How does profound acidosis lead to cardiac arrest?

Physiological Mechanisms of Profound Acidosis Leading to Cardiac Arrest

Profound acidosis causes cardiac arrest through multiple mechanisms including decreased myocardial contractility, reduced response to catecholamines, electrolyte disturbances, and vasodilation leading to hemodynamic collapse.

Primary Mechanisms of Acidosis-Induced Cardiac Dysfunction

• During cardiac arrest, there is a complex low-perfusion state characterized by venous and tissue hypercarbic and metabolic (lactic) acidosis, creating a vicious cycle 1
• Profound acidosis develops rapidly in cardiac tissue during arrest with intramyocardial pH dropping significantly (from 7.27 to 6.88 in experimental models) 2
• CO2 production during ischemia is the predominant mechanism accounting for myocardial hydrogen ion increases during cardiac arrest, with cardiac vein PCO2 increasing from 57 to 158 mm Hg in experimental models 2
• Severe acidosis directly impairs myocardial contractility, reducing the force of cardiac contractions 3

Cellular and Molecular Effects

• Acidosis shifts the oxyhemoglobin saturation curve, inhibiting oxygen release to tissues, further worsening cellular hypoxia 4
• Acidosis reduces the binding of catecholamines (like adrenaline/epinephrine) to receptors, decreasing their effectiveness during resuscitation 4
• Acidosis may inactivate simultaneously administered catecholamines during resuscitation attempts 3
• Severe acidosis can compromise coronary perfusion pressure by reducing systemic vascular resistance 4

Electrolyte Disturbances

• Acidosis is often accompanied by hyperkalemia, which can cause cardiac arrhythmias and further impair cardiac function 5
• Patients with cardiac arrest demonstrate complex electrolyte abnormalities including hyperkalemia, hypochloremia, and hyperphosphatemia that contribute to the acidotic state 5
• The cause of metabolic acidosis in cardiac arrest is multifactorial - while lactate is the strongest determinant (-11.8 mEq/L contribution), other factors like strong ion gap (-7.3 mEq/L) and phosphate (-2.9 mEq/L) also play significant roles 5

Vicious Cycle of Acidosis and Cardiac Dysfunction

• Acidosis creates a vicious cycle where decreased cardiac output leads to tissue hypoperfusion, which worsens acidosis, further impairing cardiac function 1
• Increased release of CO2 from ischemic tissues and reduced CO2 transport from tissues to lungs result in profound tissue acidosis during cardiac arrest 1
• Struggling against restraints (as in excited delirium cases) can produce severe lactic acidosis (pH as low as 6.25) leading to cardiac arrest 6

Compensatory Mechanisms

• Some compensatory mechanisms naturally attenuate acidosis severity, including hypochloremia (+4.6 mEq/L alkalinizing effect), hyperkalemia (+3.6 mEq/L), and hypoalbuminemia (+3.5 mEq/L) 5
• Adequate ventilation can counterbalance hypercarbic and metabolic acidemia by creating concurrent hypocarbic arterial alkalemia during early phases of CPR 1

Clinical Implications

• Severe acidosis is compatible with survival if rapidly corrected - cases of survival have been documented with pH as low as 6.46 and 6.61 7, 6
• The best method of reversing acidosis associated with cardiac arrest is to restore spontaneous circulation through effective CPR and early defibrillation when indicated 3
• Sodium bicarbonate is not recommended for routine use in cardiac arrest but may be considered in cases of profound acidosis (pH < 7.1 and base deficit < -10) 3, 4
• When bicarbonate is indicated, the recommended initial dose is 1 mEq/kg given as a single bolus slow intravenous injection 3, 8

Prevention and Management

• Effective basic life support, adequate ventilation with oxygen, and high-quality chest compressions are the mainstays of preventing and treating the acidosis of cardiac arrest 4
• In special situations like preexisting metabolic acidosis, hyperkalemia, or tricyclic antidepressant overdose, bicarbonate administration may be beneficial 3, 4
• Rapid achievement of return of spontaneous circulation is the most effective way to reverse the acidosis of cardiac arrest 4