These are caused by the premature discharge of a ventricular ectopic focus which produces an early and broad QRS complex.

Extrasystoles are usually normal, being common in middle age. The patient may feel an occasional missed beat.

Patients with no ischaemic heart disease or cardiomyopathy have an excellent prognosis.

Ventricular extrasystoles after myocardial infarction are associated with increased mortality.

They may be the presenting feature of viral myocarditis.

Ventricular ectopics are common and usually of no clinical significance (1):

• often arise from specific sites such as the right ventricular outflow tract and can affect people of all ages
  • may be completely asymptomatic and discovered incidentally on a routine ECG or they are experienced as 'missed beats'
  • in structurally normal hearts, they are not dangerous and can be difficult to suppress with medication which is thus best avoided
  • in the presence of significant structural heart disease, frequent ectopy marks an increased risk of sudden cardiac death and specialist advice should be sought (1)
  • rarely they have the potential to induce ventricular fibrillation particularly if they coincide with the T wave of a preceding beat - this is described as the 'R on T phenomenon'
    
    
• ventricular bigeminy
  • this occurs when a ventricular premature beat follows each normal beat
    • click here for an ECG displaying ventricular bigeminy
      
      
• ventricular trigeminy
  • occurs when there is a ventricular premature beat following two normal beats
    • click here for an ECG displaying ventricular trigeminy

Reference:

1. BHF Factfile (March 2005). Ventricular Arrhythmias.