VENTRICULAR PREMATURE COMPLEXES
A premature depolarization that originates in the ventricular myocardium or terminal Purkinje fibers is termed a ventricular premature complex (VPC). The impulse is initiated outside of the specialized conduction system, activating the nearby myocardium via slow, myocyte-to-myocyte transmission. The impulse then engages the His-Purkinje system to depolarize the remaining myocardium. We know that this type of transmission produces a wide QRS complex (≥0.12 seconds) with secondary T wave abnormalities that are directed opposite to that of the dominant R wave (Figure 19-1).
Ventricular premature complexes.
VPCs occur early relative to the cycle length of the normal sinus rhythm. One of the defining characteristics of VPCs is that they arise independently of the sinus mechanism; therefore, they are electrically unrelated to any preceding P wave. The coupling interval refers to the time between the onset of the VPC and the QRS of the preceding complex. Ectopic complexes from one focus tend to have the same coupling interval, whereas those from multiple locations have different coupling intervals (Figure 19-2).
Ventricular premature complexes with two different morphologies and coupling intervals.
In most circumstances, the VPC does not conduct retrograde to depolarize and reset the SA node, so the sinus mechanism continues without interruption. You usually can’t see the P wave because it is hidden in the ventricular complex, so we depict the missing P wave on our laddergram as a “virtual” or “hidden” depolarization. The VPC renders the ventricles (and likely the AV junction) refractory, so the sinus depolarization cannot generate a QRS complex. After the VPC, the next P wave and related QRS complex appear as expected. This fully compensatory pause between the VPC and the normal sinus complex is another of the characteristic features of ventricular ectopy (Figure 19-3). Remember that you can check for a fully compensatory pause by using your calipers to confirm that the R-R (or P-P) interval containing the VPC is exactly twice that of the normal cycle length. If so, the post-ectopic pause is fully compensatory.
A fully compensatory pause is one where the sum of the R-R intervals before and after the premature complex equals twice that of the native sinus cycle.
On occasion, the ventricular depolarization can “reverse course” through the specialized conduction system to depolarize the atria. If so, a retrograde inverted P wave (P´) may be seen in leads II, III, and aVF. Alternatively, the P´ wave may be hidden within the ST segment or T wave of the VPC. If the retrograde impulse travels through the atria to depolarize the SA node, the ...