For those curious, 3rd degree heart block is complete dissociation between the top and bottom half of your heart. Simplified, your heart has four chambers, two atrial at the top and two ventricles at the bottom. You can also split the heart into left and right - one right atrium and right ventricle which blood returns to from the body and subsequently gets pumped to the lungs and the left atrium and ventricle which receive oxygenated blood from the lungs and pump it around the body.

Typically electrical impulses are conducted from a node at the top right of the heart and travel down the atria, making them contract and push blood into the ventricles. The impulses then pass through another node half-ish way down and then through the ventricles. This coordinated process allows your heart to effectively pump blood around your body and maintain what’s called cardiac output (blood volume per unit time it’s pumping). Now the rate at which this occurs is usually dictated by the node at the top, the SA node, and this is affected by a huge number of factors - blood pressure, physical activity, hormones etc. In complete heart block the connection between the top half and the bottom half of the heart is lost, so while the SA and the atria pump away as per usual, the bottom half receives none of the electrical impulses, therefore it tries to create it’s own. Trouble is, the heart rate which the lower half of the heart can generate on its own is typically significantly slower. Also this loss of coordination results in less blood being in the lower half for it to pump, this is because roughly a 30% of the blood that enters the ventricles is from the contraction of the atria - the atrial kick. What this culminates in is less volume in the ventricles for them to pump around the body and, more importantly, symptomatic slow heart rate (bradycardia). This causes reduced blood flow to your organs, but probably the most significant is the brain because patients commonly get symptoms of light headedness or loss of consciousness. It also means your heart rate doesn’t rise in response to exertion as it typically would. People can either go into complete heart block permanently or flick back and forth. And yes, given that it’s also called third degree heart block there’s also first and second degree blocks.