The 5 H’s and 5 T’s are mnemonics used in advanced resuscitation protocols. These ten factors represent the most common underlying, reversible causes of cardiac arrest. They provide a structured framework for medical professionals to quickly identify and address systemic issues that caused the heart to stop beating. Rapidly recognizing and treating these causes is paramount. Patient survival often depends on correcting the root problem while simultaneously performing chest compressions.
The Critical Context of Reversible Causes
These diagnostic tools are especially pertinent in cases of Pulseless Electrical Activity (PEA) or asystole, which are non-shockable cardiac rhythms. In PEA, the heart’s electrical system generates a rhythm, but the muscle cannot contract forcefully enough to produce a detectable pulse or adequate blood flow. This mechanical failure signals a profound systemic problem or a massive obstruction that must be relieved. Ventricular fibrillation (VF) or ventricular tachycardia (VT) involve chaotic electrical rhythms that can be treated with defibrillation. For PEA and asystole, defibrillation is ineffective because the electrical problem is secondary to a fundamental lack of oxygen or a mechanical obstruction. Therefore, the 5 H’s and 5 T’s guide the care team to the underlying systemic failure, which must be corrected to restore circulation.
Defining the 5 H Factors
The five H factors primarily represent metabolic, environmental, or volume-related issues that impair the heart’s ability to function at a cellular level.
Hypovolemia
Hypovolemia refers to a significant loss of blood or fluid volume within the circulatory system. This loss can be due to external hemorrhage, severe dehydration, or internal bleeding, such as a ruptured aortic aneurysm. When total blood volume drops critically, the heart has insufficient fluid to pump, leading to decreased cardiac output and eventual cardiac arrest. Clinical signs include a history of fluid loss, a rapid heart rate, and low blood pressure.
Hypoxia
Hypoxia is a condition where the body is deprived of an adequate oxygen supply. A lack of oxygen causes heart muscle cells to switch to anaerobic metabolism, depleting energy stores and leading to cellular dysfunction. Even if the heart’s electrical system functions, the muscle cannot generate the mechanical force needed to pump blood, resulting in PEA. Interventions focus on ensuring a clear airway, providing high-concentration oxygen, and confirming proper ventilation to correct the deficit.
Hydrogen ion (Acidosis)
Acidosis is an excessive buildup of hydrogen ions, leading to a low blood pH. This can be caused by respiratory failure (retaining carbon dioxide) or metabolic issues (producing too much acid, such as lactic acid). Severe acidosis directly depresses the heart muscle’s contractility and reduces its responsiveness to medications like epinephrine. Diagnosis is typically confirmed using an arterial blood gas test to distinguish between respiratory and metabolic causes.
Hypo-/Hyperkalemia
Hypo-/Hyperkalemia refers to dangerously low or high levels of potassium in the blood. Potassium is an electrolyte that plays a crucial role in maintaining the electrical stability and contraction of the heart muscle. Both extremes can trigger life-threatening arrhythmias. Hyperkalemia often presents on an electrocardiogram (ECG) with tall, peaked T-waves, while hypokalemia may present with flattened T-waves.
Hypothermia
Hypothermia is an abnormally low core body temperature, typically defined as below 95°F (35°C). As the body temperature drops, the heart muscle becomes irritable and susceptible to lethal arrhythmias like ventricular fibrillation. Hypothermia slows down the body’s metabolism and impairs the effectiveness of resuscitation drugs and defibrillation. The primary treatment is active core rewarming, and resuscitation efforts are often prolonged until the patient is warmed.
Defining the 5 T Factors
The five T factors generally represent mechanical or pharmacological problems that obstruct blood flow or poison the heart muscle, leading to circulatory collapse.
Tension Pneumothorax
A tension pneumothorax occurs when air accumulates in the chest cavity, typically due to a one-way valve effect from an injury. The trapped air builds up pressure, causing the lung to collapse and pushing the heart and great vessels to the opposite side. This severe pressure prevents blood from returning to the heart, causing obstructive shock and cardiac arrest. Immediate signs include absent breath sounds, distended neck veins, and a deviated trachea, prompting urgent needle decompression.
Tamponade (Cardiac)
Cardiac tamponade is a mechanical compression of the heart caused by a rapid accumulation of fluid, usually blood, in the sac surrounding the heart (pericardium). This pressure prevents the heart chambers from filling completely during diastole, severely reducing the volume of blood the heart can pump. The resulting obstructive shock leads quickly to cardiac arrest. Diagnosis is often suggested by Beck’s triad—low blood pressure, distended jugular veins, and muffled heart sounds—and confirmed by bedside ultrasound.
Toxins
Toxins, or poisoning, represent the pharmacological causes of cardiac arrest. An overdose or exposure to certain substances interferes with the heart’s function. These agents can disrupt the heart’s electrical system, impair contractility, or cause severe metabolic derangements. Common culprits include opioids, tricyclic antidepressants, and certain beta-blockers. The immediate approach involves supportive care, CPR, and, if possible, the administration of a specific antidote.
Thrombosis (Pulmonary)
Pulmonary thrombosis, or a massive pulmonary embolism (PE), is the blockage of the pulmonary artery by a blood clot. This blockage creates overwhelming resistance to blood flow from the right side of the heart, causing acute right-heart failure. The resulting circulatory collapse is a sudden cause of cardiac arrest. Diagnosis is suspected in patients with clotting risk factors, and treatment often requires clot-dissolving medications (thrombolytics).
Thrombosis (Coronary)
Coronary thrombosis is the formation of a blood clot in a coronary artery, causing an acute myocardial infarction (MI). This event cuts off the blood supply to the heart muscle, leading to rapid cell death and electrical instability. The resulting damage and electrical chaos can trigger ventricular fibrillation and subsequent cardiac arrest. This condition often causes a shockable rhythm and requires immediate intervention, typically via urgent cardiac catheterization.