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The strong electric currents used for heating purposes in welding and steel industries set up related magnetic fields (generally 0.1--10 mT and 50 Hz). Such fields 10--50 Hz, 0--40 mT) were used to induce visual light phenomena, magnetophosphenes. Threshold values for magnetophosphenes were determined as a function of magnetic field frequency as well as colour and intensity of the background illumination. A typical sensitivity maximum was found at 20--25 Hz. Differences between volunteers with normal colour vision and colour defective ones were observed. The frog retina was exposed to the same type of fields. Retinal activity, induced by the fields, was registered from the ganglion cell layer by means of microelectrode technique. The results indicate that magnetophosphenes are generated in the retina and in the same channels that are normally propagating signals induced by light of different qualities. The findings may be of guidance when formulating threshold limit values for extremely low frequency electromagnetic fields in industry.

The adhesion between the eye and a contact lens was recorded when influenced by water with varying salt concentrations. Both active removal and spontaneous loss of hard and soft contact lenses applied to the eye were investigated. In environments with saline less than 9 0/00 the risk of a soft contact lens falling out from the eye is almost negligible. In environments with saline greater than or equal to 9 0/00 the risk increase since adhesion in active removal decreased considerably. In reality, based upon the investigations of spontaneous loss of contact lenses in an underwater environment, well fitted soft lenses still do not however fall out of the eye. It should therefore be possible to use well fitted soft contact lenses without any significant risk even in working conditions which may include splashing of water upon the eye and regardless of the various salt concentrations of naturally occurring water in Sweden. For hard contact lenses, independent of salt concentration, adhesion in the underwater environment is extremely low and such lenses cannot therefore be used on an unprotected eye in conjunction with, for example, swimming. The risk of them falling out under splashing with water upon the eye is clearly greater than for soft lenses.

To evaluate the possibility of respiratory-volume measurement using photoplethysmography (PPG), PPG signals from 16 normal volunteers are collected, and the respiratory-induced intensity variations (RIIV) are digitally extracted. The RIIV signals are studied while respiratory volume is varied. Furthermore, respiratory rate, body posture and type of respiration are varied. A Fleisch pneumotachograph is used as the inspired volume reference. The RIIV and pneumotachograph signals are compared, and a statistical analysis is performed (linear regression and t-tests). The key idea is that the amplitude of the RIIV signal is related to the respiratory volume. The conclusion from the measurements is that there exists a relationship between the amplitude of the RIIV signal and the respiratory volume (R = 0.842, s = 0.428, p0.005). Absolute measurements of the respiratory volume are not possible from the RIIV signal with the present set-up. The RIIV signal also seems to be affected by respiratory rate and type. More knowledge about respiratory parameters and improved sensor and filter design are required to make absolute measurements of volumes possible.

Objective.A new photoplethysmographic (PPG) device for respiratoryand heart rate monitoring has been evaluated in the neonatal care units at theUniversity Children's Hospital of Uppsala, Sweden. Th ...

A Windkessel model has been constructed with the aim of investigating the respiratory-volume dependence of the photoplethysmographic (PPG) signal. Experimental studies show a correlation between respiratory volume and the peak-to-peak value of the respiratory-induced intensity variations (RIIV) in the PPG signal. The model compartments are organised in two closed chambers, representing the thorax and the abdomen, and in a peripheral part not directly influenced by respiration. Cardiac pulse and respiration are created by continuous adjustment of the pressures in the affected compartments. Together with the criteria for heart and venous valves, the model is based on a set of 17 differential equations. These equations are solved for varying thoracic and abdominal pressures corresponding to different respiratory volumes. Furthermore, a sensitivity analysis is performed to evaluate the properties of the model. The PPG signals are created as a combination of peripheral blood flow and pressure. From these signals, the respiratory synchronous parts are extracted and analysed. To study some important limitations of the model, respiratory type and rate are varied. From the simulations, it is possible to verify our earlier experimental results concerning the relationship between respiratory volume and the peak-to-peak value of the RIIV signal. An expected decrease in the amplitude of the respiratory signal with increased respiratory rate is also found, which is due to the lowpass characteristics of the vessel system. Variations in the relationship between thoracic and abdominal respiration also affect the RIIV signal. The simulations explain and verify what has been found previously in experimental studies.

We have previously described a technology based on diffuse reflectance of broadband light for measuring joint articular cartilage thickness, utilizing that optical absorption is different in cartilage and subchondral bone. This study is the first evaluation of the technology in human material. We also investigated the prospects of cartilage lesion imaging, with the specific aim of arthroscopic integration. Cartilage thickness was studied ex vivo in a number of sites (n = 87) on human knee joint condyles, removed from nine patients during total knee replacement surgery. A reflectance spectrum was taken at each site and the cartilage thickness was estimated using the blue, green, red and near-infrared regions of the spectrum, respectively. Estimated values were compared with reference cartilage thickness values (taken after sample slicing) using an exponential model. Two-dimensional Monte Carlo simulations were performed in a theoretical analysis of the experimental results. The reference cartilage thickness of the investigated sites was 1.60 ± 1.30 mm (mean ± SD) in the range 0–4.2 mm. Highest correlation coefficients were seen for the calculations based on the near-infrared region after normalization to the red region (r = 0.86) and for the green region (r = 0.80).

A new method, which uses a fibre-optic probe to monitor respiratory and heart rates simultaneously, has been developed and evaluated. The results provide evidence that it is possible to monitor respiratory and heart rates using the reflection mode of photoplethysmography (PPG). The fibre-optic probe makes it possible to monitor from different sites on the patient, and the method is convenient to use. In addition, the probe is X-ray transparent, insensitive to electromagnetic interference (e.g. during MRI investigation) and may be made very light and small. Therefore the method is suitable for the observation of both adults and neonates, in hospitals as well as in other environments.

Stimulation of the renal sympathetic nerve in young pigs with biphasic pulses of current (3 mA, 800 microseconds per phase, 5, 10 and 50 Hz) produced decreases in arterial and cortical blood flow in the kidney, with the greatest decreases occurring at the highest stimulus frequencies. The decrease in cortical flow lagged that in arterial flow by 1.53-1.99 s; the delay increased with decreasing frequency but was unaffected by captopril, an angiotensin-converting enzyme-blocking agent. This result was consistent with the hypothesis that stimulation of the sympathetic nerve causes constriction first in the afferent arteriole and then in the efferent arteriole. Systemic arterial pressure increased during stimulation of the nerve; the increase was greater in intact nerves than in nerves that had been crushed proximal to the point of the stimulus, indicating that pigs do have renal afferent nerves. Pressure increased after the stimulus ended, but the increase abated or changed to a decrease after administration of captopril. The changes in flow were unaffected by administration of captopril, but were markedly reduced by the blocking agent labetalol (renal arterial flow, 77 +/- 14 per cent; cortical flow, 70 +/- 12 per cent). Thus, the observed changes in flow resulted from direct stimulation of the sympathetic nerves and not from stimulation of the renin-angiotensin system, which affects the pressure response after the stimulus.

Photoplethysmography (PPG) and laser Doppler flowmetry (LDF) were compared and evaluated. The comparison was accomplished considering differences in physical principles and geometrical and optical conditions. Changes in human skin perfusion were induced by cold and hot water provocation on limited areas of the finger and the forearm. The results showed that LDF and PPG, using red light at 630-660 nm and a commercial PPG probe, were in general equally sensitive in detecting a blood perfusion increase following a skin temperature elevation. However, we also found that PPG occasionally showed an inverse response to a skin temperature elevation, especially in finger skin. Furthermore, the study indicated that laser light is unsuitable as a light source in PPG using optical fibres of small diameter and with no fibre separation. It was also found that the physical dimensions of the probe (including the light source and photodetector) play an important role in determining the measuring volume and the quality of the signal.

A study was performed to investigate whether measurements of the evaporation rate from the skin of newborn infants by the gradient method are affected by the presence of non-ionizing radiation from phototherapy equipment or a radiant heater. The evaporation rate was measured experimentally with the measuring sensors either exposed to or protected from non-ionizing radiation. Either blue light (phototherapy) or infrared light (radiant heater) was used; in the former case the evaporation rate was measured from a beaker of water covered with a semipermeable membrane, and in the latter case from the hand of an adult subject, aluminium foil or with the measuring probe in the air. No adverse effect on the determinations of the evaporation rate was found in the presence of blue light. Infrared radiation caused an error of 0.8 g/m2h when the radiant heater was set at its highest effect level or when the ambient humidity was high. At low and moderate levels the observed evaporation rate was not affected. It is concluded that when clinical measurements are made from the skin of newborn infants nursed under a radiant heater, the evaporation rate can appropriately be determined by the gradient method.

Optical reflection spectra of the cartilage/bone interface from hip joints of cows were studied. When comparing to ultrasonic measurement, it was found that cartilage thickness could be extracted using optical reflectance spectroscopy. For thicker cartilage layers, a high reflection for the wavelengths 400-600 nm was seen, and for thinner cartilage layers, the characteristic spectra of blood and bone dominated. The optical reflectance spectra may be used to characterise cartilage, and specifically cartilage thickness, in connection with in situ diagnosis or autologous chondrocyte implantation (ACI).

A new principle for cartilage layer thickness assessments in joints is presented. It is based on the differences between the absorption spectra of cartilage and subchondral bone (containing blood). High-resolution ultrasound measurements of cartilage thickness were compared with reflection spectroscopy data from the same area of bovine hip joint condyles. A simple mathematical model allowed calculation of thickness and comparison with ultrasound data. The cartilage thickness was changed by being ground in short episodes. For thicker cartilage layers, a high degree of reflection in the 400-600 nm wavelength interval was seen. For thinner cartilage layers, the characteristics of the spectra of blood and bone dominated those of cartilage. The mean (+/- SD) thickness of intact cartilage was 1.21 +/- 0.30 mm (n = 30). In an exponential regression model, spectroscopic estimation of cartilage thickness showed a correlation coefficient of r = 0.69 (n = 182). For thinner cartilage layers (d0.5 mm), the mean model error was 0.19 +/- 0.17 mm. Results from a bi-layer Monte Carlo simulation supported the assumption of an exponential relationship between spectroscopy data and reference ultrasound data. The conclusion is that optical reflection spectroscopy can be used for cartilage layer thickness assessment.

Laser Doppler flowmetry was used to study the microvascular blood flow of the human gingiva. Power spectral density analysis of the Doppler signals indicated differences in the vascular structure/flow patterns in four tissue types that were studied. Cigarette smoking elevated the gingival blood flow during a 5-min period after which the flow came back to baseline values. The study indicates that the laser Doppler method is a convenient and noninvasive method for dynamic blood flow measurements in the gingiva. >

The authors have developed two new sensor principles for heart- and respiratory rate monitoring. One sensor (Hjartrud) is based on skin microcirculatory fluctuations in blood flow, synchronous with the heart- and respiratory rates. A second sensor (Optovent) is based on condensation of humidity from the airways at the end of an optical fibre, the properties of which change with condensation of water. Both sensor principles have been evaluated. The accuracy's are comparable with sensors more established in clinical work. By using optical fibres one can obtain electromagnetic noise immunity of great value in MR-monitoring and during surgical diathermy. The Hjartrud sensor utilises digital signal processing techniques to extract and separate the heart- and respiratory signals from noise. Both instruments are in an industrial preproduction evaluation phase.

A new respiratory monitoring device has been evaluated in the postoperative care unit at Linkoping University Hospital, Sweden. The purpose of the study was to compare the new photoplethysmographic device to more established techniques. Continuous data acquisition from 16 patients (each 1 h long), were performed. The respiratory signals from the photoplethysmographic device were compared to impedance plethysmographic signals. Conclusions from the evaluation are that the performance of the new device is comparable with the impedance method, and in some cases even more reliable.

Ten newborn infants, born at 25-41 wk of gestation and nursed in a neonatal intensive care unit or in a neonatal intermediate care unit, underwent monitoring of heart and respiratory rates with a new technique using an optical sensor, 0-60 d postnatally. The aim of this study was to compare the heart and respiratory rates recorded in infants by photoplethysmography with a probe positioned on three monitoring sites, namely the leg, buttock and interscapular region, with the rates recorded by ECG and transthoracic impedance, respectively. The recordings were compared in order to determine which individual heart beats and respiratory cycles were recorded with one or both relevant techniques. A high degree of association (r =0.99) was found between the respiratory rates recorded by photoplethysmography and by transthoracic impedance. Recordings of heart rates by ECG were also highly associated with the photoplethysmographic rates as measured at two (r = 0.99) of the three monitoring sites. It is concluded that heart and respiratory rates in infants can be monitored satisfactorily by the new method based on photoplethysmography. This method offers the advantage of recording several physiological parameters non-invasively with a single probe during the neonatal period.

A new optical sensor for respiratory rate monitoring was simultaneously compared with an acoustic sensor and a transthoracic impedance plethysmograph during normoventilation in the respiratory rate range of 9–17 breaths per minute. The response characteristics of the optical sensor were then measured during simulation of central apnoea and tachypnoea. Visual observation was chosen as the reference method for monitoring the respiratory rate. The measurements were performed in ten healthy volunteers and the respiratory signals recorded on an analogue tape and strip-chart recorder and analysed off-line. The response characteristics of the fibre optic sensor corresponded well with those of the acoustic sensor and impedance plethysmograph. All three methods responded rapidly to an apnoeic event.

Visual observation was chosen as the reference method for measuring the respiratory rate in ten healthy volunteers. The new fibre-optic and acoustic sensors were simultaneously compared with capnography and transthoracic impedance plethysmography during normoventilation in the respiratory rate range of 6-24 breaths per minute and at a fixed respiratory rate of 13 breaths per minute. In addition a simulation of central apnoea was performed. All the measurements were recorded on an analogue tape recorder and a strip-chart recorder and analysed off line. The analyses of the recordings were performed by a person who was unable to see the monitoring systems. There was no discrepancy in the results of these methods. Each of the methods responded rapidly to an apnoeic event. The new fibre-optic and acoustic sensors correlate well with more traditional methods such as capnography and transthoracic impedance plethysmography for respiratory rate monitoring.

In situations in which it may be impossible and/or unethical to evaluate pulse oximetry in humans, an in vitro model with circulating blood may be a necessity. The main objective was to develop such an in vitro model and, in this model, validate the pulse oximetry technique at various haematocrit levels. The pulsating character of arterial blood flow in a tubing system was simulated by using a specially constructed pressure-regulated roller pump. The tubing system was designed to minimise damage to red blood cells. The pulse oximeter readings (SpO2) were compared with oxygen saturation analyses by a haemoximeter (SaO2). The pulse oximetry readings were recorded at various haematocrit levels and during haemolysis in the SaO2 range 60–100 per cent. At a haematocrit level of 41–44 per cent, there was no correlation between SaO2 and SpO2 readings. After diluting the blood with normal saline to a haematocrit of 10–11 per cent, a good correlation between SaO2 and SpO2 was found. Following haemolysis, the agreement between SaO2 and SpO2 was further improved. Using the developed in vitro model, the results indicate that the accuracy of a pulse oximeter may be dependent on the haematocrit level.

The object of this study was to investigate the influence of haematocrit on the accuracy of pulse oximetry. Seven Swedish land race rabbits were studied. The oxygen saturation of haemoglobin was decreased step-wise using increasing fractions of nitrogen to the inspiratory gas. One pulse oximeter probe was attached on the front leg and another probe directly over the common carotid artery. The pulse oximeter readings (SpO2) were compared with simultaneous oxygen saturation analysis (SaO2) by a haemoximeter. The pulse oximeter measurements were performed at the haematocrit levels of approximately 40% and 11%, respectively. We found a good correlation between SpO2 and SaO2 in a wide range of the oxygen saturation, i.e. SaO2 26-100%. After haemodilution the correlation was improved in the range 86-100%, but not in the range 26-85%. No correlation between SpO2 and SaO2 was found when the sensor was attached directly over the artery during normal haematocrit levels. After haemodilution a better correlation was however obtained. These results indicate that the accuracy of pulse oximetry is dependent on the haematocrit level.

Using a method described in a previous article the transepidermal water loss (TEWL) was studied in 10 healthy newborn infants at rest and during activity. On the average TEWL was 37% higher during activity than during rest although no sweating was observed. In 9 infants placed in incubators with an ambient temperature slightly above the thermoneutral range measurements were made as the body temperature rose. TEWL was almost constant until a temperature of 37.1 degrees C was reached whereupon the water loss suddenly increased as the infant started sweating.

Electrical stimulation of nerves has become an established method in long-term treatment of patients with disturbances in the central or peripheral nervous system. This type of treatment raises questions as to the long-term consequences of implantation in terms of tissue and electrode reactions. The aim of the present study was to find out whether the structure of peripheral nerve tissue is altered after long-term contact with platinum electrodes and the way in which platinum electrodes are changed by prolonged implantation. The study was performed in six dogs which had undergone experiments in which stimulation electrodes were placed bilaterally on the carotid sinus nerves. Fibrotic changes in the nerve bundle and the perineurium were observed after 12·5 years. Accumulation of organic material on the electrode surface was revealed by Auger electron spectroscopy. The surfaces of the implanted electrodes had oxidised and had a high concentration of carbon and measurable amounts of sulphur, indicating corrosion.

The long-term properties of commercially available ECG-electrodes were studied by investigating the parameters: polarization potential, electrical impedance, adhesion, and skin reactions during a period of 7 days. As expected, the most stable polarization potentials were obtained for Ag/AgCl electrodes. Certain simple disposable electrodes showed large polarization potential variations. The most stable electrode impedance was obtained for disposable electrodes with stable adhesion and equipped with an electrode cup or similar. Unchanged adhesion and mechanical properties during the test period were shown by the disposable electrodes with a large self-adhesive collar.

The possible influence of time-varying magnetic fields on action potential in the lobster giant axon was studied. The axon membrane was excited by galvanic stimulation and the action potential was recorded intracellularly with microelectrodes. During the propagation of the action potential along the axon, alternating or pulsed magnetic fields were applied across the middle part of the axon to study whether or not magnetic fields have any effect on parameters such as conduction velocity and refractory period of the nerve fibre and amplitude, duration and shape of the action potentials. No effect on these parameters was observed under different flux densities and frequencies of the magnetic fields. When simulating the conductive properties of tissue surrounding the nerve with the aid of an external conducting loop with a load resistance, action potentials were generated which made it possible to study the threshold value of the induced eddy current for nerve excitation. Based on the results of the experiment, the influence of magnetic flux density, frequency, conductivity, induced EMF and induced eddy current density is discussed, and a method is proposed for estimating the threshold values of magnetic flux density for nerve excitation.

In order to investigate whether sweating and cutaneous vasodilatation. induced by a warm environment, is inhibited by central cold-stimulation, the evaporation rate and skin blood flow were studied by non-invasive techniques before and after the feeding of cold glucose to 17 fullterm appropriate for gestational age infants, born by elective Caesarean section, on their first day after birth. In newborn infants who were visibly sweating and had an increased skin blood flow at a body temperature above 37.1 degrees C, such central cold-stimulation quickly inhibited sweating and decreased skin blood flow. In infants with almost the same body temperature but with no sweating, this stimulation caused no change in evaporation rate or skin blood flow. The regulation of body temperature in fullterm newborn infants can thus be disturbed by central cold-stimulation, resulting in a decreased heat loss to the environment even though the body temperature is elevated.

Localized neural control of renal cortical blood flow was observed in five chloralose-anaesthetized baboons. Renal artery blood flow was measured with an electromagnetic flowmeter, while superficial renal cortical blood flow was measured with a laser Doppler flowmeter. Stimulation of all renal nerve bundles isolated produced significant reductions in total renal blood flow, but reduced the superficial renal cortical blood flow only in localized areas. Systemic arterial blood pressure did not vary significantly, which suggests that the changes in renal cortical blood flow were neurally mediated and not the result of autoregulation. The renal cortical blood flow response was delayed 1-4 s with respect to the total renal blood flow response. The localization of flow control is consistent with the neuroanatomy of the kidney, and the delayed renal cortical blood flow response is consistent with the vascular anatomy of the kidney and with previous measurements of red cell transit times through the renal cortex.

A method has been worked out by which one of the closed control circuits in the blood-pressure-regulating system of the dog can be reversibly opened and closed again with the aid of an on-line analog computer. This makes it possible to study the regulatory properties of the carotid-sinus reflex, as regards the baroreceptor function, in the regulation of the systemic blood pressure. Proportional characteristics in the analog produce marked instability in the blood-pressure regulation, while with increasing proportional-plus-derivative characteristics the stability is increased. The electronic analog can be given characteristics that produce the same step-function response in the systemic pressure as the biological reflex.

Oberg, P. A. and U. Sjostrand, Studies of blood-pressure regulation. III. Dynamics of arterial blood pressure on carotid-sinus nerve stimulation. Acta physiol. scand. 1971. 81. 96–109. An analysis of the properties of the blood pressure regulatory system has been made by means of carotid-sinus nerve stimulation. The method used involves analog simulation of the nonlinear properties of this reflex. The analysis gives the gain-frequency and the gain-phase shift curves characterizing the system under study. A resonance frequency for the system is mostly found in the frequency range 0.03–0.06 Hz. Modulation of the control signal decreases the systemic bandwidth. Non-linear simulation indicates that the mammalian blood pressure regulating system has some of the characteristics of a sampled regulatory system. The afferent information in the carotid-sinus baroreceptor reflex stabilizes the systemic blood pressure by means of its pulsating nature. Transfer functions characterizing the system under study are given. The data presented are relevant to the functional characteristics of baropacer systems.

In studies of the carotid-sinus reflex mechanisms, the effects of sinusnerve stimulation on blood pressure and heart rate in unanaesthetized normotensive dogs were studied with three different types of stimulation-impulse distribution. The stimulation pulses were employed with the dogs running on a treadmill and at rest. The studies also comprised states of acute hypertension. The results strongly support the view that there is a frequency limitation in the baroreceptor-reflex arc and that the time distribution of impulses is of importance for the central interpretation of afferent baroreceptor information and for reflex cardiovascular control.

In order to investigate whether a change in skin blood flow is associated with changes in the evaporative loss of water from the skin of fullterm appropriate for gestational age infants, we measured the evaporation rate and skin blood flow in the body temperature range 36.6 to 37.2 degrees C. Fifteen healthy, fullterm newborn infants delivered by Caesarean section were studied in a warm environment. Evaporation rate and skin blood flow were measured non-invasively without disturbing the infant. When the body temperature slowly rose because of the warm environment, the skin blood flow steadily increased. The rate of evaporation of water from the skin slowly decreased until a body temperature of 37.1 degrees C was reached. The evaporation rate then suddenly increased as the infant began to sweat. Thus, in fullterm newborn infants born by Caesarean section the skin blood flow increases in the body temperature range 36.6-37.1 degrees C but does not influence the evaporation rate from the skin.

Blood flow was examined in sciatic nerves of pentobarbital-anesthetized rats by means of laser Doppler flowmetry (LDF) and intravenous [14C]iodoantipyrine infusion. Continuous LDF signals demonstrated slow oscillations and acute, pressure-related changes in flow. The steady-state LDF signal was related linearly to nerve blood flow, as measured with [14C]iodoantipyrine, in intact nerves and nerves stripped of the epineurium. In 14 intact nerves, nerve blood flow averaged 0.27 +/- 0.03 (SE) ml X min-1 X g-1, whereas it averaged 0.13 +/- 0.01 in 5 stripped nerves. Autoradiographs of [3H]-nicotine-infused nerves and intra-arterial injection of 57Co-labeled microspheres demonstrated that flow was not uniform throughout the nerve cross section. The results indicate that LDF can be used to examine nerve blood flow in vivo, demonstrate a linear relation between the LDF signal and flow, and establish absolute values for blood flow in intact and stripped nerves of the anesthetized rat.

Factors that may influence the water transport through the skin in infants on their first day of life were studied with a method based on determination of the vapour pressure gradient in the air layer close to the skin surface. The evaporation rate from the skin was found to vary with the site of measurement, ambient humidity, temperature, activity, gestational age and nutritional status at birth. Differences related to maturity were shown to decrease with postnatal age.

Blood flow in the small intestine was assessed in 48 patients by laser Doppler flowmetry. Mucosal and serosal flowmeter signals were compared during 'resting' conditions, vascular occlusion, and reactive hyperemia. Serosal flowmeter recordings were compared with the total blood flow of a bowel segment as measured by venous collection. The magnitudes of the mucosal (n = 49) and serosal (n = 49) flowmeter signals were comparable throughout the whole range of flowmeter signals (r = 0.97; p less than 0.001). A correlation coefficient of 0.95 (n = 51; p less than 0.001) was obtained between serosal flowmeter signals and total blood flow during 'resting' and reduced blood flows. During vasodilatation after a vascular occlusion, blood flow was underestimated by the flowmeter. A calibration curve could be constructed for approximate interpretation of the flowmeter signals in absolute flow units. The present study underlines the potential of laser Doppler flowmetry in the assessment of blood flow in the human small intestine.

Hammarlund, K., Nilsson, G. E., oberg, P. A. and Sedin, G. (Department of Paediatrics, University Hospital, Uppsala, and Department of Biomedical Engineering, Linkoping University, Linkoping, Sweden). Transepidermal water loss in newborn infants. V. Evaporation from the skin and heat exchange during the first hours of life. Acta Paediatr Scand, 69:385, 1980.—The amount of water evaporated from the skin was studied in 10 healthy newborn infants from their first minute of life, while being taken care of in the delivery room, and in 11 infants treated in incubators from their 30th min of life. The heat lost by evaporation, radiation and convection was calculated. Evaporation from the skin was very high during the first minutes after birth and was the main cause of heat loss during the first 15–30 min of life. Thereafter the amount of heat lost depended on the conditions under which the infant was nursed. Higher convective and radiative heat losses were found in delivery rooms than in incubators.

The effect of sinus nerve stimulation on the blood pressure and heart rate was studied in dogs. Electrical stimulation of the sinus nerves of both sides was given both in the form of 2 types of intermittent impulse trains and as impulses of constant frequency. The intermittent impulse bursts, which were synchronized with the ECG, comprised 5–50 imp which coincided with systole (150 ms). Comparisons were made between stimulation with a constant frequency and the intermittent types, with the same number of impulses per cardiac cycle. It was found that the maximal blood pressure reduction was already reached at 5–10 imp per cardiac cycle. The difference between stimulation with a constant frequency and the intermittent types, with the same number of impulses per cardiac cycle, is negligible as regards blood pressure reduction but the intermittent type has a greater effect on the heart rate. Sinus nerve stimulation in massive haemorrhage and the influence of the amplitude of the stimulation signal were also studied.

A method is described by which the influence of time-varying magnetic fields on excitable tissue can be studied in a reproducible way. The retina of the frog (Rana temporaria) was exposed to a time-varying magnetic field. The field-generated electrical activity of the ganglion cells was recorded by a microelectrode. The method opens up new possibilities for the study of effects of low-frequency magnetic fields on excitable tissues.