The Hs and Ts: Reversible Causes of Cardiac Arrest

During CPR we should consider and correct reversible causes of cardiac arrest these factors are typically remembered as the Hs and Ts.

A variety of disease processes can lead to a cardiac arrest; however, they usually boil down to one or more of the "Hs and Ts";


  • Hypoxia (give oxygen).
  • Hypovolaemia (correct with IV fluids).
  • Hypothermia (especially consider in cases of drowning - check with a low-reading thermometer).
  • Hyperkalaemia (or hypokalaemia, hypocalcaemia, acidaemia, or other metabolic disorder). ECG may be characteristic of hyperkalaemia. Give IV calcium chloride for hyperkalaemia, hypocalcaemia and calcium-channel blocking drug overdose.
  • Hydrogen Ions* (acidosis).


  • Tension pneumothorax (consider if trauma or previous attempts to insert a central venous catheter).
  • Tamponade (cardiac) - particularly in cases of trauma.
  • Toxins or Tablets - consider reversal agents. See toxbase or the BNF. 
  • Thromboembolism (coronary or pulmonary) - consider thrombolytic drugs but these may take up to 90 minutes to work.
  • Trauma*.

*Hydrogen Ions and Trauma are recent additions to the advanced life support guidelines on reversible causes. Consideration should also be made to hypoglycemic cardiac arrest. 


The Advance Life Support Manual (7th Edition) by the Resuscitation Council is the recommended pre-reading for the Advance Life Support course.

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