Thursday, June 16, 2011

ECG Interpretation Review #23 (Wide Tachycardia - WCT - VT vs SVT)

Is the rhythm below in Figure 1 sinus tachycardia? It was obtained from a hemodynamically stable man (BP=120/80 mmHg) with new-onset palpitations.

Figure 1 – Lead II rhythm strip from a patient with palpitations. BP = 120/80. Is this sinus tachycardia?

INTERPRETATION:  The lead II rhythm strip shows a regular tachycardia at about 150/minute. Although the red arrow suggests sinus tachycardia – We are uncertain about QRS width, and therefore do not know how to proceed …
  • The “good news” – is that the patient is stable (BP=120/80). This means that there is time to confirm the diagnosis before beginning treatment.
  • The “less good” news – is that we do not yet know what the rhythm is …
The first steps (given that this patient is hemodynamically stable) – are to: i) obtain more history (History of underlying heart disease? of tachyarrhythmias such as documented SVT or VT? ) – and ii) obtain a 12-lead ECG during the tachycardia (being ready to cardiovert the patient if he at any time decompensates).

The 12-lead ECG during tachycardia is shown below (Figure 2):

Figure 2 – 12-lead ECG during tachycardia. Does this provide the answer?

The 12-lead ECG obtained during tachycardia clearly reveals that the QRS complex is wide!
  • The small upright deflection in lead II (red arrow) is not a P wave after all.  Instead – the vertical red time lines show this deflection to be the initial part of the QRS complex! 
  • All other leads show the QRS to be very wide (>0.16 second).
  • The rate is about 150/minute.  The rhythm is regular. No P waves. 
IMPRESSION: This is a regular WCT (Wide Complex Tachycardia) with no sign of atrial activity. As emphasized in our KEY List below (Table 1) – always presume VT until proven otherwise! Statistically – the odds that a regular WCT without definite atrial activity is VT are at least 80%.  This goes up to over 90% if the patient also has underlying heart disease.

Table 1 – Common Causes of a Regular WCT without P waves.  

KEY Points in the Assessment of this Regular WCT:
  • VT  is by far the most common cause of a WCT when there is either no atrial activity or uncertain atrial activity. Although possible that a WCT may be due to supraventricular tachycardia with either preexisting bundle branch block or aberrant conduction – Always presume VT until proven otherwise. Treat accordingly.
  • Not all patients in VT immediately decompensate. Some may remain hemodynamically stable despite ongoing VT for hours, or even days …
  • Obtaining  a 12-lead tracing during tachycardia may be invaluable for honing in on the etiology of a tachycardia.  In this case, there are a number of clues that overwhelmingly suggest VT as the diagnosis.  These include: i) marked QRS widening in all other leads; ii) bizarre QRS axis during the tachycardia (extreme left axis deviation); iii) bizarre QRS morphology (not resembling any form of organized bundle branch block); iv) markedly negative QRS complex in left-sided lead V6 (which virtually always has more than a tiny r wave of positive activity); v) delayed R-to-S-nadir (VT very likely when there is an RS complex in any precordial lead with an interval from onset of the R until deepest point of the S >0.10 sec. - as it is in V5 and V6); and vi) concordance of the QRS in precordial leads (all precordial lead QRS complexes are predominantly negative). BOTTOM Line: “12 leads are better than one”and - a WCT without definite P waves should always be presumed to be VT until proven otherwise (regardless of whether or not the patient is hemodynamically stable).
- See Sections 08.0, 09.0 in our ACLS-2013-ePub -

Saturday, June 4, 2011

ECG Interpretation Review #22 (AV Block - What Type?)

The lead II rhythm strip below was obtained from a patient on telemetry. Is there AV block?  If so - what type?
Figure 1 - What type of AV block do you suspect? (Reproduced from ACLS-2013-ePub).
- Note - Enlarge by clicking on Figures - Right-Click to open in a separate window.
INTERPRETATION: The rhythm is irregular. The QRS complex is narrow, indicating a supraventricular mechanism. P waves of similar morphology are present - but they do not all seem to be conducting. This suggests that some form of AV block may be present. The most striking finding on this tracing is the presence of group beating (alternating short-long cycles). Recognition of "group beating" should make one suspect that a type of Wenckebach conduction may be present.  Looking closer:
  • As noted - P waves of similar morphology are seen in this lead II rhythm strip.  However, the P-P interval varies.  This suggests that the underlying rhythm is sinus arrhythmia.
  • Beats are being dropped (ie, one would definitely expect the P waves following beats #2 and #4 to conduct). Complete (3rd degree) AV block is unlikely because the ventricular response is so variable). Thus, this tracing most probably represents a type of 2nd degree AV block (See ECG Blog Review #20). The narrow QRS and the presence of group beating suggest Mobitz I as the prime suspect.
  • The KEY lies with analysis of beats #1, #3, and #5.  On close inspection, it can be seen that QRS morphology of each of these beats differs slightly from that of beats #2, #4, and #6 (the latter 3 beats having a shorter R wave and slightly deeper S wave). The PR interval preceding beat #5 is definitely too short to conduct. This implies that this beat (and probably also the other two beats like it [beats #1 and #3]) are junctional escape beats. Supporting this contention is the fact that the R-R interval of the two escape beats that are seen on this tracing (the R-R interval between beats #2-3 and #4-5) is the same, and corresponds to a junctional escape rate of 38 beats/minute. Subsequent rhythm strips on this patient confirmed our suspicion that the arrhythmia was indeed 2nd degree AV block, Mobitz type I (AV Wenckebach).
COMMENT: This tracing is an excellent example of how in addition to group beating, recognition of subtle differences in QRS morphology between junctional and sinus conducted beats may also provide a major clue to diagnosis.
  • FINAL POINT: It is important to appreciate that the emergence of escape beats in this tracing is appropriate - and that without them the ventricular response would be even slower.  However, their presence makes definitive diagnosis of the conduction disturbance a difficult task from this tracing alone.

  - See also ECG Blogs #19, 20, 21 - and Section 20.0 in our ACLS-2013-ePub -