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Biomedical subjects

A K Evans

Publications and source records attributed to A K Evans.

7 recordsLinked to original sources
PubMed2001

The impact of meconium ileus on the clinical course of children with cystic fibrosis.

The present study was designed to compare the clinical course of children diagnosed with cystic fibrosis (CF) in infancy due to the presence of meconium ileus (MI) with children diagnosed by way of a newborn screening programme (non-MI). A matched case-control study design was used. Matching was performed on the basis of sex and date of birth. All children born in New South Wales, Australia after 1980 and who had attended the CF clinic at The Children's Hospital at Westmead since diagnosis were included as possible cases or controls. Parameters pertaining to the clinical course were compared in 39 matched pairs. MI children had a significantly worse pulmonary status. The forced expiratory volume in one second was 16.3 +/- 5.2% higher (p<0.001, n=21 pairs) and the forced vital capacity value 10.5 +/- 4.7%, higher (p<0.05, n=21 pairs) in non-MI children. The difference between the pairs (18.6 +/- 4.4 MI and 20.5 +/- 3.4 non-MI) in the Shwachman chest radiograph score was statistically significant (p<0.05, n=39 pairs). There were no significant differences in any other assessed parameters, such as height, weight, the presence of liver function abnormalities, the frequency of hospitalization or airway microbial colonization. Meconium ileus may be an early indication of a more severe phenotype of cystic fibrosis. This was suggested by the significantly lower pulmonary function found in children with a history of meconium ileus compared to age- and sex-matched children who did not have meconium ileus.

Case-Control Studies↗
PubMed1982

The QRST deflection area of electrograms during global alterations of ventricular repolarization.

Changes of refractory period and QRST deflection area in cardiac electrograms due to localized myocardial warming and altered cycle length were determined. Localized myocardial warming consistently resulted in increased QRST deflection area which was highly correlated with reduction of refractory period. Similar reductions of refractory period by decreased cycle length were associated with insignificant changes of small QRST areas and significant reductions of larger QRST areas. The different effect of local thermal alteration of repolarization and global alteration due to cycle length changes is experimental evidence that QRST deflection area depends on differences in duration of ventricular repolarization properties. The finding of decreased deflection area with decreased cycle length is also evidence of decreased disparity of repolarization properties of normal myocardium at rapid heart rates.

Animals↗
PubMed1981

Redundancy reduction for improved display and analysis of body surface potential maps. I. Spatial compression.

The Karhunen-Loeve technique of random process representation was investigated as a method of quantitatively characterizing body surface potential maps. One hundred ninety-two lead body surface potential maps from 124 normal subjects and 97 patients with independently documented heart disease were used in the study. Each map frame in QRS and ST-T of 34 maps in a test set was represented as a linear sum of orthonormal distributions derived from the covariance matrix estimated from all QRS frames in the 221 training maps. A 16:1 reduction in spatial data of the test set was achieved with rms errors of 45 and 21 microV in QRS and ST-T, respectively. Results suggest that 12 independent waveforms, derived from the 192 measured ECGs, may be used in place of those 192 ECGs. In addition to providing a convenient and familiar method of display for map data, the technique puts the data in an appropriate form for quantitative statistical analysis.

Cardiomegaly↗
PubMed1981

Redundancy reduction for improved display and analysis of body surface potential maps. II. Temporal compression.

This paper describes use of the Karhunen-Loeve expansion to identify and reduce temporal redundancy in electrocardiographic body surface potential maps (192 body surface leads recorded simultaneously at 1 kHz/channel for approximately 600 msec). Temporal data compression of about 20 to 1 was obtained with accurate representation of the original data. Use of separate sets of orthonormal basis functions for QRS and ST-T provided a more accurate representation than the basis derived from QRST. Combined with the spatial compression described in the preceding paper, overall map data compression of about 320 to 1 was obtained without significant loss of accuracy of representation or map appearance. With both spatial and temporal compression the 100,000 numbers which typically comprise a single cardiac complex were accurately represented by 216 coefficients. Using basis functions derived from a single cardiac complex were accurately represented by 216 coefficients. Using basis functions derived from a training set of 221 maps, the estimated average rms error of representation was 60 microV during the ST-T. For 34 test maps which were not part of the training set, measured average errors were 64 microV during the QRS and 23 microV during the ST-T. This technique provides a basis for quantification of the diagnostic content of maps and automated classification of maps.

Adult↗
PubMed1980

Ventricular recovery properties and QRST deflection area in cardiac electrograms.

We measured QRST deflection areas in cardiac surface electrograms of dogs and determined their relation to changes of refractory period induced by localized myocardial warming. Refractory period changes and QRST deflection areas were highly correlated with an average correlation of 0.95 over multiple activation sequences. For a single activation sequence, refractory period change was also highly correlated with ST-T deflection area, but over multiple activation sequences this correlation was reduced and the pooled variance of ST-T deflection areas was over 18 times greater than that of QRST area. No relation between QRST deflection area and the absolute value of refractory period over various cardiac sites was evident. Findings suggest that a particular set of ventricular recovery properties cannot be defined on the basis of QRST deflection areas, but that changes of recovery properties can be recognized on that basis.

Action Potentials↗
PubMed1979

Clinically practical lead systems for improved electrocardiography: comparison with precordial grids and conventional lead systems.

The use of limited leads for estimating total body surface potential distributions was investigated as a practical solution to the problem associated with extensive electrocardiographic sampling used in surface potential mapping. Two practical, limited lead sets of 32 leads each were derived and contrasted to a set of 30 precordial leads similar to those used in ST-segment and QRS mapping for estimating infarct size, and to a set of nine leads simulating those used in conventional 12-lead examinations. The two arrays, one of which excluded posterior sites for use in recumbent patients, showed little difference in ability to estimate 192 lead measured maps (average rms voltage error of 35 muV and average correlation coefficient of 0.97). The 30- and 9-lead arrays consistently showed twice the voltage (72 muV) and poorer pattern estimation (average correlation coefficient of 0.91) than either of the 32 lead arrays. These findings indicate the need for 20-35 properly located electrodes for accurate total body surface potential estimation. They also show that there is no difference in the abilities of a 30-lead precordial array and conventional leads to estimate maps.

Arrhythmias, Cardiac↗