The largest portions of the heart, the ventricules, are the areas most dangerous problems occur. Since it is the largest part of the muscle, it does the most work to move the blood. In a normal contraction the impulse passes through the ventricles quickly, about 0.04 to 0.12 seconds as we measure it on the ECG, (Electrocardiogram.) An interval larger then those can be seen when a block occurs somewhere along the bundle branches, causing a wider qrs then normal.
It is possible to have a normal sinus rhythm with a bundle branch block. The p-r interval would be normal and the rate within 60 to 100 beats a minute but the qrs is longer then 0.12 seconds.
The ventricle may contract earlier then expected in the normal cycle without seriously interrupting the normal sequence. Many such pvc's are benign and can just be watched. The wave forms will be wider then normal for a qrs and generally occur with a compensatory pause. Interpolated pvc's are when there is no compensatory pause. The dangerous part is learning which are ok and which are bad. Don't think that by viewing this page you will be ready to distinguish those subtle differences, it takes more time then I have here to help you with that. We can at least define the various rhythms you will see.
Remember to always evaluate the patient first! The other information is just data, the patient comes first!