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In patients who are symptomatic but stable or who have one borderline symptom of instability (e.g., dizzy or anxious with a low blood pressure), certain actions may aid. Patients with regular narrow‐complex tachydysrhythmias between 120 and 140 per minute are likely to have sinus tachycardia and require no antidysrhythmic treatment. Stable patients with regular narrow‐complex tachydysrhythmias at 140 per minute or greater should have vagal stimulating maneuvers performed to assess and hopefully terminate the rhythm. Sometimes, this maneuver uncovers sinus P waves, clarifying the sinus or atrial etiology. When P waves are seen, treatment is directed at the cause, not the rhythm.

Those with minor symptoms (e.g., isolated subjective dizziness or palpitations) do not require field treatment beyond vagal maneuvers. For those with symptoms that are more prominent and with regular narrow‐complex tachydysrhythmias at 140 per minute or greater, give adenosine (6‐12 mg as a rapid IV bolus from a mid‐arm or more proximal site, followed with a flush) [1–3, 8]. The smaller initial dose (6 mg) is effective about 60% of the time. It should be repeated within 2 minutes at the higher dose if no effect is seen. If adenosine causes slowing followed by a return to tachycardia, repeat or larger doses will not help. The cause is a non‐reentrant source, often an atrial rhythm, either atrial tachycardia, fibrillation, or flutter.

Adenosine is effective in 85% to 90% of patients with regular narrow‐complex tachydysrhythmias. The drug has a duration of effect of 20 seconds or less, and recurrence of a narrow‐complex tachydysrhythmia may occur in 10% to 58% of cases. Patients commonly complain of transient chest pain, dyspnea, or flushing during adenosine treatment. Some patients may experience bradycardia or asystole after adenosine. It usually lasts only seconds, but it may require temporary external pacing if prolonged. Contrary to popular belief, adenosine can occasionally terminate VT, although most such patients are unaffected [11].

Verapamil (2.5‐5 mg IV initially, followed by 5–10 mg in 15 minutes, if unsuccessful) and diltiazem (0.15 mg/kg initially, followed by 0.20‐0.25 mg/kg in 15 minutes, if unsuccessful) will terminate 85% to 90% regular narrow‐complex tachydysrhythmias [12, 13]. However, both can cause hypotension and congestive heart failure, though diltiazem may have slightly lower rates of this in equipotent doses. Because of these disadvantages, many prefer to use adenosine in the field.

Whenever giving adenosine, verapamil, or diltiazem in the field, EMS clinicians must be certain that the QRS duration is less than three small boxes (0.12 seconds). This will help avoid the hemodynamic collapse that can occur with these drugs in VT or atrial fibrillation with an accessory pathway. Most patients tolerate the transient effects of adenosine, often “fooling” clinicians into thinking no harm is possible if given in error. The potential harm is real, albeit much less frequent than with calcium channel blockers. If hypotension occurs after IV verapamil or diltiazem in the absence of bradycardia, treatment with saline infusions, IV calcium salts (5‐10 mL of a 10% calcium chloride solution), or vasopressors (i.e., norepinephrine or epinephrine) should be given.

Many wide‐complex tachydysrhythmias are erroneously classified in the field as narrow (up to 20% of cases). Therefore, many medical oversight physicians prefer adenosine to treat all regular and symptomatic narrow‐complex tachydysrhythmias, avoiding the risks associated with giving a calcium channel blocker to a patient with wide‐complex tachydysrhythmia. For those patients with chaotic narrow‐complex tachydysrhythmia, atrial fibrillation is the likely rhythm. If mildly symptomatic and stable, no field treatment is required. An example is an elderly patient with an irregular narrow‐complex tachydysrhythmia at a rate of 130/minute, complaining of weakness. Although rapid atrial fibrillation may contribute to the symptoms, no field treatment is needed in the absence of other clear signs or symptoms of decompensation. Those with instability deserve immediate countershock with 50 to 100 J. If transport is prolonged and the patient has either borderline symptoms or a rate of 140 to 180/minute, metoprolol (5‐10 mg intravenously) or diltiazem (0.15‐0.25 mg/kg intravenously) will control the ventricular rate in 85% to 90% cases of rapid atrial fibrillation [10, 11].

One pitfall in the treatment of stable narrow‐complex tachydysrhythmia is very rapid rates. When the ventricular rate is greater than 220/minute, the risk of decompensation rises and the ability to detect irregularity is limited [2]. Therefore, all adults with a very fast regular narrow‐complex tachydysrhythmia (heart rate >220/minute) should be either cardioverted with 100 J or treated with adenosine plus prepared for cardioversion. If the rate rises greater than 250/minute, cardioversion is the best choice given the risk of deterioration. Irregular narrow‐complex tachydysrhythmia greater than 220/minute deserves countershock promptly as previously noted (50‐100 J).