PVCs: What are they? What causes them?

‘’Student, Can you Press Print Please; I can see a Bunch of PVCs on the Patient’s ECG!’’

..PVCs? What are they? Should I be concerned?..

A PVC (Premature Ventricular Contraction) is exactly what it says on the tin. It is a ventricular contraction, that occurs EARLIER than expected and/or wanted! Remember, a whole variety of ‘pacemaker’ cells within the cardiac conducting system are capable of spontaneously depolarising and initiating an impulse; it is just that NORMALLY, this is the job of the SA (Sino-Atrial) Node to initiate, override and rate-control the conduction cycle. A PVC is a result of a premature beat arising from somewhere in the ventricles of the heart; a ventricular ectopic impulse.

..Ahhh…..but why? Why or how does that happen?..

Usually, impulses that originate from other pacemaker cells in the heart are suppressed by more rapid impulses originating from above them. But… if the intrinsic rate of depolarisation decreases from the top to the bottom of the heart (fastest at the SA Node and slowest within the ventricles), then in theory this should never happen? Well, If an ectopic impulse manages to depolarise EARLY enough PRIOR to the arrival of the next Sinus impulse….that premature ectopic beat may ‘capture’; meaning that the impulse conducts through the ventricles to produce a premature contraction; bypassing the His-Purkinje system and self depolarising DIRECTLY!

..They look a bit wide and strange? Why do they look like that?..

Due to the interventricular conduction delay as a result of a ventricular ectopic focus (and bypassing the His-Purkinje system; self depolarising), a PVC causes a premature asynchronous contraction of the ventricles, producing a QRS complex that is both WIDE (more than 120ms, or three small 0.04s squares (0.12s), from the beginning of the QRS complex to the end) and ABNORMAL in morphology (flippin’ strange lookin’).

When we say that a PVC is premature, it means that it occurs EARLIER than expected, where the next impulse would have been. There is also generally a ‘Compensatory Pause’ following the PVC; meaning the next ‘normal’ impulse arrives after an interval which is double the preceding R-R interval (it’s basically where it ‘should’ be if one beat was skipped!). You may also see some Discordant ST segment and T wave changes (All this means, is if the majority of the QRS was below the isoelectric line, the ST segment may be elevated and the T wave upright – opposites. And also, for example, if the majority of the QRS was above the isoelectric line, the ST segment may be depressed and the T wave inverted – opposites…. Discordance in a nutshell for you right there!). You may also witness ‘Retrograde Capture’, should the impulse ‘backtrack’ into the atria from the ventricles (inverted or late P waves!)

..Woah woah woah…Let’s keep this simple shall we. Just tell me what I’m looking for?..

  ~ A Wide QRS complex that is both Bizarre in Morphology and Earlier than Expected during an ECG trace ~

..Should I be worried? What causes PVCs?..

PVCs can be a normal variant; we can all get a strange ‘palpitation’ feeling every now and then. It’s when they’re more regular that they can be of concern and potentially require further investigation. There are a whole range of reasons as to what causes PVCs; I’ll leave an image I created at the end of this post.

PVCs are graded using the ‘Lown’s Grading of PVCs’ from Grade I – V. The first three grades are generally benign and do not need treatment UNLESS the patient is symptomatic (they MUST be haemodynamically stable). The problem with the prehospital phase of care, is that often we can be with the patient for a very little amount of time; especially if it is an emergency call and the patient requires rapid transportation to definitive care. We do not have the resources to stay with the patient for hours on end, counting the amount of PVCs they have over a long period of time! Besides.. who would want to do that? It must be meal break by now!

What we can do though, is to take note of any PVCs during our phase of care, noting down or printing any traces to be used as clinical evidence in the patient’s continuation of care.

Looking at the below table, ‘frequent’ PVCs in Grade I or II are < 30 or > 30 in one hour. For the prehospital phase of care, we may only be physically monitoring the patient continuously for ~30 minutes (obviously, job dependant), so this could raise some alarm bells if you’re seeing quite a few PVCs within minutes. They need documenting and potentially need investigation within the hospital environment, especially if your patient is symptomatic. The PVCs you are seeing could be the result of a whole range of reasons (see end image for reference).

Lown’s Grading of PVCs

Grade I   – Unifocal and infrequent PVCs (< 30 per hour)

Grade II  – Unifocal and frequent PVCs (> 30 per hour)

Grade III – Multifocal

Grade IV – > 2/3 consecutive beats – Couplets or Triplets (Salvos is another term sometimes used for Triplets)

Grade V   – ‘R on T’ Phenomenon

screen-shot-2016-10-11-at-20-08-42
PVC

Terms Explained

Unifocal -> Impulses originate consistently from a single point within the ventricles, therefore the QRS morphology is identical.

Multifocal -> Impulses originate from more than just a single focus within the ventricles, therefore there are multiple QRS morphologies.

‘R on T’ Phenomenon -> An event of the conduction cycle where a PVC may land on the terminal portion of the T wave, during the ‘relative’ refractory period. This is a time where cardiac cells are approaching full repolarisation and have SOME ability to depolarise following a new stimulus. This can lead to life-threatening ventricular arrhythmia; notably, Ventricular Tachycardia (VT) and Ventricular Fibrillation (VF).

PVC – Clinical Significance

Can be a normal ECG finding that doesn’t usually require investigation or treatment

Frequent or Symptomatic PVCs can be the result from a multitude of causes (see below)

In patients with underlying heart disease or accessory pathways (e.g WPW), PVCs may trigger a tachyarrhythmia

In patients with a prolonged QTc (particularly >500ms), PVCs may trigger life-threatening arrhythmias such as Torsades de Pointes

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