H's and T's of Cardiac Arrest

You may have heard about the H's and T's of cardiac arrest. Maybe yes maybe no. In this article, we have made it simple and clear for you regarding the topic.

Many different traumatic and medical conditions can lead to cardiac arrest in both adults and children. This includes electrical abnormalities, inherited disorders and structural changes in the heart.

It is of great importance to determine and treat the cause of cardiac arrest. Fortunately, many causes of cardiac arrest are reversible. These conditions are often referred to by the mnemonic “H’s and T’s of cardiac arrest”.

The H’s and T’s of cardiac arrest is a mnemonic used to help recall the major contributing factors to pulseless arrest including PEA, Asystole, Ventricular Fibrillation, and Ventricular Tachycardia.

H's and T's of Cardiac Arrest

Without taking much time lets start with the H"s

The H's

Hypovolemia

Hypovolemia basically refers to a lack of circulating body fluids, principally blood volume. This is usually caused by some form of bleeding, anaphylaxis, or pregnancy with a gravid uterus.

Severe burns can also lead to hypovolemia. Hypovolemia from blood loss is a leading cause of death in traumatic cardiac arrest. External blood loss is usually obvious (e.g., trauma, hematemesis, hemoptysis), but may be more challenging to diagnose when occult (e.g., gastrointestinal bleeding or rupture of an aortic aneurysm)

Peri-arrest treatment includes giving IV fluids preferably warmed crystalloids and blood transfusions, and controlling the source of any bleeding either by direct pressure for external bleeding, or emergency surgical techniques such as esophageal banding, gastroesophageal balloon tamponade, thoracotomy in cases of penetrating trauma or significant shear forces applied to the chest, or exploratory laparotomy in cases of penetrating trauma, spontaneous rupture of major blood vessels, or rupture of a hollow viscus in the abdomen.

Hypoxia

Hypoxemia is low levels of circulating oxygen in the blood, which can lead to hypoxia at the tissues. A lack of oxygen delivery to the heart, brain, and other vital organs.

In most cases, hypoxemia is a consequence of asphyxia, which accounts for most of the non-cardiac causes of cardiac arrest. The conditions that can cause hypoxia are:

- Airway obstruction
- Anemia
- Asthma
- Central hypoventilation – brain or spinal cord injury
- Chronic obstructive pulmonary disease
- Drowning
- Impaired alveolar ventilation from neuromuscular disease
- Pneumonia
- Tension pneumothorax
- Trauma or traumatic asphyxia.

Rapid assessment of airway patency and respiratory effort must be performed. If the patient is mechanically ventilated, the presence of breath sounds and the proper placement of the endotracheal tube should be verified.

Treatment may include providing oxygen, proper ventilation, and good CPR technique. In cases of carbon monoxide poisoning or cyanide poisoning, hyperbaric oxygen may be employed after the patient is stabilized.

Hydrogen ions (acidosis)

When you hear of hydrogen ions you think of something to do with acids right? In this case, acidosis can be either metabolic or respiratory.

An abnormal pH in the body as a result of lactic acidosis which occurs in prolonged hypoxia and in severe infection, diabetic ketoacidosis, renal failure causing uremia, or ingestion of toxic agents or overdose of pharmacological agents, such as aspirin and other salicylates, ethanol, ethylene glycol, and other alcohols, tricyclic antidepressants, isoniazid, or iron sulfate.

Acidosis of any kind is most likely detrimental to the circulation as it causes peripheral vasodilatation, negative inotropy and impaired oxygen uptake in the lungs.

An arterial blood gas is a quick and accurate method to determine if a patient is acidotic.

This can be treated with proper ventilation, good CPR technique, buffers like sodium bicarbonate, and in select cases may require emergent hemodialysis.

Hyperkalemia or hypokalemia

Both excess and inadequate potassium can be life-threatening.

Electrolyte abnormalities can cause cardiac arrhythmias or cardiac arrest, and life-threatening arrhythmias are associated most commonly with potassium disorders, particularly hyperkalemia. Potassium is an electrolyte that plays a role in maintaining the normal contraction of the myocardium. If levels become too high or too low, cardiac arrest may ensue.

Evaluation of serum potassium must take into consideration the effects of changes in serum pH. When serum pH decreases (acidemia), serum potassium increases because potassium shifts from the cellular to the vascular space; the process that is reversed when serum pH increases (alkalemia).

Causes of hypokalemia include excessive vomiting/diarrhea or the use of diuretics. Chronic kidney disease can also lead to potassium loss.

Treatment may include a controlled but rapid infusion of potassium. Hyperkalemia may be caused by kidney disease, diabetes and as a side effect of certain drugs.

Hyperkalemia can be treated by administering sodium bicarbonate or calcium chloride or by performing dialysis.

Hyperkalemia:

Hyperkalemia is the most common electrolyte disorder associated with cardiac arrest. It is usually caused by impaired excretion by the kidneys, drugs or increased potassium release from cells and metabolic acidosis with hyperkalemia occurring in up to 10% of hospitalized patients.

There is no steadfast numeric limit universally used to define hyperkalemia, but 5.5 mmol-1 is commonly recognized. As the potassium concentration increases above this value the risk of adverse events increases and the need for urgent treatment increases.

The main causes of hyperkalemia are:

Renal failure (i.e., acute kidney injury or chronic kidney disease)
Drugs (e.g., angiotensin-converting enzyme inhibitors (ACE-I), angiotensin II receptor antagonists (ARB), potassium-sparing diuretics, non-steroidal anti-inflammatory drugs, beta-blockers, trimethoprim)
Tissue breakdown (e.g., rhabdomyolysis, tumor lysis, hemolysis)
Metabolic acidosis (e.g., renal failure, diabetic ketoacidosis)
Endocrine disorders (e.g., Addison’s disease)
Diet (may be the sole cause in patients with advanced chronic kidney disease)

The treatment for hyperkalemia involves five key strategies:

Cardiac protection
Shifting potassium into cells
Removing potassium from the body
Monitoring serum potassium and blood glucose
Prevention of recurrence

Hypokalemia:

Hypokalemia is defined as a serum potassium level <3.5 mmol-1 and severe hypokalemia is serum potassium <2.5 mmol-1. Hypokalemia is the most common electrolyte disturbance seen in up to 20% of hospitalized patients. It increases the incidence of arrhythmias and sudden cardiac death.

The main causes of hypokalemia include:

Gastrointestinal loss (e.g., diarrhea)
Drugs (e.g., diuretics, laxatives, steroids)
Renal losses (e.g., renal tubular disorders, diabetes insipidus, dialysis)
Endocrine disorders (e.g., Cushing’s syndrome, hyperaldosteronism)
Metabolic alkalosis
Magnesium depletion
Poor dietary intake

Treatment of hypokalemia depends on the severity and the presence of symptoms and ECG abnormalities.

The best course of action is the gradual replacement of potassium to normal serum levels. In an emergency, intravenous potassium is warranted, with the knowledge that many patients who are hypokalemic are also hypomagnesemia.

Repletion of magnesium stores will facilitate more rapid correction of hypokalemia and is recommended in severe cases of hypokalemia

A common presentation of hyperkalemia is in the patient with end-stage renal disease who has missed a dialysis appointment and presents with weakness, nausea, and broad QRS complexes on the electrocardiogram.