21
Cardiac Arrest Rhythms

Amita Sudhir¹ and William J. Brady²

¹ Department of Emergency Medicine, University of Virginia School of Medicine, Charlottesville, VA, USA

² Departments of Emergency Medicine and Medicine, University of Virginia School of Medicine, Charlottesville, VA, USA

In a cardiac arrest, there are several possible rhythms that may result in loss of spontaneous perfusion, including asystole, pulseless electrical activity (PEA) rhythm presentations, pulseless ventricular tachycardia (VT), and ventricular fibrillation (VF).

Asystole

Asystole is a complete lack of electrical activity in the heart, resulting in pulselessness. Primary asystole occurs when there is a problem with the electrical system of the heart and no electrical impulses can be generated. It can be caused by a structural defect affecting the sinoatrial (SA) node, atrioventricular (AV) node, or other part of the conduction system; acute myocardial infarction or non‐ischemic disease such as a tumor or cardiac trauma may be the cause. Primary asystole usually begins as a bradycardia caused by heart block, then proceeds to asystole. Alternatively, a patient who is dependent on his or her pacemaker may develop primary asystole if the pacemaker malfunctions and stops working.

Secondary asystole is caused by factors outside the heart’s electrical system that affect its ability to generate depolarization. Electrolyte abnormalities and acidosis resulting from systemic illnesses can cause this type of asystole; toxicologic issues also can ultimately produce asystole. Untreated VF or pulseless VT can also result in this type of asystole.

Electrocardiographic Manifestations

In either type of asystole, the electrocardiogram (ECG) demonstrates the complete absence of cardiac electrical activity (Figure 21.1) – in essence, the ECG is a “flat line.” The clinician should determine that the equipment, including ECG cables, is functioning appropriately. Furthermore, asystole should be determined in three separate leads since fine VF can mimic asystole in certain ECG leads.

Pulseless Electrical Activity

PEA features the unique combination of no discernible cardiac mechanical activity (i.e. a “pulseless” state) with a persistent, identifiable cardiac electrical activity (i.e. the cardiac rhythm). Electromechanical dissociation, an older term used for PEA, is another descriptive phrase for this malignant cardiac presentation – the complete dissociation of electrical and mechanical activity of the cardiovascular system.

PEA is usually caused by two major mechanisms (Table 21.1):

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