What are the Hs (hypovolemia, hypoxia, hydrogen ions, hypokalemia, hyperkalemia, hypothermia) and Ts (tamponade, tension pneumothorax, toxins, thrombosis) to consider in cardiac arrest?

The Hs and Ts of Cardiac Arrest

During cardiac arrest resuscitation, clinicians should systematically evaluate and address the reversible causes known as the "Hs and Ts" to improve survival outcomes and reduce mortality. 1, 2

The Hs (5 causes)

• Hypovolemia: Inadequate circulating blood volume

  • Common causes: hemorrhage, severe dehydration, third-spacing
  • Management: Rapid fluid resuscitation, blood products as needed, control bleeding source

• Hypoxia: Insufficient oxygen delivery to tissues

  • Common causes: airway obstruction, respiratory failure, ventilation-perfusion mismatch
  • Management: Ensure patent airway, provide supplemental oxygen, consider advanced airway

• Hydrogen ion (acidosis): Severe acid-base disturbance

  • Common causes: respiratory acidosis (CO₂ retention), metabolic acidosis (lactic acidosis, diabetic ketoacidosis)
  • Management: Optimize ventilation, treat underlying cause, rarely consider bicarbonate for severe acidosis

• Hypo-/Hyperkalemia: Potassium disturbances

  • Hypokalemia: Causes cardiac conduction disturbances and neuromuscular dysfunction 3
  • Hyperkalemia: Can lead to life-threatening cardiac arrhythmias
  • Management: IV potassium for severe hypokalemia; calcium, insulin/glucose, and albuterol for hyperkalemia

• Hypothermia: Core body temperature below 35°C

  • Common causes: environmental exposure, submersion, metabolic disorders
  • Management: Active rewarming, continue resuscitation until patient is warm
  • Caution: Hypothermia can cause hypokalemia through intracellular shift of potassium 4

The Ts (5 causes)

• Tamponade (cardiac): Fluid accumulation in pericardial space causing cardiac compression

  • Common causes: trauma, malignancy, pericarditis, post-cardiac surgery
  • Management: Pericardiocentesis, surgical intervention if necessary

• Tension pneumothorax: Air in pleural space causing mediastinal shift and decreased venous return

  • Common causes: trauma, barotrauma from mechanical ventilation, underlying lung disease
  • Management: Needle decompression followed by chest tube placement

• Toxins: Drug overdose or toxic exposure

  • Common causes: opioids, beta-blockers, calcium channel blockers, antidepressants
  • Management: Specific antidotes when available, supportive care, consider extracorporeal removal

• Thrombosis (pulmonary): Obstruction of pulmonary arteries by emboli

  • Common causes: deep vein thrombosis, immobility, hypercoagulable states
  • Management: Consider thrombolysis, anticoagulation, mechanical thrombectomy

• Thrombosis (coronary): Acute coronary syndrome leading to myocardial ischemia

  • Common causes: atherosclerotic plaque rupture, coronary vasospasm
  • Management: Consider immediate coronary angiography and intervention

Implementation During Resuscitation

When managing cardiac arrest, incorporate evaluation for these reversible causes into your resuscitation algorithm:

1. Begin high-quality CPR immediately (100-120 compressions/min, depth of at least 2 inches) 1, 2

2. While continuing CPR, systematically consider and address potential Hs and Ts:

   • Assess for hypovolemia (history, skin turgor, mucous membranes)
   • Ensure adequate oxygenation and ventilation
   • Check laboratory values for acidosis and electrolyte abnormalities
   • Measure core temperature
   • Evaluate for cardiac tamponade (bedside ultrasound if available)
   • Assess for tension pneumothorax (breath sounds, chest wall movement)
   • Review for potential toxin exposure or overdose
   • Consider pulmonary and coronary thrombosis based on history and risk factors

3. Integrate this assessment with the standard ACLS algorithm, including rhythm checks every 2 minutes and appropriate medication administration 1, 2

Clinical Pearls and Pitfalls

• Don't delay CPR to complete the Hs and Ts assessment; perform it concurrently with resuscitation efforts
• Use the PCARR construct (Preload-Contractility-Afterload-Rate and Rhythm) as a complementary framework to identify causes 5
• Consider using point-of-care ultrasound during pulse checks to rapidly identify certain causes (tamponade, hypovolemia, PE)
• Remember that multiple causes may coexist in a single patient
• Avoid premature termination of resuscitation, as intact survival has been reported following prolonged resuscitation 6

By systematically addressing these reversible causes during cardiac arrest management, you can significantly improve the chances of achieving return of spontaneous circulation and favorable neurological outcomes.