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Biomedical subjects

P Rolfe

Publications and source records attributed to P Rolfe.

At least 91 records · Page 5Linked to original sources
PubMed1988

Analysis and assessment of errors in a parallel data acquisition system for electrical impedance tomography.

Three possible data collection configurations for electrical impedance tomography are considered. These are serial current but parallel voltage measurement or full parallel operations. Measurement errors due to stray capacitances, common mode signals and contact impedance are considered. It is concluded that a parallel system offers advantages in terms of either speed or signal to noise ratio by may give larger common mode errors.

Electric Conductivity↗
PubMed1988

Cerebral electrical impedance: do indices derived from it provide information on cerebral blood flow in the neonate?

Cerebral electrical impedance was compared with simultaneous estimations of cerebral blood flow (CBF infinity) using the 133Xe clearance technique on 28 occasions in 9 neonates who had changes in PaCO2 and hence presumably cerebral blood flow between measurements. Various indices derived from the impedance waveform which have been reported in the adult to correlate with blood flow were analysed for their relationship with CBF infinity. Percentage change of the two variables was calculated and three indices showed a significant correlation using linear regression. They were peak amplitude (delta Zp), delta Zp times heart rate (delta Zp x HR), and backslope projection. The regression equation for the index with the closest correlation was delta Zp x HR = 0.57CBF infinity + 3.4 with r = 0.68 (p = 0.003) and standard error of the estimate = 29%. Some cerebral electrical impedance indices did correlate with cerebral blood flow changes under the conditions of this study and provide accurate information on the direction of changes but because of the wide confidence limits cannot be used for quantification of cerebral blood flow changes.

Blood Gas Monitoring, Transcutaneous↗
PubMed1987

Continuous non-invasive beat-by-beat blood pressure (B.P.) measurement in the newborn.

Intermittent measurement of blood pressure (BP) is frequently performed in newborn babies under intensive care using an inflatable cuff encircling either the arm or leg. This paper describes work aimed at achieving continuous beat-by-beat measurement of the arterial pressure waveform by means of a finger cuff, the pressure of which is controlled by a feedback arrangement utilising a photoplethysmograph. Preliminary results are encouraging, but improvements to cuff design are still needed.

Blood Pressure Determination↗
PubMed1987

Mass spectrometer system for continuous skin-surface and intravascular blood gas measurement of maternal-fetal respiration in labour.

This paper describes the performance characteristics of a dual-inlet mass spectrometer suitable for continuous monitoring of maternal and fetal blood gases using intra-arterial mass spectrometer catheters in the pregnant ewe and skin-surface mass spectrometer probes during human labour. Skin-surface measurements in newborn and adult sheep bore a close and easily interpretable relationship to intra-arterial measurements. The performance of the skin-surface mass spectrometer probe on the human forearm compared favourably with a commercially available transcutaneous electrochemical system for the measurement of oxygen and carbon dioxide but the mass spectrometer was capable of measuring additional gases such as nitrogen and argon.

Animals↗
PubMed1987

Laser Doppler velocimetry: the problem of fibre movement artefact.

The problem of fibre movement artefact in laser Doppler velocimetry recordings of skin blood flow has been investigated. Beat frequency spectra were obtained for a stationary target, unheated skin, heated skin (43.5 degrees C), with and without blood flow occluded for various induced amounts of fibre movement. Possible methods for either eliminating, or obtaining an indicator of, fibre movement artefact were considered. The solution proposed is to incorporate additional fibres terminating within the transducer to permit an index of fibre movement to be obtained that is independent of blood flow.

Adult↗
PubMed1987

Impedance imaging in the newborn.

We have been investigating the use of electrical impedance methods for the study of cerebral haemodynamics in the newborn for many years. Of particular interest has been the early detection of intraventricular haemorrhage, which is a major cause of death or handicap in low birthweight infants. Several problems exist in obtaining representative impedance measurements from the newborn, most notably movement artefact, respiratory-based modulation and a blood-flow related pulsatile component. Movement artefact is by far the most significant problem; in order to perform long-term monitoring we have developed an effective algorithm for rejection of corrupted data. To remove the respiratory component, which can be 1-2% of the impedance measurement, we link to the output of a respiratory monitor (standard in intensive care units) as a means of synchronising measurements to a fixed point in the respiratory cycle. Similarly, to remove the blood-flow pulsatile component, measurements are gated from the R-wave of the recorded ECG. Our total system consists of a front-end measurement system that passes image data over a serial link to a host computer. Data storage, image reconstruction and display is performed on the host, which can be any of a wide range of personal computers. The front-end contains the impedance measurement circuit, electrode switching electronics and a microprocessor for control, and is of a sufficiently small size to fit into the incubator next to the baby. Incorporating a microprocessor into the front-end produces a very flexible system that has many benefits, including: asynchronous operation from host, 'intelligent' pre-processing of measurements, command driven operation from host, etc. Software development for the front-end is performed on the host with program down-load for interactive debugging. Details of the front-end electronics and software, system performance, preliminary clinical results and other application areas of the impedance imaging technique to the care of the newborn are presented.

Cerebral Hemorrhage↗
PubMed1987

New fontanometer for continuous estimation of intracranial pressure in the newborn.

Intracranial pressure was estimated by a new pneumatic applanation fontanometer in babies in intensive care. A close correlation with cerebrospinal fluid pressure was found on 35 separate occasions. In 12 control babies the mean (SD) fontanelle pressure was 5.2 (2.2) cm H2O, in eight with a hypoxic-ischaemic encephalopathy it was 12.6 (4.3) cm H2O, and in nine with post-haemorrhagic hydrocephalus it was 10.8 (4.3) cm H2O. In four babies monitored continuously over seven days the correlation remained good. The fontanometer enables pressure to be monitored accurately, continuously, and non-invasively in neonates at risk of raised intracranial pressure.

Brain Diseases↗
PubMed1985

Laser Doppler measurements of blood flow in skin tissue.

Measurements of blood flow in skin tissue have been made using a low power helium neon laser, with a pair of multi-modal plastic optic fibres to transmit the laser radiation to the skin, one similar fibre to collect the scattered laser radiation, and photodiode detection. Before transmission to the skin surface one of the laser beams is frequency shifted by reflection from a moving corner cube, enabling the experimenter to distinguish between low frequency blood flow dependent signals having a Doppler shift origin, and those low frequency signals generated by other sources of intensity fluctuation such as fibre movement, number fluctuations and laser amplitude modulation.

Biomedical Engineering↗
PubMed1984

Continuous, noninvasive measurement of fetal oxygen and carbon dioxide levels in labor by use of mass spectrometry.

Clinical evaluation of the continuous, simultaneous measurement of fetal scalp surface oxygen and carbon dioxide partial pressures by mass spectrometry was undertaken for 52 labors. The mass spectrometer (MM8-80, V.G. Gas Analysis, Winsford, England) was easy to operate and had good long-term stability. The mean drifts for both oxygen and carbon dioxide over the study periods were less than 2 mm Hg. The mean (+/- SD) cervical dilatation at the time of transducer application was 6.1 (+/- 1.9) cm and the mean (+/- SD) duration of the studies was 169 (+/- 122) minutes; 10.5% of the transducer applications were unsuccessful. Falls in fetal scalp surface oxygen levels and rises in carbon dioxide levels were more frequent with late than with variable and with variable than with early fetal heart rate decelerations and with increasing severity and frequency of decelerations. Fetal scalp surface pressure changes also occurred with fetal heart rate variability changes, including some related to behavioral state changes. There was not a constant reciprocal relationship between oxygen and carbon dioxide changes, and fetal heart rate patterns were not related to actual blood gas levels. Fetal scalp surface measurements were related to both fetal blood sample and umbilical artery results. Trends in both oxygen and carbon dioxide levels during the course of labor were compared and related to other fetal variables, and most of the time the scalp surface measurements were an accurate guide to systemic blood gas levels. Maternal oxygen administration resulted in significant increase in fetal scalp surface oxygen levels, and on two of eight occasions it also led to decreases in fetal carbon dioxide levels. Scalp surface gas measurement by means of mass spectrometry is a powerful new method of intrapartum fetal monitoring, which should increase the precision of fetal surveillance as well as allow the accurate assessment of both established and new methods for optimizing labor and delivery.

Carbon Dioxide↗
PubMed1984

Electrochemical aspects of intravascular oxygen transducers.

The factors which limit the useful lifetime of miniaturized electrochemical oxygen transducers for use in neonatal intensive care are described. These factors arise from the use of the conventional silver-silver chloride reference electrode. Four possible means of improving the useful lifetime are described, namely the use of ion-permeable membranes, guard electrodes, potential-time programmes and nickel(II) hydroxide reference electrodes.

Blood Gas Analysis↗
PubMed1984

A comparison between electrical impedance and strain gauge plethysmography for the study of cerebral blood flow in the newborn.

This study investigates the possibility of using pulsatile transcephalic impedance changes, delta Z, for the continuous monitoring of cerebral blood flow, CBF, in the sick newborn infant. The performance of the impedance method is compared with the measurement of cranial flow, Fc, using a strain gauge for detecting the predicted changes in cerebral blood flow when the baby breathes 2% CO2 in air, or O2. In all five studies in which measurements were compared in air and CO2 the expected increases in Fc and delta Z were seen. In six of seven studies comparing measurements in air and O2 the expected fall in the variables was seen, indicating reduced cerebral blood flow. The problems of both these methods are discussed and we conclude that the impedance method is as good as the strain gauge method for detecting changes in cerebral blood flow.

Carbon Dioxide↗
PubMed1983

An appraisal of techniques for studying cerebral circulation in the newborn. Report of a mini-symposium held in September 1982.

The study of cerebral circulation in ill pre-term infants is important both to improve our understanding of the physiological aspects of cerebral blood flow control, and to provide further insight into the aetiology of intracranial intra-ventricular haemorrhage. Progress in this field has been hampered by the absence of measurement techniques which must be safe and clinically applicable whilst at the same time providing results with sufficient quality for the confident development of hypotheses. The symposium was held in order to evaluate critically the measurement techniques which might be considered for use in the sick, human newborn infant.

Cerebrovascular Circulation↗
PubMed1983

Cerebral haemodynamics in newborn babies studied by electrical impedance. Preliminary results.

We report studies in which the amplitude of the cardiac-synchronous change in transcephalic impedance, delta Z, has been used in the investigation of cerebral haemodynamics in newborn babies. Three clinical situations, in which cerebral blood flow might be expected to change, have been studied. The impedance signal, delta Z, decreased in size during a tension pneumothorax, increased slightly with feeding, and, during an exchange transfusion of an asphyxiated baby, there was a consistent increase in the size of the signal during infusion of blood and a decrease in size as blood was withdrawn. This method may be useful for the continuous study of cerebral haemodynamics in the preterm baby over prolonged periods.

Cerebrovascular Circulation↗