In the high-stakes environment of cardiac arrest management, healthcare providers rely on structured algorithms to save lives. While high-quality CPR and defibrillation are the cornerstones of resuscitation, they are often not enough. This is where the H’s and T’s of ACLS come into play.
These mnemonics represent the potentially reversible causes of cardiac arrest. Identifying and treating these factors quickly can mean the difference between a return of spontaneous circulation (ROSC) and a poor outcome.
Whether you are preparing for your ACLS certification or refreshing your skills for a code team response, understanding the pathophysiology behind the H’s and T’s is critical.
What Are the H’s and T’s in ACLS?
The H’s and T’s are a memory aid used during the Advanced Cardiovascular Life Support algorithm, specifically during the systematic approach to pulseless arrest (PEA and Asystole). They serve as a checklist to help providers rapidly identify underlying triggers that, if corrected, may restore a perfusing rhythm.
The 5 H’s
- Hypovolemia
- Hypoxia
- Hydrogen ion (Acidosis)
- Hypo-/Hyperkalemia (Electrolyte abnormalities)
- Hypothermia
The 5 T’s
- Tension pneumothorax
- Tamponade (Cardiac)
- Toxins
- Thrombosis (Pulmonary Embolism)
- Thrombosis (Coronary)
Note: While hypoglycemia is not traditionally one of the primary H’s, many clinicians include it as a critical consideration during resuscitation.
| Reversible Cause | Primary Clinical Clue | Key ACLS Intervention |
| Hypovolemia | Flat neck veins, tachycardia | Rapid IV/IO fluid bolus |
| Hypoxia | Cyanosis, low SpO2 | Oxygen, airway management |
| Hydrogen Ion | History of DKA or Renal Failure | Ventilation (Bicarb for specific cases) |
| Hyperkalemia | Peaked T-waves, wide QRS | Calcium, Insulin + Glucose |
| Hypothermia | Exposure history, J-waves | Active internal/external rewarming |
| Tension Pneumo | JVD, tracheal deviation | Needle decompression |
| Tamponade | Muffled heart sounds, JVD | Pericardiocentesis |
| Toxins | Pupil changes, specific OD signs | Antidotes (Naloxone, etc.) |
| Thrombosis (PE) | Sudden SpO2 drop, right heart strain | Fibrinolytics (tPA) |
| Thrombosis (MI) | ST-segment changes | PCI / Coronary Angiography |
A Deep Dive into the H’s
The H’s largely represent metabolic and environmental disturbances that depress the heart’s ability to function.
Hypovolemia
Hypovolemia is the most common reversible cause of PEA (Pulseless Electrical Activity). It results from a severe lack of intravascular volume, leading to inadequate preload and cardiac output.
- Common Causes: Hemorrhage (trauma or internal), dehydration, severe burns, or third-spacing of fluids.
- Clinical Clues: Flat neck veins, tachycardia (early), hypotension, and a history of fluid loss.
- Treatment: Rapid fluid resuscitation with isotonic crystalloids or blood products. In the context of ACLS, administering fluid boluses is prioritized over vasopressors if hypovolemia is suspected.
Hypoxia
Hypoxia occurs when oxygen supply to the tissues is insufficient. Prolonged hypoxia leads to myocardial dysfunction and eventually cardiac arrest.
- Common Causes: Airway obstruction, respiratory failure, near-drowning, or aspiration.
- Clinical Clues: Cyanosis, absent breath sounds, or difficulty ventilating the patient.
- Treatment: Secure the airway immediately. Ensure high-flow oxygen and verify proper endotracheal tube placement. This emphasizes why BLS certification skills are the foundation of all advanced life support.
Hydrogen Ion (Acidosis)
Severe acidosis (low pH) depresses myocardial function and makes the heart less responsive to catecholamines (like epinephrine) and defibrillation.
- Common Causes: Severe sepsis, diabetic ketoacidosis (DKA), renal failure, or prolonged cardiac arrest (lactic acidosis).
- Clinical Clues: A history of diabetes or renal disease, acidic blood gas values.
- Treatment: Focus on ventilation to blow off CO2 (respiratory acidosis) and consider sodium bicarbonate only in specific scenarios (e.g., hyperkalemia or TCA overdose), as routine use is not recommended by current AHA guidelines.
Hypo-/Hyperkalemia
Potassium imbalances are life-threatening electrolyte emergencies.
- Hyperkalemia (High Potassium): Often seen in renal failure patients. It causes peaked T-waves and wide QRS complexes on the ECG, eventually leading to a sinusoidal waveform. Treatments include Calcium Chloride/Gluconate (cardiac membrane stabilizer), insulin with glucose, and sodium bicarbonate to shift potassium into cells.
- Hypokalemia (Low Potassium): Can cause ventricular fibrillation or flat T-waves and U-waves. Treatment involves careful potassium replacement.
Hypothermia
Hypothermia is defined as a core body temperature below 30°C (86°F). The cold heart is irritable and unresponsive to defibrillation and drugs.
- Clinical Clues: Rigid chest, cold skin, and slow capillary refill.
- Treatment: Do not pronounce death until the patient is warm. Active core rewarming (warm IV fluids, bladder/gastric lavage) is required. If the core temp is <30°C, limit defibrillation attempts to 3; hold drugs until temp >30°C.
A Deep Dive into the T’s
The T’s are largely structural or toxicological obstructions that prevent the heart from pumping effectively.
Tension Pneumothorax
This is a life-threatening condition where air accumulates in the pleural space under pressure, collapsing the lung and shifting the mediastinum. This shift obstructs venous return to the heart.
- Clinical Clues: Absent breath sounds on one side, tracheal deviation (late sign), distended neck veins (JVD), and difficulty ventilating.
- Treatment: Immediate needle decompression (usually 2nd intercostal space, mid-clavicular line) followed by chest tube insertion.
Tamponade (Cardiac)
Cardiac tamponade occurs when fluid accumulates in the pericardial sac, compressing the heart and preventing diastolic filling.
- Common Causes: Trauma, aortic dissection, post-MI rupture, or malignancy.
- Clinical Clues: Beck’s Triad (JVD, hypotension, muffled heart sounds), PEA arrest.
- Treatment: Pericardiocentesis (needle aspiration of the pericardial sac) or emergency thoracotomy.
Toxins
Drug overdoses or poisoning can induce cardiac arrest. ACLS providers must think like detectives here.
- Common Causes: Opioids, TCA (Tricyclic antidepressants), Beta-blockers, Calcium channel blockers, Cocaine, or Digoxin.
- Clinical Clues: Pupils (pinpoint vs. dilated), specific ECG patterns (e.g., wide QRS in TCA overdose), and scene clues (empty pill bottles).
- Treatment: Specific antidotes (e.g., Naloxone for opioids, Calcium for Calcium channel blockers, Glucagon for Beta-blockers). Activated charcoal may be considered if the airway is protected and time permits.
Thrombosis (Pulmonary Embolism)
A massive PE blocks blood flow to the lungs, causing acute right heart failure.
- Clinical Clues: Sudden onset of shortness of breath, pleuritic chest pain, signs of DVT, severe hypoxia, and PEA with narrow QRS complexes.
- Treatment: Fibrinolytic therapy (tPA) is the primary intervention for arrest due to suspected PE. Surgical embolectomy is an alternative in centers with capability.
Thrombosis (Coronary)
Acute Coronary Syndrome (ACS) leading to massive myocardial infarction can result in sudden cardiac death.
- Clinical Clues: Chest pain prior to arrest, ST-segment changes on ECG, or new Left Bundle Branch Block.
- Treatment: While the immediate priority is CPR and defibrillation for VF/pVT, the post-cardiac arrest care involves immediate coronary angiography. For detailed heart rhythms, review our guide on interpreting cardiac rhythms.
How to Remember the H’s and T’s
During a code, cognitive load is high. A simple way to remember these is to categorize them:
- “Things inside the vessels/heart:” Hypovolemia, Hyperkalemia, Thrombosis (Coronary/Pulmonary), Tamponade.
- “Things affecting the body globally:” Hypoxia, Hypothermia, Hydrogen ions, Toxins.
- “Things outside the heart pressing on it:” Tension Pneumothorax, Tamponade.
The Role of Point-of-Care Ultrasound (POCUS)
Modern ACLS guidelines increasingly utilize Point-of-Care Ultrasound (POCUS) during pulse checks to identify the H’s and T’s.
- No heart movement? Think Tamponade (fluid in sac) or Hyperkalemia (still heart).
- Right ventricular strain? Think Pulmonary Embolism.
- Collapsing IVC? Think Hypovolemia.
It is vital to remember that ultrasound should not interrupt chest compressions. It should be performed during the rhythm check pulse check.
Conclusion
Mastering the H’s and T’s of ACLS transforms a responder from a robot following an algorithm into a clinician solving a complex medical puzzle. While high-quality CPR buys time, identifying and treating these reversible causes is what ultimately saves the patient.
To see how these causes fit into the larger life-saving sequence, you should review the full ACLS Algorithm to understand the timing of interventions.
Frequently Asked Questions (FAQs)
What are the reversible causes of cardiac arrest?
The reversible causes of cardiac arrest are commonly remembered as the H’s and T’s. The H’s are Hypovolemia, Hypoxia, Hydrogen ions (Acidosis), Hypo-/Hyperkalemia, and Hypothermia. The T’s are Tension pneumothorax, Tamponade, Toxins, Thrombosis (Pulmonary), and Thrombosis (Coronary).
What is the difference between the H’s and T’s?
The H’s primarily refer to metabolic, electrolyte, or environmental conditions that affect the body’s chemistry (e.g., low oxygen, low volume, cold temperature). The T’s primarily refer to physical obstructions or structural issues (e.g., blocked vessels, air in the chest, fluid around the heart).
What is the most common H and T in PEA?
Hypovolemia is widely considered the most common reversible cause of PEA. It is easily treatable with fluid resuscitation. Among the T’s, Coronary Thrombosis (MI) and Pulmonary Embolism are frequent causes.
Why do we check the H’s and T’s in ACLS?
We check the H’s and T’s because standard CPR and defibrillation treat the symptom (the arrest) but not the cause. If the underlying cause (like a tension pneumothorax or a drug overdose) is not fixed, the patient will not achieve Return of Spontaneous Circulation (ROSC) despite high-quality CPR.