VT or not VT?

A fast heart rate with a wide QRS complex (greater than 120 ms) can be scary. Here’s the crucial question to consider when you encounter wide complex tachycardia (WCT):   Is it VT or not VT?  The distinction is clinically important.

If it's not VT it is usually a supraventricular mechanism such as atrial fibrillation, atrial flutter or SVT with aberrant conduction (bundle branches not working properly at tachycardic rates). So it's mainly a binary differential diagnosis between VT and supraventricular wide complex tachycardia (SVWCT). However, a couple of very infrequent exceptions should be noted for which the diagnostic rules to be described herein may not apply:  Some WCTs in Wolff Parkinson White syndrome and suraventricular mechanisms with severe metabolic disturbances, eg TCA overdose. Otherwise for most purposes, in the binary differential diagnosis between VT and SVWCT certain rules are helpful. Through the years many proposed rules and algorithms have been put forth. This has led to some confusion. However, finally they have all been compiled in one place in the review linked below: 

Wide Complex Tachycardia Differentiation: A Reappraisal of the State‐of‐the‐Art

Here is a graphic from the review that highlights some of the key findings, any of which, if present, favor VT. Most of these findings have high specificity but low sensitivity. 

Figure 2 from the review summarizes the various algorithms and other approaches:

Some key points to be gleaned from figures one and two are summarized here:

AV dissociation

This finding has high specificity but low sensitivity. This is when P waves can be seen independently of the QRS complexes. If this is seen it is strong evidence for VT. There are some caveats. First, many cases of VT, (in some series up to half ) exhibit retrograde conduction to the atria (in other words, AV association ). Whan AV dissociation is present, particularly at faster VT rates, it can be difficult to see because of interference from the wide QRS complexes and the ST segments and T waves. 

Morphologic criteria

Does the tachycardia have a typical bundle branch block appearance? If not, it is more likely VT. This test is difficult to apply unless one knows the ins and outs of bundle branch block morphology. (The details of BBB morphology are covered in the archived lecture series).

QRS duration

If the QRS is a greater than 160 milliseconds it favors VT. (If the QRS in V1 is upright, greater than 140 milliseconds favors VT). In general, the wider the QRS the more it favors VT. 

Chest lead concordance 

In the chest leads, (V1-6) if the QRS complexes are all upright (monolithic R waves) or are all pointing down (QS complexes) VT is the diagnosis.

Frontal plane (limb lead) axis

Right upper quadrant axis ( -90  to -180) strongly favors VT. Left upper quadrant and left lower quadrant axis (normal ) less so.

Ventricular activation velocity

If the rate of voltage change of the first 40 milliseconds is less than or equal to that of the last 40 milliseconds of the QRS complex, VT is favored. In addition, if the time from the beginning of an R wave to the nadir of an S wave greater than 60 milliseconds it favors VT. (In the Brugada algorithm that cut off is 100 milliseconds). 

Algorithms

Several algorithms have been proposed and have good test characteristics. In the paper the Brugada algorithm, the Vereckei algorithm, the limb lead algorithm, the Griffith algorithm, and the RWPT algorithm are described.

There are also point scoring systems that perform well. These are discussed in the paper. Another, someone novel method, is the Bayesian approach which will be discussed in a separate post.