Overview
The San Francisco Syncope Rule (SFSR) is a bedside tool developed for adults presenting to the emergency department with syncope—a sudden, transient loss of consciousness with rapid recovery. Its purpose is to help clinicians identify a subset of patients who have a higher probability of short-term serious outcomes (classically framed around the first week after the index visit), so that disposition decisions (observation, monitoring, admission, or focused workup) align better with risk.
Unlike additive scores that sum many small effects, the SFSR is built as a screening rule: it uses a small number of binary findings, and the rule is considered positive if any single criterion is present. That design prioritizes sensitivity for identifying risk rather than fine-grained grading across a continuous scale.
Why syncope is challenging in the ED
Syncope is common and often benign (vasovagal syncope, situational triggers, orthostatic symptoms), but it can also be the first manifestation of arrhythmia, structural heart disease, acute coronary syndrome, pulmonary embolism, hemorrhage, or other time-sensitive conditions. The emergency evaluation therefore balances two competing needs: avoid unnecessary hospitalization for low-risk presentations, and avoid missing patients who will deteriorate shortly after discharge.
Clinical decision rules help standardize which features have been associated with adverse trajectories in cohort studies. The SFSR focuses on features that are often available early in the ED course: history elements, triage vital signs, a basic hematocrit, and the presenting electrocardiogram.
What the rule is trying to predict
The rule was derived to predict short-term serious outcomes after an ED visit for syncope. Depending on the reporting framework, “serious outcomes” typically include events such as life-threatening arrhythmia, myocardial infarction, need for urgent interventions for structural heart disease, pulmonary embolism, stroke, intracranial hemorrhage, significant hemorrhage or transfusion, and other major conditions that require escalation of care or hospitalization. The exact endpoint definitions vary across studies, but the clinical intent is consistent: identify patients whose syncope may be a marker of a dangerous process rather than a benign reflex faint.
The CHESS mnemonic: criteria and clinical meaning
The five criteria are commonly remembered with the mnemonic CHESS. Each item is intended to be assessed in the context of the ED evaluation for syncope, not as an isolated laboratory curiosity.
C — History of congestive heart failure
Prior heart failure increases concern for cardiogenic mechanisms, reduced reserve, and arrhythmia substrates. In the SFSR framework, a documented history of congestive heart failure flags the patient as higher risk because the syncopal episode may be related to perfusion or rhythm instability in a patient with known ventricular dysfunction or valvular burden.
H — Hematocrit < 30%
A low hematocrit raises suspicion for anemia from chronic disease or acute blood loss, and anemia can contribute to symptoms that mimic syncope or unmask hemodynamic instability. In the acute setting, it also prompts consideration of whether bleeding is ongoing or occult, even when the patient’s primary complaint is framed as syncope.
E — Abnormal ECG
An abnormal electrocardiogram is a broad label by design: it can include rhythms other than normal sinus, conduction disease, repolarization abnormalities suggesting ischemia, or other changes judged clinically significant by the treating clinician. The practical point is that the ECG is not treated as a “checkbox normal” if subtle but important abnormalities are present. When a prior ECG exists, new changes compared with baseline are particularly relevant.
S — Shortness of breath
Dyspnea at presentation suggests cardiopulmonary compromise that may not be fully explained by a simple vasovagal episode. It broadens the differential toward heart failure exacerbation, ischemia, pulmonary embolism, and other processes where syncope may be one component of a broader syndrome.
S — Systolic blood pressure < 90 mmHg at triage
Hypotension at triage is a direct hemodynamic signal. Even if transient, it raises questions about volume status, sepsis, bleeding, cardiogenic shock, autonomic failure, or medication effects. The rule uses triage systolic blood pressure as an early, objective measure of circulatory stress.
| Letter | Criterion | Why it matters |
|---|---|---|
| C | CHF history | Cardiac substrate and higher risk cardiogenic causes |
| H | Hematocrit < 30% | Anemia, possible bleed, reduced oxygen delivery |
| E | Abnormal ECG | Electrical instability or ischemia |
| S | Shortness of breath | Cardiopulmonary symptoms beyond “simple syncope” |
| S | Triage SBP < 90 mmHg | Objective hypotension |
How to apply the rule operationally
In practice, clinicians collect the five inputs in parallel with the standard syncope history, focused physical examination, and initial testing. The rule is positive if any single CHESS criterion is present. The number of positive criteria can be informative (more positives may reflect a sicker patient), but the derivation and common use of the SFSR emphasize the any-hit interpretation rather than a scored gradient.
- Be explicit about what counts as “abnormal ECG.” Use local criteria and cardiology input when ECG interpretation is ambiguous.
- Use triage blood pressure as defined by your workflow; avoid re-labeling the criterion using later ED vitals unless the rule is explicitly adapted.
- Integrate context: anticoagulation, pregnancy, exertional syncope, focal neurologic deficits, chest pain, or persistent altered mental status may warrant evaluation pathways even if CHESS is negative.
Interpreting a positive versus negative rule
A positive SFSR indicates that the patient carries at least one feature that was associated with short-term serious outcomes in derivation work. That does not automatically mandate admission in every health system, but it often supports a lower threshold for monitored observation, specialist review, and additional testing when the overall presentation is consistent with elevated risk.
A negative SFSR (no CHESS criteria) suggests that the patient lacked those particular risk features at the time of assessment. That can be reassuring in a structured risk-stratification framework, but it is not a guarantee of safety. External validation studies have reported that performance varies by population, setting, and outcome definitions, including sensitivity that may be lower than initially hoped in some cohorts. This is why the rule is best used as one input alongside clinician judgment, shared decision-making, and return precautions.
How the SFSR compares conceptually to other syncope approaches
The emergency medicine literature on syncope includes several rules and scores that differ by endpoints (for example, short-term serious outcomes versus longer-term mortality), variables (e.g., troponin, telemetry findings, imaging), and intended use (screening versus risk grading). The SFSR is notable for being compact and early-available, but it is not interchangeable with every other syncope tool. Teams should choose the instrument that matches their clinical question, local evidence, and disposition pathways.
Limitations and responsible use
The SFSR can miss important diagnoses if clinicians treat it as a standalone diagnostic test. It does not replace orthostatic vital signs when indicated, a careful history for cardiac symptoms, neurologic evaluation when warranted, or institution-specific protocols for high-risk features outside the rule. Additionally, the “abnormal ECG” component requires interpretation skill and may vary between readers.
Finally, the rule is not designed for every patient with transient loss of consciousness. Conditions that mimic syncope (seizure, intoxication, hypoglycemia, stroke with altered awareness) may require different frameworks entirely.
Educational use only. The San Francisco Syncope Rule supports clinical reasoning and communication; it does not replace individualized assessment, serial re-evaluation, or local standards of care.