Patients with complete heart block, or disrupted electrical conduction in their hearts, are at risk for life-threatening rhythm disturbances and heart failure. The condition is currently treated by implanting a pacemaker in the patient's chest or abdomen, but these devices often fail over time, particularly in infants and small children who must undergo many re-operations. Researchers at Children's Hospital Boston have now taken preliminary steps toward using a patient's own cells instead of a pacemaker, marking the first time tissue-engineering methods have been used to create electrically conductive tissue for the heart. Results appear in the July issue of the American Journal of Pathology (published online on June 19).

In complete heart block, electrical signals cannot pass from the heart's upper chambers (atria) to the lower chambers (ventricles), leading to heart failure. In normal hearts, electrical impulses move first through the atria, then pause at the atrioventricular (AV) node. Then, after a short delay that allows the ventricles to fill with blood, the AV node releases the impulses, which move through the ventricles causing them to contract. In this way, the beats of the atria and ventricles are synchronized.

Investigators led by Douglas Cowan, PhD, a cell biologist in Children's Department of Anesthesiology, Perioperative and Pain Medicine, wanted to create a biological substitute for the AV node that would work in patients who have defective atrioventricular conduction. "The idea was that rather than using a pacemaker, we could create an electrical conduit to connect the atria and ventricles," Cowan says.
Cowan's team, including first author Yeong-Hoon Choi in Children's Department of Cardiac Surgery, obtained skeletal muscle from rats and isolated muscle precursor cells called myoblasts. They "seeded" the myoblasts onto a flexible scaffolding material made of collagen, creating a 3-dimensional bit of living tissue that could be surgically implanted in the heart.

The cells distributed themselves evenly in the tissue and oriented themselves in the same direction. Tested in the laboratory, the engineered tissue started beating when stimulated electrically, and its muscle cells produced proteins called connexins that channel ions from cell to cell, connecting the cells electrically.

When the engineered tissue was implanted into rats, between the right atrium and right ventricle, the implanted cells integrated with the surrounding heart tissue and electrically coupled to neighboring heart cells. Optical mapping of the heart showed that in nearly a third of the hearts, the engineered tissue had established an electrical conduction pathway, which disappeared when the implants were destroyed. The implants remained functional through the animals' lifespan (about 3 years).

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