This section will show a series of problems where the emphasis is on description, analysis and diagnosis of ECG rhythms. Treatment (and mechanism) are only relevant if they help with the diagnosis or description.
Rhythm Case R_0004: More than meets the eye
The initial rhythm strips (R_0004_A) taken at 1100 from a 80 year old woman with dizziness and near syncope
Describe the ECG changes and the rhythm(s) present in R_0004_A.
The rhythm strips (R_0004_B) of the patient taken 30 minutes after the initial tracing are shown below.
Describe the ECG changes and the rhythm(s) present on the tracings.
A quick look at the first rhythm (R_0004_A) shows two P waves before each ventricular complex; this suggests second degree 2:1 AV block. Thirty minutes later the patient develops ventricular tachycardia (R_0004_B). After cardioversion and drug treatment have stabilized the patient we review the rhythms and our initial diagnosis, and ask ourselves “Is that all there is?”
Analysis of R_0004_A
The ventricular complexes have a Rs morphology, with a slight variation in their shape (e.g. compare the second Rs complex with the fourth Rs complex in the upper strip). The duration of the Rs complexes is within normal limits (0.08 - 0.10 seconds). There is a slight irregularity in the ventricular rhythm (the R-R interval ranges from 1160-1360 milliseconds) and the ventricular rate is about 60 beats per minute. There is no group beating of the ventricular complexes (which would suggest Wenckebach block).
When we look at the P waves note that:
- There are two P waves before each ventricular complex
- The conducted P waves (1,3,5,7 etc) have a slightly variable PR interval (the range is between 0.12 - 0.20 seconds, but the most common interval is 0.16 seconds) and have slightly different shapes (compare P3, P7, P15 and P17 with P11 and P19). The interval between the conducted P waves is variable e.g 1280 milliseconds between P1 and P3, 1160 milliseconds between P5 and P7, 1320 milliseconds between P15 and P17.
- The non conducted P waves (2, 4, 6, 8 etc) vary slightly in their shape (compare P4 and P8) and their location relative to the T wave: at the top of the T wave (P12, P18), blending with the downstroke of the T wave (P2, P14, P24), and separating from the T wave (P4, P6, P8, P10, P16, P20, P22).
It is unlikely that the non conducted P wave in second degree 2:1 AV block would display such variability, so the most likely explanation is non conducted atrial bigeminy.
The different shapes of the P waves (conducted and non conducted) suggests that the P waves have different origins within the atria i.e. there is a wandering atrial pacemaker (WAP).
Other examples of non conducted atrial bigeminy and their outcome are shown in the following rhythm strips.
Geek Alert: Compare the P-P intervals of the P waves that encompass each Rs complex (these intervals are coloured green) with the P-P intervals that are not separated by a Rs complex in R_0004_A. In five cases the P-P intervals that encompass a Rs complex are shorter than the (non Rs) P-P interval that immediately precede and follow them.
Ventriculophasic sinus arrhythmia (VPSA) is a phenomenon in which phasic variations in sinus rate occur in relation to the QRS complex. Typically, the P-P intervals encompassing a QRS complex are shorter than those without an intervening QRS complex. The previous examples have shown regular VPSA in non conducted atrial bigeminy and in second degree 2:1 AV block. Intermittent VPSA is present in R_0004_A
Analysis of R_0004_B
We begin by marking the start of visible P waves with a red arrow. The second strip contains the most P waves, so these have been numbered and the P-P intervals measured. There are two groups of P waves: those with a short P-P interval (of 600-640 milliseconds) and those with a longer P-P interval (of 1380-1440 milliseconds). The midpoint of the two longer P-P intervals (marked by a green arrow) is about where a P wave would be expected to appear, and these points coincide with a Rs complex (which would thus obscure the P wave).
The Rs complexes are indicated by an asterisk, and the relationship between the P waves and the Rs complexes in the second strip indicates AV dissociation. There are slight differences in the Rs complexes compared to those seen in R_0004_A: the complexes in R_0004_B are wider (0.12 seconds) and have a somewhat different shape (a notch on the downstroke of the R wave). In the second strip the ventricular rhythm is slightly irregular, and the ventricular rate is about 60 beats per minute. The rate favours a junctional escape rhythm.
Is there any link between the presence of non conducted atrial bigeminy in the initial strip and the presence of AV dissociation in the current strips? In his book Rhythm Quizlets Self Assessment (Lea and Febiger 1987) the late ECG expert Henry JL Marriott included an example of a non conducted atrial premature beat that was followed by AV block and ventricular asystole that lasted several seconds and then resolved.
Marriott’s example suggests that the non-conducted atrial bigeminy in this case might have directly affected conduction through the AVN and caused AV dissociation, rather than the two processes being caused by a single disease process involving the AVN.
We can now summarize the rhythms in R_0004_B as follows:
- AV dissociation is present, with a junctional escape rhythm (best seen in the second strip)
- A single ventricular ectopics (D) is seen close to the T wave of a junctional beat at the start of the third strip
- Two R on T ventricular ectopics (A and B) initiate two episodes of brief (600 milliseconds) and self limited polymorphic ventricular tachycardia
- A third R on T ventricular ectopic (E) triggers polymorphic ventricular tachycardia that continues to the end of the rhythm strip.
Rhythm Case R_0005
A broad complex tachycardia is present, with a regular rhythm and a rate of about 214 beats per minute. P waves are not clearly visible, although an occasional small hump (red asterick) might be due to a P wave. The complexes have a sine wave pattern, although small notches (blue arrows) are seen on the downstroke of some of the complexes. Subtle electrical alternans is present (best seen in the lower strip). Capture beats and fusion beats are not seen.
The following terms could be applied to the above rhythm: a broad complex tachycardia (BCT), monomorphic ventricular tachycardia or ventricular flutter