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Biomedical subjects

D C MacGregor

Publications and source records attributed to D C MacGregor.

At least 37 records · Page 2Linked to original sources
PubMed1979

The porous-surfaced electrode: a new concept in pacemaker lead design.

Three major problems which may be encountered with endocardial pacemaker electrodes are a lack of stable position, a chronic increase in stimulation threshold, and a diminishing magnitude of the sensed endocardial signal. These problems are particularly manifest in the atrium. Having previously shown that porous metal surfaces can support stable tissue ingrowth in both bloodstream and soft tissue environments, we set out todetermine the performance of porous-surfaced endocardial pacing electrodes in the atrial position. In two groups of six dogs each, J-shaped atrial leads with Elgiloy electrode tips (2.3 mm. in diameter, 2.3 mm. in length), having either conventional smooth surfaces (control) or porous surfaces (20 to 50 micron particle size) produced by powder metallurgy techniques, were positioned in the right atrial a-pendage. Stimulation thresholds and P-wave amplitude were repeatedly measured until the dogs were put to death 30 w-eks following implantation. The presence or absence of electrode fixation was observed and the atrial tissue reaction was examined grossly and by both light and scanning electron microscopy (SEM). The porous-surfaced electrodes demonstrated superior long-term stimulation thresholds which, at 30 weeks, averaged less then one third of those in the control group. In addition, the porous group showed a small but significant improvement in the amplitude of the sensed P wave. None of the smooth-surfaced electrodes showed fixation, and the tissue reaction consisted of a thick layer of granulation and fibrous tissue on the underlying endocardium, widely separating the electrode from the myocardium. In contrast, all of the porous-surfaced electrodes were fexed to the endocardium by fibrous tissue ingrowth into the surface pores. This tissue fixation of the electrode tip in close proximity to underlying myocytes explains their superior performance.

Animals↗
PubMed1978

Left ventricular receptors inhibit brain serotonin neurons during coronary artery occlusion.

Acute coronary artery ligation in pargyline-treated rats decreased serotonin and increased 5-hydroxyindoleacetic acid in the medulla and posterior hypothalamus. Lidocaine applied topically to the left ventricle completely prevented these alterations. No changes in serotonin were observed in the other brain regions examined. These data suggest a reflex inhibition of bulbar and hypothalamic serotonergic nerves by left ventricular receptors following acute coronary artery occlusion in the rat.

Animals↗
PubMed1978

Computer-assisted reporting system for the follow-up of patients with prosthetic heart valves.

The implantation of large numbers of prosthetic heart valves carries with it the responsibility for continual reassessment of all aspects of patient management. Experience with more than 2,000 prosthetic valve operations since 1963 led to the development of a comprehensive computer-assisted data collection, management and reporting system. Over a 5 year period, data forms were developed for the detailed documentation of preoperative, intraoperative and postoperative information. These were designed in the form of checklists suitable for direct computer entry with use of mark-sense document readers. Special emphasis was placed on preoperative assessment of ventricular function, valve selection, intraoperative myocardial preservation, postoperative rehabilitation and prosthetic valve-related complications. This system makes possible rapid computer generation of a variety of reports to the referring physician regarding the individual patient and to the clinical investigator in relation to patient group statistics. Also, questionnaires to patients of physicians, or both, to update patient data can be produced by the computer at appropriate intervals after valve surgery. Experience indicated that a computer-assisted methodology is the only practical way to provide adequate follow-up of large groups of patients. Additionally direct access to relevant information helps to create an environment in which essential research can be carried out in the face of a demanding clinical practice.

Follow-Up Studies↗
PubMed1978

Self-conversion of supraventricular tachycardia by rapid atrial pacing.

The use of pacemakers in the treatment of tachycardias is one of the most exciting and rapidly expanding applications of cardiac pacing. One of the more recent developments in this field has been the use of patient-activated radio frequency transmitted rapid atrial stimulation (RAS) in the treatment of paroxysmal supraventricular tachycardia (PSVT). Based on the previously established ability of asynchronous atrial pacing to interrupt a variety of re-entrant supraventricular rhythm disturbances, this modality of treatment is gaining increasing applicability in patients with PSVT associated with debilitating symptoms or other severe cardiovascular consequences in whom standard pharmacological regimens have either failed or are impossible to maintain for indefinite periods. This report describes our experience with five patients who underwent implantation of RAS units. The detailed electrophysiological studies required to ensure success and avoid any possible future complications are described. Over a follow-up period of four months to four years (mean 16 months) very few problems arose in the use of these units which have immeasurably improved the quality of life of the recipients. Our experience with RAS units has led to a few suggestions for future improvement and these are outlined in this report. The excellent patient acceptance and the reliability of this technique in terminating episodes of PSVT should, in the future, render RAS the treatment of choice in certain selected patients suffering from this common disorder.

Adult↗
PubMed1978

Permanent transvenous atrial pacing.

Atrial pacing has electrophysiologic and hemodynamic advantages for patients with symptomatic bradyarrhythmias and intact atrioventricular conduction or with certain refractory tachyarrhythmias. Permanent atrial pacing has been achieved in 29 patients followed up for periods of up to 7 years at two institutions. At Sunnybrook Medical Centre, Toronto, 16 patients have had coronary sinus electrodes introduced pervenously using standard bipolar or special unipolar catheters. Initial pacing thresholds were acceptably low (mean, 2.0 mA); chronic thresholds in three patients were similar. At the Toronto General Hospital, 13 patients had endocardial J-electrodes, bipolar or tined unipolar, inserted in the right atrial appendage. Initial thresholds were low (mean, 1.3 mA) and P-wave voltage was adequate (3.4 mV) for pacemakers with standard sensitivity. In both series, conventional R-inhibited, asynchronous or rate-programmable units have been employed. Radiofrequency self-conversion pacemakers were used in three patients. Preliminary His bundle studies were done in 10 patients; the others were tested by rapid atrial pacing during insertion of electrodes. Early and late electrode stability in both series was excellent; one electrode became dislodged from the coronary sinus position and one from the right atrial appendage.

Adult↗
PubMed1976

Aortocoronary bypass graft in dogs: late histological changes.

Late histological changes occurring in aortocoronary bypass vein grafts were studied by lignt and electron microscopy in three dogs killed one, two and three years after grafting. The changes consisted of intimal thickening due to a proliferation of modified smooth muscle cells (myointimal hyperplasia) and replacement of most of the medial smooth muscle by fibrocytes. Serial angiography in the dogs did not reveal progression of the intimal thickening after one month.

Animals↗
PubMed1976

Cardiorespiratory failure secondary to peripheral pulmonary emboli. Survival following a combination of prolonged extracorporeal membrane oxygenator support and pulmonary embolectomy.

A 19-year-old woman developed cardiorespiratory failure from multiple, peripheral pulmonary emboli apparently developing over the preceding 3 weeks. She was not considered to be an operative candidate. However, when 3 days of intravenous heparin infusion and 30 hours of membrane oxygenator support failed to improve the pulmonary pathology, pulmonary embolectomy was performed. The membrane oxygenator support had to be continued for 34 hours following the operation before it was successfully discontinued. The patient made a complete recovery.

Adult↗
PubMed1975

Ischemic contracture of the left ventricle. Production and prevention.

Ischemic contracture of the left ventricle ("stone heart") is a recognized complication of prolonged periods of interruption of the coronary circulation during open-heart surgery. We have examined the effects of moderate hypothermia (28 degrees C.) and preoperative beta-adrenergic blockade (propranolol, 0.5 mg. per kilogram; 1.0 mg. per kilogram) on contracture development during ischemic arrest of the heart. Four groups of 8 dogs each were placed on total cardiopulmonary bypass, and ischemic arrest of the heart was produced by cross-clamping the ascending aorta and venting the left ventricle. Intramyocardial carbon dioxide tension was continuously monitored by mass spectrometry. When anaerobic energy production ceased, as indicated by a final plateau in the intramyocardial carbon dioxide accumulation curve, the ischemic arrest was terminated and the contractile state of the heart observed. These results are given in the text. We conclude that beta-adrenergic blockade delays, but does not prevent, the onset of ischemic contracture of the left ventricle under normothermic conditions. Moderate hypothermia appears to prevent this complication completely.

Animals↗