The action potential can be divided into five phases: the resting potential, threshold, the rising phase, the falling phase, and the recovery phase. We begin with the resting potential, which is the membrane potential of a neuron at rest.

Potassium ions are important for RMP because of its active transport, which increase more its concentration inside the cell. Its outward movement is due to random molecular motion and continues until enough excess negative charge accumulates inside the cell to form a membrane potential.

The activity of the Na+/K+-pump influences the membrane potential directly and indirectly. Thus, the maintenance of a normal electrical function requires that the Na+/K+-pump maintain normal ionic concentrations within the cell.

When the electrical signal of a depolarization reaches the contractile cells, they contract. When the repolarization signal reaches the myocardial cells, they relax. Thus, the electrical signals cause the mechanical pumping action of the heart.

Repolarization typically results from the movement of positively charged K+ ions out of the cell. Repolarization is a stage of an action potential in which the cell experiences a decrease of voltage due to the efflux of potassium (K+) ions along its electrochemical gradient.

There is no distinctly visible wave representing atrial repolarization in the ECG because it occurs during ventricular depolarization. Because the wave of atrial repolarization is relatively small in amplitude (i.e., has low voltage), it is masked by the much larger ventricular-generated QRS complex.

As the human Ta wave of atrial repolarization occurs during the PR segment and QRS complex, it is not observed and recorded widely in sinus rhythm subjects by the standard 12-lead ECG (1).

When the cardiac rhythm is regular, the heart rate can be determined by the interval between two successive QRS complexes. On standard paper with the most common tracing settings, the heart rate is calculated by dividing the number of large boxes (5 mm or 0.2 seconds) between two successive QRS complexes into 300.

Count the number of RR intervals between two Tick marks (6 seconds) in the rhythm strip and multiply by 10 to get the bpm.

Normal intervals PR interval (measured from the beginning of the P wave to the first deflection of the QRS complex). Normal range 120 – 200 ms (3 – 5 small squares on ECG paper). QRS duration (measured from first deflection of QRS complex to end of QRS complex at isoelectric line).

The pictorial explanation of this method is shown here. Another quick way to calculate the rate is based on the entire ECG being 10 seconds. By counting the number of QRS complexes and multiplying by six, the number per minute can be calculated — because 10 seconds times six equals 60 seconds, or 1 minute.

2. The Six Second Method: Get 6 seconds of ECG tracing (i.e. 30 big boxes) and count the number of R waves that appear within that 6 second period and multiply by 10.

An abnormal EKG can mean many things. Sometimes an EKG abnormality is a normal variation of a heart’s rhythm, which does not affect your health. Other times, an abnormal EKG can signal a medical emergency, such as a myocardial infarction (heart attack) or a dangerous arrhythmia.

The ECG paper speed is ordinarily 25 mm/sec. As a result, each 1 mm (small) horizontal box corresponds to 0.04 second (40 ms), with heavier lines forming larger boxes that include five small boxes and hence represent 0.20 sec (200 ms) intervals.

Each small square represents 0.04 seconds of time. 5 small squares equal 0.20 seconds of time. When you are trying to calculate the heart rate with the six second rule, you must count out enough LARGE squares to equal 6 seconds. Therefore, 30 large squares would equal 6 seconds.

Electrocardiogram waves, intervals, and segments. On standard calibration, each large box has sides of 0.5 cm. On the horizontal axis, each large box represents 0.2 seconds, and each smaller box represents 0.04 seconds.

The first measurement is known as the “P-R interval” and is measured from the beginning of the upslope of the P wave to the beginning of the QRS wave. This measurement should be 0.12-0.20 seconds, or 3-5 small squares in duration.