Heart and circulatory diseases cause more than 160,000 deaths in the UK yearly. This is an average of 460 deaths daily, one every 3 minutes.

In England alone, over 6.4 million people are said to be living with cardiovascular illnesses.

This is why some first aiders are also trained to perform Advanced Cardiovascular Life Support (ACLS).

This trains them to properly address and manage any urgent and emergent life-threatening situation that might cause (or have caused) cardiac arrest.

ACLS also involves identifying and addressing the reversible causes of cardiac arrest. For the first aider to easily remember these causes, they’re taught about the 4 Hs and Ts.

Let’s learn more about this in this article. Read on!


The 4 Hs and 4 Ts

The Reversible Causes of Cardiac Arrest: 4 Hs, 4 Ts

Cardiac arrest happens when the heart stops beating. When this occurs, the heart also loses its ability to pump blood throughout the body, thus interrupting blood flow.

Cardiopulmonary resuscitation (CPR) and defibrillation are mainly the treatment for cardiac arrest. 

But, according to Truhlar et al. (European Resuscitation Council, Guidelines for Resuscitation), early recognition, proper management, and calling for help can greatly benefit the patient. 

Performing CPR as a resuscitation attempt is meant to help oxygen and blood circulate in the body. 

Defibrillation, on the other hand, intends to address cardiac arrhythmias, including non-perfusing ventricular tachycardia.

These medical procedures are taught to first aiders and first responders. But, aside from these, they also learn how to identify the reversible causes of cardiac arrest.

With this knowledge, there is a greater chance of saving a person suffering from cardiac arrest and minimising the probability of sudden cardiac death.

Hypoxia (Low Levels of Oxygen)

Hypoxia is one of the leading causes of cardiac arrest. This occurs when there is a sudden drop of oxygen in body tissues.

Oxygen is needed to help organs and tissues perform their specific function, and it’s also required to produce energy.

Prolonged hypoxia can cause damage to the organs, including your heart and brain.

A person suffering from this condition may experience:

  • Restlessness
  • Headache
  • Confusion and anxiety
  • Rapid heart rate and breathing
  • Shortness of breath
  • Slow heart rate
  • Bluish skin, nails, and lips

If you identify a patient with hypoxia, it’s vital to stabilise them IMMEDIATELY.

Providing adequate ventilation and proper oxygenation can aid the air in circulating correctly. Performing high-quality CPR may also aid with such a situation.

When adequate amounts of oxygen are restored, lethal cardiac rhythm is also avoided.

Hypovolemia (Shock)

Hypovolemia occurs when the body loses body fluid (blood or water). When there is a severe loss of fluid (i.e., blood loss), the heart stops pumping blood, and other organs stop functioning.

The common cause of hypovolemia is severe burns and injury, internal bleeding, vasodilation, vomiting, and diarrhoea.

It’s also common among people suffering from illnesses with a sudden onset and those who are critically ill.

When an individual is suffering from this condition, the following symptoms are experienced:

  • Dry skin and mouth
  • Dizziness
  • Difficulty of breathing
  • Rapid
  • Excessive sweating
  • Pale or bluish skin and lips

Rapid infusion of crystalloids or blood products is infused to treat hypovolemia. This addresses the original cause of the condition (blood loss). 


Metabolic disturbances (such as hyperkalemia, hypokalemia, hypoglycemia, and hypocalcemia) are also reversible causes of cardiac arrest.

Hyperkalemia is characterised by abnormally high levels of potassium in the body. This occurs when its levels escalate above 5.5 mmol/L.

While potassium helps in maintaining a normal contraction of the myocardium, too much potassium can damage the heart and cause a heart attack.

Underlying causes of hyperkalemia include kidney disease, metabolic acidosis, diabetes, drugs, and endocrine disorders.

You can treat this by giving the patient sodium bicarbonate or calcium chloride.

Hypokalemia, on the other hand, is the opposite of hyperkalemia. This is characterised by a drop in potassium levels (below 2.5 mmol/L).

Common underlying causes of this condition are diarrhoea, renal losses, metabolic alkalosis, magnesium depletion, and kidney disease.

This is recognised when the electro diagram shows flattened T-waves, prominent U-waves, or widened QRS complex. 

Treatment for hypokalemia depends on its severity, as well as the diagnosis of symptoms and ECG abnormalities.

But, commonly, potassium is gradually replaced with normal serum levels. In emergencies, intravenous potassium is also administered, given that most hypokalemic patients also have hypomagnesemia.

In addition to abnormal potassium levels, hypoglycemia is another reversible cause of cardiac arrest. This involves low blood sugar in the body.

For situations like these, proper treatment should raise the patient’s blood sugar levels. You can provide food rich in protein, fat, and complex carbohydrates or sugary drinks.

Hypocalcemia is another condition that can cause cardiac arrest. It is characterised by low levels of calcium in the blood.

Vitamin D and Vitamin C supplementation and oral calcium pills are usually the treatment for this condition. In severe cases, the patient is administered intravenous calcium gluconate.


According to Brown, Brugger, and Boyd (New England Journal of Medicine), over 1,500 people die from accidental hypothermia in the United States. 

In England and Wales, on the other hand, 330 deaths were associated with such a condition between 2013 and 2018. 

Hypothermia is a condition where the core body temperature drops below 35 °C. Some professionals use the Swiss Staging System to classify hypothermia:

  • Hypothermia I – mild (core temperature 32–35 °C)
  • Hypothermia II – moderate (core temperature 28–32 °C)
  • Hypothermia III – severe (core temperature 24–28 °C)
  • Hypothermia IV – cardiac arrest (core temperature below 24 °C)
  • Hypothermia V – death (core temperature below 13.7 °C)

As the temperature decreases, sinus bradycardia (slow heartbeats) occurs, conversely resulting in atrial fibrillation.

This is followed by ventricular fibrillation (abnormal heart rhythm) and, finally, asystole (flatline).

Before the patient undergoes VF, you must stabilise them by external warming with forced warm air or minimally invasive procedures like warm intravenous infusion.

Thrombosis (Coronary or Pulmonary)

Coronary thrombosis occurs when the clotted blood blocks the coronary artery. Conversely, this also clots blood flow to other body parts, including the heart and brain.

Such a condition often results in a heart attack (myocardial infarction) or stroke.

Coronary thrombosis is usually the cause of cardiac arrest OUTSIDE of the hospital. Diagnosis is quite tricky, especially if the patient is already in cardiac arrest.

However, if the heart’s rhythm is VF, the thrombosis is likely caused by coronary artery disease.

The specific treatment for this type of thrombosis can be coronary angiography, primary percutaneous coronary intervention, and other related procedures.

In cases like this, you can do high-quality CPR if the facility has access to a medical team trained to perform mechanical or hemodynamic support and rescue PPCI with ongoing CPR.

Pulmonary embolism (thrombosis), on the other hand, occurs when a pulmonary artery in a patient’s lungs is blocked.

Most cases of pulmonary embolism originate from deep venous thrombosis. This means that the clotted blood from the legs and other parts of the body is carried to the lungs through blood flow. 

Preceding cardiac arrest due to pulmonary embolism may be identified through the following symptoms:

  • Dyspnea
  • Pleuritic or substernal chest pain
  • Low extremity swelling
  • Cough
  • Hemoptysis

Cardiac arrest caused by pulmonary embolism can be treated by administering fibrinolytic, surgical embolectomy and percutaneous mechanical thrombectomy.

Tension Pneumothorax

Tension pneumothorax is also one of the reversible causes of cardiac arrest. 

Tension pneumothorax develops when there is air buildup in the pleural space. Such a buildup will cause a shift in the mediastinum, and venous return to the heart is obstructed. 

In turn, this condition can rapidly lead to cardiovascular collapse and death

A diagnosis of tension pneumothorax can be identified through clinical examinations. Symptoms of this condition can include:

  • Respiratory distress
  • Absent unilateral breath sounds on auscultation
  • Subcutaneous emphysema
  • Tracheal deviation from the affected side
  • Jugular venous distention

The treatment for tension pneumothorax can either be needle compression or thoracostomy with chest tube placement.

Tamponade (Cardiac)

Cardiac tamponade happens when fluid builds up in the pericardium. Conversely, this results in the heart’s compression and cardiac arrest

This reversible cause of cardiac arrest is usually caused by trauma in the chest (i.e., gunshot or inflammation). 

A tamponade can be recognised if you observe signs like a narrowing pulse pressure, muffled heart sounds, and distended neck veins. It can also be identified with narrow QRS complexes.

The treatment for cardiac arrest caused by traumatic or non-traumatic tamponade is thoracotomy or pericardiocentesis.


Self-poisoning with toxins can also cause cardiac arrest due to airway obstruction and respiratory arrest. 

Typically, this is seen as a prolonged QT interval on the ECG. At the same time, distinct signs and symptoms can be seen depending on the specific toxin. 

Toxic cardiac arrest may be rare, according to Gunja and Graudins (Emergency Medicine of Australia). But, patients suffering from such a condition can BENEFIT from a proper resuscitation attempt.

Certain drugs that are usually involved in overdose are benzodiazepines, opioids, tricyclic antidepressants, local anaesthetics, beta-blockers, and calcium channel blockers:

  • Benzodiazepine overdose is medicated by administering flumazenil to the patient. Flumazenil is an antagonist drug of benzodiazepines. 
  • Opioids overdose, on the other hand, can be treated through their antagonist, naloxone. This can help preserve the respiratory effects of opioid overdose. 
  • Tricyclic antidepressants can similarly cause ventricular arrhythmias. Such a condition can be treated by giving sodium bicarbonate to the patient. 
  • Local anaesthetic systemic toxicity (LAST) happens due to inadvertent vascular injection. This, in turn, causes seizures, bradycardia, asystole or ventricular tachyarrhythmias. When a LAST is recognised in a patient, the best medication to provide is benzodiazepines. They are good anticonvulsant drugs without causing cardiac depression. 
  • Overdose on beta-blockers, on the other hand, may be difficult to treat. Hence, it’s likely to cause cardiac arrest. But, a study showed that glucagon, high-dose insulin and glucose, lipid emulsions, phosphodiesterase inhibitors, extracorporeal and intra-aortic balloon pump support, and calcium salts were said to help in such a condition.
  • Finally, calcium channel blockers and other short-acting drugs can rapidly progress into cardiac arrest. Treatment for this condition involves the administration of calcium chloride. 

The Bottom Line

These reversible causes of cardiac arrest are as important as understanding the protocols of Advanced Cardiac Life Support.

Having a thorough understanding of these reversible causes of cardiac arrest is ESSENTIAL for first aiders and first aid responders in providing appropriate treatment for patients. 

Hence, learning about the 4 Hs and Ts can help determine and treat the cause of pre-arrest and cardiac arrest.

It’s key for optimal care and response that will likely result in positive outcomes. 

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