Syncope and the EP Evaluation
When a patient loses consciousness, the electrophysiologist's first question is whether the heart caused it. The answer determines everything that follows.
Syncope is a transient loss of consciousness caused by global cerebral hypoperfusion. The brain needs continuous perfusion; even a few seconds of interrupted flow can drop consciousness. The patient goes down, cerebral perfusion restores (usually because they are now horizontal), and they wake up.
For the electrophysiologist, every syncope evaluation starts with one question: is this cardiac? Cardiac syncope carries a mortality risk. Reflex syncope, the most common kind, is frightening but almost never fatal. The entire diagnostic workup exists to separate these two categories as quickly and reliably as possible.
This chapter follows the syncope evaluation from mechanism to diagnostic test. We start with the reflex pathways, move through cardiac causes, and end with the two tools the EP lab brings to the problem: the tilt-table test and the electrophysiology study.
Reflex (Neurocardiogenic) Syncope
Reflex syncope is the most common cause of transient loss of consciousness across all age groups. The classic form, vasovagal syncope, accounts for the majority of cases in young, otherwise healthy patients.
The mechanism is a paradox. The patient is upright. Blood pools in the lower extremities, reducing venous return. The left ventricle, now relatively underfilled, contracts vigorously around a small volume. This vigorous squeeze stimulates ventricular mechanoreceptors (C-fibers), which send a powerful afferent signal to the brainstem via the vagus nerve. The brainstem interprets this signal as hypertension and responds with a sudden surge in parasympathetic tone and a withdrawal of sympathetic drive. Heart rate drops. Peripheral resistance collapses. Blood pressure falls. The brain loses perfusion. This reflex arc is called the Bezold-Jarisch reflex.
The paradox: the body is hypotensive and underfilled, yet the reflex treats the situation as if blood pressure were too high. The result is a hemodynamic crash in a patient who was already on the edge.
The dominant response is bradycardia. Heart rate drops precipitously, sometimes to asystole lasting several seconds. The blood pressure fall is secondary to the rate drop.
The dominant response is hypotension from loss of peripheral vascular resistance. Heart rate may not change significantly. The patient drops their blood pressure without a dramatic bradycardia.
Both components occur together: bradycardia and hypotension simultaneously. This is the most common pattern seen on tilt-table testing. The hemodynamic collapse is often abrupt.
Young, healthy patients are particularly susceptible because their vigorous left ventricular contraction produces a stronger mechanoreceptor stimulus. A long period of standing, dehydration, warm environments, and emotional stress all amplify the trigger by reducing preload further.
Carotid Sinus Hypersensitivity
The carotid sinus sits at the bifurcation of the common carotid artery, packed with baroreceptors that monitor arterial stretch. In a normal reflex arc, increased arterial pressure stretches these receptors, signaling the brainstem to reduce heart rate and blood pressure. The system works as a negative feedback loop, preventing dangerous spikes in pressure.
In carotid sinus hypersensitivity, this reflex is exaggerated. Minimal mechanical stimulation of the carotid sinus (turning the head, shaving the neck, a tight collar) triggers a disproportionate vagal response. The threshold for activation is pathologically low.
Carotid sinus massage produces a ventricular pause >3 seconds. The vagal surge suppresses sinus node firing and AV conduction simultaneously. This subtype responds to pacing.
Carotid sinus massage produces a systolic blood pressure drop ≥50 mmHg without significant bradycardia. The mechanism is sympathetic withdrawal causing peripheral vasodilation. Pacing alone does not prevent syncope in this subtype.
Carotid sinus hypersensitivity is predominantly a condition of older men. The diagnosis requires reproducing the patient's typical symptoms during carotid sinus massage, performed with the patient upright whenever possible. A positive haemodynamic response alone, without symptom reproduction, does not confirm the diagnosis.
Cardiac Syncope from Arrhythmia
Cardiac syncope is the category that matters most, because it carries real mortality risk. The mechanism is straightforward: the arrhythmia reduces cardiac output below the threshold needed for cerebral perfusion. The causes divide cleanly into two groups.
Too Slow
Sinus node dysfunction: Prolonged sinus pauses or sinus arrest. The sinus node stops firing, no escape rhythm emerges promptly, and the patient loses consciousness during the gap.
AV block: High-degree or complete AV block. The atrial impulse fails to reach the ventricles. If the ventricular escape rhythm is slow (<30 bpm) or delayed in onset, cerebral perfusion drops and syncope follows. Infranodal block is more dangerous because ventricular escape rhythms are unreliable.
Too Fast
The specific rhythms below are each explored in their own chapters across Volumes V through X. What matters here is their shared ability to drop cardiac output fast enough to cause syncope.
Ventricular tachycardia: The most dangerous cause. Fast VT (rates >200 bpm) reduces diastolic filling time so severely that cardiac output crashes. Even slower VT (150-180 bpm) can cause syncope in patients with poor LV function, because the already weakened ventricle cannot compensate for the reduced filling.
SVT with hemodynamic compromise: Rapid SVT (AVNRT, AVRT, atrial flutter with 1:1 conduction) can cause syncope, especially at onset before compensatory reflexes engage. Pre-excited atrial fibrillation conducting rapidly over an accessory pathway is a life-threatening variant.
Syncope in a patient with structural heart disease (reduced ejection fraction, prior myocardial infarction, hypertrophic cardiomyopathy, arrhythmogenic cardiomyopathy) must be presumed cardiac until proven otherwise. Structural disease provides the substrate for VT. In this population, syncope may be the only warning before sudden cardiac death. The threshold for invasive evaluation should be low.
Two variables determine whether a tachycardia causes syncope: rate and duration. A brief run of VT at 180 bpm lasting three seconds may cause presyncope. Sustained VT at the same rate for thirty seconds will likely produce loss of consciousness. VT at 250 bpm may cause syncope within a few beats. The interaction between rate and ventricular function is what determines hemodynamic tolerance; a normal LV may tolerate rates that a failing ventricle cannot.
Tilt-Table Testing
The tilt-table test is a provocation study designed to reproduce the hemodynamic conditions that trigger reflex syncope. It stresses the autonomic nervous system under controlled conditions, forcing the heart and vasculature to respond to sustained orthostatic challenge.
1. Supine baseline (5-20 min): The patient rests flat. Continuous ECG monitoring, beat-to-beat blood pressure recording, and symptom assessment establish baseline hemodynamics.
2. Head-up tilt (70 degrees, 20-45 min): The table tilts to 60-70 degrees. This pools 500-800 mL of blood in the lower extremities. The patient remains strapped to the table with continuous monitoring. We watch for progressive heart rate and blood pressure changes.
3. Isoproterenol provocation (optional): If the passive tilt phase is negative, low-dose isoproterenol (1-3 μg/min) is infused while the patient remains tilted. Isoproterenol increases myocardial contractility and heart rate, amplifying the ventricular mechanoreceptor stimulus that drives the Bezold-Jarisch reflex. This increases the sensitivity of the test at some cost to specificity.
What We See
A positive tilt-table test requires both a hemodynamic response and reproduction of the patient's clinical symptoms.
What to Keep in Mind
The tilt-table test is most useful when clinical suspicion for vasovagal syncope is moderate and the history alone is not diagnostic. It helps classify the response type (cardioinhibitory vs. vasodepressor), which can guide therapy. In patients where the history is classic (young patient, prolonged standing, prodrome of warmth and nausea, witnessed pallor), the test may add little to what we already know.
The EP Study for Unexplained Syncope
When syncope remains unexplained after initial evaluation, and the clinical profile suggests a cardiac cause, the electrophysiology study provides direct interrogation of the conduction system and arrhythmia substrate. The decision to perform an EP study is driven by three clinical features: structural heart disease, an abnormal resting ECG, and failure of non-invasive testing to establish a diagnosis.
The EP study for syncope evaluates three systems, each with specific endpoints.
We pace the right atrium at progressively faster rates (typically 100-150 bpm) for 30-60 seconds at each rate, then abruptly stop. The sinus node recovery time (SNRT) is the interval from the last paced beat to the first spontaneous sinus beat. The corrected SNRT (cSNRT) subtracts the baseline sinus cycle length, isolating the recovery component.
A cSNRT >550 ms is abnormal and suggests intrinsic sinus node dysfunction. A very prolonged cSNRT (>1000 ms) or a prolonged pause with escape failure strongly suggests that sinus node disease could be the cause of syncope.
Incremental atrial pacing: We pace the atrium at progressively faster rates and observe the AV conduction response. The point at which AV block occurs (the Wenckebach cycle length) is recorded. Wenckebach block at cycle lengths >500 ms (rates <120 bpm) suggests abnormal AV nodal reserve.
HV interval: Measured from the His bundle recording at baseline. A normal HV (35-55 ms) means the infranodal system is healthy. An HV >70 ms is significantly prolonged and associated with increased risk of progression to high-degree AV block. An HV >100 ms, or the development of infra-Hisian block during atrial pacing, is an indication for pacemaker implantation.
Programmed ventricular stimulation: We deliver a drive train of paced beats from the right ventricular apex (and sometimes the RV outflow tract), followed by one, two, or three premature extrastimuli at progressively shorter coupling intervals. The goal is to test whether a sustained monomorphic VT can be initiated.
Positive result: Induction of sustained monomorphic VT (≥30 seconds or requiring termination due to hemodynamic compromise) is a clinically significant finding. In a patient with structural heart disease and unexplained syncope, inducible sustained monomorphic VT strongly suggests that VT was the cause of the clinical event. This finding typically leads to ICD implantation.
Non-specific result: Induction of polymorphic VT or ventricular fibrillation with very aggressive stimulation protocols (three extrastimuli at very short coupling intervals) is less specific. This may be an artifact of the protocol rather than evidence of clinical arrhythmia substrate.
A completely normal EP study in a patient with syncope and structural heart disease does not fully exclude a cardiac cause. The negative predictive value of the EP study is imperfect (approximately 85%). In these cases, an implantable loop recorder for long-term monitoring may be the next step.
The Evaluation Algorithm
The flowchart below summarizes the decision pathway from initial presentation to definitive testing. The first branch separates high-risk features from low-risk features. Structural heart disease and abnormal ECG drive the pathway toward the EP lab. A classic reflex history drives it toward tilt-table testing. Unexplained cases flow to prolonged monitoring.
Structural heart disease and abnormal ECG drive toward EP study. Classic reflex history drives toward tilt-table test. Unexplained syncope with normal ECG leads to prolonged monitoring (ILR).
Key Takeaways
- Syncope is transient loss of consciousness from global cerebral hypoperfusion; the central EP question is always whether the cause is cardiac, because cardiac syncope carries mortality risk.
- Vasovagal syncope results from the Bezold-Jarisch reflex: ventricular mechanoreceptor stimulation triggers paradoxical vagal activation, producing bradycardia, hypotension, or both (cardioinhibitory, vasodepressor, or mixed types).
- Carotid sinus hypersensitivity produces an exaggerated baroreceptor response; the cardioinhibitory subtype (pause >3 seconds) is the form that responds to pacing.
- Syncope in a patient with structural heart disease must be presumed cardiac until proven otherwise; the combination of reduced EF and syncope raises concern for VT and may warrant an EP study and ICD.
- The tilt-table test (70-degree tilt for 20-45 minutes, optional isoproterenol) reproduces reflex syncope under controlled conditions; a positive test requires both a hemodynamic response and symptom reproduction.
- The EP study for syncope tests three systems: sinus node recovery time (cSNRT >550 ms = abnormal), HV interval (>100 ms or infra-Hisian block = pacemaker), and VT inducibility (sustained monomorphic VT = ICD in structural heart disease).