Reversible Causes of Cardiac Arrest: Hs and Ts

What Are the Hs and Ts?

Related Video – One Quick Question: What are Hs and Ts?

Sudden cardiac death occurs in less than one minute from the onset of symptoms.¹ It can even happen to a healthy, asymptomatic person. That phenomenon makes it difficult to classify the primary cause of mortality in sudden cardiac arrest. However, there are transient causes of cardiac arrest that are reversible, such as:

• Myocardial ischemia and reperfusion
• Hemodynamic dysfunction
• Electrolyte imbalance
• Changes in physiologic pH or partial-pressure oxygen
• Effects of central and peripheral neurophysical action
• Transient effects of toxins and drugs
• Alcohol

The Hs and Ts mnemonic is a valuable tool to help healthcare providers remember the reversible causes of cardiac arrest:

• Hs: Hypovolemia, hypoxia, hydrogen ion (acidosis), hypo- or hyperkalemia, and hypothermia
• Ts: Tension pneumothorax, tamponade (cardiac), toxins, and thrombosis (pulmonary or cardiac)

Related Video – Introduction to the Hs and Ts

Hypovolemia

Excessive fluid loss due to severe diarrhea is a common cause of hypovolemia. 

Viral or bacterial pathogens can cause diarrhea from acute gastroenteritis. It can also be due to an autoimmune process, such as celiac disease, where gluten in the diet damages the intestinal villi and its lining. Extensive intestine dysfunction causes irritable bowel syndrome, which is a chronic disease. Some medications also cause diarrhea.

Fluid loss from diarrhea is a common cause of hypovolemia.

Excessive sweating leads to excessive fluid loss as well. Common causes include heatstroke in the older population where the risk of mortality is high. Prolonged exercise or exposure to a hot climate is also a risk factor. 

Traumatic blood loss is also a common cause of hypovolemia.²

To correct hypovolemia, intravenous fluid resuscitation is recommended.

Related Video – Hs and Ts – Hypovolemia

Hypoxia

Hypoxia occurs when there is a lack of oxygen perfusion to tissues. Ventilation problems are common causes of hypoxia.³ Examples are acute exacerbations of chronic obstructive pulmonary disease (COPD) and asthma. Foreign body obstruction or other causes of significant upper airway obstruction, such as croup or epiglottitis, cause hypoxia as well.

Hypoxia is treated by clearing the airway obstruction, such as by using bronchodilators and corticosteroids for asthma, COPD, croup, and epiglottitis, or extracting the foreign body obstruction. 

Treating hypoxia also requires oxygen supplementation via a nonrebreather oxygen mask or nasal cannula if the patient is breathing spontaneously. Treatment may necessitate the use of positive-airway pressure or mechanical ventilation if the patient has abnormal breathing.

Foreign body obstruction, or choking, can lead to hypoxia.

Related Video – Hs and Ts – Hypoxia

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Hydrogen Ion (Acidosis)

Carbon dioxide is acidic. Respiratory acidosis is due to the increased serum concentration of carbon dioxide due to a decreased respiratory drive from the patient. Correcting respiratory acidosis can be achieved by providing positive-pressure ventilation.⁴

Metabolic acidosis can be due to intestinal losses from diarrhea or kidney problems. Sodium bicarbonate can temporarily correct metabolic acidosis while the provider diagnoses and treats the underlying cause of metabolic acidosis.

Acidosis is broadly categorized as respiratory acidosis and metabolic acidosis.

Related Video – Hs and Ts – Hydrogen Ions

Hypokalemia or Hyperkalemia

Hypokalemia and hyperkalemia are electrolyte imbalances that pertain to serum potassium concentration.⁵ Potassium is an electrolyte that myocytes (cardiac cells) need to contract. Sudden increases or losses of potassium can cause sudden cardiac death. Diarrhea, vomiting, and the use of diuretic medications can cause hypokalemia.

The kidneys excrete potassium. Patients with severe kidney disease can have an abnormal increase in potassium excretion.

Cells store potassium. When there is massive cellular destruction, potassium will leak into the systemic circulation. Also, low levels of insulin depress the activity of the sodium-potassium adenosine triphosphate (ATPase) pump in the cell walls. In this instance, potassium does not enter the cell and eventually increases serum potassium concentration or hyperkalemia. 

Metabolic acidosis causes a shift of potassium from the cells into the systemic circulation in exchange for extracellular hydrogen ions. That also causes hyperkalemia.

To correct hyperkalemia, providers identify and treat the underlying cause. If there is kidney failure, hyperkalemia may be treated with hemodialysis. If there are low levels of insulin, administering exogenous insulin corrects hyperkalemia. If there is metabolic acidosis, sodium bicarbonate is a solution.

Related Video –  Hs and Ts – Hypo-Hyper Kalemia

Hypothermia

Hypothermia is a core body temperature of less than 36.0^°C. As soon as the core temperature drops below 30^°C, the body experiences bradycardia, which compromises cardiac output and eventually causes cardiac arrest.

Patients at risk are those exposed to an extremely cold environment. Cardiopulmonary resuscitation and defibrillation are ineffective if the core temperature is below 30^°C; hence, rewarming is a priority.

Read: Asystole and Pulseless Electrical Activity Algorithm

Tension Pneumothorax

When the integrity of the lungs is compromised, air will leak into the pleural space.⁶ The increase in air pressure compresses the mediastinal structure and the contralateral lung. When mediastinal structures are compressed, venous return to the heart is decreased. 

Treatment involves decreasing the air pressure in the mediastinum. Needle decompression in minor cases or closed tube thoracostomy in severe cases is the treatment of choice.

Related Video – Hs and Ts: Tension Pneumothorax

Tamponade, Cardiac

Cardiac tamponade is the accumulation of fluid or blood within the pericardium, which causes an increase in pressure between the pericardial space and myocardium, restricting the heart’s motion. That accumulation of fluid can be exudative or transudative secondary to an infection or inflammation. Pericardiocentesis or emergency thoracotomy in severe cases is used to relieve fluid accumulation.

Cardiac tamponade is the accumulation of fluid within the pericardium.

Related Video – Hs and Ts: Cardiac Tamponade

Toxins

A common cause of reversible sudden cardiac arrest is the ingestion of toxins, such as recreational or prohibited drugs, or an overdose of certain prescribed or ethical drugs. 

Prolonged QT interval is a common finding. Supportive care reverses this presentation. Likewise, an antidote or reversing agent must be given to prevent the development of sudden cardiac arrest due to toxins. For example, the effects of a narcotic overdose with opiates can be treated with naloxone.⁷

Thrombosis, Pulmonary

Pulmonary embolism is caused when a thrombus travels from the body and lodges in the pulmonary arteries or its branches. Patients at risk for thrombus formation, such as patients with chronic venous insufficiency, nonambulant patients, or those with cardiac dysrhythmia, are susceptible to pulmonary embolism. 

Patients experiencing pulmonary embolism will report chest pain and sudden shortness of breath. It is treated by fibrinolytic therapy, anticoagulant therapy, or embolectomy.

In pulmonary embolism, a clot travels from the body to the right chambers of the heart, then lodges in the lungs.

Related Video – Hs and Ts – Pulmonary Embolism

Thrombosis, Coronary

Coronary thrombosis occurs when a thrombus restricts blood flow inside a coronary artery.⁸ If the perfusion is large enough, cardiac arrest will likely occur. Treatment involves angioplasty or coronary artery bypass graft surgery.

A coronary thrombus is a blood clot in the coronary artery that can lead to restricted blood flow.

This article summarizes the conditions that make up the Hs and Ts mnemonic. The Hs and Ts mnemonic is useful for remembering the reversible causes of cardiac arrest.