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Because asymptomatic or incidental dysrhythmias usually require no direct treatment, the prehospital focus shifts to those dysrhythmias associated with symptoms. These patients are classified based on the severity of symptoms as either stable or unstable. Although many patients have symptoms attributable to the change from a “normal” rhythm, most tolerate these well and are stable. However, unstable patients are likely to suffer harm or deteriorate. EMS clinicians must identify these unstable patients and intervene promptly.

Unstable patients have signs and symptoms of impaired end‐organ perfusion due to the rhythm disturbance [2]. A few brief historical questions and physical examination features identify these patients early in their evaluations.

 Hypotension–often defined as a systolic blood pressure below 90 mmHg, a mean arterial pressure <65 mmHg, or a drop of systolic blood pressure >40 mmHg from a known baseline.

 Cardiac dysfunction–manifest as chest pain, shortness of breath, or rales, each signifying inadequate myocardial perfusion or function.

 Altered consciousness–from mild agitation or somnolence to obtundation or coma, signifying central nervous system hypoperfusion.

Delayed capillary refill and lowered skin temperature can indicate poor perfusion. However, the subjective nature of these observations and multiple other potential causes limit their use in the field.

Assessing instability is usually a continuum, not an “all‐or‐none” phenomenon. Either a single severe sign or symptom, or multiple milder findings, is diagnostic of an unstable patient and rhythm. A single mildly abnormal finding suggests “borderline” stability.

The blood pressure is the simplest method of assessing circulatory adequacy, but it alone may be insufficient in accurately classifying patients. A patient with a systolic blood pressure of 60 mmHg is always unstable. Another patient with a blood pressure of 90 mmHg systolic, rales, and a depressed sensorium is also unstable. If awake and with no rales, chest pain, or other symptoms, the patient with a systolic blood pressure of 90 mmHg occupies a borderline position due to the singular finding; the patient may also report that this is his or her baseline blood pressure. Similarly, agitation suggests mild central nervous system hypoperfusion and borderline stability, whereas coma is associated with more profound derangement and instability.

In the absence of instability, each patient can receive a more complete evaluation. However, do not prolong the total prehospital interval for a detailed effort. Unstable patients need rapid therapy, usually with electrical interventions such as external countershock/cardioversion or pacing. Symptomatic but stable or borderline unstable patients can be initially treated with pharmacologic agents, with electrical devices nearby in the case of deterioration. The more extreme the sign or symptom of instability (e.g., coma versus mild anxiety), the more intensive the needed initial treatment will be.