Understanding Hs and Ts in Cardiac Arrest

Understanding the Hs and Ts in cardiac arrest is helpful in these situations. These are called “reversible causes” because they can be fixed with the right treatment during resuscitation.

It’s important to recognise and treat these causes quickly, as doing so can greatly improve the chances of getting the heart working again (called return of spontaneous circulation (ROSC)) and saving the patient.

The reversible causes are commonly remembered using the mnemonic “Hs and Ts”.

About 66% of cardiac arrests happen in men, and 34% in women. This difference may be due to biological and lifestyle factors.

Cardiac arrest is a serious medical emergency where the heart stops beating properly, causing blood to stop flowing around the body.

Medical professionals use a simple memory tool called the 4Hs and Ts to quickly check what might have caused the cardiac arrest.

The survival rate for cardiac arrest remains low, with recent data showing only about 8% of patients surviving 30 days after an out-of-hospital cardiac arrest in England.

Survival can be as high as 90% if treatment starts within the first minutes after sudden cardiac arrest, but the rate drops by about 10% each minute without intervention.

This can quickly lead to death unless the person receives immediate help, such as cardiopulmonary resuscitation (CPR) and defibrillation.

A useful tool for healthcare workers to identify and treat causes of cardiac arrest is the “4 Hs and 4 Ts” mnemonic, which helps find treatable reasons for the arrest.

Each year in the UK, about 30,000 to 35,000 people experience a cardiac arrest outside of the hospital, with some estimates going as high as 60,000. In hospitals, over 10,400 patients had a cardiac arrest in 2020/21, which is about 1 in every 1,000 hospital visits.

These numbers show cases where efforts were made to restart the heart, but the actual number may be higher, as not all cardiac arrests get treated. The COVID-19 pandemic also affected these numbers, especially in hospitals.

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What is Cardiac Arrest?

Before we look at the causes ( the 4 Hs and Ts), let’s understand what cardiac arrest is. It happens when the heart suddenly stops pumping blood around the body. This leads to a loss of consciousness, absence of normal breathing, and no signs of blood circulation.

This is different from a heart attack, where blood flow to part of the heart is blocked but the heart usually keeps beating.

Unlike a heart attack, which is caused by a blockage in blood flow to the heart, cardiac arrest is an electrical problem (called arrhythmias) that causes the heart to stop pumping blood effectively.

Cardiac arrest can happen to anyone, with or without heart conditions, and needs immediate emergency care to avoid death.

 In-hospital cardiac arrests have better outcomes, with about 26% of patients surviving and being discharged, compared to 11% for out-of-hospital cases. [Cleveland Clinic]

For patients who survive the initial hospital stay after an out-of-hospital cardiac arrest, the estimated survival rates are:

  • 82.8% at 3 years
  • 77.0% at 5 years
  • 63.9% at 10 years
  • 57.5% at 15 years.

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The 4 Hs of Cardiac Arrest

The “Hs” in cardiac arrest are used to remember the reversible causes. These causes include:

  • Hypoxia: Not enough oxygen in the body, caused by blocked airways or breathing problems.
  • Hypovolemia: Low blood volume due to blood loss or dehydration, leading to shock.
  • Hypothermia: A very low body temperature, below 30°C (86°F), affecting heart function.
  • Hypo-/Hyperkalemia: Unusual potassium levels in the blood, which can cause heart rhythm problems.

Addressing these causes is important as they can be treated to improve chances of survival. Although, there is a new addition which is the Hydrogen Ion (acidosis).

So in some cases, you would have 5Hs of cardiac arrest instead of four Hs.

The 5 Hs are used to remember causes of cardiac arrest that can be treated. They are: Hypovolemia (low blood volume), Hypoxia (not enough oxygen), Hydrogen ion (acidosis – too much acid in the blood), Hypo/Hyperkalemia (too low or too high potassium), and Hypothermia (low body temperature).

We will stick with 4H here.

Hydrogen Ion (acidosis) means having too much acid in the blood, which can stop the heart. You can treat this by checking acid levels, helping the person breathe properly, and possibly giving sodium bicarbonate.

So let’s go back to the main 4 Hs.

1. Hypoxia ( Lack of oxygen)

What is it?

Hypoxia means the body isn’t getting enough oxygen. Think of oxygen as the fuel your body needs to work properly.

Why does it matter?

  • The body’s cells need oxygen to stay alive
  • The heart works harder when oxygen is low
  • Brain damage can happen quickly without oxygen

Common causes:

  • Choking
  • Drowning
  • Severe asthma attacks
  • Lung problems
  • Blocked airways

What helps:

  • Quick action to clear airways
  • Giving oxygen
  • Treating the underlying breathing problem
  • CPR to move oxygen around the body

2. Hypovolemia (Low Blood Volume)

What is it?

This means there isn’t enough blood flowing through the body. Think of it like a car running low on oil.

Why does it matter?

  • The heart can’t pump properly without enough blood
  • Organs don’t get what they need
  • Blood pressure drops dangerously low

Common causes:

  • Bleeding (internal or external)
  • Severe vomiting or diarrhea
  • Burns
  • Dehydration
  • Heavy sweating without drinking enough

What helps:

  • Giving fluids through an IV
  • Blood transfusions if needed
  • Finding and stopping any bleeding
  • Treating the cause of fluid loss

3. Hyperkalemia or Hypokalemia (Potassium Problems)

What is it?

These are problems with potassium levels in the blood – either too high (hyper) or too low (hypo).

Why does it matter?

  • Potassium helps control heart rhythm
  • Wrong levels can make the heart beat irregularly
  • Can stop the heart from working properly

Common causes:

  • Kidney problems
  • Some medicines
  • Severe dehydration
  • Too many potassium supplements
  • Not eating enough (for low levels)

What helps:

  • Medicines to fix potassium levels
  • Kidney treatments if needed
  • Changing problem medications
  • IV fluids
  • Diet changes for the future

4. Hypothermia (Too Cold)

What is it?

Body temperature drops too low, below 35°C (95°F).

Why does it matter?

  • Cold hearts don’t beat properly
  • Body processes slow down
  • Can cause confusion and poor decisions

Common causes:

  • Cold water exposure
  • Being outside in cold weather
  • Cold operating rooms
  • Certain medications
  • Some medical conditions

What helps:

  • Gentle warming
  • Warm fluids through IV
  • Removing wet clothing
  • Warming blankets
  • Preventing further heat loss

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The 4 Ts of Cardiac Arrest

The ‘Ts’ in cardiac arrest are used to remember possible causes and what to do in an emergency situation. These include:

  • Tension Pneumothorax: Air trapped in the chest, causing pressure on the lungs and heart.
  • Tamponade (Cardiac): Fluid around the heart, which prevents it from pumping properly.
  • Toxins: Poison or drugs that affect the heart’s ability to function.
  • Thrombosis: Blood clots in the heart or lungs, blocking blood flow and causing heart failure.

The 4 Ts are part of the mnemonic “4 Hs and 4 Ts”, which is used to help remember reversible causes of cardiac arrest. 

1. Tension Pneumothorax (Collapsed Lung)

What is it?

Air gets trapped around the lung, squashing it and the heart.

Why does it matter?

  • Puts pressure on the heart
  • Makes it hard to breathe
  • Can stop blood from returning to the heart

Common causes:

  • Chest injuries
  • Medical procedures
  • Lung disease
  • Ventilator problems
  • Sometimes happens on its own

What helps:

  • Needle decompression
  • Chest tube insertion
  • Surgery if needed
  • Treating underlying causes
  • Close monitoring

2. Tamponade (Cardiac)

What is it?

Fluid builds up around the heart, squeezing it so it can’t pump properly.

Why does it matter?

  • Heart can’t fill properly
  • Less blood gets pumped out
  • Can quickly become life-threatening

Common causes:

  • Heart surgery complications
  • Chest trauma
  • Some cancers
  • Heart inflammation
  • Blood-thinning medication problems

What helps:

  • Draining the fluid
  • Surgery if needed
  • Treating the underlying cause
  • Careful monitoring
  • Preventive measures

3. Thrombosis (Coronary – Heart Attack)

What is it?

Blood clots block arteries in the heart.

Why does it matter?

  • Parts of the heart muscle die
  • Can trigger dangerous rhythms
  • May lead to heart failure

Common causes:

  • Coronary artery disease
  • High blood pressure
  • High cholesterol
  • Smoking
  • Family history

What helps:

  • Clot-busting drugs
  • Opening blocked arteries
  • Heart procedures
  • Blood thinners
  • Lifestyle changes

4. Thrombosis (Pulmonary – Blood Clot in Lungs)

What is it?

Blood clots block arteries in the lungs.

Why does it matter?

  • Less oxygen gets into the blood
  • Heart works too hard
  • Can cause sudden collapse

Common causes:

  • Long periods of not moving
  • Recent surgery
  • Some medical conditions
  • Birth control pills
  • Long flights

What helps:

  • Blood-thinning medicines
  • Clot-dissolving drugs
  • Surgery in some cases
  • Prevention strategies
  • Early treatment
Hs and Ts Cardiac Arrest infographics for reversible cause

Why The Knowledge of Hs and Ts Matters

The knowledge of Hs and Ts matters because it helps healthcare providers quickly identify and treat reversible causes of cardiac arrest, improving the chances of survival.

Now it’s time to know how it helps each individual within the industry.

1. Healthcare Workers

  • Helps make quick decisions
  • Guides treatment choices
  • Improves patient outcomes
  • Makes teamwork better
  • Saves precious time

2. First Responders

  • Helps spot dangerous situations
  • Guides initial treatment
  • Knows when to call for help
  • Better handover to hospital
  • More lives saved

3. Everyone

  • Understanding what’s happening
  • Knowing when to seek help
  • Better prevention
  • Quicker recognition of problems
  • Better support for others

Prevention Tips: Saving Lives

How do you prevent cardiac arrest? There are things you need to do and some you need to avoid. We have group tips to survive cardiac arrest into general health tips and specific prevention tips below:

General Health

  • Regular check-ups
  • Healthy lifestyle
  • Know your risks
  • Take medicines properly
  • Stay active safely

Specific Prevention

  • Stay hydrated
  • Keep warm/cool as needed
  • Move regularly on long trips
  • Know your medical conditions
  • Watch for warning signs

When to Seek Help

Get immediate medical help if you notice:

  • Frequent Chest pain.
  • Trouble breathing.
  • Feeling very weak.
  • Fainting.
  • Fast or irregular heartbeat.

Remember that the survival rate for cardiac arrest remains low, with recent data showing only about 8% of patients surviving 30 days after an out-of-hospital cardiac arrest in England.

Though,  it’s important to note that survival rates have improved over the past decade, with some areas in the UK reporting rates as high as 10.8% for out-of-hospital cardiac arrests.

Importance of the 4 Hs and 4 Ts

The 4 Hs and 4 Ts framework helps healthcare providers quickly identify treatable causes of cardiac arrest. This leads to:

  • Better diagnosis: By following the framework, doctors and nurses can quickly assess the situation.
  • Targeted treatment: It allows for specific treatments aimed at the underlying cause.
  • Faster action: The mnemonic helps prioritise the most important steps to take in the situation.

Most people who have a cardiac arrest are over 70 years old, with half of all patients being in that age group.

Only 9% of patients are under 40. The average age of people who have a cardiac arrest outside of the hospital is different in each country, ranging from 51 years in the United Arab Emirates to 78 years in Japan.

How to Use the 4 Hs and 4 Ts in Cardiac Arrest Situations

Using the Hs and Ts in actual cardiac arrest situations is very important for finding and treating causes of the arrest that can be fixed. To use this approach effectively:

  1. Quick assessment: When helping someone in cardiac arrest, think about the Hs and Ts quickly to see if any of these could be causing the problem.
  2. Prioritise treatments: Based on what the patient looks like and their history, treat the most likely causes first.
  3. Hypovolemia (low blood volume): Look for signs of bleeding or dehydration. If you suspect this, give IV fluids as quickly as possible.
  4. Hypoxia (lack of oxygen): Make sure the airway is clear and the person is getting enough oxygen. Check that the chest is rising and that you can hear both sides of the chest when breathing.
  5. Hydrogen ion (acidosis): Make sure the person is breathing well to avoid too much acid in the blood. You might need to give sodium bicarbonate if metabolic acidosis is suspected.
  6. Hyper/hypokalemia (too much or too little potassium): If possible, get a blood test to check potassium levels. Treat with calcium or potassium through an IV if needed.
  7. Hypothermia (low body temperature): Check the person’s temperature. If it’s below 30°C, avoid giving certain drugs. If it’s between 30-34°C, make sure to give medicines less often and in smaller amounts.
  8. Tension pneumothorax (air in the chest): If you suspect this, perform a needle decompression and insert a chest drain if possible.
  9. Toxins (poisoning or drug overdoses): Think about if the person might have taken drugs or been poisoned. If possible, give the right antidotes.
  10. Cardiac tamponade (fluid around the heart): Look for signs of shock. If needed, perform a procedure to drain the fluid from around the heart.
  11. Thrombosis (blood clots in the heart or lungs): If a clot is suspected in the lungs or heart, consider giving clot-dissolving treatment.

Remember, high-quality CPR and quick defibrillation are still the most important actions in managing cardiac arrest. The Hs and Ts should be used along with these to improve the chances of survival.

The Five Hs and Ts in Cardiac Arrest Management

The Five Hs and Ts in cardiac arrest are a helpful mnemonic to remember the possible causes of cardiac arrest that can be treated. These are important for healthcare providers to consider during resuscitation.

The Five Hs

  1. Hypovolemia: A major loss of blood, often due to heavy bleeding or severe dehydration.
  2. Hypoxia: A lack of oxygen in the body, which can be caused by blocked airways, drowning, or serious lung diseases.
  3. Hydrogen ion (acidosis): Too much acid in the body, which can happen due to breathing problems or other health issues.
  4. Hyper-/hypokalemia: Imbalances in potassium levels, which can seriously affect the heart’s ability to beat properly.
  5. Hypothermia: A dangerously low body temperature, which can affect the heart and overall body function.

The Five Ts

  1. Tension pneumothorax: Air trapped in the chest, putting pressure on the lung and heart, affecting how well the heart pumps.
  2. Tamponade (cardiac): Fluid around the heart, which makes it harder for the heart to pump blood.
  3. Toxins: Drugs or poisons that can cause the heart to stop.
  4. Thrombosis (coronary): A heart attack caused by blood clots blocking the arteries of the heart.
  5. Thrombosis (pulmonary): A blood clot in the lungs, which stops blood from flowing properly.

Healthcare providers should quickly check for these possible causes during resuscitation to improve the chances of saving the patient and ensuring better recovery.

Cardiac Arrest vs Heart Attack in Hs and Ts

Cardiac arrest and heart attack are different heart emergencies, but both can be serious. The “Hs and Ts” are primarily associated with cardiac arrest, not heart attacks.

However, Some factors from the Hs and Ts list, like lack of oxygen (hypoxia) or blood clots (thrombosis), can be related to or contribute to a heart attack.

Cardiac Arrest

  • Definition: The heart suddenly stops beating.
  • Cause: An electrical problem in the heart.
  • Symptoms: The person may suddenly pass out, stop breathing, and have no pulse.
  • Immediacy: Happens quickly, with no warning.
  • Survival: Needs immediate CPR and use of a defibrillator (AED) to survive.

Heart Attack

  • Definition: Blood flow to part of the heart muscle is blocked.
  • Cause: A problem with blood flow, often from blocked arteries.
  • Symptoms: Chest pain, trouble breathing, sweating, and it can happen slowly.
  • Progression: The heart usually still beats during a heart attack.
  • Treatment: Needs quick medical help to restore blood flow.

Key Differences

  • Nature of the problem: Cardiac arrest is caused by an electrical problem, while a heart attack is caused by a blockage in blood flow.
  • Heart function: In cardiac arrest, the heart stops. In a heart attack, the heart usually keeps beating.
  • Onset: Cardiac arrest happens suddenly, while a heart attack may have warning signs and develop slowly.
  • Immediate response: Cardiac arrest needs immediate CPR and AED use, but a heart attack needs medical help while the person might stay conscious.

Although different, a heart attack can sometimes lead to cardiac arrest, and both can increase the risk of other heart problems.

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Conclusion

The 4 Hs and 4 Ts provide a systematic way for healthcare professionals to assess and treat the underlying causes of cardiac arrest. While it’s mainly used by medical professionals, knowing these causes helps others understand what’s happening and why certain treatments are given.

Remember, quick action and early help make a big difference in survival rates.

By using this method, survival chances improve, and care becomes more effective. It remains an essential part of managing cardiac arrest in emergency situations.

This information is for understanding only. Always call emergency services if someone has a cardiac arrest or shows warning signs.

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Studies show that mobile medical teams only perform a full evaluation in 75% of cases, and 20% don’t use the 4 Hs and 4 Ts in cardiac arrest as advised.

This shows that more training is needed to make sure healthcare providers follow this approach in every cardiac arrest situation to improve patient outcomes.

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