Dumb question but I'm a newbie. If a prolonged qti is a risk for torsades and hypocalcemia is causes a prolonged at would it be appropriate to give calcium or is it presumed that if you have torsades its hypomagnesium(sp) also we do not have mag so would Amil be appropriate? And could someone please explain overdrive pacing for torsades to me?
If the QT prolongation is a congenital thing you'd want to definitely get the HR above 100 in order to shorten your QTc.
The term "overdrive" pacing is at times somewhat of a misnomer (when you have the option of VVI, AAI, or DDD mode) because a lot of the time, you "overdrive" pace at a rate slower than the actual tach, but that's ok because we're pacing asynchronously. With the TCP, you're wanting to pace as fast or faster than the rhythm. Never done it with an external pacemaker, only with the epicardial wires or tvp in place, so someone else who's done it with a TCP may be of more help in that regard. Anywho, the point of overdrive pacing is to allow us to tap into the tachycardia's circuit, interfere, and eventually cause the entrainment of the rhythm.
This is a gross oversimplification but picture this:
Think back to those old western movies where a wild mustang takes off super fast across a valley (usually with some poor damsel in distress aboard). Then the cowboy takes off on his horse and runs equally as fast or sometimes faster, catches up to the wild mustang, tosses a rope around it and then gradually slows it down to where it is now under control and the cowboy's horse is now setting the pace, eventually slowing down to a manageable, less dangerous speed.
I don't know to what you're referring when you say "Amil."
As far as the other stuff, without writing two pages of cardiac action potential (which i assume everyone here already has a good grasp on), the basic premise is that a lack of calcium (and usually potassium as well) means it takes cells longer to repolarize which is identified as a prolonged QT. This can cause early afterdepolarizations (inward depol current is > outward repol current in late phases). Mag lowers the amplitude of the EADs (presumably) via the sodium-potassium ATPase. Vene can expound on this in great detail I'm sure.
You can play with your Nernst equation and see how the varying degrees of your intracellular and extracellular ions affect potential.