In any case where there is uncertainty always assume that the
rhytm is v tach. One reason is that sustained v tach may be stable
for a while , but tends to deteriorate into ventricular
fibrillation after a period of time. Unfortunately that period of
time is often very short , making the differentiation more
difficult because of time constraints. The other reason to assume v
tach is that treatment as SVT especially with calcium channel
blockers may be lethal if the rhythm is in fact v tach V tach can
be categorized in 3 ways : 1 . Sustained or non sustained (over 30
seconds is sustained) 2 . Monomorphic or polmorphic 3 . Pulseless v
tach or v tach with pulses With pulseless v tach always think ACLS
course of the American Heart Association . The treatment is exactly
the same as for pulseless v fib For patients with a pulse and wide
complex tachycardia the differentiation needs to be attempted. This
can be easy , difficult or sometimes even impossible from the ECG
tracing alone . To reiterate when in doubt think v tach . One of
the many usefel guides to differentiating the two rhythms is the
clinical context . A patient with any history of MI and also a
patient with no history of previous tachyarrythmia points you to v
tach .The young patient with a history of multiple episodes of
tachyarrythmia which resulted in simple treatment and release from
emergency rooms in the past likely does have SVT again. The full
story on ECG differentiation of v tach from supraventricular
tachycardia with "aberrent conduction" or in simple English : SVT
with coexisting bundle branch block , is beyond the scope of this
answer. But there are a number of points that are myths , real old
myths , canards that just fail to die . One myth is that the
patient with v tach will tolerate the arrythmia better than the one
with SVT ; FALSE! These patients can be stable for a while after
initally seen , but deteriorate into vfib at some later time.The
SVT patient can tolerate the rhytm of v tach just as poorly as the
SVT patient! The ability to tolerate a tachyarrythmia depends on
the rate, the size of the heart , and the severity of the
underlying condition causing the arrythmia ; not whether it is
ventricular or supraventricular. The ECG provides many helpful
clues to differentiate the two rhythms . For example, and this is
one example only , concordant complexes are almost always v tach .
Concordant meaning all the chest leads from V1-V6 show
predominantly upright complexes , or else all show predominantly
negative complexes But for the nonexpert clinician always think v
tach unless certain it is SVT . And hold off on the calcium
blockers! Calcium blockers are useful in treating SVT but can be
lethal in v tach ,as previously stated. If you really can't tell ,
either treat as v tach , or if at all possible get an expert
opinion . Not always possible before treating as v tach , because
if the patient is unstable time may not permit this , and if the
patient is pulseless time will definitely not permit this. . But in
the stable patient , in whom calcium blocker treatment is being
considered for presumed SVT hold off- get an expert opion first if
in any doubt whatsoever.
