The width of a square on the ECG represents time
The count method to determine the heart frequency. The second QRS complex is between
75 and
60 beat per minute. This heartbeat is between that, around 65 beats per minute.
What is the heart rate?
To answer this question, determine the time between two QRS complexes. Previously, the ECG was registered on a paperstrip transported through an ECG writer at the speed of 25 mm/second. Nowadays, digital ECG registration is common however, the method of determining the frequency remains the same. The ECG paper has a grid with thick lines 5 mm apart (= 0,20 second) and thin lines 1 mm (0,04 second).
There are three simple methods to determine the heart rate (HR)

 The square counting method is ideal for regular heart rates. Use the sequence 3001501007560504337. Count from the first QRS complex, the first thick line is 300, the next thick line 150 etc. Stop the sequence at the next QRS complex. When the second QRS complex is in between two lines, take the mean of the two numbers from the sequence or use the finetune method listed below.
 Count the small (1mm) squares between two QRS complexes. Hense, the ECG paper runs with 25 mm/sec through the ECG writer, therefore:This method works well in case of tachycardia (>100 beats/minute)
 Non regular rhythms are best determined with the "3 second marker method" Count the number of QRS complexes that fit in 3 seconds (some ECG writers register this period on the ECG paper). Multiply this number by 20 and find the number of beats/minute.

The 'square counting' method can be finetuned with the following sequence
300 
250 
214 
187 
167 
150 
136 
125 
115 
107 
100 
94 
88 
83 
79 
75 
71 
68 
65 
62 
60

What changes the frequency of the heart?
A number of factors change the heart frequency including:
 the (para) sympathic nerve system.
 The sympathic system e.g. epinephrin (=adrenalin) increases the atrioventricular conduction and contractility. (the fight, fright, flight reaction)
 The parasympathic system (nervus vagus) e.g. acetycholin decreases the frequency and atrioventricular conduction. The parasympathic system effects mainly the atria.
 Cardiac filling increases the frequency.
 Also arrhythmias influence heart rate.