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Biomedical subjects

J C Cowan

Publications and source records attributed to J C Cowan.

At least 37 records · Page 2Linked to original sources
PubMed1992

Cardiac rupture after thrombolytic therapy: the use of aprotinin to reduce blood loss after surgical repair.

Emergency cardiac surgery after recent thrombolytic therapy is associated with increased blood loss. A patient underwent emergency repair of a ruptured left ventricle after intravenous streptokinase treatment for acute coronary occlusion. High dose aprotinin was given during the operation to reduce the expected blood loss. Surgical repair was successful without bleeding complications. Total postoperative blood loss was 365 ml.

Aged↗
PubMed1992

Comparison of unipolar and bipolar ventricular paced evoked responses.

OBJECTIVES: To study the differences between endocardial bipolar and unipolar ventricular paced evoked responses and surface electrocardiograms. PATIENTS: 10 patients with conduction system disease awaiting insertion of a permanent pacemaker were studied with temporary ventricular pacing from the right ventricular apex. MAIN OUTCOME MEASURE: Comparison of the durations of the QRS complexes and QTa and QTe intervals of the endocardial bipolar paced evoked response and the surface electrocardiogram with those of the reference unipolar paced evoked response. RESULTS: By comparison with the unipolar reference, the mean durations of the QRS complexes of the bipolar signal and the surface electrocardiogram were 41.8% and 132.1% respectively. The mean QTa interval was 85.9% and 112.2% respectively and the mean QTe interval was 86.9% and 109.5% respectively. All these differences were significant. The amplitudes of the unipolar QRS complexes and T waves were significantly larger than those recorded in the bipolar configuration. CONCLUSIONS: Differences between the unipolar and bipolar ventricular paced evoked responses are significant. The time course of the unipolar signal is closer to that of the surface electrocardiogram. This indicates that the unipolar paced evoked response does not reflect local electrophysiological events, as has been suggested previously.

Aged↗
PubMed1991

Torsades de pointes ventricular tachycardia and terodiline.

We report five patients (four women, one man; mean age 74 years, range 55-87) taking terodiline who had torsades de pointes ventricular tachycardia associated with prolongation of the QT interval. The QT interval was normal before the introduction of terodiline in three patients and returned to normal after terodiline withdrawal in four. Atrioventricular conduction disturbance or bradycardia were evident in all patients. Terodiline should be prescribed with caution to elderly patients.

Aged↗
PubMed1991

Contraction-excitation feedback in an ejecting whole heart model--dependence of action potential duration on left ventricular diastolic and systolic pressures.

STUDY OBJECTIVE: Contraction-excitation feedback may play a role in arrhythmogenesis in heart failure. The aims of this study were to determine whether contraction-excitation feedback has a significant effect on action potential duration within a physiological range of pressures, and to investigate the individual effects of left ventricular end diastolic and peak systolic pressures on action potential duration. DESIGN: A new model was developed for studying contraction-excitation feedback in a physiologically ejecting isolated heart preparation. Hearts were perfused via the left atrium, ejecting against an aortic afterload. By varying left atrial and aortic pressures, left ventricular end diastolic pressure and left ventricular peak systolic pressure were controlled independently. Intracellular potentials were recorded from the epicardium. EXPERIMENTAL MATERIAL: Hearts (n = 33) were obtained from guinea pigs weighing 300-350 g. MEASUREMENTS AND MAIN RESULTS: Increasing left ventricular end diastolic pressure from 0.3 to 1.1 kPa (2.2 to 8.3 mm Hg), at constant left ventricular peak systolic pressure, shortened action potential duration by 11.3(SEM 1.5) ms (p less than 0.0005). Action potential duration achieved a new steady state within 30 s of a change in end diastolic pressure. The changes were fully reversible. The effects of left ventricular peak systolic pressure variation at constant left ventricular end diastolic pressure were less marked. Increasing left ventricular peak systolic pressure from 10.3 to 13.1 kPa (75 to 100 mm Hg) shortened action potential duration by 2.1(0.7) ms (p less than 0.01). Reduction of aortic pressure below 8 kPa had variable effects on action potential duration, reflecting the development of ischaemia. CONCLUSION: The results show the existence of contraction-excitation feedback in a physiologically ejecting whole heart preparation and suggest that raised end diastolic pressure may contribute to arrhythmogenesis in heart failure.

Action Potentials↗
PubMed1991

Antiarrhythmic therapy and survival following myocardial infarction.

Arrhythmias remain a major cause of late mortality following myocardial infarction. They arise due to fibrosis within the infarct, which creates the conditions of slow conduction necessary for re-entry. In individual patients who have already manifested a malignant arrhythmia, antiarrhythmic drug therapy, guided by invasive electrophysiological testing, is of proven benefit in prolonging survival. By contrast, when used on a population basis, antiarrhythmic drug therapy has proved singularly ineffective. This is illustrated by the recent Cardiac Arrhythmia Suppression Trial (CAST) study--far from improving survival, antiarrhythmic therapy increased mortality. The use of antiarrhythmic drugs on a population basis is therefore fundamentally flawed. Hemodynamic intervention provides an alternative strategy in arrhythmia prevention. Hemodynamic changes may influence electrophysiological parameters and arrhythmogenesis in a number of ways. First, myocardial stretch may contribute to arrhythmogenesis through contraction-excitation feedback. Second, hemodynamic changes can influence ventricular remodeling following infarction, which may be an important determinant of subsequent arrhythmogenesis. Hemodynamic intervention, therefore, represents a promising new approach to arrhythmia prevention following myocardial infarction.

Anti-Arrhythmia Agents↗
PubMed1990

Surgery for control of recurrent life-threatening ventricular tachyarrhythmias within 2 months of myocardial infarction.

Twenty-seven patients (mean age 57 +/- 7 years) underwent surgery for control of recurrent drug-refractory ventricular tachyarrhythmias (uniform ventricular tachycardia alone in 9 patients, ventricular tachycardia and ventricular fibrillation in 15 and ventricular fibrillation alone in 3) within 2 months of acute myocardial infarction. The mean number of major arrhythmic episodes per patient was 15 (range 2 to 200) and of drug failures 4 +/- 2. Left ventricular function was severely impaired in the majority (ejection fraction 29%; range 14% to 47%) and 18 patients (66%) had a left ventricular aneurysm. Endocardial resection guided by a combination of endocardial activation mapping during tachycardia and fragmentation mapping during sinus rhythm was performed in all patients. All electrically abnormal left ventricular endocardium was excised. Eight patients (29.6%) died within 30 days of surgery. Death was not related to age, time of surgery after infarction, ventricular function, bypass time or type of arrhythmia. Patients requiring emergency surgery had a higher early postoperative mortality rate than did those undergoing planned surgery (43% versus 15%). During a follow-up period of 32 +/- 20 months, there have been no arrhythmic deaths and only three patients (16%) have required antiarrhythmic drug therapy. When required in the early weeks after infarction, surgery for ventricular arrhythmias offers a high cure rate at a risk related to the patient's preoperative arrhythmia frequency, which in turn relates to the risk of arrhythmic death.

Adult↗
PubMed1989

Surgery for ventricular tachyarrhythmias based on fragmentation mapping in sinus rhythm alone.

Intraoperative arrhythmia activation mapping underlies the impressive success of surgery for sustained uniform ventricular tachycardia. Unstable arrhythmias and those intraoperatively noninducible, however, are not amenable to activation mapping and strategies for dealing with them are poorly defined. We propose that fragmentation mapping in sinus rhythm can be used to direct surgery in such situation. In 21 (33%) of 64 patients operated upon at this unit, intra-operative arrhythmia mapping was impossible because of non-inducibility in 17 (27%) and unstable morphology in 4 (6%). Endocardial resection was performed in all areas showing 'fragmented' local electrograms (greater than 100 ms duration at 30-300 Hz filtering). Mean patient characteristics included: age 51 years; LV ejection fraction 32%; major arrhythmic episodes 16 (range 2-200); antiarrhythmic drug failures, 4. There were 5 (24%) early postoperative deaths (heart failure 3; sudden 1; metabolic 1) and 1 early arrhythmia recurrence. There were 3 late non-arrhythmic deaths and 1 further arrhythmia recurrence during follow-up of 23 +/- 19 months. Both patients with documented postoperative arrhythmic episodes were controlled on previously ineffective antiarrhythmic drug therapy. Fragmentation mapping in sinus rhythm successfully extends the surgical option to arrhythmias previously considered inoperable. The results compare favourably with those for arrhythmias in which surgery was directed by activation mapping.

Adolescent↗
PubMed1989

Do electrode and lead design differences for permanent cardiac pacing translate into clinically demonstrable differences? (Comparison of sintered platinum and activated vitreous and porous carbon electrodes).

A randomized prospective study was undertaken to compare the electrical performances of three permanent, endocardial, tined pacing leads with different electrode designs--sintered platinum, vitreous carbon, and porous carbon. Ninety-nine patients received one of the leads (S80 31; 423S 32; S100 36). Acute R wave amplitude and ST elevation of the native endocardial electrogram, voltage threshold, impedance, and current flow at four pulse durations (0.25-1.0 msec) were measured. Voltage thresholds were measured noninvasively at each of four pulse durations at 2 days and 1, 3, and 6 months after implantation. No significant differences were found in sensing properties, or current flow at threshold at 0.5 msec pulse duration. The 423S lead had a significantly higher impedance at threshold and both a higher impedance and lower current flow at 5 V. No significant differences in threshold voltages were found between the three leads at any pulse duration, at any of the assessed times after implantation. Six-month thresholds for the S80, 423S, and S100 leads were 1.18 +/- 0.35, 1.17 +/- 0.29, and 1.06 +/- 0.38 V respectively at 0.5 msec pulse duration. Differences between 'high performance' pacing leads need to be of a greater order of magnitude before they can be exploited to give any real clinical advantage to patients.

Aged↗
PubMed1988

Importance of lead selection in QT interval measurement.

The influence of lead selection on QT estimation in the 12-lead electrocardiogram was assessed in 63 patients (21 control subjects, 21 with anterior myocardial infarction, 21 with inferior myocardial infarction). QT estimates varied between leads. The variation was greater in patients with myocardial infarction than in control subjects (mean dispersion of QT: control subjects, 48 +/- 18 ms [+/- standard deviation]; anterior myocardial infarction, 70 +/- 30 ms; inferior myocardial infarction, 73 +/-32 ms). The maximum QT in any lead (QTmax) was determined and the deviation of each lead from this maximum value calculated. In all 3 groups, anteroseptal leads (V2 or V3) provided the closest approximation to QTmax. Interlead variability was found to be mainly due to variation in timing of the end of the T wave, rather than the onset of the QRS complex. The variability due to leads was considerably greater than the variability due to cycles, observers or measurement error. Implementation of a variety of current lead selection practices resulted in widely divergent estimates of QT interval. It is concluded that there is a need for standardization of lead selection practice for QT measurement. If measurements are confined to one or a few leads, anteroseptal leads provide the closest approximation to QTmax.

Electrocardiography↗
PubMed1988

Sequence of epicardial repolarisation and configuration of the T wave.

Epicardial activation and repolarisation sequences were investigated in patients with upright or inverted T waves in left ventricular leads of the surface electrocardiogram. Fifteen patients were studied: 10 were undergoing coronary artery bypass grafting (upright T waves) and five aortic valve replacement (four patients with T inversion). Monophasic action potentials were recorded intraoperatively from eight to 10 left ventricular sites in each patient. In patients with upright T waves there was an inverse relation between the duration of the monophasic action potential and the activation time (mean slope -1.44). As a consequence, activation and repolarisation proceeded in opposite directions. Dispersion of repolarisation time (14 ms) was less than dispersion of activation time (23 ms). In patients with T wave inversion caused by aortic stenosis there was no relation between the duration of action potential and activation time; the repolarisation sequence resembled the activation sequence, and the dispersion of repolarisation time was greater than the dispersion of activation time (31 and 26 ms respectively). These results show that there are epicardial repolarisation gradients in man and that these are related to the configuration of the T wave. In patients with upright T waves an inverse relation between the duration of the action potential and the activation time reduces the dispersion of the repolarisation time. When the T wave was inverted this relation was no longer found and the dispersion of repolarisation increased.

Action Potentials↗
PubMed1987

Prevention of tolerance to nitroglycerin patches by overnight removal.

This investigation assesses the extent of tolerance development with nitroglycerin patches and whether tolerance might be prevented by overnight patch removal. On commencing therapy, active patches significantly prolonged exercise time (3.5 hours after patch application) in comparison with placebo, with an accompanying reduction in ST-segment depression at maximal common workload. Patients then received continuous or 12-hour-daily intermittent patch therapy, in a double-blind fashion, for 7 days. Exercise testing was repeated before and after active patch application, on the eighth day of each treatment phase. During continuous therapy, beneficial effects on exercise time and ST depression were abolished. By contrast, during intermittent therapy, prolongation of exercise time and reduction in ST-segment depression still occurred, on testing 3.5 hours after active patch application. These results confirm previous studies showing a high degree of tolerance during continuous therapy with nitroglycerin patches and suggest that tolerance can be prevented by 12-hour-daily intermittent therapy.

Administration, Cutaneous↗
PubMed1987

Flecainide--an intravenous infusion regimen.

An intravenous infusion regimen for flecainide was studied in 12 patients. Therapeutic drug levels were rapidly established and maintained throughout the 48-h infusion period. Two patients were withdrawn because of possible drug related problems, but in general the regimen seemed both effective and well tolerated.

Aged↗
PubMed1987

Epicardial repolarization mapping in man.

A simple method was developed for recording epicardial monophasic action potentials intraoperatively in man. Potentials were recorded rapidly and reliably from multiple epicardial sites using a hand-held probe. The feasibility of repolarization mapping was assessed in 30 patients. It was possible to record technically adequate signals from most left ventricular and posterior right ventricular sites (success rates 72% and 61%, respectively), but it was more difficult to achieve satisfactory recordings over the free wall of the right ventricle (success rate 36%). The onset of cardiopulmonary bypass resulted in a transient rise in action potential duration of approximately 30-40 ms in the first minutes of bypass. An approximate steady state was achieved thereafter. Reproducibility of the method was assessed by mapping a small number of sites repeatedly. The 95% confidence limits for an estimate of repolarization time, based on a single potential, were +/- 21 ms. With 4 probe applications at each site, the confidence limits of the estimated site mean were reduced to +/- 8 ms. Based on this approach, statistically significant differences between sites were demonstrated in 9 of 10 patients, confirming that the method is sensitive enough to provide information on regional differences in repolarization. Intraoperative epicardial repolarization mapping has been shown to be a practical possibility. The technique holds promise as a means of assessing the role of dispersion of repolarization in arrhythmogenesis in man.

Action Potentials↗
PubMed1986

Use and misuse of a digoxin assay service.

In a study of the use of a digoxin assay service and its influence on clinical management 285 assay requests were audited over 12 weeks. For 67 (24%) there was no clear clinical indication for the request and for 140 the period between the last dose of digoxin being given and the blood sample being taken was either unknown or inappropriate. Treatment in 64 patients (22%) was changed after the assay result was received [corrected]. 24 of these changes bore no relation to the original clinical indication for requesting the assay, suggesting that such changes were based on the assay result alone. Of samples collected within six hours after the last dose, 15 of 69 (22%) led to a reduction in treatment compared with 10 of 116 (9%) taken after six hours (p less than 0.025), thereby highlighting the danger that incorrectly timed samples may lead to inappropriate clinical decisions.

Clinical Laboratory Techniques↗