When a person experiences cardiac arrest, their heart stops pumping blood effectively. While many associate cardiac arrest with the need for an electrical shock, not all heart rhythms respond to this treatment. Some rhythms are classified as “non-shockable,” meaning a defibrillator will not be effective in restoring normal heart function. These rhythms require different, targeted interventions to improve the chances of survival.
Understanding Non-Shockable Rhythms
Non-shockable rhythms are heart patterns observed during cardiac arrest where defibrillation is not indicated. The two primary non-shockable rhythms are Asystole and Pulseless Electrical Activity (PEA). Asystole, often called “flatline,” signifies a complete absence of electrical activity in the heart. On an electrocardiogram (ECG), it appears as a flat line.
Pulseless Electrical Activity (PEA) presents differently, as there is organized or semi-organized electrical activity visible on the ECG. Despite this electrical activity, the heart is not effectively pumping blood, resulting in no palpable pulse. In PEA, the electrical signals are either too weak or too disorganized to produce a mechanical contraction strong enough to circulate blood throughout the body.
The Science Behind Why Defibrillation Fails
Defibrillation is a medical procedure that delivers an electrical shock to the heart. Its purpose is to “reset” or stop chaotic, disorganized electrical activity, such as ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT). This allows the heart’s natural pacemaker to potentially resume a normal, organized rhythm. Defibrillation is not intended to “jump-start” a heart that has no electrical activity.
For Asystole, defibrillation is ineffective because there is no electrical activity present in the heart to “reset.” Delivering a shock to an electrically silent heart would be futile and consume valuable time during a cardiac arrest.
For PEA, defibrillation also fails because the underlying problem is not chaotic electrical activity. While electrical signals are present, they are not the rapid, uncoordinated type that defibrillation is designed to correct. The core issue in PEA is a mechanical failure of the heart to contract effectively and pump blood, despite some electrical signals.
Responding to Non-Shockable Rhythms
Since defibrillation is not an option for non-shockable rhythms, immediate and sustained interventions are necessary. High-quality cardiopulmonary resuscitation (CPR) is the primary treatment. Continuous chest compressions manually circulate blood to the brain and other vital organs, providing temporary support until underlying issues can be addressed.
Medication administration is another important component of managing non-shockable rhythms. Epinephrine, also known as adrenaline, is typically administered to improve blood flow and potentially enhance heart function. This medication helps to constrict blood vessels and increase blood pressure, aiding in perfusion during resuscitation efforts.
An important aspect of managing non-shockable rhythms involves identifying and treating the reversible causes that led to the cardiac arrest. These causes are often remembered using mnemonics like the “H’s and T’s.” Successfully treating these underlying conditions is often important for achieving a return of spontaneous circulation and improving patient outcomes.
Reversible Causes: The H’s and T’s
The “H’s” include:
- Hypovolemia (low blood volume)
- Hypoxia (lack of oxygen)
- Acidosis (excess acid in the blood)
- Hyperkalemia or hypokalemia (electrolyte imbalances)
- Hypothermia (low body temperature)
The “T’s” refer to:
- Toxins (drug overdose or poisoning)
- Cardiac tamponade (fluid around the heart)
- Tension pneumothorax (collapsed lung with pressure buildup)
- Thrombosis (blood clots in the heart or lungs)