ACLS Provider: Course

Heart Rhythms

Accurate interpretation of heart rhythms is a cornerstone of Advanced Cardiac Life Support (ACLS). The ability to quickly and correctly identify these rhythms on an electrocardiogram (ECG), whether displayed on a cardiac monitor or a printed ECG strip, is crucial for initiating appropriate and timely interventions during cardiac emergencies. This lesson provides a comprehensive overview of key heart rhythms encountered in ACLS, focusing on differentiating between shockable and non-shockable rhythms, as well as addressing bradycardia and tachycardia, conditions requiring distinct management approaches.

Rhythm Categories:

In ACLS, cardiac rhythms are broadly classified into two primary categories: Shockable Rhythms and Non-Shockable Rhythms. This fundamental distinction dictates the initial treatment strategy. Shockable rhythms (Ventricular Fibrillation and Pulseless Ventricular Tachycardia) are treated with defibrillation, while non-shockable rhythms (Asystole and Pulseless Electrical Activity) require cardiopulmonary resuscitation (CPR) and medication. Additionally, specific attention is given to:

  • Bradycardia: A slow heart rate (typically <50 bpm) that may require intervention if causing symptoms.
  • Tachycardia: A fast heart rate (typically >100 bpm) that is managed based on the patient's stability and the QRS complex width on the ECG.

Shockable Rhythms:

  • Ventricular Fibrillation (VF):
    • ECG Characteristics: VF is characterized by chaotic, disorganized electrical activity in the ventricles. This results in a completely irregular baseline without identifiable P waves, QRS complexes, or T waves. The ECG tracing appears as erratic, undulating waves of varying amplitude and morphology. This chaotic electrical activity prevents effective ventricular contraction, leading to no cardiac output.
    • Clinical Context: VF is the most common initial rhythm observed in sudden cardiac arrest. It represents a complete loss of coordinated ventricular contraction, resulting in no effective cardiac output and thus no palpable pulse. Immediate defibrillation is essential to restore organized electrical activity and potentially restore a perfusing rhythm.
    • Management: Defibrillation is the definitive treatment for VF. The goal is to deliver a controlled electrical shock to depolarize the myocardial cells simultaneously, allowing the heart's natural pacemaker (the sinoatrial node) to regain control and restore a normal heart rhythm. After delivering one shock, high-quality CPR should be resumed immediately for 2 minutes, followed by a rhythm check. This approach maximizes the chances of successful defibrillation and ROSC (Return of Spontaneous Circulation).
  • Pulseless Ventricular Tachycardia (pVT):
    • ECG Characteristics: pVT is characterized by a rapid ventricular rate (typically >100 bpm) with wide QRS complexes (≥0.12 seconds). There are no discernible P waves. The QRS complexes are usually uniform in morphology (monomorphic pVT) but can sometimes vary (polymorphic pVT). Critically, in pVT the patient has no palpable pulse.
    • Clinical Context: Although there is organized electrical activity in the ventricles in pVT, the contractions are ineffective in generating adequate cardiac output. Therefore, like VF, pVT is treated as a shockable rhythm due to the absence of a pulse and lack of effective circulation.
    • Management: The management of pVT is identical to that of VF: immediate defibrillation, followed by high-quality CPR for 2 minutes, then a rhythm check. Epinephrine is administered if defibrillation is unsuccessful in converting the rhythm.

Non-Shockable Rhythms:

  • Asystole:
    • ECG Characteristics: Asystole is characterized by a flatline or near-flatline ECG tracing, indicating the complete absence of electrical activity in the heart. There are no P waves, QRS complexes, or T waves. It is essential to confirm asystole in multiple leads to rule out fine ventricular fibrillation (VF), which can sometimes mimic asystole. Fine VF may be treatable with defibrillation, so careful assessment is crucial.
    • Clinical Context: Asystole often represents the end-stage of cardiac arrest and is associated with a very poor prognosis. It can result from prolonged hypoxia, severe acidosis, or other underlying conditions that have exhausted the heart's electrical and mechanical reserves.
    • Management: Defibrillation is not indicated in asystole. Delivering an electrical shock to a heart with no electrical activity will not be effective. The primary interventions are high-quality CPR and the administration of epinephrine (1 mg IV/IO every 3–5 minutes). It is crucial to identify and treat any reversible causes of cardiac arrest using the “H’s and T’s”:
      • H’s: Hypovolemia, Hypoxia, Hydrogen ion (acidosis), Hypo/hyperkalemia, Hypothermia
      • T’s: Tension pneumothorax, Tamponade (cardiac), Toxins, Thrombosis (pulmonary or coronary)
  • Pulseless Electrical Activity (PEA):
    • ECG Characteristics: PEA is defined as the presence of organized electrical activity on the ECG (e.g., normal sinus rhythm, supraventricular tachycardia, wide QRS complexes) but without a corresponding palpable pulse. This means that the heart's electrical system is generating a rhythm, but the heart muscle is not contracting effectively or at all.
    • Clinical Context: In PEA, the electrical activity is present, but there is a mechanical problem with the heart's ability to pump blood. This can be caused by various underlying conditions that impair cardiac function, such as severe hypovolemia (low blood volume), massive pulmonary embolism (blood clot in the lungs), cardiac tamponade (fluid buildup around the heart), or tension pneumothorax.
    • Management: The management of PEA focuses on high-quality CPR, epinephrine administration (1 mg IV/IO every 3–5 minutes), and most importantly, identifying and treating the underlying cause. The “H’s and T’s” are crucial in guiding the search for reversible causes and directing appropriate therapy. Addressing the underlying cause is the key to potentially restoring a perfusing rhythm.