H's and T's

The importance of consideration, diagnosis and treatment of potential causes of a cardiac arrest should be considered during any resuscitation by the provider, as an underlying cause that is contributing to the arrest can be corrected and improve outcomes. Consideration of potentially reversible causes should continue during the active management of the resuscitation.

Common reversible causes of cardiac arrest are an important part of the knowledge base for an ACLS Provider, and in order to make it easier for a healthcare provider in an emergent resuscitation to consider all causes, the mnemonic “H’s and T’s” was created, with division of the potential reversible causes into groups that are named beginning with either H or T, as below, and are discussed in the journal Circulation as Cardiac Arrest in Special Situations.


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The H’s and T’s: Potentially Reversible Causes of Cardiac Arrest Rhythms


Hypoxia: associated with asthma, COPD, CHF. Ensure adequate oxygen delivery.

Hypovolemia: replace fluids or blood as needed. Look for sources of fluid loss, including bleeding or inadequate intake (nursing home patients). DKA also leads to dehydration because of osmotic diuresis.

Hydrogen Ion (Acidosis): Anerobic metabolism produces lactate which can cause metabolic acidosis. Respiratory acidosis is the result of underventilation and accumulation of carobn dioxide. Determination is made by analysis of ABG’s and serum bicarbonate.

Hyper/hypokalemia: Life threatening electrolyte disturbances are discussed in Section 12 of the 2010 Guidelines Publication in Circulation. Potassium is critical to the function of sodium potassium channels in the heart cells and an imbalance must corrected, but treatment for hyperkalemia is beyond the scope of this course. However, immediate action to stabilize the myocardium can be accomplished by infusion of 10% Calcium Chloride over2 to 5 minutes (50 to 1000 mg). Shift of potassium intracellularly can be accomplished by use of sodium bicarbonate, glucose plus insulin and with nebulized albuterol. Slower excretion of potassium can be achieved by using furosemide for diuresis, kayexelate or dialysis.

Hypothermia: Accidental hypothermia is treated depending upon the severity of the hypothermia. A core temperature should be obtained, and because the cardiac muscle is irritable under conditions of severe hypothermia, movement should be careful and a trial of CPR with active rewarming to 1 or 2 degrees increase in temperature before resuming CPR is reasonable. The treatment of cardiac arrest in hypothermia is discussed in detail in a blog on ACLScertification.com


Toxins: Any number of ingestions may cause cardiac arrest, as well as environmental exposure to certain chemicals. There are often toxidromes, or sets of symptoms that reflect the class of toxin exposure and that may aid in diagnosis of cardiac arrest due to an unknown toxin that can potentially be treated. In the case of uncommon toxic ingestion or exposure, poison control is a valuable and timely resource, but treatment of commonly involved medications that include digoxin, beta-blockers, calcium channel blockers and tricyclic antidepressants are well understood by emergency physicians When treating a victim of cardiac arrest as a result of exposure to an environmental toxin, decontamination is critical and you must ensure the safety of the scene as you could become a victim of exposure.

Tamponade (Cardiac): Cardiac tamponade occurs when fluid build up in the pericardial sac. If this occurs rapidly, the heart is unable to compensate for the reduction in atrial and ventricular filling and hemodynamic collapse follows with low output shock.

Cardiac tamponade is diagnosed clinically by muffled heart sounds, hypotension and JVD. Electrical alternans may be seen on ECG, reflecting the movement of the heart within the pericardial sac.

Treatment is pericardiocentesis, which involves decompression of the pericardial sac. This should be diagnosed and treated rapidly. It has been suggested that emergency thoracotomy may produce better outcomes in trauma arrest or pre-arrest when compared to pericardiocentesis, particularly if the blood in the pericardium has clotted.

Tension pneumothoraxTension  pneumothorax occurs when air enters the pleural space but has not route of escape, resulting in collapse of the lung on the affected side with tracheal deviation and hypotension that occurs from reduced return of blood flow to the heart. Because tension pneumothorax results in hemodynamic collapse, it is a true emergency but is reversible with a simple needle thoracotomy, performed by placing an 18 gauge angiocath in the second intercostal space in the midclavicular line.

Thrombosis: Pulmonary Embolism: Cardiac arrest caused by a PE may present as pulseless electrical activity (PEA), and if there is presumption or knowledge of a PE, the use of fibrinolytics during CPR may improve survival to discharge and long-term neurological function. Percutaneous mechanical thromboembolectomy or surgical embolectomy have been used successfully.

Thrombosis, coronary: Coronary thrombosis, if massive, will cause an acute myocardial infarction with sudden death depending upon the area served by the blocked artery. ST segment elevation is typically seen, and treatment with fibrinolytic medications may improve survival by restoring flow, although if the patient is in cardiac arrest, flow may not be restored rapidly enough to improve outcome and routine fibrinolytic therapy may not be successful when used during resuscitation from sudden cardiac death due to coronary thrombosis.