Help with reading strips!

I assume you are referring to EKG- right? (there are other types of 'strips')

Quick answer - it depends on which way the electric current is flowing. If it is going away from the lead you are looking at, it goes down (negative) -- if it is coming toward the lead, it goes up. The general direction of the current is also called the 'vector'.

In order to accurately interpret an EKG rhythm strip, you need to know which lead it is. In some leads, it is normal for the QRS to be negative.

Sorry - I hit 'send' before I answered the second part of your question re PACs & PVCs.

PAC is a contraction that begins in the atria - but maybe not in the sinus node. So the p-wave generally looks different than the 'normal' p-waves. It may also be earlier (hence "premature") than expected. You can figure this out by 'marching out' the p-waves on the underlying rhythm. There may be no difference at all in the QRS, because that part of the conduction is no different than the normal rhythm.

PVCs always look different than the underlying rhythm. There is no P-wave because this complex begins in the ventricles instead of the atria. Generally, the QRS will be wider than normal and look very different. It is possible for PVCs to originate from different places - in this case, this is called "multi-focal PVCs". The direction of the PVS (positive or negative) will depend on where it originates in relation to the lead that is being used.

Don't worry - I have taught this for eons and most students don't really understand it until their 3rd course. Hang in there!