Prehospital Academy Lesson 13.1
Pathophysiology of Shock
Shock is not merely a drop in blood pressure; it is a profound state of hypoperfusion where cellular oxygen delivery fails to meet metabolic demand. This lecture explores the Fick Principle, the transition to anaerobic metabolism, and the microscopic failure of the sodium-potassium pump that leads to irreversible organ damage.
The Logistics of Perfusion: The Fick Principle
Adequate tissue oxygenation depends on the Fick Principle, which requires the seamless integration of three components:
Onloading: Sufficient atmospheric O2 and functional alveolar-capillary gas exchange.
Transport: Adequate hemoglobin concentrations and a functional "pump" (cardiac output) to move oxygenated blood.
Offloading: The ability of O2 to leave the RBC at the tissue level and move into the mitochondria.
A failure in any of these components whether from hemorrhage (transport), airway obstruction (onloading), or cyanide poisoning (offloading)�results in the shock state.
Metabolic Failure: The ATP Gap
The transition from Aerobic to Anaerobic metabolism is the hallmark of shock.
The Energy Crisis: Aerobic metabolism produces 36 units of ATP; anaerobic produces only 2. This 95% loss of energy causes the Sodium-Potassium Pump to fail.
Cellular Edema: Without ATP, sodium stays inside the cell, pulling water with it. The cell swells, the lysosomes rupture, and the cell eventually auto-digests (lyses).
Lactic Acidosis: The byproduct of anaerobic metabolism is lactic acid. As the blood pH drops (acidosis), clotting enzymes become inactive, and cardiac contractility fails, leading to the "Triad of Death."
Mean Arterial Pressure (MAP) and Organ Perfusion
Systolic blood pressure is often "falsely normal" in early shock due to compensation. Mean Arterial Pressure (MAP) is the superior marker of tissue perfusion.
MAP = [(2 x Diastolic) + Systolic] / 3
A MAP of at least 60-65 mmHg is required to maintain the glomerular filtration rate in the kidneys and perfusion to the cerebral cortex.
The Microcirculation: Shunting and Washout
In Compensated Shock, the body uses pre-capillary sphincters to "shunt" blood away from the skin and GI tract toward the brain and heart.
Capillary Stagnation: During prolonged shock, the sphincters relax (due to acidosis), but the post-capillary sphincters remain closed. Blood "pools" and clots in the capillaries.
The Washout Phase: In decompensated shock, the post-capillary sphincters finally fail, dumping micro-clots, potassium, and lactic acid into the systemic circulation. This "washout" can cause sudden cardiac arrest during resuscitation.
Clinical Predictor: The Shock Index
The Shock Index (HR / Systolic BP) is a powerful tool for identifying "occult" or hidden shock. A normal index is 0.5-0.7. If the index is > 0.9, the patient is likely in a state of significant hypoperfusion, even if their blood pressure appears normal.
Critical Thinking Questions
Explain why a "normal" blood pressure can be a dangerous distractor in a trauma patient with a heart rate of 130. Use the **Shock Index** to justify your answer.
Describe the role of the **Sodium-Potassium Pump** in cellular survival. Why does its failure lead to irreversible shock?
Calculate the **MAP** for a patient with a BP of 90/60. Are they perfusing their kidneys?
What is the "Washout Phase," and why might a patient's condition drastically worsen just as you begin aggressive fluid resuscitation?
Contrast the **Onloading** and **Transport** components of the Fick Principle. How would you treat a failure in each?