Search PubMed⌕ Search

Biomedical subjects

P Rolfe

Publications and source records attributed to P Rolfe.

At least 73 records · Page 4Linked to original sources
PubMed1991

Effects of hypoxaemia and bradycardia on neonatal cerebral haemodynamics.

Near infrared spectroscopy has been used to assess the effects of bradycardia and hypoxia on the cerebral circulation in the premature neonate. The technique is well tolerated and can be applied in almost any infant. Continuous monitoring of changes in cerebral oxygenated, deoxygenated, and total haemoglobin is possible. Total haemoglobin is analogous to cerebral blood volume; thus information on circulatory changes as well as oxygenation state can be obtained. Twenty five babies had cerebral monitoring carried out using this technique. During episodes of hypoxia, both spontaneous and induced, impairment of haemoglobin oxygenation within the brain was detected together with an overall increase in the total mean haemoglobin concentration, which was 0.8 x 10(-2) mmol/l. Bradycardia with apnoea also led to impairment of cerebral oxygenation, and to a rapid fall in the concentration of total mean haemoglobin to 1.4 x 10(-2) mmol/l, which was followed in some cases by an increase to above the resting value on recovery of the heart rate to a mean of 0.7 x 10(-2) mmol/l. These disturbances to total haemoglobin concentration represent abnormalities of cerebral blood volume that may be implicated in the pathogenesis of neonatal cerebral injury.

Bradycardia↗
PubMed1990

The non-invasive monitoring of cerebral tissue oxygenation.

The instrument drift was found to be less than 0.004 OD/hour and from measurements on glass filters of 8 optical density units, a coefficient of variation of 0.01 over the 30 second averaging time was observed. The instrument is sufficiently sensitive to enable monitoring of changes in the cerebral oxygen saturation level of haemoglobin and to enable changes in the concentration of cyt aa3 (oxidised form), to be measured with reasonable confidence. It is of the utmost importance in NIRS investigations to be certain of the specificity of the technique, and it is vital that reliable determinations of the amount of cytochrome aa3 in brain are made in addition to measurements of the extinction coefficients. This is still a matter of considerable debate, not only in the isolation and properties of the multisubunit structured membrane bound protein, which many enzymologists have been investigating for the last 50 years, (Keilin and Hartree, 1939; Brunori et al, 1981) but also in relating to in-vivo versus in vitro comparisons. An additional point for consideration is the validity of employing multiplier coefficients derived from the rat brain, as several groups have done (Wyatt et al 1986, Ferrari et al 1985,) and applying them to the human brain and to the infant brain. There may be significant differences in the activity of the enzyme, and the profound physiological effects which will arise during the end point fluorocarbon studies: the presence of fetal haemoglobin must also be considered (Carta et al., 1987). The clinical determination of the optical path length is a considerable problem (Cope etal., 1988,) and once again estimates of the pathlength correction factor made in animals and dead fetuses may not be valid for living human tissue. Results from our animal studies indicate that NIRS can be used to monitor changes in the oxygenation level of Hb and, in extreme hypoxia, changes in the level of the redox state of cyt aa3 can be reliably measured and are well within the sensitivity of the instrument. The results indicate that under small change in saturation the redox state of cyt aa3 appears to be unaltered. It may be that under normal physiological conditions the redox state of aa3 appears to be apparently unchanged under episodes of mild hypoxia of short duration.

Algorithms↗
PubMed1990

Continuous cerebral electrical impedance monitoring in sick preterm infants.

Cerebral electrical impedance (dZ) and intra-arterial blood pressure were measured continuously during the first 48 h after birth in 26 sick ventilated preterm infants with a birth weight less than 1500 g. The aim was to establish whether any patterns of dZp or the variability of either blood pressure or dZp would allow identification of those infants who developed intraventricular haemorrhage (IVH), periventricular leucomalacia (PVL) or a poor neurological outcome. IVH and PVL were diagnosed by ultrasound image obtained every 6 h. Cerebral electrical impedance recordings were unsuitable for analysis in three patients and a further three died within 14 h of birth. In the remaining 20 patients, no step changes that may have been related to the onset of IVH or PVL were evident and whilst three patterns of dZp were identified, they were not useful in distinguishing between normal infants or those who developed IVH, PVL or had a poor neurological outcome. Using multiple linear regression, the coefficient of variation of dZp was significantly associated with both IVH and outcome, as was the coefficient of variation of blood pressure. Continuous measurement of cerebral electrical impedance, whilst technically feasible in sick preterm infants, was not found useful as a method of identifying those who developed IVH, PVL or had a poor neurological outcome.

Blood Pressure Determination↗
PubMed1990

In vivo chemical sensors for intensive-care monitoring.

There is a need for rapid assessment of a patient's biochemical status during intensive care so that therapies may be optimised. Chemical sensors for key species have the potential to allow continuous in vivo monitoring, and some progress is being made with certain sensors. Gases, ions and certain catabolites such as glucose and urea may be measured with devices based on mass spectrometric, electrochemical or optical principles. The physical form, and size of sensors must be matched to the measurement site, which can include the airway, the intravascular space, tissue and the skin surface. Electrochemical sensors for measurement of O2, pH and glucose have been the most widely used to date, although fibre-optic devices are currently attracting considerable interest. Invasive sensors still suffer from the problem of poor biocompatibility, particularly devices used in arteries and veins. Noninvasive methods may be successful in certain circumstances and in some patient groups, but peripheral measurements are often significantly influenced by circulatory phenomena such as shock. Further research is required if these limitations of both invasive and noninvasive sensors are to be overcome and continuous chemical monitoring is to be established as a routine clinical technique.

Biocompatible Materials↗
PubMed1990

[Laser spectroscopic assessment of induced hyperoxia--an animal experiment in lambs].

The noninvasive determination of biochemical parameters has become an important aspect of intensive care medicine. The newly developed monitors for laser reflectometry provide the possibility of spectroscopic monitoring. The equipment consists of a near-infrared data collection unit and a personal computer. The four laser diodes emit light at wavelengths of 775, 805, 845 and 904 nm. By analyzing the changes in optical density during laser irradiation of biological tissue, information is obtained about the relative changes in the concentration of hemoglobin and the blood volume. In animal experiments with ten fetal lambs we evaluated the reliability of near infrared laser spectroscopy. Fetal hyperoxia was achieved by means of an extracorporeal circuit with interposition of a membrane oxygenator (0.8 m2, Scimed). During the induced hyperoxia the laser spectroscopic tracings showed a rise in the HbO2 signal with a synchronous decrease in the HbR signal. Additionally, the spectroscopic pattern showed a characteristic initial rise in the intracerebral blood volume, which stabilized after 4 minutes. We found a significant correlation between the intermittently measured PO2 values of the arterial blood samples and the laser spectroscopic HbO2 and HbR signals (r = 0.87, and r = -0.82, respectively; p less than 0.001). Furthermore, hyperoxia was indicated by the laser system with a short lag time. We conclude that laser spectroscopy is a reliable method with a high potential for clinical routine use in intensive care, as it provides noninvasive continuous information at comparatively low costs using portable monitors.

Animals↗
PubMed1989

Effect of infusion rate of indomethacin on cerebrovascular responses in preterm neonates.

Cerebrovascular responses were studied in preterm infants by Doppler ultrasound and cerebral electrical impedance for one hour after intravenous indomethacin infusion for patent ductus arteriosus. Indomethacin in a dose of 0.2 mg/kg body weight was infused over five minutes in one group of infants (20 doses) and over 20 minutes in a second group of infants (16 doses). There were no significant differences between the two groups in birth weight (mean = 1068 g, range 569-1950), gestational age (mean 28.4 weeks, range 25-31), or postnatal age (mean 18.1 days, range 6-42). There was a significant reduction in both the Doppler mean flow velocity in the anterior cerebral artery (mean (SE) -20 (4.2)%) and peak amplitude of cerebral electrical impedance (-26 (3.9)%) within two minutes of starting the indomethacin infusion over five minutes. There was no significant change after the infusion over 20 minutes. There were no significant changes in blood pressure or carbon dioxide tensions after infusion at either rate. The results suggest that infusion of indomethacin over five minutes caused a potentially deleterious reduction in cerebral blood flow. No such reduction occurred when it was infused over 20 minutes.

Blood Flow Velocity↗
PubMed1988

Doppler ultrasound signal analysis based on the TMS320 signal processor.

Doppler ultrasound is an established method for the study of haemodynamics. Considerable improvement in accuracy and resolution can be achieved by utilizing advanced data processing techniques. Such a system has been developed and used to assess the cardiac component of the Baroreceptor Reflex in adults and to examine cerebral blood flow in neonates.

Blood Flow Velocity↗
PubMed1988

Current developments in non-invasive measurement of arterial blood pressure.

Two new types of non-invasive method for measuring arterial blood pressure recently developed by us are reviewed. Both of the methods are based on the characteristics of the pressure-volume relationship in the artery. One is the volume-oscillometric method; and the other is the volume-compensation method, based on the vascular unloading principle. Both methods employ photoelectric plethysmography for detection of arterial volume changes in the biological segment. The volume-oscillometric method can measure systolic and mean arterial pressure, and is applicable to long-term ambulatory monitoring. The volume-compensation method allows the beat-by-beat measurement of systolic and diastolic pressure and the recording of the pressure waveform continuously and non-invasively. This paper discusses the measurement principle and evaluates the accuracy of each method as compared with direct measurements. Preliminary descriptions of newly designed instruments based on these two methods, and a few examples of the indirect pressure recordings, are also described. The results obtained show that the non-invasive methods for measuring arterial pressure presented here appear promising for use not only in physiological studies, but also in clinical practice and research laboratories.

Animals↗
PubMed1988

Review of chemical sensors for physiological measurement.

The continuous measurement of chemical species is important for physiological and biochemical research and for critical care medicine. Chemical sensors are being developed for the measurement of a wide range of analytes, especially gases, vapours, ions, catabolites, drugs and hormones. The mechanical form of these sensors may be adapted for use in single cells, tissue fluid and blood vessels, on the skin surface, in the respiratory and gastrointestinal systems. Optical and electro-chemical principles are now used widely for sensor design, and advances in electronic fabrication methods and in optical fibres, sources, and detectors, have been important.

Chemistry, Clinical↗
PubMed1988

Flexible approach to amperometric oxygen determination.

In vitro and in vivo results obtained from a novel flexible amperometric oxygen sensor are reported. The sensor is fabricated using thin film deposition techniques and is operated by the application of a pulsed waveform. Development of the sensor was undertaken in order to produce a device that is capable of being sited at the interface of a wound and an overlying wound dressing. Oxygen determinations in such an environment would aid in gaining an understanding of the role of oxygen in wound healing and the type of wound dressing that would provide an environment conducive towards wound healing. In vitro data indicate that linearity of response is good although other performance characteristics are irreproducible. In vivo response to oxygen has been observed 50 h after insertion into a porcine sham wound. Expected trends were followed when changes to the oxygen regime of the wound space were effected, but absolute values of oxygen tension are difficult to state with certainty. This may be due to poor calibration stability and inadequate sealing of the sensor from the surrounding environment.

Animals↗
PubMed1988

Ion-selective sensors for assessment of the fetus.

Frequent measurement of pH and PCO2 levels of fetal blood during labour provides an insight into the presence and nature of fetal asphyxia, effectiveness of in utero resuscitation and the need for operative delivery. A pH measuring system utilizing a hydrogen ion-selective polymer membrane built into a fetal probe suitable for making intermittent measurements on the fetal scalp during labour is described. Probe design, construction of electrodes and preliminary results obtained using the system in vitro are presented.

Amines↗
PubMed1988

Development of a sensor for non-invasive intracranial pressure measurement in the newborn.

A new system has been developed for the non-invasive monitoring of intracranial pressure (ICP) across the anterior fontanelle in the newborn. The system comprises a very small pneumatic sensor linked via two vinyl tubes to an instrument where the ICP value is displayed. The sensor is simply bonded to the anterior fontanelle, using industrial collodion normally used for attaching EEG electrodes. The sensor body is injection moulded in semi-flexible polyurethane with a very thin, compliant membrane bonded to its front surface. Using these manufacturing techniques the sensors are made to be disposable thus minimizing the risk of cross infection. ICP characteristics can now be recorded continuously in a wide range of neonates and the evaluation of currently used therapeutic treatments for lowering elevated ICP can be carried out safely and accurately.

Electrodes↗
PubMed1988

Aspects of instrumentation design for impedance imaging.

This paper takes the circuit for a general purpose impedance imaging data measurement system and analyses the importance of several components. For high accuracy it is shown that stray capacitances in the input circuitry are the major problem although the use of a phase sensitive demodulator can minimise this problem. It is shown that harmonic distortion in the constant current generator can cause errors which are, however, reduced if a linear multiplying demodulator is used rather than a switching demodulator.

Mathematics↗