Neurocardiogenic Syncope and Rate Drop Response
Neurocardiogenic Syncope
Understanding neurocardiogenic syncope is helpful for the EP exam, focusing on the causes rather than specific manufacturer solutions.
Medtronic Rate Drop Response (RDR)
The Medtronic RDR is designed to detect rate drops associated with neurocardiogenic syncope (Vasovagal syncope (VVS)) and provide high rate pacing to reduce symptoms. The algorithm works as follows:
Heart Rate Increase: Preceding VVS, there's an increase in heart rate (HR), identified by a programmed "Top" rate.
Heart Rate Drop: The HR rise is followed by a fall, identified by a programmed "Bottom" rate.
Width Beats: The HR must fall to the "Bottom" rate within a programmed number of "Width" beats.
Confirmation Beats: The HR must remain below the "Bottom" rate for a programmed number of "Confirmation" beats to activate the algorithm.
Benditt, et al: Clinical experience with Thera DR Rate-Drop Response pacing algorithm in carotid sinus syndrome and vasovagal syncope. pp. 832 - 839
Tilt Testing
Tilt testing is essential to properly program the RDR, requiring at least one tilt test cycle.
The algorithm's effectiveness depends on:
An initial rise in HR to or above the "Top Rate."
A rapid fall in HR.
A subsequent fall in blood pressure (BP) resulting in symptoms.
The presence of the above conditions (1-3) at all syncopal episodes.
Syncope not occurring until the HR has dropped by 20 to 30 bpm.
Neurocardiogenic Syncope Subtypes (Dr. Richard Sutton, 1993)
Type 3 (Pure Vasovagal Response): Unlikely to benefit from RDR, as syncope occurs before significant bradycardia.
Type 2b (Cardioinhibitory): Most likely to respond to pacing therapy.
IDR almost certainly.
DDI with hysteresis possibly, since BP falls after significant bradycardia.
Type 2a (Cardioinhibitory): HR falls below 40 bpm, but blood pressure falls before HR.
The patient may receive some small benefit for RDR
Full resolution of symptoms is highly unlikely
Type 1 (Mixed Response): Bradycardia occurs after the drop in BP, but HR stays above 40 bpm.
The patient may receive some small benefit from RDR
Most likely will require some form of pharmaceutical therapy
Reduction of symptoms rather than-resolution of symptoms is the goal.
Type 1a (Mixed Response): BP falls simultaneously with or slightly after the HR; profound symptoms occur even if HR remains above 40 bpm. HR fall is often preceded by increased oscillation or respiratory variation. These patients may benefit from RDR.
Ideal Patients for RDR Therapy
Must have an initial heart rate rise with tilting (or before syncopal episodes).
The heart rate must begin to fall before the blood pressure during tilt testing.
The response must be reproducible and runs true on each occasion of tilt testing or spontaneous syncope.
Rapid pacing (AAI/DDI) abolishes or markedly ameliorates the symptoms.
Gammage MD; A useful screen for Rate-Drop Response. pp. 329–831
Complexity of Neurocardiogenic Syncope
Neurocardiogenic syncope is a complex condition precipitated by several factors, including inappropriate slowing of HR due to:
Sudden augmentation of efferent vagal activity.
Arteriolar dilatation by sudden reduction of sympathetic activity.
Van Lieshout, el al: Neural Circulatory Control in vasovagal syncope. pp: 753 - 763
These factors are regulated by arterial baroreflexes (located in the Aortic Arch and Carotid Sinus) and cardiac reflexes. Baroreceptor discharge causes excitation of resting parasympathetic output to the SA node and resting inhibition of sympathetic output to the heart and peripheral circulation.
Increasing HR alone may not reduce symptoms; atropine injection has historically shown no symptom reduction.
Quan KJ, Carlson MD, Thames MD: Mechanisms of heart rate and arterial blood pressure control: Implications for the pathophysiology of neurocardiogenic syncope. pp: 764 -774
References
PACE: Neurocardiogenic Syncope: An International Symposium. March 1997, Volume 20, No. 3, Part II, pages 751-860. Futura.