An electrocardiogram - an ECG - or the dutch and german version of the word - elektrokardiogram or EKG, is a tool used to visualize “gram” the electricity “electro” that flows through the heart “cardio”. An ECG tracing specifically shows how the depolarization wave moves during each heartbeat - which is a wave of positive charge - looks from the perspective of different sets of electrodes. This particular set of electrodes is called lead II, with one electrode on the right arm and the other on the left leg, so essentially when the wave’s moving toward the left leg electrode, you get a positive deflection, like this big positive deflection correspond to the wave moving down the septum. To read an ECG it’s really helpful to first understand what a normal sinus rhythm looks like.
To do that, let’s look at a single heartbeat on an ECG - from the viewpoint of lead II. In a healthy heart, everything starts at the sinoatrial node, or SA node, - which is a little patch of tissue in the wall of the right atrium full of pacemaker cells. When one of these pacemaker cells depolarizes, a wave of positive charge spreads outward and overall it moves from the SA node towards the apex of the heart, so it aligns pretty nicely with the lead II vector.
Now, the heart muscle cells, are often described as having a functional syncytium because even though each one cell has its own cell membrane - the cells also have tiny connections or openings between them. That means that during a depolarization wave, ions can flow right from one cell to the next. Now - it’s also important to mention that the depolarization wave moves at two different speeds through the heart. In the pacemaker cells, which are special types of cardiomycocytes laid out like highways through the heart, it moves really fast, but it moves more slowly through the rest of myocytes which do the contracting - the depolarization wave moves through those like a car moving through congested small streets.
The ECG measures out changes in time on the X-axis, where one small box is 0.04 s, and voltage on the Y-axis, with each small box being 0.1 mV, sometimes called 1mm voltage on the Y-axis, with each small box being 0.1mV sometimes called 1mm, with zero being called the “isoelectric line”. Every time there’s a positive or negative deflection away from this line - the isoelectric line. So a depolarization wave starts in the SA node, then goes through atrial intranodal tracts, also called Bachmann's bundle, over to the left atrium so that both atria basically depolarize together. The overall direction of that depolarization wave is in the same direction as the lead II vector, so there is a positive deflection called the P wave.