulmonary reflexes play an important role in controlling efferent sympathetic nerve activity to the kidneys and modulating sodium and water excretion. Results Starting from fixed RNS afferents to the NTS and random synaptic weight values, the sympathetic efferents converged to the optimal values. When learning was completed, the output from the chemoreceptors became zero because the sympathetic efferents led to normal partial pressures of oxygen and carbon dioxide. Conclusions We introduce here a simple simulating computational theory to study, from a neurophysiologic point of view, the sympathetic development of cardiovascular regulation due to feedback signals sent off by cardiovascular receptors. The model simulates, too, how the NTS, as emergent property, acts as a comparator and how its rostral afferents behave as set point.