ECG Blog #61 — What is Vagotonic Block?

The series of rhythm strips shown in Figures 1,2,3, and 4 were obtained from a young adult recovering from recent surgery.

• How would you interpret these tracings?
• Is the conduction disturbance due to 2nd degree AV block Mobitz Type I (AV Wenckebach)? – or to Mobitz II? – or to complete (3rd degree) AV block?
• Is this patient likely to need a pacemaker?

Figure 1 – First in a series of Holter tracings obtained from a young adult with recent surgery (See text).

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Figure 2 – Second in a series of 4 tracings (See text).

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Figure 3 – Third in a series of 4 tracings (See text).

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Figure 4 – Fourth in a series of 4 tracings (See text).

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INTERPRETATION: Clinical assessment of what we are seeing over this series of 4 tracings is best accomplished by first noting all findings, and then putting them together into a single cohesive explanation.

• Figure 1 – The QRS complex is narrow. The R-R interval is the same between beats #2-3 and #3-4 – but it varies for other beats on this tracing. This suggests that beats #3 and #4 (which each end a longer R-R interval cycle) may be junctional escape beats. Supporting this supposition is at least transient AV dissociation (there seems to be no relation between P waves and the QRS in the middle of the tracing) – and – slightly different QRS morphology for beats #3 and #4 compared to the other beats on the tracing (the S wave seems deeper for both beats #3 and 4). The P-P interval varies greatly throughout the tracing (See arrows in Figure 5) – much more so than one would normally expect for simple sinus arrhythmia. Finally – Beats #1 and #5 may be conducting, albeit with a long PR interval (the PR interval appears to be the same for beats #1 and #5). If this were true (that beats #1 and #5 are conducting) – then there would be a Wenckebach cycle with progressive lengthening of the PR interval preceding beat #2 compared to #1 – with non-conduction of the P wave that notches the T wave of beat #2. That said - interpretation of Figure 1 is complex and clearly atypical from the usual appearance of 2nd degree AV block, Mobitz Type I .

Figure 5 – Red arrows highlight the marked P-P irregularity seen in Figure 1. Variation in the P-P interval is clearly more marked than is normally seen with simple sinus arrhythmia.

• Figure 2 – Interpretation of this second tracing is even more complex than was the rhythm strip in Figure 1. The QRS complex is narrow, with slight variation in QRS morphology that is difficult to characterize (the QRS is seen to vary more in lead V1 than in lead II). The P-P interval is even more irregular than it was in Figure 1. It is difficult to determine IF the reason P wave morphology in lead V1 is changing is due to shift in the site of the atrial pacemaker (?) – vs acceptable variation in P wave morphology as a result of baseline artifact. It is equally difficult to determine if beats #2, 3 and 4 make up a Wenckebach cycle (with progressive PR interval lengthening until a beat is dropped) vs some other random occurrence accounting for the change in P wave and QRS morphology with marked variation in P-P interval. More questions are raised by Figure 2 than are answered .

• Figure 3 – We know beat #2 is not conducting (the PR interval preceding beat #2 is clearly too short for conduction). We wonder if beats #3 and #5 are sinus conducted with 1st degree AV block (both have similar albeit prolonged PR intervals). Although at first glance one might think beat #1 is conducted in the same manner – measurement (with calipers) of the PR interval preceding beat #1 reveals slight-but-real lengthening compared to the PR interval preceding beats #3 and 5. The cause for this is uncertain from assessment of Figure 3 alone.

• Figure 4 – More of the same variation is seen in this fourth and final tracing. We suspect beats #2 and 4 are conducting (they have similar PR intervals to each other, and to other beats we thought were conducting on previous tracings). That said – we are once again at a loss to explain the variation in QRS morphology and marked change in both P-P and R-R intervals.

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IMPRESSION: The usual rules for rhythm interpretation and assessment of AV blocks and AV dissociation are not closely followed here. Instead – We see dramatic variation in P-P interval and elements resembling Mobitz Type I AV block, higher-grade 2nd degree AV block, and AV dissociation. That said – there IS one phenomenon that readily explains ALL findings seen in this series of 4 tracings – which is Vagotonic AV Block.

• The literature is scant regarding vagotonic AV block. This unusual conduction disturbance occurs far less often than other forms of AV block – which may in part account for the difficulty in recognizing it.

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VAGOTONIC AV BLOCK:

Clinical characteristics of vagotonic AV block include the following:

• Presence of a condition consistent with increased vagal tone (the patient in this case is in the early post-operative period). Other situations in which enhanced vagal tone may be seen include persistent vomiting; performance of a medical procedure; athletic training. Vagotonic AV block may also be seen in healthy adults during normal sleep (and especially in individuals with sleep apnea).
• Sinus arrhythmia which may be marked – and which especially slows just prior to and during periods of AV block.
• Mixed forms of AV block including Mobitz I 2nd degree AV block – 2nd degree AV block with 2:1 AV conduction — Mobitz II — and periods of high-grade or even complete AV block.
• Frequent switching back-and-forth between two or more of the above forms of AV block, often punctuated by periods of P-P irregularity, sinus rate slowing, and sometimes unexpected change in the PR interval.

KEY Point: Fortunately — the clinical course of vagotonic AV block in otherwise healthy adults without underlying heart disease is most often benign. It does not generally progress to significant AV conduction system disease — and pacing is rarely needed. That said — more severe, symptomatic cases not responding to simple measures should be referred.

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SUMMARY: It is well to be aware of the phenomenon of Vagotonic AV block. This conduction disturbance may be transiently seen during Holter monitoring of otherwise healthy individuals during sleep or in a situation of increased vagal tone (ie, persistent vomiting).  It should be thought of in such situations when serial tracings reveal marked and unpredictable P-P interval variation with mixed and atypical AV block conduction disturbances (such as those illustrated in Figures 1-thru-4). The course is usually benign, such that pacing is rarely needed – but persistent symptomatic cases may merit referral.

• A related arrhythmia to be aware of is Vagotonic AFib! You may occasionally see a younger-to-middle-aged adult (occurs most often in males) — who through participation in endurance sports or activities, develops AFib at a relatively slower rate. Vagotonic AFib most often occurs at night; at rest; after eating; or following an episode of coughing or vomiting (all potentiating causes of vagal tone).  
• CLICK HERE — for My Comment in the November 5, 2019 post on Dr. Smith's ECG blog, in which we discuss a case of Vagotonic AFib in detail.

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• ACKNOWLEDGMENT: My appreciation goes to Dr. Faisal Qadir (from Karachi, Pakistan) – for allowing me to use this ECG and this clinical case.

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- For more information - GO TO:

1) See Section 20.0 (from ACLS-2013-ePub) on the Basics of AV Block. The part on Vagotonic Block begins in Section 20.30. 

2) Zysko D, Gajek J, Kozluk E, Mazurek W: ECG Characteristics of AV Block Induced by Tilt Testing. Europace 11:225-230, 2009.

3) Brignole M: Different ECG Manifestations of the Cardioinhibitory Vasovagal Reflex. Europace 11:144-146, 2009.
4) Korantzopoulos et al: Markedly Prolonged AV Block with Ventricular Asystole During Sleep. S Med J 102:872-873, 2009. You have to READ this 2 page Letter (beginning on page 872) to believe it — a 25 second period of vasovagally-induced ventricular asystole during sleep!

5) Jaiswal S, Aldave APN, Wool KJ: Ventricular Standstill (VS): An Uncommon Electrophysiological Abnormality Caused by Profound Vagal Tone. N Am J Med Sci 6(4):178-180, 2014. (Prolonged asymptomatic periods of VS!).