Supraventricular Tachycardia (SVT) is an abnormally fast heart rhythm, typically defined as a heart rate over 150 beats per minute, originating above the ventricles (in the atria or AV node). This rapid rate shortens the time the heart has to fill, reducing the amount of blood pumped to the body. Advanced Cardiac Life Support (ACLS) provides standardized guidelines for healthcare professionals managing this condition. The ACLS algorithm requires rapid assessment to determine the patient’s stability, which dictates the specific treatment sequence. Management differs significantly based on whether the patient is clinically stable or unstable.
Initial Assessment Determining Stability
Initial patient contact involves a rapid assessment following Airway, Breathing, and Circulation (ABC) principles to ensure immediate life threats are addressed. Establishing cardiac monitoring and obtaining intravenous access are simultaneous priorities. The decision between immediate electrical intervention or pharmacological therapy depends entirely on the patient’s hemodynamic stability.
A patient is considered unstable if the rapid heart rate causes signs of poor perfusion, meaning the body is not receiving enough oxygenated blood. Indicators of instability include acute signs of shock (such as cool, clammy skin or altered mental status), hypotensive blood pressure, ongoing ischemic chest discomfort, or acute heart failure. If these signs are present and caused by the fast heart rate, the patient requires immediate synchronized cardioversion.
A stable patient may experience palpitations, mild shortness of breath, or anxiety, but maintains adequate blood pressure and mental clarity. A 12-lead electrocardiogram (ECG) is obtained quickly to confirm the diagnosis, specifically looking for the narrow QRS complex characteristic of SVT.
Managing SVT When the Patient is Stable
When the patient is stable, treatment focuses on non-invasive and pharmacological methods before electrical intervention. The first step involves vagal maneuvers, which stimulate the vagus nerve to slow conduction through the AV node. The preferred technique is the modified Valsalva maneuver, where the patient forcibly exhales against a closed airway for 10 to 15 seconds.
Immediately following the strain, the patient is quickly repositioned supine with passive leg elevation to increase venous return. If vagal maneuvers fail to convert the rhythm to a normal sinus pattern, the ACLS protocol moves to the administration of a specific medication.
The primary drug is adenosine, which induces a transient block in the AV node, effectively interrupting the re-entry circuit causing the SVT. Due to its short half-life, it requires a rapid administration technique. The medication must be injected as a rapid intravenous (IV) push over one to three seconds through a port closest to the heart.
Immediately after the push, a rapid 20 mL normal saline flush is administered to ensure the entire dose reaches the central circulation quickly. The initial dose of adenosine is 6 mg. If the rhythm does not convert within one to two minutes, a second dose of 12 mg is administered using the same rapid push and flush technique. The team should anticipate a brief period of asystole or profound bradycardia immediately following administration, which is expected and usually resolves quickly.
If two doses of adenosine fail, the ACLS algorithm suggests alternative pharmacological agents. These secondary options include an intravenous calcium channel blocker (such as diltiazem or verapamil) or a beta-blocker. If pharmacological treatment remains unsuccessful, synchronized cardioversion is then considered for the stable patient.
Managing SVT When the Patient is Unstable
For the patient exhibiting signs of instability, the priority shifts to immediate synchronized electrical cardioversion, bypassing the attempts at vagal maneuvers and drug administration. The instability indicates that the heart rate is causing systemic compromise, making immediate rhythm termination the most time-sensitive action. The goal of this electrical therapy is to deliver a controlled electrical shock to the heart muscle to momentarily stop the abnormal electrical activity and allow the heart’s natural pacemaker to restart the rhythm.
Synchronized cardioversion is distinct from defibrillation because the electrical discharge is timed to occur precisely on the peak of the QRS complex, known as the R-wave. This synchronization prevents the shock from being delivered during the vulnerable repolarization phase of the cardiac cycle, which could otherwise induce a more dangerous rhythm like ventricular fibrillation. The device is placed into “sync” mode, and the machine marks the R-waves to ensure proper timing.
If the patient is conscious and time permits without delaying the procedure, a rapid-acting sedative may be administered to minimize discomfort. For narrow-complex tachycardias like SVT, the recommended initial energy setting for a biphasic defibrillator is 50 to 100 Joules. If the first shock fails to convert the rhythm, the energy level is progressively increased in a stepwise manner for subsequent attempts.