Introduction
During cardiac arrest resuscitation, high-quality CPR and timely medications are essential. However, successful resuscitation often depends on identifying and correcting the underlying cause of the arrest.
The Advanced Cardiovascular Life Support (ACLS) framework uses the H’s and T’s mnemonic to help clinicians systematically evaluate reversible causes of cardiac arrest.
These conditions are particularly important when managing non-shockable rhythms such as pulseless electrical activity (PEA) and asystole, although they may also contribute to ventricular fibrillation and pulseless ventricular tachycardia.
Understanding the H’s and T’s is a fundamental ACLS skill for healthcare professionals involved in emergency cardiovascular care.
What Are the H’s and T’s?
The H’s and T’s represent potentially reversible causes of cardiac arrest.
The H’s
- Hypovolemia
- Hypoxia
- Hydrogen ion excess (Acidosis)
- Hypo-/Hyperkalemia
- Hypothermia
The T’s
- Tension Pneumothorax
- Cardiac Tamponade
- Toxins
- Pulmonary Thrombosis
- Coronary Thrombosis
Prompt recognition and treatment may improve the likelihood of Return of Spontaneous Circulation (ROSC).
The H’s Explained
1. Hypovolemia
Hypovolemia refers to severe intravascular volume depletion.
Common causes include:
- Hemorrhage
- Gastrointestinal fluid losses
- Severe dehydration
- Trauma
Clinical Clues
- Hypotension before arrest
- Tachycardia
- Flat neck veins
- History of bleeding
Treatment
- Rapid fluid administration
- Blood products when indicated
- Control of ongoing bleeding
2. Hypoxia
Hypoxia remains one of the most common reversible causes of cardiac arrest.
Common causes include:
- Airway obstruction
- Respiratory failure
- Severe pneumonia
- Drowning
- Pulmonary disease
Clinical Clues
- Low oxygen saturation
- Cyanosis
- Respiratory distress before arrest
Treatment
- Airway management
- Supplemental oxygen
- Ventilation support
- Correction of underlying respiratory pathology
3. Hydrogen Ion Excess (Acidosis)
Severe metabolic or respiratory acidosis may impair cardiac function.
Potential causes include:
- Septic shock
- Renal failure
- Prolonged cardiac arrest
- Severe respiratory failure
Clinical Clues
- Low arterial pH
- Elevated lactate
- Severe physiologic compromise
Treatment
- Improve perfusion
- Correct underlying cause
- Optimize ventilation
4. Hypokalemia and Hyperkalemia
Potassium abnormalities can cause life-threatening arrhythmias.
Hypokalemia
May produce:
- Ventricular arrhythmias
- QT prolongation
- Cardiac instability
Hyperkalemia
May produce:
- Bradycardia
- Conduction disturbances
- Cardiac arrest
Treatment
Depends on the underlying abnormality and laboratory findings.
5. Hypothermia
Hypothermia may significantly depress cardiac activity.
Common causes include:
- Environmental exposure
- Cold-water immersion
- Severe trauma
Clinical Clues
- Low core body temperature
- Altered mental status
- Bradycardia
Treatment
- Active rewarming
- Supportive care
- Correction of associated abnormalities
The T’s Explained
1. Tension Pneumothorax
Tension pneumothorax occurs when air accumulates under pressure within the pleural space.
Clinical Clues
- Absent breath sounds
- Tracheal deviation
- Distended neck veins
- Sudden cardiovascular collapse
Treatment
Immediate decompression followed by definitive chest drainage.
2. Cardiac Tamponade
Cardiac tamponade results from fluid accumulation within the pericardial sac.
Clinical Clues
- Hypotension
- Elevated jugular venous pressure
- Muffled heart sounds
Treatment
Emergency pericardiocentesis.
3. Toxins
Drug overdose and poisoning may precipitate cardiac arrest.
Examples include:
- Opioids
- Calcium channel blockers
- Beta blockers
- Tricyclic antidepressants
Treatment
- Antidotes when available
- Supportive care
- Consultation with toxicology specialists
4. Pulmonary Thrombosis
Massive pulmonary embolism can obstruct pulmonary blood flow and cause sudden cardiac arrest.
Clinical Clues
- Sudden dyspnea
- Right heart strain
- Known venous thromboembolism
Treatment
- Thrombolytic therapy when appropriate
- Advanced circulatory support in selected patients
5. Coronary Thrombosis
Acute coronary occlusion remains one of the most common causes of sudden cardiac arrest.
Clinical Clues
- Chest pain before arrest
- ST-segment elevation
- Known coronary artery disease
Treatment
- Rapid reperfusion
- Coronary angiography
- Percutaneous coronary intervention when indicated
Why Are the H’s and T’s Important in PEA?
Pulseless electrical activity often occurs because an organized electrical rhythm is present but the heart cannot generate effective mechanical contraction.
In many cases, a reversible H or T is responsible.
Successful treatment depends on identifying and correcting the underlying cause.
Key Takeaways
- The H’s and T’s represent reversible causes of cardiac arrest.
- They are especially important in PEA and asystole.
- Early recognition may improve ROSC and survival.
- Clinicians should actively evaluate for reversible causes during every cardiac arrest resuscitation.
- Understanding the H’s and T’s is a core ACLS competency.
Continue Your ACLS Education
Mastering the H’s and T’s helps healthcare professionals identify potentially reversible causes of cardiac arrest and improve resuscitation outcomes. Understanding these concepts is an essential component of ACLS training and emergency cardiovascular care.