http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12892 The capacity of impedance cardiography (IC) to measure stroke volume (SV) accurately has been examined under widely varying conditions of SV, thoracic size, haemotocrit (Hct), heart rate (HR) and myocardial inotropy in dog models in experimental complete heart block, and in man and rabbits. In vivo blood resistivity (p) is essentially constant at 135 ± l.OΩ.cm over a wide range of Hcts (26%-66%) in mammalian species. Using in vivo p in the Kubicek formula the accuracy of a single estimate of SV is about 3% of the true SV, over a range of SV from 1.0-120.0 ml, of HR from 60 to 300 min-1, of resting to maximal myocardial inotropy, and of thoracic anatomy, e.g. rabbit, dog, and adult man. The technique overreads at heart rates less than 60 min⁻¹, and underreads slightly at high inotropic states, as in maximal exercise. Tile accuracy of the method compares favourably with the clinical right heart thermodilution method used in our laboratory (i.e. better than ±9% of true SV). 2013-05-16T04:41:35.700Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12890 The role of central (CNS) 5-hydroxytryptamine (5-HT) nerves in the thermal and arterial chemoreflex control of sympathetic innervation of ear skin vessels was studied in groups of unanesthetized normal, CNS 5-HT deplete (intracisternal 5,7-dihydroxytryptamine injection) and pontine rabbits. Ear artery flow (and conductance) was measured by a Doppler transducer. The high vasomotor tone in the ear skin of normal animals at rest was inhibited by the arterial chemoreflex and warm environment. It was absent, however, at rest in CNS 5-HT deplete animals. In pontine rabbits the resting high vasomotor tone was not inhibited by the arterial chemoreflex or a warm environment. These data suggest that bulbospinal 5-HT nerves have a major excitatory influence on tonic sympathetic activity to ear vessels, and that the central inhibition of this activity by arterial chemoreflex and environmental warmth involves suprabulbar influences and a transmitter other than 5-HT. 2013-05-16T04:41:22.347Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12763 The Kubicek thoracic cylinder model of impedance cardiography (IC) for measuring beat-by-beat stroke volume (SV) was evaluated in controlled studies using the electromagnetic flowmeter (FM) as the reference technique. Assuming the validity of the Kubicek equation for stroke volume calculation, IC stroke volume was found to be a linear function of EM values at any one haematocrit over a wide range of SV, but the slope of the relationship fell as haematocrit fell. Experiments using the same equation in dogs, in which blood resistivity in vivo (pt) was made the dependent variable, and the EM-derived value was usedfor stroke volume, showed that pt was almost constant over a wide range of haematocrits. These findings were supported by studies in man and rabbit where Fick and thermodilution-derived values were used for stroke volume. When these data were applied to normotensive and hypertensive human subjects with normal hearts and lungs in controlled studies at rest, during tilting, with drug therapy and on exercise, IC measured stroke volume and cardiac output with a variability at least as good as the 9–11% acceptable for clinical use. This conclusion applied to thoracic configurations of d sizes and shapes from adult man to the neonate. In chronic disease states, while assessments of relative changes are valuable, absolute data are questionable. Further research is required under these conditions, as it is also for other models of IC, which are based on different asswnptions. 2013-05-16T03:35:32.482Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12700 The steroid induction agent, Althesin, infused intravenously in light anesthetic doses in otherwise unsedated man (84 μg kg⁻¹ min⁻¹) and rabbit (140 μg kg⁻¹ min⁻¹) causes similar autonomic and somatic effects. In the rabbit, the rise in heart rate (mainly due to central vagal blockade) and the selective depressant effects on respiratory rate are independent of CNS 5-hydroxytryptamine and noradrenaline. The rise in arterial pressure and the fall in hindlimb conductance is dependent on CNS 5-hydroxytryptamine and noradrenaline synthesizing neurons, which are probably arranged in series. These findings provide a working hypothesis for the mechanisms of action of Althesin on central cardiopulmonary controls in man. 2013-05-16T01:59:47.457Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12701 Contrasting hypotheses of mammalian thermoregulation were tested in unanesthetized rabbits in relation to the role of the central nervous (CNS) monoamines 5-hydroxytryptamine (5-HT) and noradrenaline (NA) in regulating lung function and ear skin blood flow (Doppler flowmeter). Normal rabbits and rabbits with CNS depletion of 5-HT and NA (caused by the neurotoxins 5,7-dihydroxytryptamine and 6-hydroxydopamine) were studied in an airconditioned chamber at ambient temperatures of 12 °, 22 ° and 35°C. The results suggest that CNS 5-HT plays an excitatory role in the heat conservation mechanism of cold-induced ear skin vasoconstriction, and that this effect is inhibited by CNS NA to cause heat dissipation during heat stress. Both CNS 5-HT and NA appear to exert a mild inhibitory restraint on ventilation even during heat stress. The data support the theory that CNS 5-HT is concerned with heat conservation and CNS NA with heat loss mechanisms in the cutaneous circulation, and that both monoamines moderate heat loss through panting. CNS monoamine-dependent thermoregulation in the rabbit thus resembles the model postulated for the cat, dog and monkey rather than, as previously proposed, for the sheep and goat. 2013-05-16T01:55:26.581Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12747 Due to a unique vascular network, bronchial blood flow may be determined by the opening of anastomotic channels during perfusion pressure changes as well as by neurohumoral mechanisms. For example, previous studies from this laboratory have shown that a rise in aortic pressure, or a sigh followed by apnoea, result in dramatic rises (< 1s time-constant) in bronchial flow and conductance, which are not diminished by conventional autonomic blockade. These data suggest that unusual mechanical effects may indeed be involved, or that unknown reflex pathways are active. In order to define further the potential neurohumoral factors regulating bronchial flow, the present studies were designed to examine the role of a- and a-adrenoceptor, and cholinoceptor control. While the presence of such receptor mechanisms in the bronchial bed are likely the data were derived from indirect techniques for estimating airway vascular activity in anaesthetized, open-chested dogs. By contrast, in this study the continuous-wave Doppler flowmeter was used to examine the dynamic effects of these receptor control systems on bronchial flow in the conscious dog. 2013-04-08T06:51:37.053Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12691 Several new techniques have been introduced and/or refined over the past decade for the non-invasive measurement of stroke volume (SV) and cardiac output (CO) in man which have clear implications for the assessment of performance characteristics of athletes hitherto not possible. The CO₂ rebreathing method, however, still suffers from relatively poor accuracy at rest, and gated pool radionuclide scanning requires high levels of medical technological expertise under confined laboratory conditions. Impedance cardiography (IC) does not suffer from these disadvantages. We have evaluated the accuracy of impedance cardiography against accepted invasive methods e.g. thermodilution (TD), dye dilution (DD), the electromagnetic flowmeter (EM), in models where different degrees of experimental control could be applied. This approach was necessary due to the empirical aspects of the Kubicek formula for the calculation of stroke volume using IC. In addition, we have assessed the accuracy of the independent methods in clinical usage, as the assessment of accuracy of impedance cardiography would depend to some extent on the accuracy of the independent method used. Using regression analysis of simultaneous estimates of SV and CO, the accuracy of TD (against EM) in rabbits is 1.0% (S.E. of a single CO estimate expressed as % of mean CO (492 ± 3.5 ml), and that of IC SV measurements in the dog (against EM) is 1.7% (22.2 ± 0.4ml). In man, the accuracy of right heart thermodilution SV measurements (against DD) is 3.9% (75.4 3.0ml), and that for IC SV measurements (against TD) is 2.1 % (58.2 ± 1.2ml). These results indicate that both the invasive TD, and the non-invasive IC systems used in our laboratories have comparable and satisfactory accuracies for SV measurement, and suggest that under conditions of normal hearts and lungs, IC is useful for accurate and continuous non-invasive stroke volume measurements in exercising man, provided certain conditions of usage are satisfied. 2013-03-25T01:32:23.629Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12678 In four ballet schools in the Newcastle region of NSW, Australia, the prevalence of menstrual dysfunction was examined in relation to changes in cardiorespiratory, endocrine and metabolic function. Thirty four dancers (mean age 15.4 plus/minus 0.4 yr) and 31 non-dancers (mean age 15.5 plus/minus 0.7 yr) were investigated by: 1) questionnaire; 2) a test to determine peak exercise oxygen uptake; and 3) a prolonged exercise test (conducted at days 1-6 of the follicular phase of the menstrual cycle, or more than 90 days from the preceding menses, in amenorrhoeics). 71 per cent of the dancers, including one primary and two secondary amenorrhoeics, reported menstrual irregularity, compared to 19 per cent of the non-dancers (p less than 0.001). 47 per cent of the dancers had a measured cycle length of greater than 38 days, compared to 3 per cent of the non-dancers (p less than 0.001). Cardiorespiratory function was not notably different between the two groups, however the endocrine profiles were clearly different. The hormone levels suggest that the hypothalamus and pituitary functionally respond to the lower E2 levels, and, by increasing gonadotrophin production, elevate T and P production, but not E2. The menstrual irregularity of the teenage ballet dancer is therefore possibly due to the changing E2/T ratio secondary to a block in the conversion process of androgen to oestrogen within the ovary. 2013-03-21T22:34:09.133Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12668 The accuracy of impedance cardiography for measurement of stroke volume (SV_z) has been controversial. This could be, in part, due to the use in the Kubicek formula of the bench-determined blood resistivity (p)-haematocrit (Hct) relationship, which does not take into account factors such as complex blood-velocity movements. In the present study in vivo, p was calculated in dog, rabbit and man from the rearranged Kubicek formula, p = (SV. Zo²)/(L². dZ/dt max T); the stroke volumes used in the calculation being derived at different Hcts in the different species from the independent techniques of electromagnetic flowmeter, direct Fick, and direct Fick and thermodilution, respectively. The in vivo p-Hct relationship is linear, inverse and nearly constant over the range of Hcts tested (dog 26–62%, man 31–48%, rabbit 37%). No significant difference exists between meanin vivo p values determined for the three species at corresponding haematocrits despite different thoracic anatomy and circulatory dynamics. Thus, the best estimate of mean p in vivo within the normal Hct range is 135Ωcm, the use of which will result in a SV_z accuracy of better than ± 10%. 2013-03-21T03:23:17.124Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:2994 Summary: 1. The regional coronary circulation is under the control of local metabolic and myogenic factors, but is also influenced by autonomic systems, including sympathetic and parasympathetic nerves. 2. General anaesthetic agents influence not only local control through changes in metabolic demand, but also neural control through suppression of autonomic influence. 3. Anaesthetic agents have differing effects on reflex control systems, which are dependent on coronary territory and ventricular rate. 4. Effects of anaesthesia should be taken into account when interpreting results in anaesthetized models of coronary control. 2012-12-17T00:20:02.614Z ]]>