Heart and circulatory diseases cause around 170,000 deaths in the UK every year, according to the British Heart Foundation. That is around a quarter of all UK deaths, an average of 460 a day, or one every three minutes.
More than 8 million people are living with cardiovascular disease across the UK, including around 7 million people in England. For more on the national picture, see the latest UK heart and circulatory diseases statistics.
Some of those arrests can be reversed if the underlying cause is found and treated. That is the job of Advanced Life Support (ALS) — the in-hospital and pre-hospital standard taught by Resuscitation Council UK to doctors, nurses, paramedics and other healthcare professionals. Outside the UK the same body of practice is often called Advanced Cardiovascular Life Support (ACLS).
ALS teams are trained to manage any urgent and emergent life-threatening situation that might cause (or have caused) cardiac arrest, using drugs, advanced airway management, manual defibrillation and diagnostics that are not available to a lay rescuer.
Part of that training is identifying and treating the reversible causes of cardiac arrest, which clinicians remember as the 4 Hs and 4 Ts.
Who this article is for: this is advanced, clinical material aimed at healthcare professionals and at people studying towards ALS, ILS or an equivalent qualification. It is background reading, not a substitute for an accredited ALS course, and none of it replaces basic life support. If you are a workplace first aider or a member of the public, the actions that save lives are the ones in the basic life support sequence: call 999 for any unresponsive person, start chest compressions, and use a defibrillator as soon as one is available.
Last reviewed: September 2026 — checked against Resuscitation Council UK Guidelines 2025.
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 early recognition, prompt management and calling for help remain the interventions that change outcomes. The current UK authority is Resuscitation Council UK Guidelines 2025: Special circumstances, published on 27 October 2025, alongside the 2025 Adult advanced life support guidelines. UK training materials have had to comply with the 2025 guidelines since January 2026, and they replace the 2021 and 2015 editions that older articles still quote.
The 2025 general recommendation is direct: follow the standard ALS algorithm, and always address hypoxia, hypovolaemia, electrolyte disorders, hypothermia, cardiac tamponade, tension pneumothorax, thrombosis and toxins. Where appropriate, treating a reversible cause takes priority even if chest compressions are briefly interrupted.
Resuscitation Council UK also reports the current UK picture. Around 115,000 out-of-hospital cardiac arrests are reported to UK ambulance services each year, with resuscitation attempted in roughly 43,000 people; about 80 per cent have a cardiac cause. Overall 30-day survival after an out-of-hospital arrest is 9.5 per cent in England and 9.6 per cent in Scotland. In hospital, the National Cardiac Arrest Audit recorded 12,755 cardiac arrests in 2023-24, of which 25.8 per cent of patients survived to discharge — 52.9 per cent where the first rhythm was shockable, but only 11.5 per cent where it was asystole. Those non-shockable rhythms are exactly where finding a reversible cause matters most.
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).
Hyperkalemia/Hypokalemia/Hypoglycemia/Hypocalcemia
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. Resuscitation Council UK treats 6.0–6.4 mmol/L as moderate and anything above 6.5 mmol/L as severe.
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.
Treatment works on three fronts: shift potassium into the cells, protect the myocardium, and remove potassium from the body. RCUK 2025 recommends 10 units of soluble insulin with 25 g of glucose intravenously for moderate and severe hyperkalaemia, with nebulised salbutamol (10–20 mg) as an adjunct, and sodium zirconium cyclosilicate 10 g orally to remove potassium. Where there are ECG changes, give 10 mL of 10% calcium chloride IV over 5 minutes (or 30 mL of 10% calcium gluconate over 10 minutes). In hyperkalaemic cardiac arrest, give 10 mL of 10% calcium chloride and 50 mmol of sodium bicarbonate intravenously, through separate lines or with a flush between them, and consider dialysis or ECPR if the initial attempt is unsuccessful.
Hypokalemia, on the other hand, is the opposite of hyperkalemia: an abnormally low potassium level. RCUK does not manage it by a single number — treatment is guided by the severity of the deficit and by whether there are symptoms or ECG abnormalities.
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 replaced gradually back to normal serum levels, correcting any magnesium deficit at the same time. In hypokalaemic cardiac arrest, RCUK 2025 recommends 20 mmol of potassium chloride IV or IO over 2–3 minutes, followed by 10 mmol over 2 minutes, then rechecking the potassium level and adjusting the infusion rate.
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. Oral glucose — a sugary drink or glucose gel, followed by longer-acting carbohydrate — is only appropriate for someone who is fully conscious and able to swallow safely. Never give food or drink to a patient with reduced consciousness or an unprotected airway. In that situation, and in cardiac arrest, blood glucose is corrected with intravenous glucose or intramuscular glucagon.
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.
Hypothermia
Accidental hypothermia is uncommon in the UK but disproportionately important, because a hypothermic patient can survive a prolonged arrest with a good neurological outcome. A study of England and Wales identified 330 deaths associated with accidental hypothermia over the six years from 2013 to 2018 — a historical figure, but one that illustrates how rarely most clinicians meet it.
Hypothermia is a condition where the core body temperature drops below 35 °C. Resuscitation Council UK asks clinicians to measure core temperature with a low-reading thermometer, and to use the Swiss Staging System only when core temperature cannot be measured. The 2025 guidelines use four stages, not the five that older articles list:
- Stage I (mild) – conscious and shivering, estimated core temperature 35–32 °C
- Stage II (moderate) – impaired consciousness such as confusion or lethargy, shivering reduced or absent, estimated core temperature 32–28 °C
- Stage III (severe) – unconscious but with a pulse and breathing present, estimated core temperature 28–24 °C
- Stage IV (deep) – vital signs absent, estimated core temperature below 24 °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 deteriorates into VF, stabilise them with external warming with forced warm air or minimally invasive procedures such as warmed intravenous infusion.
The 2025 guidelines also changed how hypothermic arrest is managed. Transfer patients in hypothermic cardiac arrest, and those at imminent risk of it, directly to an ECPR centre for rewarming with veno-arterial ECMO. Below 28 °C, delayed or intermittent CPR is acceptable where continuous CPR is not feasible. If VF persists after three shocks, hold further defibrillation attempts until the core temperature is above 30 °C. Adrenaline accumulates below 30 °C, so withhold it until that temperature is reached and then lengthen the interval to 6–10 minutes while the patient is between 30 and 35 °C.
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.
Toxins
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 managed first by supporting the airway, breathing and circulation. Flumazenil, the benzodiazepine antagonist, is not a routine treatment for the comatose overdose patient: it is reserved for reversing sedation after a single-agent benzodiazepine ingestion in someone with no history of or risk factors for seizures, because it can precipitate seizures and arrhythmias in dependence or mixed overdose.
- 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. The specific treatment is 20% lipid emulsion: an initial intravenous bolus of 1.5 mL/kg followed by an infusion at 15 mL/kg/hour, with boluses repeatable twice at 5-minute intervals and the rate increased to 30 mL/kg/hour if needed, up to a maximum of 12 mL/kg. Stop the local anaesthetic, give a reduced adrenaline dose (1 mcg/kg or less rather than a 1 mg bolus), treat seizures with benzodiazepines, and consider prolonged resuscitation and ECPR.
- 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 the Advanced Life Support algorithm itself. Under RCUK Guidelines 2025 they should be identified and treated without delay, and treating them can take priority over uninterrupted chest compressions where that is clinically appropriate.
A thorough understanding of them is essential for the clinical teams who deliver advanced life support — resuscitation team members, paramedics, and anyone working towards ILS or ALS. It is not a workplace first aid skill: for a lay rescuer, the life-saving actions remain early recognition, calling 999, high-quality chest compressions and early defibrillation.
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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