Biomedical subjects

Philip Langley

Publications and source records attributed to Philip Langley.

3 recordsLinked to original sources

PubMed2002

Errors in repolarization measurement using magnetocardiography.

Multichannel magnetocardiography (MCG) noninvasively measures variations in magnetic field strength from many sites at the body surface, potentially providing useful regional information about ventricular repolarization. MCGs contain features similar to ECGs, and although errors associated with repolarization measurement have been quantified for ECGs, no comparative data exists for MCGs. In this study, errors in manual measurement of repolarization interval in the MCG were determined. Sixteen MCG channels and three ECG leads were recorded simultaneously in eight healthy subjects. Each recording was displayed in a random order on a computer screen, in presentations with different noise levels, time display widths, and amplitude display heights. In total, manual measurement of repolarization intervals in 2,048 (eight subjects x 16 channels x eight presentations x two repeats) MCGs were made by each of four analysts. Measured repolarization intervals were reduced by 3 ms when noise was added and by a further 3 ms when this noise was doubled. Intervals were shortened by 9 ms when the time display width was doubled and by a further 10 ms when the display width was doubled again. Measurements increased by 7 ms for a doubling of amplitude display height, equivalent to a doubling of T wave height. There were also consistent differences between analysts; amounting to a greatest mean difference of 24 ms. Display characteristics, added noise, and different analysts thus affect manual repolarization interval measurements in MCG. The errors detected demonstrate the importance of a standard presentation for repolarization measurement in the MCG.

Electrocardiography↗

PubMed2002

Quantification of T wave shape changes following exercise.

T wave shape is increasingly used to provide insights into cardiac repolarization and, although shape is known to change as heart rate changes, there are no published quantitative clinical data. The aim of this study was to quantify these changes. Heart rate (HR), T wave amplitude, and two measures of T wave symmetry (SRarea--ratio of areas about the peak, SRtime--ratio of centrality of peak), were quantified over a period of 360 seconds following exercise in 20 healthy human subjects. As HR decreased, in all subjects the T wave became more asymmetrical (SRarea 20/20, SRtime 20/20, P < 0.0001). Resting reference HR and symmetry ratios were (mean +/- SD), HR 63 +/- 10 beat/min, SRarea 1.68 +/- 0.25, and SRtime 2.13 +/- 0.39. Fifty seconds postexercise, HR was significantly higher than reference at 92 +/- 11 beat/min (P < 0.0001), and symmetry ratios were significantly less at SRarea 1.04 +/- 0.15, SRtime 1.24 +/- 0.36 (P < 0.0001). Significant differences in HR and both symmetry ratios remained at 300 seconds postexercise. Amplitude increases had returned to their reference values at 300 seconds. T wave shape was significantly more symmetrical at higher HRs. These findings confirm qualitative reports of shape changes.

Adult↗

PubMed2002

Effect of changes in heart rate and in action potential duration on the electrocardiogram T wave shape.

The mechanisms responsible for changes in T wave symmetry and amplitude with changes in heart rate and action potential duration were investigated. A computer model of normal left ventricular repolarization was used to simulate the T waves on the surface 12-lead ECG. The effect of heart rate changes was studied by varying the ratio between dispersion of repolarization (Disp) and action potential repolarization duration (APRD). With constant dispersion. as heart rate increases, APRD decreases and the ratio Disp/APRD increases. T waves were simulated while varying the Disp/APRD ratio from 3.6% to 100%. The T wave symmetry ratio measured from the areas either side of the peak (SRarea), the symmetry ratio from the times either side of the peak (SRtime) and the T wave amplitude (Tamplitude) were calculated from each simulated ECG. SRarea decreased from 1.42 to 0.77, SRtime from 1.75 to 1.04 and the Tamplitude increased from 0. 19 mV to 2.30 mV. The stability of results with variation in model characteristics was also investigated, by moving the heart +/- 20 mm on all three axes, rotating the heart axes by +/- 10 and by modifying all constants defining the action potential by +/- 5% and +/- 10%. T wave amplitude was sensitive to changes in heart position, as the heart was moved towards the body surface. However, T wave shape changed very little with heart position or rotation, with the SD of SRarea varying by less than 0.05 over an SRarea range of 0.65 for different values of Disp/APRD ratio. We have shown from our model that cardiac T waves increase in amplitude, and become more symmetric with their peaks becoming central as APRD shortens with increasing heart rate, agreeing with clinical observations. These results help to explain the T wave shape changes which occur when heart rate increases.

Action Potentials↗