Asystole

Excerpt

Asystole Overview

Asystole, informally referred to as "flatline," signifies a complete cessation of the heart's electrical and mechanical activity. The condition frequently begins as a nonperfusing ventricular dysrhythmia, specifically ventricular fibrillation or pulseless ventricular tachycardia (pVT). Pulseless electrical activity (PEA) may likewise progress to asystole. Those with sudden cardiac arrest presenting with asystole as the initial rhythm have an extremely poor prognosis. Return of spontaneous circulation (ROSC) is less often achieved when asystole is the initial cardiac rhythm than a shockable rhythm after out-of-hospital cardiac arrest (OHCA). Patients with asystole as the initial cardiac rhythm after OHCA are less likely to survive after 30 days than if other rhythms are initially detected. Asystole represents the terminal rhythm of a cardiac arrest.

Prolonged resuscitation efforts in a patient in asystole are unlikely to provide a medical benefit in OHCA. Termination of resuscitation efforts should be considered for such individuals in consultation with online medical direction, as local protocols allow. The American College of Emergency Physicians and the National Association of Emergency Medical Services Physicians support emergency medical services protocols allowing providers to cease resuscitation efforts in cases where continued interventions and hospital transport offer no chance of patient survival.

The Heart's Electrical Circuity

Like a complex circuit, the heart's electrical conduction system coordinates the spread of electrical impulses to initiate muscle contractions and propel blood. The sinoatrial node in the right atrium acts as the natural pacemaker, initiating the electrical impulse. This impulse travels through internodal pathways to reach the atrioventricular node lying between the atria and ventricles. The atrioventricular node delays the signal for coordinated atrial contraction before ventricular activation. The impulse is then transmitted through the His bundle, a muscular bridge that splits into right and left bundle branches. These branches further divide into Purkinje fibers, which spread the electrical wavefront throughout the ventricles, triggering synchronized contraction of the heart chambers and blood expulsion.

Myocardial voltage-gated channels involved in cardiac pacing include sodium, potassium, and calcium channels. Sodium channels initiate the depolarization phase of the cardiac action potential. Sodium channel activity corresponds to ventricular contraction and the QRS complex on the electrocardiogram (ECG). Potassium channels contribute to repolarization, helping to restore the resting membrane potential. Potassium channel activation corresponds to ventricular relaxation and the T wave on ECG. Calcium channels are involved in both depolarization and repolarization phases and in regulating intracellular calcium levels, which are essential for excitation-contraction coupling and myocardial contraction. Ventricular calcium channel activity contributes to the action potential's plateau phase and the QRS complex. Similar voltage channels exist in the atria. Atrial depolarization is responsible for the P wave on ECG (see Image. Normal Sinus Rhythm on Electrocardiography).

The heart's electrical conduction system is disrupted in cardiac dysrhythmias. This disruption could be due to various factors, including severe ischemia with subsequent myocardial death, electrolyte imbalances hindering electrical flow, or direct myocardial damage from trauma or toxins. The consequence is that the electrical wavefront fails to propagate through the system, leaving the heart in a state of complete electrical and mechanical silence.