This page is intended as a supplementary resource to the didactic knowledge found in a paramedic program. This will cover the basic anatomy and physiology of cardioversion as well as the differences between electrical, chemical, synchronized, and unsynchronized cardioversion. This page is not intended to be a guide for treatment of a tachycardic patient as it will only speak briefly of other treatments including fluid administration, vagal maneuvers, and pharmacological agents. This page will give basic guidelines and information on cardioversion without bias towards a single device or manufacturer. Always refer to local protocols when using a new or unfamiliar monitor/defibrillator. This page will not go over the dosing of monophasic vs. biphasic energy administration but will speak about both briefly. If you are on this page to learn the sequence of events to cardiovert, skip to How to Cardiovert.

General Cardioversion[edit | edit source]

Cardioversion is the forced conversion of one cardiac rhythm to another with the intent of treating a patient's symptoms or reducing risk of injury or death. Cardioversion is one of the skills paramedics must be confident in as rapid assessment, recognition, and treatment of dangerous cardiac rhythms is paramount to good patient care. Cardioversion may be achieved in a variety of manners ranging from administration of antidysrhythmic medications such as adenosine or amiodarone, fluid administration, non-invasive measures such as vagal maneuvers, or electrical cardioversion. This page will focus on electrical cardioversion.

Electrical vs. Pharmacological Cardioversion[edit | edit source]

Your patient is complaining of dizziness and lethargy and their heart rate is too fast to count manually. You place them on a 4 lead and find that they are in monomorphic ventricular tachycardia. With the exception of their heart rate, all other vital signs are within normal limits. Do you shock this patient or give medication? This question is answered easily by the ACLS algorithm for tachycardia (this patient is in stable, monomorphic tachycardia)ː hang amiodarone or attempt adenosine if you expect aberrant SVT.

The choice between electrical and pharmacological cardioversion can be difficult for some providers to understand, so this section will briefly discuss the differences between electrical and pharmacological cardioversion.

Pharmacological Cardioversion[edit | edit source]

Pharmacological cardioversion is often the first line treatment for a stable tachycardia (assuming a fluid bolus is not indicated and after vagal maneuvers if indicated). There are a whole host of medications that will interact with a whole host of hormones, transmitters, gates, channels, nerves, and other biologic and chemical structures to produce a drop in heart rate to a safe level. Because these medications are tailored to some extent to have certain effects they can be used to treat very specific abnormalities in cardiac rhythm or the underlying cause of a cardiac rhythm. Agents including but not limited to adenosine, lidocaine, procainamide, amiodarone, magnesium sulfate, metoprolol, and diltiazem can and have been used to treat a whole plethora of different tachycardias from pulsing VT and TDP to SVT or even sinus tachycardia. Due to the fact that they are much less painful that electrical cardioversion, medications are generally used first for stable patients to minimize patient distress. Unfortunately, some patients may not respond to medication or may be too time sensitive or unstable for medication to be indicated.

Electrical Cardioversion[edit | edit source]

Electrical cardioversion is the easiest to explain as it essentially is a pulse of energy delivered from one defibrillation pad to another with the goal of depolarizing the myocardium enough that a normal pacemaker will take over the cardiac rhythm. Generally, electrical cardioversion is considered safer than pharmacological cardioversion as there is no need to worry about the contraindications, allergies, medication interactions, etc. that come with administration of medication. Additionally, electrical cardioversion is much faster to apply and administer than medication which may require specialized vascular access, administration methods, and preparation. The main downside to electrical cardioversion is that it can be painful for the patient and may be dangerous if not performed correctly. Due to its speed and relative safety, electrical cardioversion is generally used for unstable patients or patients who are unresponsive to initial medication.

Physiology and Rhythms of Electrical Cardioversion[edit | edit source]

Figure 1. SVT overview (CardioNetworks ECGpedia)

This section will briefly speak about some of the most common rhythms that paramedics see that can require cardioversion. If you are unfamiliar with basic rhythm analysis and etiology, look at the EKG Rhythm Interpretation page of this course for examples of basic cardiac rhythms.

Shock Physiology[edit | edit source]

If you are unfamiliar with the mechanisms of cardiac cells such as depolarization, repolarization, etc. look at the Physiology of the Heart for a basic overview of the heart. The goal of electrical cardioversion is to restore normal electrical flow to the cardiac conduction system by depolarizing the myocardium. While many tachycardias are produced by underlying conditions such as increased muscular oxygen demand (e.g. working out), dehydration, or hypovolemia there are tachycardias that can be caused by abnormal structural components (e.g. AVRT) or insult to the myocardium (e.g. pulsing VT) where the cause of the tachycardia cannot be directly treated. In these cases and cases where prolongation of the tachycardic rhythm places the patient in increased danger, an electrical shock is delivered in an attempt to fully depolarize the myocardium, including the aberrant focus or foci, and allow the normal cardiac conduction system to take over. The shock is meant to travel through the entire myocardium, so the provider should attempt to place the pads as close to manufacturer's recommendation as possible to ensure a maximal amount of myocardium is depolarized.

Supraventricular Tachycardias[edit | edit source]

A supraventricular tachycardia by definition is a tachycardia whose originating focus is above the ventricles. This is different than what most paramedics learn in class, where SVT is a rhythm characterized by a rate above 150 bpm with no discernable P waves. We will talk about this definition of SVT later in this passage, but realizing that sinus tachycardia, atrial fibrillation with rapid ventricular response, MAT, and atrial flutter are all technically supraventricular tachycardias is important for any provider, as (barring sinus tachycardia) all of these rhythms may be cardioverted depending on patient presentation and local protocol (for example, atrial fibrillation is frequently cardioverted in hospital due to concern of clots but many EMS protocols refrain from electrical cardioversion of the rhythm unless the patient displays signs of cardiovascular compromise).

The "SVT" treated with cardioversion, adenosine, and vagal maneuvers in the prehospital and hospital setting is actually two separate cardiac rhythms that are very similar. These rhythms are known as AV re-entry tachycardia (AVRT) and AV nodal re-entry tachycardia (AVNRT); a graphical representation of these rhythms can be found in Figure 1. In both rhythms, conduction takes place in a circular motion (i.e. re-entrant motion) that allows for a faster rhythm than the SA node would normally allow; because the myocytes will depolarize and be refractory to further stimulus, the SA node may not be able to take over without external assistance. In AVRT, there is an accessory path between the and ventricles (remember the Bundle of Kent from Wolff-Parkinson-White syndrome) that allows for a circuit of electricity to bypass normal conduction. AVNRT is simply a circuit of electricity caught within the AV node its fast and slow pathways that propagates a ventricular response with every cycle. It is not required for paramedic students to be able to differentiate between AVRT and AVNRT, but it is helpful to know they exist, as the accessory pathway that is always present with AVRT is responsible for the danger of using adenosine for "SVT" with concurrent atrial fibrillation history.

Ventricular Tachycardias[edit | edit source]

A ventricular tachycardia by definition is a tachycardia whose originating focus is within the ventricles. Ventricular tachycardias are classified by their wave morphology which is either monomorphic (i.e. everything looks the same) or polymorphic (i.e. there are significant differences between the waveforms). Ventricular tachycardias are fairly easily identified by their wide, abnormal appearance but can be easily confused with AVRT/AVNRT with aberrant conduction. While supraventricular tachycardias are by convention always pulsing (pulseless rhythms are generally called PEA), ventricular tachycardias can be further subdivided into pulsing and pulseless rhythms, with both being identified by the underlying rhythm (e.g. pulsing monomorphic VT or pulseless Torsades de Pointes). If the patient is in any pulseless rhythm, CPR should be immediately started.

Narrow complex vs. Wide Complex[edit | edit source]

A narrow complex rhythm is defined as a rhythm with a QRS duration of less than 0.12 sec (< 120 ms) while a wide complex rhythm is one where the QRS duration is greater than 0.12 sec (> 120 ms). All supraventricular tachycardias (barring aberrant conduction) should fall within the category of "Narrow Complex" while all ventricular tachycardias are considered "Wide Complex". Using the nomenclature "Narrow Complex" and "Wide Complex" allows providers to bypass the need to diagnose the rhythm and focus on treatment. When utilizing electrical cardioversion, the distinction of narrow vs wide complex will occasionally determine initial and subsequent energy levels for shocks dependent on local protocol. As always, follow the monitor/defibrillator manufacturer's recommendation and local protocol when determining energy level.

Synchronized vs. Unsynchronized[edit | edit source]

Cardioversion and defibrillation both involve charging the defibrillator and shocking the patient but are different procedures. Where defibrillation involves shocking the patient as soon as possible, cardioversion is generally synchronized to the patient's electrical rhythm to prevent R on T phenomenon and lethal arrhythmias. Synchronized cardioversion involves utilizing the cardiac monitor's computer to track the QRS complexes of the patient's rhythm to provide a shock that does not fall on a T wave. If you are cardioverting anything other than a polymorphic wide complex rhythm, you must synchronize the monitor to the patient's cardiac rhythm. An unsynchronized cardioversion is essentially the same thing as a defibrillation of the same energy level and should not be used without medical control and contact with a more experienced or specialized provider except in extreme cases allowed by protocol.

Monophasic vs. Biphasic Energy[edit | edit source]

There are at current date two main ways to provide electrical energy to a patient's heart via transcutaneous electrical cardioversionː via a 1) monophasic and 2) biphasic waveform. The intricacies of the waveforms and differences in waveform presentation (such as Zoll's Rectilinear waveform) are above the scope of this page and will not be discussed. The main difference between a monophasic and biphasic waveform is that while a monophasic waveform moves energy from one electrode to another via the patient's heart, a biphasic waveform will reverse the charge and run the energy back through the patient's heart. There has been research showing that biphasic waveforms may be superior to monophasic waveforms in providing enough electrical energy to perform the desired procedure while mitigating the negative effects of electrical therapy such as burns. Additionally, the research has indicated that a biphasic waveform will generally need less energy to achieve the same level of effect as a monophasic waveform at lower energy levels; so far this efficiency has not been shown to transfer to higher energy levels. Many of the most common EMS defibrillators are now utilizing biphasic energy (e.g. Lifepak 15, Zoll X series, Philips Tempus Pro) although energy levels for both defibrillation and cardioversion should still be found in local protocol or manufacturer's recommendations.

How to Cardiovert[edit | edit source]

This section will discuss the physical actions necessary to successful cardioversion for both a Lifepak 15 and Zoll X series monitor. Always refer to manufacturer's recommendations to find the optimal way to use your operation's devices. Consider sedation prior to cardioversion or pain management post-cardioversion.

  1. Turn on the monitor/defibrillator and connect the patient to some sort of electrical monitoring (pads or 4/5/12 lead EKG).
  2. Determine patient requires electrical cardioversion (use local protocol or medical control for choice between electrical and pharmacological cardioversion).
  3. Place pads if not already placed in Step 1. Choose anterior-posterior or anterior-lateral placement based upon protocol/medical control.
  4. Synchronize the monitor to the patient's electrical rhythm by pressing the "Sync" button on a Lifepak 15 or selecting "Sync" on the Zoll X series.
  5. Confirm that a synchronization marker is present above each and every R wave on the patient's rhythm.
  6. Select energy dosage and charge device. Clear patient.
  7. Press and hold shock button until device discharges.
  8. Repeat each step as needed (for example, unless otherwise set a Zoll X series will automatically drop from sync mode after a shock has been delivered).

Documentation[edit | edit source]

  • "Patient found to be in unstable wide complex tachycardia at 180 bpm. Patient hypotensive with pale, cool and diaphoretic skin signs and weak, thready radial pulse. Patient denies CP/SOB. Patient cardioverted at 100 J to resolution of wide complex tachycardia into sinus rhythm at 72 bpm with strong radial pulse. Skin signs improve and BP increases to 110/72."

Self Assessment[edit | edit source]

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