Biomedical subjects

D H Evans

Publications and source records attributed to D H Evans.

At least 91 records · Page 5Linked to original sources

PubMed1996

The effect of geometrical spectral broadening on the estimation of mean blood velocity using wide and narrow ultrasound beams.

If an ultrasound beam uniformly insonates the cross section of a blood vessel then the Doppler signal can be analyzed to give a frequency proportional to the spatial mean blood velocity. This is also possible if the beam can be thought of as negligibly thin compared to the blood vessel radius, centrally placed, and the blood velocity profile is axisymmetric and monotonic, although the analysis takes a different form. The immunity of these mean velocity estimators to broadening of the ideal frequency spectrum is studied. If the broadening of a frequency component is such that its mean frequency, weighted by intensity, is unchanged then the analysis with a uniformly insonating beam still leads to the correct mean velocity. In contrast, for any such broadening, the analysis if the beam is negligibly thin produces an underestimate of the mean velocity. Error expressions are derived for idealized cases and some practical results given.

Blood Flow Velocity↗

PubMed1996

Analysis of cerebral blood flow autoregulation in neonates.

The dynamic response of cerebral autoregulation to spontaneous changes in arterial blood pressure (ABP) is described by the relationship between cerebral blood flow velocity (CBFV) and resistance-area product (RAP). CBFV was measured with Doppler ultrasound in the middle cerebral artery and ABP with an intra-arterial catheter in 66 neonates. Spontaneous changes in mean ABP were automatically detected and the maximum derivative was used to synchronize the coherent averaging of corresponding CBFV and RAP transients. These were classified into two groups corresponding to intact (group A) or impaired (group B) autoregulation. The cross correlation between RAP and CBFV indicates a significant relationship with a time delay of 5 s for group A. The frequency response of RAP was estimated by the cross spectra with CBFV. Groups A and B present a similar amplitude spectra but the phase spectra of group A lags that of group B. The impulse responses of the two groups are also markedly different and were used to simulate the velocity response to a 5% step change in ABP. Impulse responses were also obtained for four different levels of pCO2 showing that hypercapnia leads to an impulse response similar to that of group B (impaired autoregulation). This method can be used to extend the usual dichotomic classification adopted in clinical studies of autoregulation.

Blood Flow Velocity↗

PubMed1995

DNA strand transfer catalyzed by the 5'-3' exonuclease domain of Escherichia coli DNA polymerase I.

A protein which promotes DNA strand transfer between linear double-stranded M13mp19 DNA and single-stranded viral M13mp19 DNA has been isolated from recA- E.coli. The protein is DNA polymerase I. Strand transfer activity residues in the small fragment encoding the 5'-3' exonuclease and can be detected using a recombinant protein comprising the first 324 amino acids encoded by polA. Either the recombinant 5'-3' exonuclease or intact DNA polymerase I can catalyze joint molecule formation, in reactions requiring only Mg2+ and homologous DNA substrates. Both kinds of reactions are unaffected by added ATP. Electron microscopy shows that the joint molecules formed in these reactions bear displaced single strands and therefore this reaction is not simply promoted by annealing of exonuclease-gapped molecules. The pairing reaction is also polar and displaces the 5'-end of the non-complementary strand, extending the heteroduplex joint in a 5'-3' direction relative to the displaced strand. Thus strand transfer occurs with the same polarity as nick translation. These results show that E.coli, like many eukaryotes, possesses a protein which can promote ATP-independent strand-transfer reactions and raises questions concerning the possible biological role of this function.

Amino Acid Sequence↗

PubMed1995

Bioactivation of aromatic amines by recombinant human cytochrome P4501A2 expressed in Ames tester strain bacteria: a substitute for activation by mammalian tissue preparations.

The most widely used bioassay in genetic toxicology is the Ames test, which combines a bacterial mutagenicity assay (reversion of Salmonella typhimurium histidine-auxotrophic tester strains) with an exogenous bioactivation system (hepatic postmitochondrial supernatant or "S9"). The enzymatic activities of S9 prepared from the tissues of experimental animals are difficult to control. We show that the requirement for S9 can be obviated by the engineered expression of enzymes of bioactivation within the bacterial cell. With this strategy, reactive metabolites are produced inside the bacterial cell, proximate to the genetic target. Species boundaries can be crossed, and chimeric or mutant enzymes can be studied. We have constructed an Ames tester strain, expressing both aromatic amine N-acetyltransferase and human cytochrome P4501A2, which detects aromatic amine mutagenicity in the absence of S9.

Acetyltransferases↗

PubMed1995

Measurement of blood flow velocity in the basal cerebral circulation: advantages of transcranial color-coded sonography over conventional transcranial Doppler.

Unlike conventional transcranial Doppler (TCD), transcranial color-coded sonography (TCCS) enables imaging of the basal cerebral arteries using color-flow ultrasonography and correction for the angle of insonation when determining blood flow velocities. We present hemodynamic data from 20 normal subjects, each studied with TCD and TCCS. Velocities derived using TCCS with angle correction were significantly greater than those derived using TCD in all vessels (mean velocities [cm/sec; mean with 95% confidence intervals)--anterior cerebral artery: TCD 48 (45-50), TCCS 62 (58-66), p < .0001; middle cerebral artery: TCD 61 (58-64), TCCS 70 (66-74), p < .0001; posterior cerebral artery: TCD 43 (41-46), TCCS 54 (50-57), p < .0001; basilar artery: TCD 40 (34-45), TCCS 45 (38-52), p < .01. Pulsatility index values were significantly greater in all arteries when determined by TCCS, and resistance index values were significantly greater except in the basilar artery. Correcting for the angle of insonation using TCCS may enable estimation of blood flow velocities closer to the "true" values than those derived using conventional TCD.

Adult↗

PubMed1995

Comparison of cerebral blood flow velocity estimation with cranial ultrasound imaging for early prediction of outcome in preterm infants.

Seventy-four low birth weight infants underwent serial measurements of cerebral blood flow velocity (CBFV) using Doppler ultrasound together with ultrasound imaging of the brain. Surviving infants were examined and assessed using the Bayley scales of mental and motor development at 18 months post-term. There were no significant differences in mean CBFV between normal and impaired infants. Infants with abnormal neurological signs (n = 6) or developmental delay (n = 3) did not show the usual steady rise in CBFV during the first few days of life. Four of nine had a rise, then a fall in CBFV compared with 4 of 31 with complete data in the normal group. This difference is significant (chi 2 = 4.3, p = 0.03). The magnitude of the percentage increase between day 1 and day 3 was also smaller in the abnormal group (median 0% vs. 39%, Mann-Whitney p = 0.03). A structural abnormality seen on the cranial ultrasound image was a better predictor of adverse outcome than an abnormal CBFV pattern, with a better sensitivity and specificity (66% and 97% for imaging compared to 44% and 87% for CBFV). The addition of serial ultrasound Doppler measurements of CBFV did not improve the prediction of outcome obtained using ultrasound imaging alone.

Blood Flow Velocity↗

PubMed1995

Volumetric blood flow measurement by simultaneous Doppler signal and B-mode image processing: a feasibility study.

A system is described for the estimation of volumetric flow in arteries in real time by the simultaneous processing of the audio output of a commercial ultrasound scanner carrying the Doppler signal, and the video output carrying B-mode images of the lateral view of the artery. These images are processed to provide estimates of instantaneous internal vessel diameter. Combination with the relevant estimates of spatial mean velocity provides estimates of instantaneous volumetric flow. The major limitations of the system are the low rate of production of the B-mode images (17 Hz), the limited computing power and the need to tilt the transducer to produce an acceptably low Doppler angle. This tilting diminishes the quality of the imaging echoes from the arterial walls, making diameter measurement difficult. In principle, this approach to flow measurement represents an improvement on the conventional duplex technique where a single estimate of diameter is used. In practice, its value is limited by current transducer capabilities.

Adult↗

PubMed1995

Bias in mean frequency estimation of Doppler signals due to wall clutter filters.

Wall clutter filters used in Doppler ultrasound systems give rise to a bias in the mean frequency estimation. This effect has been explored for two second order filters: the initialised infinite impulse response (IIR)-type filter, and the finite impulse response (FIR) echo-canceller filter. These are used in conjunction with two mean frequency estimation algorithms: the FFT-based intensity-weighted-mean algorithm, and the autocorrelation algorithm. The bias is shown to be caused by the finite settling times of the filters. The IIR filter, although having a much better frequency response than the FIR filter, has a greater bias associated with it. A compromise between bias and frequency response is suggested and a simple method of bias correction is described.

Algorithms↗

PubMed1995

Effects of nonuniform insonation by catheter-tipped Doppler transducers on velocity estimation.

Studies of the coronary arteries using intravascular Doppler catheters have frequently been performed to assess the local circulation in various clinical situations. The parameters which are correlated with clinical observations are usually calculated from the intensity weighted mean velocity (IWMV) or the maximum velocity extracted from the Doppler power spectrum. It is known that these estimates are affected by nonuniform insonation of the blood vessel by the ultrasonic beam. The sample volume of a commercially available Doppler catheter has been theoretically and experimentally studied, and the effects of the nonuniform sampling of the vessel by the ultrasound beam examined. The results show that the velocity estimates are dramatically affected by both the position of the catheter within the artery, and the blood velocity profile. It is concluded that the maximum velocity estimator is more reliable than the IWMV estimator, but that results obtained with this estimator must also be treated with extreme caution.

Blood Flow Velocity↗

PubMed1995

Statistical bias and variance in blood flow estimation by spectral analysis of Doppler signals.

The stochastic nature of the Doppler signal is discussed as a source of variability and bias in estimation of mean blood velocity and flow performed using the Discrete or Fast-Fourier Transform. The estimators studied are those appropriate when the ultrasound beam is assumed to be wide enough to insonate the cross-section of the vessel uniformly, and assumed to be narrow enough to insonate only a diameter. Approximate expressions are derived theoretically for the biases and variances of these statistics when the Doppler power spectrum is uniform. For more complex spectra numerical evaluation is carried out by computer. Results for the double-sided spectra obtained from mixed flow are inferred from the single-sided cases. Typically, an estimate of instantaneous mean velocity has very little bias added (< 1%) but might have a standard error of approximately 10% of its mean value, and an estimate of flow in one cardiac cycle a standard error of approximately 1% of its mean value.

Bias↗

PubMed1995

Volumetric blood flow calculation using a narrow ultrasound beam.

Spectral analysis of the Doppler signal from a negligibly thin ultrasound beam directed through the centre of the blood vessel can, for certain forms of velocity profile, produce a statistic deemed to be proportional to spatial mean blood velocity. A statistic recently derived for the case of an axisymmetric, monotonic velocity profile is generalised to the situation where the profile is axisymmetric and made up of a central monotonic section inside a region of reverse flow which is itself symmetric. Using the theory of oscillating flow in a rigid tube, instantaneous velocity profiles were derived for samples of actual cardiac cycles recorded from various arterial sites. In every case the profiles satisfied the assumptions for the statistic to be valid throughout considerable regions of each cycle. The resulting error in the use of the statistic to measure the volumetric flow in a cycle had a magnitude less than 0.01% for the carotid, radial and brachial artery recordings, was typically small and positive (+2.5%) for the common femoral artery, and was typically negative (0 to -8%) for the superficial femoral artery, with outlying value of -16%.

Adult↗

PubMed1995

Estimation of critical closing pressure in the cerebral circulation of newborns.

We have developed a method to estimate the critical closing pressure (CrCP) of the cerebral circulation based on the intrinsic variability of arterial blood pressure (BP) around stable values of mean arterial pressure (MAP). A consecutive cohort of 33 premature newborns was studied at 6, 12, 24, 48 and 72 hours of age. Cerebral blood flow velocity (CBFV) was measured with Doppler ultrasound in the middle cerebral artery and BP was recorded in the abdominal aorta or in a peripheral artery. Continuous measurements lasting five minutes were recorded on digital magnetic tape and signals were digitized at a rate of 200 samples/seconds for processing on a digital computer. Mean values of BP (mBP) and CBFV (mBV) were computed for each cardiac cycle and CrCP was determined as the pressure axis intercept of the regression line of mBV as a function of mBP using 100 sequential heart beats. The resistance-area product (RAP) was obtained from the slope of the regression line. For 57 records (30 patients) the mean +/- SD values of CrCP and RAP were 23.9 +/- 11.6 mmHg and 4.07 +/- 1.83 x 10(4) kg.m-2.s-1, respectively. CrCP has a highly significant correlation with mean arterial pressure (p < 0.001) but RAP has not. Neither CrCP nor RAP are significantly correlated (p > 0.05) with PO2, PCO2, pH, haematocrit, gestational age, birthweight, postnatal age, heart rate on Pourcelot's resistance index. Our results suggest that cerebral perfusion pressure should be defined as MAP-CrCP for normal values of intracranial pressure.

Blood Pressure↗

PubMed1995

A microcomputer-based system for coronary artery Doppler studies.

Intra-vascular Doppler catheters are becoming popular as research tools for studies of the coronary circulation. Velocity estimates can be obtained from Doppler signals sampled with such catheters, and used to calculate various derived parameters which can then be correlated with the haemodynamic situation under investigation. This note discusses the requirements necessary for a system to perform the acquisition of the high-frequency Doppler signals obtained from catheter-tipped transducers, and a system developed to capture, process and store Doppler, ECG and pressure signals is described. Software used to calculate parameters from the stored waveforms is discussed and examples of the system outputs presented.

Coronary Vessels↗

PubMed1995

Circulatory effects of ventilator rate and end-expiratory pressure in unparalysed preterm infants.

Impairment of cerebrovascular autoregulation may be important in the pathogenesis of ischaemic brain injury in preterm infants. A previous study in ventilated preterm infants paralysed with pancuronium showed that changes in cerebral blood flow velocity (CBFV) were related to concomitant changes in arterial blood pressure. In a similar study in unparalysed infants, changes in CBFV in response to changes in ventilator rate or end-expiratory pressure were independent of associated changes in the arterial blood pressure. These results emphasize the importance of avoiding large swings in blood pressure in paralysed infants. Whether alternative paralysing agents have similar effects warrants further study.

Blood Pressure↗

PubMed1995

Electron spectroscopic imaging of encapsidated DNA in vaccinia virus.

We have used electron spectroscopic imaging to locate the phosphorus in vaccinia DNA in situ in unstained, ultrathin sections of virions. The phosphorus of the DNA backbone appeared to form a halo on the core periphery surrounding a phosphorus-impoverished central element. These results constrain models for how DNA could be packaged into mature vaccinia particles.

DNA, Viral↗

PubMed1995

Cerebral autoregulation dynamics in premature newborns.

BACKGROUND AND PURPOSE: Autoregulation of cerebral blood flow is easily disrupted, and loss of this normal physiological reflex may worsen the neurological outcome for patients undergoing intensive care. We studied the response of cerebral blood flow velocity to changes in mean arterial blood pressure. METHODS: Cerebral blood flow velocity was measured with Doppler ultrasonography in one middle cerebral artery for 5-minute periods in 33 babies of gestational age < 33 weeks admitted to a neonatal intensive care unit. Two methods of evaluating autoregulation were developed. The first used linear regression analysis of blood flow velocity on blood pressure. Records were classified as showing loss of autoregulation if the regression slope was greater than a critical value. A minimum change in mean arterial blood pressure of 5 mm Hg and a critical slope of 1.5%/mm Hg were found to be adequate criteria for the classification of records by the regression method. The second method used coherent averaging, a technique similar to that used in recording evoked potentials. Spontaneous transient increases in blood pressure were automatically detected, and the instant corresponding to its maximum rate of rise was used to synchronize averages of the blood pressure and blood velocity transients. The resulting coherent averages were classified into two groups based on the morphology of the cerebral blood flow velocity average. RESULTS: Whereas the regression method allowed the classification of only 51 of 106 records, the coherent average method classified 101 of 106 (95.3%) of the records available. For 51 records that were classified by both methods, there was agreement in 42 cases (82.3%). The coherent average of all records classified as having an active autoregulation showed cerebral blood flow velocity returning to baseline much earlier than blood pressure, suggesting that autoregulation was taking place within 1 to 2 seconds. This pattern was absent in records in which autoregulation was classified as absent. CONCLUSIONS: Computerized coherent averaging of the cerebral blood flow velocity response to spontaneous blood pressure transients offers a promising new method for noninvasive bedside assessment of autoregulation in patients undergoing intensive care. The time course for autoregulation, when present, is in agreement with that reported in adults.

Blood Flow Velocity↗

PubMed1995

Interpretation of embolic phenomena during carotid endarterectomy.

BACKGROUND AND PURPOSE: Air and particulate emboli are a major source of morbidity during carotid endarterectomy (CEA); however, amplitude overload and poor time resolution have restricted the ability of transcranial Doppler ultrasound to differentiate between the two. METHODS: We have now overcome these two limitations by (1) rerouting embolic signals away from the audio frequency amplifier to avoid amplitude overload and (2) substituting the Wigner distribution function for the fast Fourier transform to improve time and frequency resolution. Thus, we can now accurately determine embolic duration and embolic velocity, the product of which is the sample volume length (SVL). This measurement represents the physical distance over which an embolic signal can be detected. The underlying hypothesis was that air reflected more ultrasound and would therefore be detected over a greater SVL. RESULTS: The median SVL (interquartile range) for 75 in vitro air emboli was 1.97 cm (range, 1.70 to 2.35) compared with 0.27 cm (range, 0.16 to 0.43) for 185 particulate emboli detected during the dissection phase of CEA. Off-line analysis on an additional 560 embolic signals detected during different phases of CEA suggested that 46 of 143 (32%) of emboli immediately after shunt insertion were particulate, as were 19 of 33 (58%) occurring during shunting, 28 of 78 (36%) after restoration of flow in the external carotid artery, 23 of 251 (9%) after restoration of flow in the internal carotid artery, and 55 of 55 (100%) of those emboli detected during the early recovery phase. CONCLUSIONS: This development provides objective physical criteria upon which embolus characterization (particulate/air) can be based. This could have major implications for future patient monitoring with respect to modification of surgical technique and pharmacological intervention.

Embolism, Air↗

PubMed1995

Localisation and characteristics of natriuretic peptide receptors in the gills of the Atlantic hagfish Myxine glutinosa (Agnatha).

Specific binding of iodinated natriuretic peptides 125I-ANP and 125I-CNP was examined in the gill of the Atlantic hagfish Myxine glutinosa by tissue section autoradiography, saturation and competition analysis of binding to membrane preparations, affinity cross-linking, followed by SDS-PAGE and guanylate cyclase assays. Autoradiographs showed specific, saturable binding on the respiratory lamellar epithelium. In vitro analysis of the binding sites demonstrated that 125I-ANP bound to two receptor sites with the same affinity (Kd = 15.4 +/- 1.6 pmol l-1; Bmax = 45.9 +/- 3.0 fmol mg-1 protein). 125I-CNP bound to high- and low-affinity receptor sites; variables for the high-affinity site (Kd = 12.9 +/- 4.7 pmol l-1; Bmax = 23.4 +/- 6.5 fmol mg-1 protein) did not differ from those for the 125I-ANP sites. The low-affinity site had an apparent Kd and Bmax of 380 +/- 80 pmol l-1 and 120 +/- 21 fmol mg-1 protein, respectively. All receptors had an apparent molecular mass of approximately 150 kDa, with no indication of a mammalian type NPR-C at a lower apparent molecular mass. 1 nmol l-1 unlabelled rANP and 20 and 30 nmol l-1 unlabelled pCNP and C-ANF, respectively, competed for 50% of 125I-ANP sites. 0.1 nmol l-1 rANP and pCNP and 8 nmol l-1 C-ANF competitively inhibited 50% of 125I-CNP binding. Both rANP and pCNP stimulated cyclic GMP production, although rANP was a more potent stimulator than was pCNP. C-ANF did not stimulate cyclic GMP production. These data suggest the existence of an ANP guanylate-cyclase-linked receptor similar to the mammalian NPR-A and an ANP/CNP receptor that may be similar to, although not structurally homologous with, the mammalian NPR-C clearance receptor.

Animals↗