Furthermore, approximately 20% of those with CPVT have a slow resting heart rate known as a sinus bradycardia. Mechanism
Excitation-contraction coupling
The arrhythmias that those with CPVT experience are caused by abnormalities in the way that cardiac muscle cells control their levels of calcium. Calcium interacts with the protein fibres or myofibrils inside the cell that allow the cell to contract, and the concentration of calcium within each cell needs to be tightly regulated. During each heartbeat, the concentration of calcium must rise to allow the muscle to contract and then fall to allow the muscle to relax, a process achieved by using a store within the cell known as the sarcoplasmic reticulum. At the start of each heartbeat, calcium is released from the sarcoplasmic reticulum through specialised channels known as ryanodine receptors. Ryanodine receptors open when the concentration of calcium near the channel increases. This happens when, in response to an electrical signal from the cell membrane called an action potential, a small amount of calcium flows across the cell membrane into the cell through L-type calcium channels, many of which are located on specialised inpouchings of the membrane called T-tubules designed to bring these surface ion channels close to the sarcoplasmic reticulum.The increase in calcium concentration triggers ryanodine receptors on the sarcoplasmic reticulum to release a puff of calcium known as a calcium spark. Each spark triggers the release of further sparks from neighbouring ryanodine receptors to create an organised rise of calcium throughout the cell known as a calcium transient.
A ventricular outflow tract obstruction is one type of congenital heart defect in which either the right or left ventricular outflow tract is blocked or obstructed. These obstructions represent a spectrum of disorders. Right side
A right ventricular outflow tract obstruction (RVOTO) may be due to a defect in the pulmonic valve, the supravalvar region, the infundibulum, or the pulmonary artery. Pulmonary atresia
Pulmonary valve stenosis
Hypoplastic right heart syndrome
Tetralogy of Fallot

Left side
A left ventricular outflow tract obstruction (LVOTO) may be due to a defect in the aortic valve, or a defect located at the subvalvar or supravalvar level. Aortic valve stenosis
Supravalvar aortic stenosis
Coarctation of the aorta
Hypoplastic left heart syndrome


== References ==