2.6 Reversible Causes of Arrest: The H's

2.6 Reversible Causes of Arrest: The H's

While high-quality CPR and early defibrillation are the cornerstones of cardiac arrest management, resuscitation cannot be successful if the underlying cause of the arrest is not identified and corrected. The ACLS guidelines utilize the mnemonic "H's and T's" to help providers systematically recall and treat the most common reversible causes of cardiac arrest. The "H's" encompass critical metabolic, respiratory, and environmental derangements: Hypovolemia, Hypoxia, Hydrogen ion (acidosis), Hypo-/Hyperkalemia, and Hypothermia. Addressing these factors concurrently with the primary algorithm is essential, particularly in cases of pulseless electrical activity (PEA) and asystole, where reversing the underlying pathology is often the only path to achieving return of spontaneous circulation (ROSC).

Hypovolemia

Hypovolemia is the loss of fluid volume in the circulatory system, leading to decreased preload, drastically reduced cardiac output, and eventual cardiovascular collapse. It is the most common cause of PEA cardiac arrest. The loss can be hemorrhagic (e.g., trauma, gastrointestinal bleeding, ruptured aortic aneurysm) or non-hemorrhagic (e.g., severe dehydration, extensive burns, anaphylaxis causing relative hypovolemia).

In the setting of cardiac arrest, hypovolemia should be suspected if the patient has a known history of bleeding, severe diarrhea/vomiting, or if the ECG shows rapid, narrow-complex tachycardia prior to the loss of pulses. Treatment requires immediate and aggressive volume replacement. For non-hemorrhagic causes, rapid infusion of isotonic crystalloids (normal saline or lactated Ringer's) is indicated. A typical initial fluid bolus is 1 to 2 liters given as quickly as possible. In cases of hemorrhagic shock, while crystalloids can be used initially to restore intravascular volume, the definitive treatment requires blood products (packed red blood cells, fresh frozen plasma, and platelets) and emergent source control, which may necessitate surgical intervention or interventional radiology procedures.

Hypoxia

Hypoxia, or severe oxygen deprivation to the tissues, is a primary cause of cardiac arrest, particularly in out-of-hospital scenarios, drowning, choking, and pediatric arrests. It leads to rapid depletion of cellular adenosine triphosphate (ATP), anaerobic metabolism, and eventually failure of the myocardial sodium-potassium pump, resulting in electrical instability and arrest.

Management of hypoxia during an arrest begins with securing the airway and ensuring adequate oxygenation and ventilation. Providers must verify that the airway is patent and free of obstructions. Interventions range from basic maneuvers (head-tilt/chin-lift, placement of an oropharyngeal or nasopharyngeal airway) to advanced airway placement (endotracheal intubation or supraglottic device). High-flow oxygen (100% FiO2) must be administered. Proper ventilation technique is critical; providers should avoid excessive ventilation rates and volumes, as hyperventilation increases intrathoracic pressure, which decreases venous return to the heart and compromises CPR hemodynamics.

Hydrogen Ion (Acidosis)

Acidosis in cardiac arrest can be respiratory (due to hypoventilation and accumulation of CO2) or metabolic (due to lactic acidosis from tissue hypoperfusion, diabetic ketoacidosis, or renal failure). Severe acidosis impairs myocardial contractility and decreases the efficacy of endogenous and exogenous catecholamines (like epinephrine).

The primary treatment for respiratory acidosis during an arrest is providing adequate, controlled ventilation to blow off excess carbon dioxide. However, metabolic acidosis is more complex. The routine administration of sodium bicarbonate during cardiac arrest is strictly not recommended by the AHA, as it can paradoxically worsen intracellular acidosis and negatively affect survival. Sodium bicarbonate is reserved specifically for arrests known or highly suspected to be caused by pre-existing metabolic acidosis, hyperkalemia, or certain toxicological emergencies, such as tricyclic antidepressant (TCA) overdose, where it helps overcome sodium channel blockade.

Hypokalemia and Hyperkalemia

Electrolyte imbalances, particularly involving potassium, are highly arrhythmogenic. Potassium is critical for maintaining the resting membrane potential of cardiac muscle cells.

• Hypokalemia: Often caused by diuretic use, gastrointestinal losses, or malnutrition. Severe hypokalemia can lead to VF, pVT, or PEA. The ECG may show flattened T waves, prominent U waves, and a widened QRS complex prior to arrest. Treatment involves the careful administration of intravenous potassium and often magnesium, which acts as a necessary cofactor for potassium uptake.
• Hyperkalemia: A frequent cause of arrest in patients with end-stage renal disease, severe crush injuries, or rhabdomyolysis. It classically presents on the ECG with tall, peaked T waves, followed by widening of the QRS complex, loss of P waves, and eventually a sine-wave pattern before progressing to asystole or PEA. The management of hyperkalemic arrest is aggressive and multi-faceted. The absolute first priority is stabilizing the myocardial cell membrane with intravenous calcium (calcium chloride or calcium gluconate). This is followed by interventions to shift potassium intracellularly, primarily using a combination of regular insulin and 50% dextrose (D50), as well as high-dose nebulized albuterol. Finally, elimination of potassium can be facilitated later via dialysis or cation exchange resins, though these are not immediate resuscitative measures.

Hypothermia

Accidental hypothermia is defined as a core body temperature below 35°C (95°F). Severe hypothermia (below 30°C or 86°F) can lead directly to cardiac arrest by depressing the myocardial conduction system, leading to bradycardia, VF, or asystole.

Resuscitating a severely hypothermic patient presents unique challenges. The myocardium becomes highly irritable and resistant to defibrillation and pharmacological therapy. Metabolism of drugs like epinephrine is drastically slowed, leading to potentially toxic accumulation if administered at standard intervals. Therefore, the ACLS guidelines recommend that if the core temperature is below 30°C, providers should focus heavily on high-quality CPR and active core rewarming while withholding IV medications and limiting defibrillation attempts. Active core rewarming techniques include warmed IV fluids, heated and humidified oxygen, pleural or peritoneal lavage with warm fluids, and ideally, extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass. Once the core temperature rises above 30°C, standard ACLS algorithms, including medication administration and repeated shocks, can be resumed. The adage "they aren't dead until they're warm and dead" underscores the importance of prolonged resuscitative efforts in hypothermic arrests.