The phrase “H’s and T’s” is a mnemonic used by emergency medical professionals in cardiopulmonary resuscitation (CPR) protocols. This list represents the most common and potentially reversible causes of cardiac arrest, particularly those presenting as Pulseless Electrical Activity (PEA) or Asystole. Identifying the underlying cause is a primary goal during advanced life support, as chest compressions alone do not fix the problem that caused the heart to fail. The H’s and T’s divide these causes into two groups: metabolic and environmental issues (the H’s) and mechanical or vascular obstructions (the T’s).
The Five H’s Metabolic and Environmental Causes
The five H’s represent chemical or thermal imbalances within the body that disrupt the heart’s ability to generate or transmit electrical signals.
Hypovolemia is a low fluid volume in the body, often due to severe bleeding or dehydration. When there is not enough blood circulating, the heart lacks the fluid pressure required to pump effectively. This leads to a profound drop in cardiac output and subsequent arrest.
Hypoxia is an inadequate supply of oxygen reaching the body’s tissues. The heart muscle, known as the myocardium, has a high need for oxygen, and a lack of it starves the cells of the energy required for contraction. This condition commonly results from issues like airway obstruction, severe asthma, or drowning, quickly leading to the heart’s electrical system failing.
Hydrogen ion excess, or acidosis, occurs when too much acid builds up in the bloodstream. This chemical imbalance impairs the effectiveness of the heart muscle and reduces its response to medications used during resuscitation. Acidosis can be caused by respiratory problems, which prevent the removal of carbon dioxide, or metabolic issues, such as the buildup of lactic acid.
Hypo/Hyperkalemia describes an imbalance of potassium in the blood. Potassium is an electrolyte that plays a direct role in generating the electrical impulse that drives the heartbeat. Levels that are too high or too low destabilize the heart’s electrical rhythm, leading to life-threatening arrhythmias and cardiac arrest.
Hypothermia refers to a significantly low core body temperature, typically below 95°F (35°C). Cold temperatures slow down all metabolic processes, including the heart’s electrical activity and muscle contraction. Severe hypothermia can slow the heart to a stop or trigger chaotic electrical activity.
The Five T’s Mechanical and Vascular Issues
The five T’s represent mechanical or circulatory obstructions that physically prevent the heart from filling with blood or block blood flow.
Tension pneumothorax occurs when air builds up in the chest cavity surrounding a lung and cannot escape. This increasing pressure pushes the entire mediastinum, including the heart and major blood vessels, to the side. This physically collapses the large veins that return blood to the heart.
Tamponade (cardiac) is a condition where fluid, usually blood, accumulates in the pericardial sac surrounding the heart. Because the sac cannot stretch quickly, the accumulated fluid compresses the heart, preventing its chambers from fully relaxing and filling with blood. This obstruction drastically reduces the amount of blood the heart can pump with each beat.
Toxins encompass the third T and refer to poisons, drug overdoses, or chemical exposures that directly impair the heart muscle or disrupt its electrical system. Many medications and illicit substances can cause heart failure by depressing the function of the heart’s pumping chambers or by inducing chaotic electrical rhythms. This category focuses on the direct pharmacological impact of a foreign substance.
The final two T’s both involve Thrombosis, or the formation of a blood clot, in two different locations.
Thrombosis (coronary) is a heart attack, where a clot blocks one of the arteries supplying the heart muscle itself. This blockage causes the heart muscle to rapidly die from lack of oxygen, leading to electrical instability and pump failure.
Thrombosis (pulmonary), also known as a massive pulmonary embolism, occurs when a large blood clot lodges in the main artery going to the lungs. This clot creates a sudden and complete blockage of the blood flow from the right side of the heart to the lungs. The heart fails due to the insurmountable back-pressure.
Why Identifying These Causes Matters
The recognition of the H’s and T’s is a core aspect of advanced emergency care because these factors represent conditions that are potentially reversible. Unlike many causes of cardiac arrest, where the heart muscle or electrical system has failed irreversibly, these ten conditions have a specific, identifiable physical or chemical cause. If the underlying problem can be quickly identified and corrected, the heart often has the ability to resume a normal, effective rhythm.
The time-sensitive nature of treatment means that emergency teams must rapidly check this list while simultaneously performing CPR. For instance, draining the fluid in a cardiac tamponade or releasing the pressure in a tension pneumothorax can restore circulation almost immediately. A delay in recognizing one of these reversible causes significantly reduces the chance of survival because chest compressions alone cannot overcome a mechanical blockage or a severe chemical imbalance. Focusing on this mnemonic allows medical personnel to move beyond general resuscitation and deliver targeted interventions.