Advanced Cardiac Life Support (ACLS) provides healthcare professionals with standardized protocols for managing life-threatening cardiac emergencies. Cardiac arrest occurs when a heart rhythm disturbance causes the heart to stop pumping blood effectively, leading to collapse and loss of consciousness. The H’s and T’s are a mnemonic used within ACLS guidelines to rapidly identify underlying causes that can be reversed. These factors must be corrected to achieve a successful outcome.
The Critical Role of Reversible Causes in ACLS
The ten H’s and T’s are prioritized because cardiac arrest often has an underlying reversible cause, especially in rhythms like Pulseless Electrical Activity (PEA) and Asystole. In these rhythms, the heart’s electrical system is not generating a shockable rhythm, making immediate defibrillation ineffective. The arrest is due to a secondary mechanical or metabolic problem, not a primary electrical malfunction. Successful resuscitation hinges on the rapid identification and reversal of these underlying issues.
The H’s: Metabolic and Environmental Factors
The five H’s represent metabolic imbalances or environmental conditions that disrupt the body’s internal chemistry, leading to cardiac arrest.
Hypovolemia, or low blood volume, is a common and treatable cause, often resulting from rapid fluid loss like severe bleeding or dehydration. This reduction in circulating volume lowers blood pressure, preventing the heart from pumping enough blood to the organs. Diagnostic clues include a history of trauma or severe fluid loss coupled with a lack of palpable pulse despite electrical activity.
Hypoxia describes a lack of adequate oxygen supply to the body’s tissues, which is a frequent non-cardiac cause of arrest. When oxygen levels drop severely, heart muscle cells cannot produce the energy needed for effective mechanical contraction. This profound oxygen deprivation causes the heart to develop Pulseless Electrical Activity (PEA) before progressing to Asystole.
Hydrogen ion excess, or Acidosis, occurs when there is an abnormal buildup of acid in the bloodstream, often resulting from prolonged low blood flow or poor ventilation. This highly acidic environment impairs the heart muscle’s ability to contract and makes it less responsive to epinephrine. An arterial blood gas (ABG) test is necessary to quickly confirm this imbalance.
Hypo/Hyperkalemia refers to dangerously low or high levels of the electrolyte potassium, which is directly involved in the electrical signaling of the heart muscle. High potassium (Hyperkalemia) can cause characteristic ECG changes, such as tall T-waves and a widening QRS complex. Conversely, low potassium (Hypokalemia) can also destabilize the heart’s electrical system.
Hypothermia is defined as a core body temperature below 95°F (35°C), which slows down all metabolic and electrical processes. Extreme cold makes the heart muscle irritable and prone to life-threatening arrhythmias. Resuscitation efforts are difficult until the patient is warmed.
The T’s: Mechanical and Vascular Obstructions
The five T’s describe mechanical or vascular obstructions that physically impede the heart’s function or block blood flow.
Toxins, or drug overdose/poisoning, can cause cardiac arrest by directly disrupting the heart’s electrical system or by suppressing the central nervous system. Prolonged QT intervals on the ECG can suggest a toxin-related issue, and treatment often involves administering an antidote or supportive care.
Tamponade (Cardiac) is where fluid or blood accumulates in the sac surrounding the heart, putting pressure on the heart chambers. This pressure prevents the ventricles from filling properly, severely reducing the amount of blood the heart can pump. Clinical signs include muffled heart sounds, low blood pressure, and distended neck veins.
Tension Pneumothorax occurs when air enters the space around a lung and becomes trapped, creating immense pressure that collapses the lung. This pressure shifts the mediastinum, kinking the major blood vessels and severely limiting the return of blood to the heart. Signs include unequal chest rise and absent breath sounds on the affected side.
Thrombosis (Coronary), commonly known as a massive heart attack, involves a blood clot blocking a coronary artery. The resulting lack of oxygen causes the heart muscle to die, leading to electrical instability and cardiac arrest.
Thrombosis (Pulmonary), or a massive pulmonary embolism, occurs when a large blood clot travels to the lungs and blocks the main pulmonary artery. This obstruction prevents blood from flowing from the right side of the heart into the lungs to pick up oxygen, causing catastrophic failure of the circulatory system.
Clinical Application and Immediate Treatment Focus
The resuscitation team works to rule out or confirm these H’s and T’s simultaneously with performing chest compressions. This diagnostic process involves obtaining a rapid history and using immediate bedside tools like point-of-care ultrasound (POCUS) to look for fluid around the heart or a collapsed lung. Urgent lab work, such as checking blood glucose and potassium levels, also helps narrow the possible metabolic causes.
Once a cause is identified, treatment involves specific, non-defibrillatory interventions tailored to the underlying problem. For instance, a collapsed lung from Tension Pneumothorax requires a needle decompression to relieve the pressure. Hypovolemia demands a rapid infusion of fluids or blood products. These targeted actions aim to correct the underlying physiological derangement, giving the heart a chance to restart effective pumping.