The H's and T's of ACLS: Reversible Causes of Cardiac Arrest

The H’s and T’s are the reversible causes a resuscitation team searches for during every arrest. On an ACLS course the team leader is expected to go through them out loud.

The H’s

• Hypovolaemia — bleeding, dehydration; look for the history and flat neck veins; give fluids or blood.
• Hypoxia — ensure effective oxygenation and ventilation.
• Hydrogen ion (acidosis) — ventilation and treating the underlying cause.
• Hypo- or hyperkalaemia — renal failure, dialysis patients and certain drugs; treatment follows local protocols.
• Hypothermia — rewarming alongside resuscitation.

The T’s

• Tension pneumothorax — decompression.
• Tamponade, cardiac — pericardiocentesis.
• Toxins — specific antidotes and expert advice.
• Thrombosis, pulmonary — massive pulmonary embolism; consider fibrinolysis.
• Thrombosis, coronary — acute coronary occlusion; see ACS.

When they matter most

In PEA and asystole, treating the cause is often the only route to survival. In refractory VF, a coronary or toxic cause should be considered.

Running the list as a team

The team leader reads the list aloud and asks the team for clues from the history, examination and tests — an example of the knowledge sharing taught in team dynamics and assessed in the Megacode. They form part of the secondary assessment in the systematic approach.

This guide summarises what the ACLS course teaches for education only. It is not a clinical protocol: follow the current American Heart Association guidelines, linked below, and your institution’s policies.

More ACLS guides: ACLS algorithms and skills

• ACLS Course in Abu Dhabi — dates, format and booking
• ACLS Algorithms Explained: Cardiac Arrest, Bradycardia, Tachycardia and Post-Arrest Care
• The ACLS Systematic Approach: BLS, Primary and Secondary Assessment
• ACLS Cardiac Arrest: VF and Pulseless VT (the Shockable Pathway)
• ACLS Asystole and PEA: The Non-Shockable Pathway
• ACLS Bradycardia Algorithm: Atropine, Pacing and Infusions
• ACLS Tachycardia Algorithm: Stable vs Unstable Tachycardia
• ACLS Post-Cardiac Arrest Care: What Happens After ROSC
• ACLS Drugs: Adrenaline, Amiodarone, Atropine, Adenosine and More
• ACLS Rhythms: The ECG Rhythms You Need to Recognise
• ACLS Airway Management: From Bag-Mask to Advanced Airway
• Waveform Capnography in ACLS: What End-Tidal CO2 Tells You
• Acute Coronary Syndromes in ACLS: Recognition and First Steps
• Acute Stroke in ACLS: Recognition, Timing and First Steps
• ACLS Megacode: How It Works and How to Pass as Team Leader
• ACLS Electrical Therapy: Defibrillation, Cardioversion and Pacing
• All ACLS guides, by topic

Other life support and first aid courses: BLS Course · PALS Course · First Aid Course · Training calendar

Frequently asked questions

What are the H's in ACLS?

Hypovolaemia, hypoxia, hydrogen ion (acidosis), hypo- or hyperkalaemia and hypothermia.

What are the T's in ACLS?

Tension pneumothorax, cardiac tamponade, toxins, and pulmonary or coronary thrombosis.

When should the H's and T's be considered?

Throughout every cardiac arrest, especially PEA and asystole.

Who goes through the list?

The team leader, aloud, with input from the team.

Are the H's and T's tested?

Yes, in the Megacode and written test.

Sources and further reading

What we teach follows the awarding body and the UAE health authorities. For the definitive position on a course or licensing question, these are the places to check.

• CPR and ECC Guidelines — American Heart Association
• Advanced Cardiovascular Life Support (ACLS) — American Heart Association

Last reviewed 21 September 2026 by the training team at Safety First Medical Services, an American Heart Association training centre at Addax Office Tower, Al Reem Island, Abu Dhabi. Call +971 55 761 9841 or email training@safetyfirstmed.ae.