The 5 H's and T's of Cardiac Arrest

The H's and T's are a framework used in healthcare to help identify potential reversible causes of cardiac arrest. Understanding what happens at the cellular level for each of these can provide insights into the mechanisms underlying cardiac arrest.

The H's:

1. Hypovolemia:
   • Cellular Level: Reduced blood volume leads to decreased perfusion of tissues. At the cellular level, this means less oxygen and nutrients are delivered to cells, resulting in cellular hypoxia and impaired metabolism.
2. Hypoxia:
   • Cellular Level: Inadequate oxygen delivery to tissues occurs. At the cellular level, this leads to a switch to anaerobic metabolism. Lactic acid builds up, causing acidosis, and energy production becomes less efficient.
3. Hydrogen Ion (Acidosis):
   • Cellular Level: Severe acidosis disrupts enzyme function, alters cell membrane permeability, and impairs ion transport. This can lead to dysfunction of cellular processes, including those essential for cardiac function.
4. Hypo/Hyperkalemia:
   • Cellular Level: Abnormal potassium levels can disrupt the electrical activity of cells, including cardiac myocytes. High potassium levels can lead to depolarization of the cell membrane, causing arrhythmias.
5. Hypothermia:
   • Cellular Level: Extremely low body temperature slows down metabolic and enzymatic activity. This can lead to impaired cellular function, including reduced effectiveness of ion pumps and channels involved in cardiac electrical activity.

The T's:

1. Tension Pneumothorax:
   • Cellular Level: Increased pressure in the pleural space compresses the heart and great vessels. This reduces venous return to the heart and impairs cardiac output, leading to reduced oxygen delivery to tissues.
2. Tamponade, Cardiac:
   • Cellular Level: Accumulation of fluid in the pericardial sac compresses the heart, impeding its ability to fill and pump effectively. This leads to reduced cardiac output and diminished oxygen delivery to tissues.
3. Toxins:
   • Cellular Level: Certain toxins or drugs can interfere with the function of ion channels in cardiac cells. This can lead to abnormal electrical activity and arrhythmias at the cellular level.
4. Thrombosis, Pulmonary:
   • Cellular Level: A blood clot in the pulmonary arteries obstructs blood flow. This reduces oxygen delivery to the heart and can lead to impaired cellular metabolism and function.
5. Thrombosis, Coronary:
   • Cellular Level: A blood clot in the coronary arteries can lead to myocardial infarction (heart attack). This results in cellular death (necrosis) due to lack of oxygen and nutrients.

How does acidosis lead to cardiac arrest?

1. Disruption of Enzyme Function: Acidosis alters the pH balance within cells. Enzymes, which are essential for numerous cellular processes, including those involved in energy production and ion transport, are highly sensitive to pH changes. In an acidic environment, enzyme activity can be impaired, leading to a decrease in overall cellular function.
2. Altered Ion Transport: Acidosis affects the movement of ions across cell membranes. This can lead to disruptions in the balance of key ions like sodium, potassium, calcium, and hydrogen ions. These ions play crucial roles in electrical signaling within cells, including cardiac myocytes. Disruptions in ion transport can lead to changes in the electrical properties of the heart, potentially causing arrhythmias.
3. Impaired Cellular Metabolism: Severe acidosis can hinder the metabolic processes that generate ATP (adenosine triphosphate), which is the primary energy source for cells, including cardiac cells. This reduction in ATP availability can lead to decreased contractility and pump function of the heart.
4. Reduced Response to Catecholamines: Acidosis can dampen the responsiveness of the heart to catecholamines like epinephrine and norepinephrine. These hormones play a vital role in regulating heart rate and contractility. Acidosis can reduce the effectiveness of these regulatory mechanisms, potentially leading to decreased cardiac output.
5. Disruption of Intracellular Signaling: Acidosis can interfere with intracellular signaling pathways that are critical for normal cellular function. This disruption can impair the coordination of cellular activities, including those required for proper cardiac function.