Supraventricular Tachycardia (SVT) is an abnormally fast heart rhythm, typically 150 to 220 beats per minute, originating in the upper chambers of the heart or the junctional area above the ventricles. This rapid electrical activity forces the heart to beat too quickly to fill properly with blood. The decision to treat SVT, and specifically whether to use an electrical shock, depends on how well the patient is tolerating the rapid heart rate. The goal of intervention is to restore a normal, lower heart rhythm to ensure effective blood pumping.
Understanding Supraventricular Tachycardia and Initial Treatment
Patients experiencing SVT often report sudden-onset symptoms such as palpitations, lightheadedness, or dizziness. When a patient is considered “stable”—conscious, with normal blood pressure, and not showing signs of organ distress—the initial approach avoids electrical intervention. The first step involves non-pharmacological methods designed to stimulate the vagus nerve, which slows electrical conduction through the heart.
The modified Valsalva maneuver, where a patient exhales forcefully against a closed airway while lying flat with their legs raised, is the most effective vagal maneuver. If these maneuvers fail to convert the heart rhythm, treatment progresses to intravenous medication. The standard first-line drug is Adenosine, administered as a rapid intravenous push, typically starting with a 6-milligram dose followed by a saline flush.
Adenosine works by temporarily blocking the electrical pathway through the atrioventricular node, often the source of the short-circuiting that causes SVT. This medication is highly effective for most types of SVT, causing only a brief pause in the heart rhythm. If these maneuvers and medications prove ineffective, a conversion to electrical therapy is then considered for a stable patient.
What Electrical Cardioversion Entails
Electrical cardioversion is a controlled medical procedure that uses a therapeutic electrical current to reset the heart’s rhythm. The process involves delivering a low-energy, synchronized electrical shock across the chest wall through specialized pads or paddles. This brief burst of energy causes all heart muscle cells to depolarize simultaneously, momentarily halting electrical activity.
Synchronization differentiates this procedure from defibrillation, which is used for chaotic rhythms like ventricular fibrillation in patients without a pulse. In cardioversion, the electrical discharge is precisely timed to avoid the heart’s vulnerable repolarization period, minimizing the risk of inducing a more dangerous rhythm. Because the patient is conscious and the shock can be painful, the procedure is performed under deep, short-acting intravenous sedation to ensure comfort.
Criteria for Immediate Electrical Cardioversion
The question of when to cardiovert is answered by the patient’s clinical status, specifically the presence of hemodynamic instability. This unstable state means the rapid heart rate is preventing the heart from pumping enough oxygenated blood to the body’s vital organs. In this emergency scenario, the risks of delaying treatment with slower pharmacological agents outweigh the risks of immediate electrical intervention.
Specific signs that mandate immediate synchronized electrical cardioversion include severe hypotension or evidence of shock. Other urgent indications are altered mental status, such as confusion or unresponsiveness, which signals poor blood flow to the brain. Acute signs of heart failure, such as severe shortness of breath or pulmonary edema, also require immediate conversion.
Active chest pain, or angina, indicating the heart muscle is not receiving enough oxygen, is another compelling reason to proceed directly to electrical therapy. When these signs of instability are present, the medical team bypasses vagal maneuvers and Adenosine entirely. The clinical urgency requires the fastest and most reliable method to restore a normal rhythm: immediate synchronized electrical shock.
What Happens After the Procedure
Following successful electrical cardioversion, the patient is closely monitored in a recovery area while the effects of the intravenous sedation wear off. The medical team confirms the heart has converted to a normal rhythm through continuous monitoring. Patients often feel groggy and may experience mild discomfort or redness on the skin where the electrode pads were placed.
Patients are instructed not to drive or make important decisions for at least 24 hours due to the lingering effects of the sedative medication. Post-procedure care often includes the use of blood-thinning medication, particularly if the SVT lasted for an unknown or prolonged duration, to mitigate the risk of stroke from blood clots. The final step involves consulting with a cardiologist to determine the underlying cause of the SVT and establish a long-term plan, which may involve maintenance antiarrhythmic medications or a curative procedure like catheter ablation.