Biomedical subjects

Siew Y Ho

Publications and source records attributed to Siew Y Ho.

3 recordsLinked to original sources

PubMed2003

The relationship between structure and function: why does reshaping the left ventricle surgically not always result in functional improvement?

Surgical strategies recently introduced to improve ventricular function have been based on the concepts of reduction of ventricular diameter, synchronization of myocardial activity, passive support of diastolic ventricular shape, and active support of systolic ventricular constriction. They have depended on several established theoretical assumptions, not all of which are totally valid. Clinical results have proved markedly variable. This is especially true for procedures designed to reduce the radius of the left ventricle. Some have reported up to 80% mortality, whereas others achieve results comparable with those for heart transplantation. Because of this, the method runs the risk to be rejected, or else, its more widespread application will be postponed until essential details concerning the basic concepts have been elucidated. It is these details which we discuss in this review.

Cardiac Surgical Procedures↗

PubMed2002

Electrical conduction in canine pulmonary veins: electrophysiological and anatomic correlation.

BACKGROUND: Paroxysmal atrial fibrillation in patients is often initiated by foci in the pulmonary veins. The mechanism of these initiating arrhythmias is unknown. The aim of this study was to determine electrophysiological characteristics of canine pulmonary veins that may predispose to initiating arrhythmias. METHODS AND RESULTS: Extracellular recordings were obtained from the luminal side of 9 pulmonary veins in 6 Langendorff-perfused dog hearts after the veins were incised from the severed end to the ostium. Pulmonary veins were paced at the distal end, the ostium, and an intermediate site. During basic and premature stimulation, extracellular electrical activity was recorded with a grid electrode that harbored 247 electrode terminals. In 4 hearts, intracellular electrograms were recorded with microelectrodes. Myocyte arrangement immediately beneath the venous walls was determined by histological analysis in 3 hearts. Extracellular mapping revealed slow and complex conduction in all pulmonary veins. Activation delay after premature stimulation could be as long as 96 ms over a distance of 3 mm. Action potential duration was shorter at the distal end of the veins than at the orifice. No evidence for automaticity or triggered activity was found. Histological investigation revealed complex arrangements of myocardial fibers that often showed abrupt changes in fiber direction and short fibers arranged in mixed direction. CONCLUSIONS: Zones of activation delay were observed in canine pulmonary veins and correlated with abrupt changes in fascicle orientation. This architecture of muscular sleeves in the pulmonary veins may facilitate reentry and arrhythmias associated with ectopic activity.

Action Potentials↗

PubMed2002

Basic and clinical electrophysiology of pulmonary vein ectopy.

In a subset of patients, atrial fibrillation is caused by rapidly firing foci that are often located in the pulmonary veins especially when fibrillation is paroxysmal. Histologic data show that myocardial tissue of the left atrial wall extends into the pulmonary venous walls. Both in dog and human pulmonary veins, arrangement of the myofibers is complex. Clinical and animal studies reveal both double potentials and fractionated electrograms in the pulmonary veins, which are related to the complex architecture of the myocardial sleeves in the veins. Such a structure supports the occurrence of reentry. As well, the reduced coupling of cells at sites with abrupt changes in fiber direction could facilitate the escape of a focus and subsequent activation of surrounding tissue. Intracellular recordings made in the pulmonary veins of guinea pig and dog hearts showed that spontaneous activity can occur. Spontaneous action potentials with phase 4 depolarization as well as early after depolarizations were observed in these animal models. In non-spontaneously active preparations, spontaneous activity could be provoked by pharmacologic interventions. The cycle length of bursts of ectopic beats arising in the pulmonary veins of man is often irregular, supporting a focal mechanism of the ectopic beats. The anisotropic characteristics of the myocardial sleeves in the veins may increase the ability of a focus to become evident.

Action Potentials↗