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Patient Care

CE Article: What the AED Is Telling Us

This article is available on our LMS for CE credit.


Upon conclusion of this course, students will be able to:

  • Describe basic cardiac physiology and the electrical, mechanical and hydraulic functions of the heart; 
  • Demonstrate comprehension of AED technology and protocols;
  • Understand and differentiate between common shockable and nonshockable cardiac rhythms;
  • Use rhythms to guide clinical decision making in Advanced Cardiac Life Support.

As a young EMT fresh out of school, I was a part of a crew performing CPR on a trauma patient with massive but controlled hemorrhage. As a BLS crew, we had attached the AED to the patient. It had performed two rhythm checks and both times advised no shock. An ALS unit arrived, and its crew placed the patient on a cardiac monitor. 

Looking at the screen, I saw what looked like a normal rhythm—as an EMT I had seen this same pattern on a regular basis but was not trained in recognition or reading. The ALS crew asked for a pulse check, and I felt for a carotid. To my surprise there was none. I reported this back and was told to continue CPR. We continued to treat the patient until arrival at the ER, where the patient was transferred and sent to surgery. 

After the call I wondered how the monitor showed a normal rhythm when the patient did not have a pulse. After discussing this with the ALS crew, I realized EMT education does not necessarily do an adequate job in explaining everything the AED is really telling us. Understanding its functions more fully can help direct our care to improve outcomes for our patients. 

Basics of the Heart

To understand all the AED is telling us, we must first understand how the heart works. The heart’s function is twofold: It assists in removing waste from the blood by providing deoxygenated blood to the lungs for removal of carbon dioxide and reoxygenation. It then pumps oxygenated blood throughout the body for utilization by the cells. Over the course of the day, the heart beats approximately 115,200 times, which translates to more than 42 million beats a year. Over the course of an average 79-year life expectancy, the heart will beat more than 3.3 billion times. This makes the heart arguably one of the most efficient pumps in history. 

For any pump to function, three basic systems must be present: electrical, mechanical, and hydraulic. A pump’s electrical system provides the energy for the mechanical system to fire. In the heart, the electrical component is provided by a redundant system of pacemakers that each have their own rate of firing. When one pacemaker fails to provide impulse, the next pacemaker picks up the slack. This creates different types of cardiac rhythms on a monitor. 

Impulses from the electrical system activate the mechanical system. On a pump this is the action that causes the fluid to flow. In the heart the mechanical system consists of the cardiac muscle. Then the final system of any fluid pump is hydraulic—the fluid itself. In the case of the heart, the hydraulic system is the blood.  A failure in any one system causes cardiac arrest. Like repairing a pump, each type of cardiac arrest requires specific treatments.

What Does the AED Do?

The AED is a machine that interprets the electrical activity of the heart. Based on a set of programmed parameters, it determines if the electrical activity of the heart is likely to be repaired by the delivery of energy. If so, this is known as a shockable rhythm. The AED will state shock advised and start charging to a manufacturer-defined energy setting. This can vary based on adult vs. pediatric AED pads and the type of waveform with which the AED delivers the shock. 

Once that energy setting is achieved, the AED will sound an alarm. In a semiautomatic AED (the kind most commonly used today), the device will tell the user to press the shock button. In a fully automated AED (less common in current practice), the shock is delivered automatically. In either case the user must ensure no one is touching the patient. 

The user has a set amount of time that varies by manufacturer to deliver the shock (typically 20–30 seconds), and if the button is not pressed, then the AED will dump its charge into an internal capacitor. If the AED detects a nonshockable rhythm or after the shock is delivered, the AED will then advise the user to start CPR and in some instances coach them through it.

After two minutes the AED will then advise the user to stand clear of the patient and reenter its analyze cycle. This continues every two minutes until the AED is turned off (only at transfer of care, switching to an advanced monitor, or termination of resuscitation efforts).

Words of caution: 

  • The AED is only able to interpret the electrical activity of the heart. We still must ensure pulse checks are done, as one of the shockable rhythms is able to have a pulse with it. 
  • Prior to delivering a shock, the user must loudly announce “Clear!” and conduct a visual scan of the patient to ensure no one is touching them.  

Shockable Rhythms

The application of energy to the heart is not a jump-start, as it is often described. Instead, the goal of energy therapy is to completely shut the heart down, thus allowing the heart’s own internal pacemaker cells to restart. This is the human equivalent of turning a computer off and back on again. After the shock is delivered, there are two possible outcomes: The rhythm may continue, or it may convert (either back to a normal rhythm or to another type of cardiac arrest rhythm). 

There are two shockable rhythms in cardiac arrest: ventricular fibrillation (v-fib) and pulseless ventricular tachycardia (pulseless v-tach). These types of out-of-hospital cardiac arrests (OHCA) are the most survivable, with a return-of-spontaneous-circulation rate of 49%, survival-to-admission rate of 48%, and survival-to-discharge rate of 29%, according to the CARES registry.

Treatment of shockable rhythms for prehospital providers is aimed at stopping the arrhythmia to allow the heart to restart. This is done by utilizing high-quality CPR, bag-valve mask (BVM) ventilations, defibrillation, and medication (epinephrine, amiodarone, lidocaine, etc.) in accordance with local protocols and American Heart Association guidelines. 

V-fib is the chaotic quivering of the ventricles of the heart. This quivering does not produce a pulse, as the ventricles are not actually contracting. This could be compared to a seizure of the heart. Common causes of v-fib are problems in electrical activity flowing through the heart or damage to muscle that prevents it from receiving the impulses properly. 

In pulseless v-tach, the pacemakers around the ventricles are not receiving impulses from the heart’s normal pacemakers. In response they attempt to provide circulation to the brain and body by firing on their own, and fast. This fast pace causes the chambers of the heart not to fill properly, resulting in decreased blood flow. As this continues the problem compounds and becomes pulseless v-tach. Common causes of pulseless v-tach include scarring of the heart, coronary artery disease, drug use, medication side effects, and electrolyte imbalances. 

It is important to understand that ventricular tachycardia can present with a pulse. In these cases the muscles are contracting with each impulse, and blood is flowing to the body. The AED cannot detect a pulse, so that makes the pulse check very important. Failure to check for a pulse may result in shocking a patient who is not actually in cardiac arrest. 

Nonshockable Rhythms

Nonshockable rhythms are ones in which the electrical system may be working properly or be completely inoperable, but in either instance electrical therapy would not be likely to restore a normal rhythm. As such the AED will not advise shock for these rhythms. 

There are two types of nonshockable rhythms, pulseless electrical activity (PEA) and asystole. PEA looks like an organized cardiac rhythm. The electrical activity would normally cause the heart to beat and blood to flow, but in the case of cardiac arrest, each impulse does not generate blood flow.

If we know the electrical activity of the heart is working appropriately, we can then surmise the problem lies within either the hydraulic (blood) or mechanical (cardiac muscle) system. Patients who present with PEA in OHCA have a ROSC rate of 39%, survival-to-admission rate of 34%, and survival-to-discharge rate of 10%. 

Problems with the hydraulic system mean there is not enough blood to circulate around the body. One main cause of hydraulic system failure is hypovolemia (loss of blood due to trauma, dehydration, or internal bleeding). To fix this condition, we must stop the blood loss. This can be accomplished with direct pressure and tourniquets for external bleeding or tranexamic acid (TXA) for internal bleeding.

We must also replace the blood; preferably this is done with blood products. However, in the civilian prehospital setting, blood products are not readily available. When blood products are not available, the provider must rely on saline or lactated Ringer’s solution, depending on local protocol. 

Problems with the heart’s mechanical system mean its muscles are not responding to the electrical impulses being received or that the muscles’ movement is restricted. Some common causes of mechanical restriction include cardiac tamponade and tension pneumothorax. To reverse these the prehospital provider must provide high-quality CPR, BVM ventilations, needle decompression (to relieve pressure in the chest for a pneumothorax), and medication (epinephrine). 

Asystole is the absence of electrical activity in the heart. If you watch medical television shows, you’ve likely yelled at the TV as they shocked a patient who had a flat line on the monitor. Patients who present in asystole during OHCA are the least likely to survive, with a ROSC rate of 19%, survival-to-admission rate of 17%, and survival-to-discharge rate of 3%. 

While there are many causes of asystole, typically it presents due to prolonged periods of v-fib or because the heart muscle has died due to lack of oxygen. The out-of-hospital treatment for asystole is to provide high-quality CPR and medications to treat reversible conditions. 


Understanding what the AED is telling us can help to guide our expectations of patient care and outcome in out-of-hospital cardiac arrest. The rhythms presented can guide us in addressing potentially reversible causes, or the “H’s and T’s” of the Advanced Cardiac Life Support world. These are hypovolemia, hypoxia, hydrogen ions (acidosis), hypo-/hyperkalemia, and hypothermia, and tension pneumothorax, trauma, tamponade, thrombosis (pulmonary), and thrombosis (coronary). 

Regardless of the cause of cardiac arrest, your treatment must always be within your scope of practice. Be sure to follow local protocols, which should include high-quality CPR to maintain adequate perfusion to the brain and heart until advanced techniques such as medication and electrical therapy can be administered.  

Resources H’s and T’s of ACLS, 

Arias E, Xu J. United States Life Tables, 2017. National Vital Statistics Reports, 2019; 68(7).  

Cardiac Arrest Registry to Enhance Survival. 2019 Annual Report, 

Delgado H. Toquero J, Mitroi C, Castro V, Lozano IF. Principles of External Defibrillators. IntechOpen, 2013; 

Mayo Clinic. Ventricular fibrillation, 

Mayo Clinic. Ventricular tachycardia, 

Oliver TI, Sadiq U, Grossman SA. Pulseless Electrical Activity. StatPearls [Internet], 2020; 

SaveaLife by NHCPS. Pulseless Electrical Activity Asystole, 

SaveaLife by NHCPS. Ventricular fibrillation and Pulseless Ventricular Tachycardia,

Robert Conklin, MPA, NRP, MICP, has been in public service for more than 18 years. He has worked as a firefighter and paramedic in inner-city, rural, and military communities. Currently he serves as a full-time civilian firefighter for the U.S Air Force at Joint Base McGuire-Dix-Lakehurst in New Jersey. He is also an EMS instructor trainer for the U.S Air Force, a paramedic and educator at Robert Wood Johnson Barnabas Health System, and a lead instructor for Penn Tactical Solutions. Reach him at 


Submitted on 12/26/2020

good explanation on the function of the AED and how it actually stops the heart first.

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