Human Physiology and Shock Mechanisms

Introduction to Trauma - Physiology of Life & Death

• Body systems are interconnected and interdependent
• Every cell and organ work together to sustain cellular energy production and maintain vital metabolic processes
• Energy powers all body functions and sustains cellular and organ functions
• Cells create energy from oxygen and glucose, storing it as ATP (adenosine triphosphate) molecules
• Without energy, cellular functions cease
• The goal is to maintain the patient's body energy production

Systems & Components - Airway

• The airway must be patent (open)
• Breathing (lungs) must supply adequate oxygen:
  • Oxygen reaches the alveoli
  • Oxygen crosses the alveolar/capillary wall
  • Oxygen enters the circulation
• Carbon dioxide (CO2) must be removed

Systems & Components - Circulation

• Circulation distributes red blood cells (RBCs)
• Circulation ensures adequate numbers of RBCs
• Circulation transports oxygen to every cell in every organ
• Oxygen is then off-loaded to fuel the metabolic processes of the cell
• CO2 is transferred from the cells to the plasma for removal by the lungs

Key Components for Energy Production

• Glucose and oxygen are required for energy production

Airway

• An open airway is critical for delivering air (oxygen) to the alveoli
• Normal air movement is from negative intrathoracic pressure as the chest expands, filling the alveoli with air.
• Exhalation results from increased intrathoracic pressure as the chest relaxes, forcing air out of the alveoli

Breathing

• When air reaches the alveoli, oxygen crosses the alveolar-capillary membrane
• Oxygen enters the red blood cells (RBCs)
• Oxygen attaches to hemoglobin for transport
• CO2 in the plasma and cells is a byproduct of aerobic metabolism and energy production
• CO2 crosses the alveolar-capillary membrane into the alveoli
• CO2 is removed during exhalation

Circulation

• Oxygen-enriched RBCs are pumped through the body to deliver oxygen to target organs
• Oxygen is off-loaded from RBCs to fuel cell metabolic processes
• CO2 is transferred from cells to the plasma for elimination through the lungs

Aerobic Metabolism

• Aerobic metabolism is the most efficient method of energy production
• It uses oxygen and glucose with chemical reactions (glycolysis and the Krebs cycle) to produce large amounts of energy
• Waste products are carbon dioxide and water

Aerobic Metabolism is Dependent Upon

• Adequate and continuous supply of oxygen
• A patent airway
• Functioning lungs
• Functional heart
• Intact vascular system
• Adequate supply of RBCs

Anaerobic Metabolism

• Anaerobic metabolism is a metabolic process that functions in the absence of oxygen
• It uses stored glucose (glycogen) for energy production
• It is capable of sustaining only short-term energy requirements
• It produces much less energy than aerobic metabolism (approximately 19-fold decrease)
• Increased lactic acid is a by-product

Shock

• Inadequate energy production required to sustain life
• Results from changing from aerobic to anaerobic metabolism
• Secondary to hypoperfusion (inadequate blood flow)
• Delivery of oxygen is insufficient to meet metabolic demands
• Decreased energy production leads to cellular and organ death

Consequences of Hypoperfusion

• Cellular hypoxia
• Decreased ATP (energy) production
• Cell dysfunction
• Lactic acid buildup
• Low pH
• Cell autodigestion
• Cellular edema
• Further loss of blood volume
• Cycle continues

Consequences of Hypoperfusion (continued)

• Inadequate ATP causes cells and organs to not function properly
• Hypothermia (decreased heat production)
• Acidosis (increased lactic acid production)
• Coagulopathy (impaired blood clotting)

Triangle of Death

• Acidosis, loss of energy, hypothermia, and coagulopathy are involved in the death cascade

Cascade of Death

• Anaerobic metabolism
• Decreased energy production
• Cellular death
• Organ death
• Patient death

Types of Shock

• Hypovolemic
• Dehydration
• Burns
• Hemorrhage
• Distributive
  • Neurogenic
  • Septic
  • Anaphylactic
  • Psychogenic
• Cardiogenic
• Pump failure (intrinsic/extrinsic)

Hemorrhagic Shock

• Most common cause of hypoperfusion after trauma (internal or external blood loss)

Pathophysiology of Shock

• Shock is progressive
• Changes in shock include:
  • Hemodynamic
  • Cellular (metabolic)
  • Microvascular
• Compensatory mechanisms (short-term)
• Compensatory mechanisms will fail without interventions

Pathophysiology of Shock (continued)

• Heart must be an effective pump
• Heart is primed by return of blood through the vena cava
• Starling's Law
• Stroke volume (SV) is the amount of blood ejected with each contraction
• Blood volume decrease leads to SV decrease
• Cardiac output (CO) decreases unless heart rate increases
• Adequate blood pressure is required to maintain cellular perfusion
• CO is a factor in maintaining blood pressure (BP)
• If CO falls, vasoconstriction occurs
• Systemic vascular resistance (SVR) increases
• Vasoconstriction leads to the ischemic phase of shock
• Early precapillary and postcapillary sphincters constrict
• Tissues become ischemic and are forced to produce energy anaerobically
• As acidosis increases, precapillary sphincters relax while postcapillary sphincters remain constricted
• Results in stagnation of blood in the capillary bed and washout, releasing micro-emboli
• Aggravates acidosis
• Causes infarction of organs by micro-emboli

Neurogenic Shock

• Associated with spinal cord injury
• Interruption of sympathetic nervous system leading to vasodilation
• Patient has normal blood volume but vascular container is enlarged, thus decreasing blood pressure

Cardiogenic Shock

• Results from external compression of the heart
• Ventricles cannot fully expand
• Less blood is ejected with each contraction
• Blood return to the heart is decreased
• Causes from trauma include:
  • Pericardial tamponade
  • Tension pneumothorax

Signs Associated with Shock Types

• Hypovolemic
• Neurgenic
• Cardiogenic

Organ Sensitivity to Hypoperfusion

• Extremely sensitive: brain, heart, lungs (minutes)
• Moderately sensitive: kidneys, liver, gastrointestinal tract (1-1.5 hours)
• Least sensitive: muscle, bone, skin (4-6 hours)

Organ System Failure

• If not recognized and promptly corrected, shock leads to organ dysfunction
• First in oxygen-sensitive organs
• Followed by other, less oxygen sensitive organs
• Cascading effect results in multi-organ dysfunction syndrome and patient death
• Failure of one major organ system results in approximately 40% mortality
• Additional organ system failure leads to mortality approaching 100%
• Acute renal failure: Results if oxygen delivery is impaired for more than 45-60 minutes
• Decreased renal output -> Reduced clearing of toxic products
• Acute respiratory distress syndrome (ARDS): damage to alveolar cells, hyper-resuscitation (fluid overload)
• Fluid leakage into interstitial spaces and alveoli
• Hematologic failure: impaired clotting cascade, hypothermia, dilution or depletion of clotting factors
• Hepatic failure: Results from prolonged shock, overwhelming infection, and decreased functioning of the immune system due to ischemia and energy production loss

Summary for Compensated Hemorrhagic Shock

• Sinus tachycardia, tachypnea, normal systolic, narrowed pulse pressure, mild anxiety, cool and pale skin

General Summary

• Cellular function depends on adequate energy production, which depends on a continuous supply of oxygen.
• Impaired oxygen supply leads to anaerobic metabolism
• Inadequate energy production for sustained cellular function results in cellular dysfunction, cell death, organ dysfunction, and eventually, patient death.
• Early recognition and prompt intervention are crucial for improving survival rates in traumatic injury victims.