H's and T's Every Healthcare Professional Must Know

Thte H's and T's: Reversible Causes of Cardiac Arrest Every Healthcare Professional Must Know

When a patient goes into cardiac arrest, every second counts. While high-quality CPR and defibrillation are essential, they may not be enough if we fail to identify and treat the underlying cause. This is where the "H's and T's" mnemonic becomes a lifesaving tool.

These 12 reversible causes of cardiac arrest represent conditions that, if identified quickly, can be treated effectively, potentially saving a patient's life. Let's explore each one in detail.

Understanding the Framework

The H's and T's mnemonic helps medical teams systematically search for treatable causes during resuscitation. While performing chest compressions and managing the airway, the team must simultaneously consider these potential culprits. Missing even one could mean the difference between successful resuscitation and tragic loss.

The Six H's

1. Hypovolemia (Low Blood Volume)

Hypovolemia occurs when there's insufficient blood volume to maintain adequate circulation. This can result from severe bleeding, trauma, dehydration, or burns.

Signs to look for:

History of trauma or bleeding

Flat neck veins

Known fluid losses

Treatment: Rapid fluid resuscitation with crystalloids or blood products, control of bleeding source, and immediate surgical intervention if needed.

2. Hypoxia (Oxygen Deprivation)

When the body's tissues don't receive enough oxygen, cardiac arrest can quickly follow. Common causes include airway obstruction, severe asthma, drowning, or respiratory failure.

Signs to look for:

Cyanosis (blue discoloration)

Difficult or failed intubation

History of respiratory problems

Treatment: Secure the airway, provide high-flow oxygen, ensure adequate ventilation, and address any obstruction.

3. Hydrogen Ion (Acidosis)

Severe acidosis disrupts the heart's electrical system and reduces the effectiveness of resuscitation medications. It can result from prolonged cardiac arrest, diabetic ketoacidosis, or severe sepsis.

Signs to look for:

Prolonged resuscitation time

Known diabetes or kidney disease

Low pH on blood gas analysis

Treatment: Address the underlying cause, ensure adequate ventilation (to blow off CO2), and consider sodium bicarbonate in specific situations.

4. Hypo/Hyperkalemia (Potassium Imbalance)

Both dangerously low and dangerously high potassium levels can cause fatal cardiac arrhythmias. Patients with kidney disease, those on dialysis, or taking certain medications are at highest risk.

Signs to look for:

Renal failure history

Characteristic ECG changes (peaked T-waves in hyperkalemia, U-waves in hypokalemia)

Recent dialysis or missed dialysis sessions

Treatment:

For hyperkalemia: calcium chloride/gluconate, insulin with glucose, sodium bicarbonate, dialysis

For hypokalemia: cautious potassium replacement

5. Hypothermia (Low Body Temperature)

When core body temperature drops below 35°C (95°F), cardiac function becomes severely impaired. Cold water drowning, exposure, or certain medical conditions can cause this.

Signs to look for:

History of cold exposure

Cold skin temperature

Immersion in cold water

Treatment: Active rewarming measures, warm IV fluids, and in severe cases, extracorporeal membrane oxygenation (ECMO). Remember: "They're not dead until they're warm and dead."

6. Hypoglycemia (Low Blood Sugar)

While hypoglycemia more commonly causes altered consciousness before arrest, severe cases can lead to cardiac arrest, particularly in diabetic patients.

Signs to look for:

Known diabetes

Recent insulin administration

Point-of-care glucose reading

Treatment: Immediate administration of IV dextrose.

The Six T's

1. Tension Pneumothorax

Air trapped in the pleural space compresses the heart and great vessels, preventing adequate blood return to the heart. This is a mechanical problem requiring immediate intervention.

Signs to look for:

Absent breath sounds on one side

Tracheal deviation

Distended neck veins

Recent central line placement or trauma

Treatment: Immediate needle decompression followed by chest tube placement.

2. Cardiac Tamponade

Fluid accumulation in the pericardial sac prevents the heart from filling properly. Causes include trauma, recent cardiac surgery, malignancy, or pericarditis.

Signs to look for:

Beck's triad (hypotension, muffled heart sounds, distended neck veins)

Recent chest trauma or cardiac procedure

Narrow pulse pressure

Treatment: Emergency pericardiocentesis or surgical pericardial window.

3. Toxins (Poisoning/Overdose)

Drug overdoses and toxic ingestions are increasingly common causes of cardiac arrest. Opioids, tricyclic antidepressants, beta-blockers, and calcium channel blockers are frequent culprits.

Signs to look for:

History of drug use or access to medications

Pinpoint or dilated pupils

Medication bottles at the scene

Characteristic ECG changes

Treatment: Specific antidotes when available (naloxone for opioids, glucagon for beta-blockers, lipid emulsion therapy for certain toxins), supportive care, and consultation with poison control.

4. Thrombosis, Coronary (Heart Attack)

Acute myocardial infarction remains one of the most common causes of cardiac arrest. A blocked coronary artery starves heart muscle of oxygen.

Signs to look for:

Chest pain prior to arrest

ST-segment elevation on ECG

Known coronary artery disease

Treatment: Emergency cardiac catheterization with percutaneous coronary intervention (PCI), fibrinolytic therapy if PCI unavailable, antiplatelet and anticoagulation medications.

5. Thrombosis, Pulmonary (Pulmonary Embolism)

A massive blood clot in the pulmonary arteries can cause sudden cardiovascular collapse. Risk factors include recent surgery, prolonged immobility, cancer, or clotting disorders.

Signs to look for:

Recent surgery or long flight

Unilateral leg swelling

Known risk factors for clotting

Right heart strain on ECG or ultrasound

Treatment: Systemic thrombolysis, catheter-directed therapy, or surgical embolectomy in appropriate candidates.

6. Trauma

Severe traumatic injuries can cause cardiac arrest through multiple mechanisms including massive bleeding, cardiac contusion, or tension pneumothorax.

Signs to look for:

Obvious trauma history

Penetrating injuries

Major mechanism of injury

Treatment: Damage control resuscitation, control of hemorrhage, emergency surgery, and addressing specific injuries like tension pneumothorax or tamponade.

Putting It Into Practice

During a cardiac arrest, the resuscitation team leader should systematically consider each of these causes while CPR is ongoing. Here's how to integrate this into your practice:

Before the arrest: Know your patient's history, medications, and risk factors.

During the arrest: Assign a team member to specifically look for reversible causes while others focus on CPR and airway management.

Use diagnostic tools: Bedside ultrasound can rapidly identify tamponade, pneumothorax, or pulmonary embolism. Point-of-care testing can identify electrolyte abnormalities and hypoglycemia.

Think systematically: Go through the H's and T's in order, checking off each one as you rule it out or address it.

The Bottom Line

The H's and T's represent hope in the midst of crisis. While not every cardiac arrest is reversible, failing to identify a treatable cause means missing an opportunity to save a life. High-quality CPR buys time, but treating the underlying cause wins the battle.

Every healthcare provider involved in emergency care should commit these 12 reversible causes to memory. Regular simulation training that incorporates searching for and treating these causes will help ensure that when the critical moment arrives, you and your team are ready.

Remember: effective resuscitation isn't just about compressions and shocks. It's about thinking critically, working systematically, and never giving up on finding that reversible cause that could bring your patient back.

Have you encountered a case where identifying one of the H's or T's made the difference? Share your experiences in the comments below. For healthcare professionals looking to deepen their emergency medicine knowledge, consider taking an advanced cardiac life support (ACLS) course that emphasizes these critical concepts.