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PEA vs Asystole: Understanding the Key Differences

Mackenzie Thompson

by Mackenzie, Life Saver, NHCPS

Co-authored by Karl Disque, D.O. RPH

posted on Apr 28, 2023 at 2:02 pm

It is crucial for healthcare providers who are new to the Advanced Cardiac Life Support (ACLS) algorithm to understand the difference between pulseless electrical activity and asystole. ACLS is a set of protocols and interventions used to treat patients in cardiac arrest or other life-threatening medical emergencies. PEA and asystole are both types of cardiac arrest that require different interventions and treatments. A clear understanding of the difference between these two rhythms is essential to provide timely and appropriate care to patients, as well as to avoid unnecessary or harmful interventions. Healthcare providers can receive training in ACLS through courses offered nationwide, such as the one available here, to gain the knowledge and skills necessary to manage these critical situations effectively. Mastery of the ACLS algorithm can help maximize patient treatment outcomes and improve the chances of survival for patients experiencing cardiac arrest.

What Happens During Asystole?

healthcare-professional-holding-a-patients-whiteboard Asystole is not a random occurrence but is usually triggered by an event, injury, or electrical shock that can stun the heart muscle, leading to secondary asystole. Alternatively, asystole may be caused by a failure of the heart’s electrical nodes to generate an action potential, which is known as primary asystole. The sinoatrial (SA) node and atrioventricular (AV) node are two of the key electrical nodes in the heart, and if they fail to trigger an action potential, primary asystole may occur. Asystole has specific causes and can be classified as either primary or secondary, depending on whether it is related to a failure of the heart’s electrical nodes or an external event.

It is important to understand that the depolarization of a cell in response to a stimulus is referred to as an action potential. In the case of the heart, the sinoatrial (SA) node initiates the depolarization process, which triggers the right atrium to depolarize. As the pulse moves downward, the right ventricle contracts, pumping blood up through the pulmonary artery and to the lungs. When the normal conduction of an action potential is blocked, it can lead to a loss of rhythm and heartbeat, resulting in asystole. Primary asystole, which is caused by a failure of the heart’s electrical nodes to generate an action potential, may occur in certain cases of bradycardia, which is a slower-than-normal heart rate, or bradyarrhythmia, which is an irregular heart rhythm that is too slow. In summary, understanding the process of depolarization and the role of the SA node in initiating it can help clarify the causes and mechanisms behind asystole, including its potential link to certain types of bradycardia or bradyarrhythmia.

According to Cleveland Clinic, asystole can be caused by a range of different forms of trauma, which is also in more detail covered by B. Hawayek et al in their scientific article. These may include conditions such as spinal cord injury, eye trauma, hypersensitive carotid sinus syndrome, glossopharyngeal neuralgia, and maxillofacial surgery. In other words, any injury or medical condition that affects the heart’s electrical conduction system or disrupts the normal functioning of its nodes can potentially lead to asystole. This highlights the importance of identifying and addressing the underlying causes of asystole, in order to provide the most effective treatment and improve patient outcomes.

Secondary asystole is a type of asystole that occurs when factors outside of the heart’s electrical conduction system fail, resulting in a failure to generate any depolarization of cardiac tissue. This means that while the heart’s electrical system may still function, it lacks the power to generate an action potential. One of the most common causes of secondary asystole is hypoxia with metabolic acidosis.

Both primary and secondary asystole can also occur after untreated ventricular fibrillation, and prior to attempts to defibrillate. This highlights why asystole is known as an “unshockable” rhythm, as attempts to defibrillate will not restore a normal heartbeat in cases of asystole.

The Role of ACLS in Restoring Normal Cardiac Function

In cardiac arrest situations, healthcare professionals often rely on medications and electrical stimulation (defibrillation) to try and encourage the return of spontaneous circulation (ROSC). This is the core focus of the Advanced Cardiac Life Support (ACLS) algorithm, which provides healthcare professionals with a structured approach to promoting ROSC. If the heart’s electrical system is unable to be moved into a rhythm that can be defibrillated, or if ROSC cannot be achieved, the individual will unfortunately not survive. By following the ACLS algorithm, healthcare professionals can optimize their efforts to restore normal cardiac function and improve the chances of a successful outcome.

Understanding a Pulseless Electrical Activity

Pulseless electrical activity, also known as PEA, may seem like an early warning sign of asystole. However, it is important to understand that these two arrhythmias are distinct from each other. While it is true that all cases of cardiac arrest that do not achieve ROSC will eventually result in asystole, PEA is a unique condition with its own set of underlying causes. PEA occurs when a major issue in the body leads to a lack of force generation by the cardiac muscle following depolarization.

Essentially, the electrical signals that initiate depolarization do occur, but the heart is not able to generate a strong enough contraction to produce a heartbeat. This can occur due to a variety of factors, and it is important for healthcare professionals to identify and address the underlying cause of PEA in order to improve the chances of achieving ROSC and preventing progression to asystole.

Exploring the Underlying Causes of PEA

PEA, or pulseless electrical activity, is typically caused by a significant cardiovascular issue, as reported by Medscape. Various factors, such as flow-restricting emboli, hypovolemia, metabolic disorders, and hypoxia, may all lead to PEA. Additionally, there is a list of potentially reversible causes of arrest known as the H’s and T’s, which include:

  • Hydrogen ion buildup, also known as acidosis, which may occur from dietary changes or problems with the endocrine system.
  • Hypo/hyperkalemia, a result of an imbalance of cellular tissues due to a buildup or decline in available potassium in the body.
  • Hypoglycemia, a condition resulting from a rapid decline in blood glucose levels.
  • Hypothermia, resulting from extended exposure to extremely cold temperatures.
  • Hypoxia, which occurs when the lungs cannot supply enough oxygen to the blood.
  • Tamponade of the heart, caused by the accumulation of fluid in the protective covering of the heart, the pericardium.
  • Tension pneumothorax, caused by changes in the chest cavity pressure that make it impossible to inhale.
  • Thrombosis of the coronary or pulmonary vessels, preventing the perfusion of blood and oxygen to the cardiac and pulmonary tissues.
  • Trauma, which may cause any of the above causes of arrest.
  • Toxins that add strain on the body, leading to the aforementioned causes of arrest, including severe allergen responses, such as anaphylactic shock.

PEA can be caused by any H or T factor, but they have a few things in common. For example, any condition that causes decreased heart contractility can lead to PEA. Even if the heart contracts, there may not be enough blood flow to support reoxygenation and provide nutrients through the coronary vessels if the blood volume is insufficient due to hemorrhage. Additionally, depletion of ATP reserves, the energy compound of cellular tissues, could lead to a failure of the heart to release after contract. ATP serves as the release mechanism when it binds to sarcomeres after an action potential occurs within the muscle. However, the most common reversible causes of PEA remain pump failure, obstruction to circulation (for example choking or trauma), and hypovolemia.

Pediatric Cardiac Emergencies: PEA and Bradycardia

The guidelines for Pediatric Advanced Life Support (PALS) have a different approach to diagnosing PEA in children compared to adults. In adults, a pulse rate below 60 beats per minute is considered bradycardia and may indicate a need for intervention, such as chest compressions. However, children have a faster heart rate than adults, so the threshold for bradycardia is higher. Thus, healthcare professionals must carefully monitor them for any signs of decompensation and onset of bradycardia.

According to the ILCOR, chest compressions, and PEA treatment should be initiated when a child’s heart rate falls below a certain threshold. The specific threshold varies by age, but it is generally higher than 60 BPM. By treating PEA early and effectively, health professionals can improve the child’s chances of achieving the return of spontaneous circulation.

Shock Initiation: PEA and Asystole Cases

doctors-doing-cpr-with-an-er-patient PEA and asystole are both rhythms in cardiac arrest that are not responsive to electric shock. The primary focus of health professionals is to identify and treat the underlying causes of arrest in order to restore a shockable rhythm. It is essential for health professionals to continue the ACLS algorithm and assess possible causes of arrest and perform differential diagnoses until a shockable rhythm is achieved. It is important to stay vigilant during the process and to monitor for any change in the rhythm from non-shockable to shockable, such as ventricular or atrial fibrillation. If a pulse is found, the EKG reading should be reviewed to confirm that the pulse is not just residual from chest compressions.

In simpler terms, the electrocardiogram (EKG) reading for ventricular fibrillation will appear as a squiggly line with no clear P-wave, QRS complex, or T-wave, but with some activity still present. When the electrical activity reaches a point beyond fibrillation and does not correspond to any recognizable rhythm, it is classified as PEA. However, any arrhythmia can lead to PEA if it fails to generate a pulse and provide adequate blood flow. In these cases, the underlying cause of the arrest is interfering with either the accuracy of the EKG or the body’s ability to maintain a regular rhythm due to factors such as trauma.

When a patient returns to a shockable rhythm, it’s crucial to administer the shock and continue with the ACLS algorithm without delay. Time is of the essence, and any delays can reduce the chances of a positive outcome. Healthcare providers must recognize and understand that both PEA and asystole are non-shockable rhythms that require immediate intervention to treat the underlying causes of cardiac arrest. It’s important to note that the administration of high-dose epinephrine and traditional vasopressors has undergone changes in recent years. However, individual healthcare facilities may follow additional protocols that differ from the recommendations of governing bodies like the ILCOR. Therefore, all providers should check with their respective employers or facilities to understand the appropriate ACLS protocol for their settings. Prompt and effective interventions following ACLS protocols can increase the likelihood of successful resuscitation and improve patient outcomes.

Vasopressor Use in Advanced Cardiac Life Support

According to the updated guidelines from ILCOR, the use of atropine during ACLS is no longer recommended when either PEA or asystole occurs. Although atropine has been used in the past and had some evidence to support its use, studies have shown that it does not provide any therapeutic benefit to patients. Therefore, it has been removed entirely from the ACLS algorithm. In contrast, standard dose epinephrine is still recommended as the vasopressor of choice during ACLS. However, alternative vasopressors, such as vasopressin, may be used in addition to or in place of epinephrine at the discretion of the healthcare provider. It is important to note that healthcare professionals who do not have the level of health provider, such as nurses, should follow the appropriate facility protocol or consult with the ordering physician, physician’s assistant, or nurse practitioner before making any treatment decisions.

There is a specific situation where high-dose epinephrine may be appropriate in treating cardiac arrest, and that is in cases of anaphylactic shock. Anaphylactic shock occurs when the immune system reacts to an allergen, such as an insect sting or food allergy. In this scenario, high-dose epinephrine can be used to combat the cause of arrest, which is the obstruction of the airway due to the allergic response. This approach can improve the chances of survival and positive outcomes for the patient. However, it is important to note that this is the only instance where high-dose epinephrine should be used in conjunction with the ACLS algorithm, and healthcare providers should be aware of the appropriate protocol for administering epinephrine in this situation.

How to Differentiate between Asystole and PEA?

Recognizing the difference between PEA and asystole in order to effectively treat a patient experiencing cardiac arrest is of crucial importance. The absence of a pulse is not the same as asystole, even though both terms have similar connotations. Asystole is a flatline EKG reading, indicating the complete cessation of all electrical activity within the heart. PEA, on the other hand, may show some random, fibrillation-like activity on the EKG, but it does not reach the level of actual fibrillation. It is crucial to recognize the difference between the two rhythms and understand how to treat someone in either condition to increase the chances of ROSC.

Share your experiences with recognizing and treating these rhythms. This can facilitate learning and promote best practices to other readers alike. Use our comments section to write your thoughts. Additionally, I recommend enrolling in a life-saving skills course, such as the one available here, to learn more about identifying and treating asystole, PEA, and other life-threatening arrhythmia.

Share your wisdom and thoughts in the comments below, or head on over to the discussion about this post on Facebook.

About Mackenzie

Mackenzie is a seasoned life saver and a multifaceted professional in the medical field. With an impressive 8-year track record in medical education, Mackenzie boasts a comprehensive set of certifications, including ACLS, PALS, BLS, and CPR, which reflect her unwavering commitment and expertise. Her significant contributions to teaching and the development of medical content underscore her profound knowledge and dedication to advancing healthcare.

Beyond her medical prowess, Mackenzie seamlessly integrates her passion for education with her proficiency in media and marketing. Her academic journey at Indiana University culminated in a degree in Media and Marketing, further solidifying her expertise in these domains.

In addition to her impressive professional achievements, Mackenzie possesses a refined taste for global exploration, photography, design aesthetics, sartorial elegance, and the culinary arts, with a particular affinity for Chinese cuisine. Currently based in the vibrant city of Manhattan, she continues to be a driving force in the medical community. She is an invaluable asset to, where she champions excellence and innovation with unwavering dedication.

Contact Mackenzie at [email protected]


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