What are the 4 H's (Hypoxia, Hypovolemia, Hyperkalemia, Hypothermia) and 4 T's (Tamponade, Tension pneumothorax, Toxins, Thrombosis) in advanced life support?

The 4 H's and 4 T's in Advanced Life Support

The 4 H's and 4 T's represent the reversible causes of cardiac arrest that must be systematically identified and treated during resuscitation to improve survival and neurological outcomes. 1

The 4 H's: Metabolic and Physiological Causes

1. Hypoxia (Inadequate Oxygenation)

• Recognition: Check oxygen saturation, observe for cyanosis, assess airway patency and chest rise 1
• Immediate Actions:
  • Establish advanced airway (endotracheal intubation or supraglottic device) 1
  • Confirm placement with waveform capnography (target PETCO2 35-40 mmHg) 1
  • Deliver 100% oxygen initially until arterial oxygen can be measured 1
  • Provide 1 breath every 6 seconds (10 breaths/min) with continuous compressions after advanced airway placement 1
  • Pitfall: Avoid excessive ventilation (>10 breaths/min) as it impedes venous return and decreases cardiac output 2

2. Hypovolemia (Severe Volume Depletion)

• Recognition: History of trauma, bleeding, dehydration, or fluid losses; flat neck veins; narrow pulse pressure on arterial line 1
• Immediate Actions:
  • Establish large-bore IV/IO access 1
  • Administer rapid fluid boluses (crystalloid or blood products if hemorrhage) 3
  • Control external bleeding with direct pressure 3
  • Consider massive transfusion protocol if traumatic hemorrhage 4
  • Clinical Context: Hypovolemia occurred in <10% of PEA arrests in one study, but remains critical to identify 3

3. Hydrogen Ion (Acidosis)

• Recognition: Prolonged arrest time, diabetic ketoacidosis history, renal failure, or toxin ingestion 1
• Immediate Actions:
  • Optimize ventilation to eliminate CO2 1
  • Ensure high-quality CPR to improve tissue perfusion 1
  • Consider sodium bicarbonate only for specific indications (severe hyperkalemia, tricyclic overdose) 1
  • Pitfall: Routine bicarbonate administration is not recommended as it may worsen intracellular acidosis 1

4. Hypo/Hyperkalemia (Potassium Imbalances)

• Hypokalemia Recognition: History of diuretic use, diarrhea, vomiting; ECG shows flattened T waves, U waves, prolonged QT 1

  • Treatment: Administer potassium chloride cautiously (10-20 mEq over 5 minutes during arrest) 1

• Hyperkalemia Recognition: Renal failure, crush injury, tumor lysis; ECG shows peaked T waves, widened QRS, sine wave pattern 1

  • Treatment Protocol:
    • Calcium chloride 10% (10-20 mL IV) or calcium gluconate 10% (30-60 mL IV) for membrane stabilization 1
    • Insulin 10 units IV with 25g dextrose (if not hypoglycemic) 1
    • Sodium bicarbonate 50 mEq IV 1
    • Consider hemodialysis for refractory cases 1

5. Hypothermia (Core Temperature <35°C)

• Recognition: Environmental exposure, cold water immersion, prolonged field time; check core temperature with esophageal or bladder probe 1, 4
• Immediate Actions:
  • Remove wet clothing and insulate patient 4
  • Continue CPR until core temperature reaches at least 32-34°C (do not terminate resuscitation prematurely) 5, 4
  • For VF/pVT in hypothermia: attempt defibrillation, but rhythm may be refractory until rewarmed 5
  • Active rewarming methods: warm IV fluids (38-42°C), forced air warming, consider extracorporeal membrane oxygenation (ECMO) for severe cases 4
  • Special Note: "Frozen is not dead" - continue resuscitation during rewarming as hypothermia can be neuroprotective 4

The 4 T's: Mechanical and Obstructive Causes

1. Tension Pneumothorax (Air Under Pressure in Pleural Space)

• Recognition: Unilateral absent breath sounds, tracheal deviation away from affected side, distended neck veins, hyperresonance to percussion 1
• Immediate Actions:
  • Perform immediate needle decompression (14-16 gauge catheter) at 2nd intercostal space, midclavicular line OR 4th-5th intercostal space, anterior axillary line 1
  • Follow with tube thoracostomy 1
  • Pitfall: Do not delay decompression waiting for chest X-ray confirmation during arrest 1

2. Tamponade, Cardiac (Pericardial Fluid Compressing Heart)

• Recognition: Recent cardiac surgery, chest trauma, malignancy, uremia; distended neck veins, muffled heart sounds (Beck's triad), narrow pulse pressure 1, 6
• Immediate Actions:
  • Consider point-of-care echocardiography if available without interrupting CPR 1
  • Perform pericardiocentesis (subxiphoid approach) 1
  • For post-cardiac surgery patients: prepare for emergency resternotomy (should occur within 5 minutes if no ROSC) 6
  • Clinical Context: In post-cardiac surgery arrests, 17-79% survive to discharge when tamponade is promptly treated 6

3. Toxins (Drug Overdose or Poisoning)

• Recognition: History of ingestion, injection drug use, occupational exposure; specific toxidromes (pupil size, skin findings, vital signs) 1
• Immediate Actions by Toxin Type:
  • Opioids: Naloxone 0.4-2 mg IV/IM/IO/intranasal 1
  • Local anesthetics: Consider IV lipid emulsion 20% (1.5 mL/kg bolus, then 0.25 mL/kg/min infusion) 1
  • Beta-blockers/Calcium channel blockers: High-dose insulin (1 unit/kg bolus, then 0.5-1 unit/kg/hr) with dextrose 1
  • Tricyclic antidepressants: Sodium bicarbonate 1-2 mEq/kg IV boluses 1
  • Cyanide: Hydroxocobalamin 5g IV over 15 minutes 1
  • Digoxin: Digoxin-specific antibody fragments 1
  • Pitfall: The evidence for IV lipid emulsion is very low quality; use only for suspected lipid-soluble drug toxicity 1

4. Thrombosis (Coronary or Pulmonary)

Coronary Thrombosis (Acute MI)

• Recognition: Chest pain history, ST-segment elevation on ECG, known coronary disease 1
• Immediate Actions:
  • Continue standard ACLS with high-quality CPR 1
  • Administer epinephrine 1 mg IV/IO every 3-5 minutes 1
  • For refractory VF/pVT: amiodarone 300 mg IV bolus (then 150 mg) OR lidocaine 1-1.5 mg/kg IV 1
  • Consider emergency percutaneous coronary intervention or fibrinolysis if ROSC achieved 5
  • Advanced Option: Consider extracorporeal CPR (ECPR) in centers with capability for potentially reversible causes 5

Pulmonary Thrombosis (Massive PE)

• Recognition: Recent surgery/immobilization, sudden collapse, known DVT, right heart strain on echo 1
• Immediate Actions:
  • Continue CPR for at least 60-90 minutes (longer than typical arrest) 7
  • Consider fibrinolytic therapy (alteplase 50 mg IV bolus) during CPR for suspected massive PE 7
  • Consider surgical embolectomy or catheter-directed therapy if available 7
  • Evidence Note: Fibrinolysis for PE-induced arrest may improve outcomes but increases bleeding risk 7

Systematic Approach During Resuscitation

While performing high-quality CPR (compressions 100-120/min, depth ≥5 cm, complete recoil, minimal interruptions), systematically work through the H's and T's: 1, 2

1. Assign team roles: One person manages airway, one performs compressions (rotate every 2 minutes), one administers medications, one reviews H's and T's 1

2. Use monitoring to guide diagnosis: 1

   • PETCO2 <10 mmHg suggests inadequate CPR quality or massive PE 1
   • Sudden increase in PETCO2 to >40 mmHg indicates ROSC 1
   • Point-of-care ultrasound (if available without interrupting CPR) can identify tamponade, hypovolemia, or PE 1

3. Prioritize based on clinical context: 3

   • Trauma patients: Think hypovolemia, tension pneumothorax, tamponade first 3
   • Post-cardiac surgery: Think tamponade, bleeding, coronary thrombosis 6
   • Medical patients: Hypoxia (23.6%), acute coronary syndrome (12.5%), and PE are most common 3

4. Don't forget less common causes: Intracranial hemorrhage (6.9%) and nonischemic cardiac disorders (8.3%) occur with similar frequency to classic H's and T's causes 3

Visual Algorithm Structure

CARDIAC ARREST IDENTIFIED
         ↓
START HIGH-QUALITY CPR + DEFIBRILLATE IF INDICATED
         ↓
SIMULTANEOUSLY ASSESS H's AND T's:
         ↓
┌────────────────────────────────────────────────┐
│  H's (Metabolic)      │  T's (Mechanical)      │
├───────────────────────┼────────────────────────┤
│  □ Hypoxia            │  □ Tension pneumothorax│
│  □ Hypovolemia        │  □ Tamponade           │
│  □ Hydrogen ion       │  □ Toxins              │
│  □ Hypo/Hyperkalemia  │  □ Thrombosis (PE/MI)  │
│  □ Hypothermia        │                        │
└───────────────────────┴────────────────────────┘
         ↓
TREAT IDENTIFIED CAUSES WHILE CONTINUING CPR
         ↓
REASSESS EVERY 2 MINUTES WITH RHYTHM CHECK
         ↓
ROSC ACHIEVED? → POST-ARREST CARE

Critical Pitfalls to Avoid

• Never delay CPR to search for reversible causes - assess and treat simultaneously 1
• Do not stop CPR for prolonged diagnostic procedures - use point-of-care testing only if it doesn't interrupt compressions 1
• Avoid premature termination in hypothermia or suspected PE - continue resuscitation longer than standard protocols 5, 7, 4
• Do not forget to rotate compressors every 2 minutes - fatigue significantly reduces compression quality 1, 2
• Avoid hyperventilation - more than 10 breaths/min decreases venous return and worsens outcomes 1, 2