Question 11

A 71 year old woman is experiencing palpitations.

R_0027A

R_0027_A is a continuous rhythm strip taken soon after arrival

+ Describe the ECG changes.


Figure 1

Figure 1 shows two rhythm strips that are continuous. The first four complexes in the upper strip are sinus beats with the following features:

  • P waves are upright
  • The R-P intervals are 400-440 msec, and the PR interval is constant at 160 msec
  • The QRS complexes are upright with a width of 120 msec
  • There is a sagging ST depression with a amplitude of 3 - 4 mm
  • The T waves are reduced to a small nubbin
  • The heart rate is about 83 beats per minute

The fifth and sixth complexes in the upper strip are atrial ectopics (P waves are not seen, the QRS morphology is unchanged, the R-R interval between the fourth and fifth complexes is 480 msec and is 360 msec beteween the fifth and sixth complexes, compared to the preceding R-R interval of 680 - 720 msec).

These atrial ectopics are followed by the development of a tachycardia with the following features (see bottom strip):

  • QRS morphology is the same as that seen in sinus rhythm
  • The R-R interval is regular at 480 msec and the ventricular rate is about 125 beats per minute
  • P waves (marked by a purple asterisk) are seen in the middle of the R-R intervals; the R-P interval is 160 - 180 msec and the PR interval is 140-160 msec

Diagnosis: Supraventricular tachycardia (SVT) initiated by atrial ectopic beats, with retrograde P waves visible in the middle of the R-R intervals.

In most cases of SVT the P waves are either not seen or are visible just after the QRS complex (RP interval less than the PR interval). Less commonly P waves are seen in the middle of the RR interval or close to the start of the QRS complex (RP interval equal to or greater than the PR interval). The underlying mechanism for these differences is shown in Figure 2.


Figure 2

Figure 2 shows the two main types of re-entry that cause SVT:

  • Re-entry involving the AVN and adjacent "fast" and "slow" pathways - AVN Re-entry Tachycardia or AVNRT (A and B in Figure 2 )
  • Re-entry involving the AVN and an accessory pathway - Atrio-ventricular Re-entrant Tachycardia or AVRT (C in Figure 2)

The most common form of AVNRT involves a slow pathway AVN fast pathway circuit (A in Figure 2 ). Retrograde conduction of the re-entry circuit into the atria produces atrial activation that either coincides with the QRS complex or inscribes a P wave just after the QRS complex.

A less common form of AVNRT involves a fast pathway AVN slow pathway circuit (B in Figure 2), producing visible P waves in the middle of the RR interval or close to the start of the QRS complex.


Figure 3 - Laddergram of the upper strip in R0027A. A green asterisk marks a atrial ectopic beat. F: Fast pathway; S: Slow pathway

R_0027B

 

R_0027_B is the  initial 12 lead ECG.

+ Describe the main changes.

The main findings are:

  • Regular broad complex tachycardia with a ventricular rate of about 136 beats per minute
  • The QRS complexes have a typical left bundle branch block morphology
  • The ST segment is elevated in Lead aVR (about 3 mm) and in Leads V1 - V3 (about 5 mm). ST elevation in Lead aVR is a (usually) reversible finding in SVT's; it is not a sign of impending occlusion of the left main coronary artery. While there is ST elevation in the V leads, there are no other positive Sgarbossa criteria to suggest acute myocardial infarction in this setting of left bundle branch block
  • Biphasic P waves are seen before each QRS complex, with a PR interval of about 120 msec. The P waves are more visible than those on the rhythm strip that preceded this ECG

Final diagnosis:

  1. AVNRT with a fast pathway-AVN-slow pathway circuit that is associated with visible (retrograde) P waves with the RP interval greater than the PR interval
  2. Left bundle branch block with ST elevation in Lead aVR and Leads V1 to V3.

R_0027C

R_0027_C shows two consecutive rhythm strips taken during and soon after carotid sinus massage (CSM).

+ Describe the main findings.

Upper strip: Carotid sinus massage (CSM) during the supraventricular tachycardia increases AVN block and interrupts the re-entry circuit. The tachycardia ceases, there is a pause of 1320 msec before sinus rhythm resumes at a rate of about 68 beats per minute.

Lower strip: Sinus rhythm is initially present at a rate of about 88 beats per minute. The 10th complex is an atrial ectopic beat that initiates a supraventricular tachycardia with a ventricular rate of about 125 beats per minute.

A retrograde P wave can just be seen distorting the upstroke of the (inverted) ST segment near the end of the strip.

R_0027D

R_0027_D shows the ECG after treatment.

+ Describe the main changes

Sinus rhythm with a ventricular rate of about 79 beats per minute Left bundle branch block with ST elevation in Leads aVR and in Leads V1 to V3. There is ST depression in the anterolateral leads

Important Points:

  • Concept of AVN re-entry tachycardia that involves the "fast - slow" pathways and produces a visible (retrograde) P wave with a RP interval greater than the PR interval
  • Two episodes of AVNRT are seen to be initiated by atrial ectopic beats, and the tachycardia is interrupted by carotid sinus massage
  • "Wide complex tachycardia" due to AVNRT in a person with a pre-existing left bundle branch block.