"Sawtooth" P waves are characteristic of atrial flutter.

A prolonged P-R interval, that does not get progressively longer is typical of a type I A-V block.

A premature junctional/nodal beat would be represented on an ECG as an inverted or absent P wave. In a premature junctional/nodal beat, the atrioventricular (AV) node is firing before the sinoatrial (SA) node. Since the AV node is at the bottom of the right atrium, the net vector of atrial depolarization is occurring in a more upward direction, causing the P wave to be absent or inverted. Note that there would be a compensatory pause after the premature beat.The answer could not be a premature atrial contraction because the P wave is absent or inverted (P wave would be normal in a premature atrial contraction (PAC)).

Ventricular fibrillation is a very serious issue that requires immediate defibrillation. Ventricular fibrillation is the result of ventricular beats originating from many different automaticity foci, so the QRS looks squiggly, very fast, and incoherent. There are no P waves since the beats are originating in the ventricles. 

A bundle branch block is a block in one of the bundle branches, causing a long QRS (>.10 seconds). It is different than a PVC in that a bundle branch block will show a P wave since the beats are still originating in the atria. Bundle Branch Blocks also tend to show a notched QRS.

In First Degree AV Block, there is a longer than normal PR interval (>.20s) due to slowed AV node conduction. Second degree, Mobitz Type 1 (Wenckebach) is progressive lengthening of the PR interval followed by a dropped QRS. Second Degree, Mobitz Type 2 is sudden dropping of the QRS (no lengthening of PR interval). Third Degree AV Block is when there is no relationship between the P and the QRS. 

The normal PR interval length is .12 - .20 seconds. The PR interval represents the time lag from the onset of atrial depolarization to the onset of ventricular depolarization. Most of the time lag occurs in the AV node, and it allows atrial systole to occur. This allows the ventricles to have time to fill before ventricular systole.

Tachycardia is an abnormally rapid heart rate of rate >100 beats per minute.

Bradycardia is defined as heart rate <60 bpm. In an endurance athlete, there tends to be higher stroke volume/higher vagal tone, so it is typical for an endurance athlete to have a resting heart rate below 60 and still meet normal cardiac output. It a non-endurance athlete, a resting heart rate of 45 bpm may warrant some investigation.

Ejection fraction is a measurement of how much blood blood the ventricles pump out with each contraction as a fraction of the end-diastolic volume. Mathematically, this is (stroke volume) divided by (end-diastolic volume). In congestive heart failure, the heart is weakened and quivers, and it has a hard time pumping blood out. An ejection fraction (EF) < 40% may be evidence of congestive heart failure.