Pathophysiology: Perfusion and Shock

Anatomy Review

• Anatomy of the cell
• The pulmonary system
• The cardiovascular system

Normal Cardiovascular Physiology and Perfusion

• Normal cellular function (metabolism) requires water, glucose and oxygen.
  • Aerobic metabolism occurs when oxygen is available to the cell in sufficient quantities.
  • Aerobic metabolism yields large amounts of energy in the form of adenosine triphosphate (ATP) and relatively low-waste products.
• The medulla oblongata (brainstem) is the center of cardiovascular control. It uses input from chemoreceptors and stretch receptors (baroreceptors) to adjust the cardiovascular system to meet metabolic demands.
• Minute ventilation delivers oxygen to the alveoli (alveolar ventilation) and removes carbon dioxide.
• Delivery of nutrients and oxygen to the cells (perfusion) is accomplished by adequate blood flow. Perfusion requires:
  • an adequate blood volume
  • an adequate supply of red blood cells (the oxygen-carrying portion of the blood)
• The heart controls the delivery of blood to the cells. It is the master pump.
• Cardiac output is the amount of blood pumped by the heart each minute. It is a function of heart rate multiplied by stroke volume (HR × SV)
• Stroke volume is the amount of blood pumped in one contraction of the heart. It is a function of the amount of blood returned to the heart (preload), the amount of systemic pressure against which the heart must pump (afterload), and the strength of contraction (contractility).
• Blood vessels contain the blood and allow it to be transported.
• The diameter of blood vessels is important to the body’s ability to maintain pressure within the system:
  • Vasoconstriction and vasodilation constantly adjust the size of the cardiovascular system container and therefore constantly adjust pressure.
  • Blood pressure is maintained when the quantity of blood is sufficient and the blood vessels are of normal diameter.
• Blood vessels are permeable and can move the liquid portion of the blood into and out of circulation.
  • Plasma oncotic pressure attracts water into the bloodstream and helps maintain normal volume.
  • Hydrostatic pressure is the force that pushes water out of the bloodstream.
  • In normal conditions, these two forces balance one another out.
• Normal metabolism is maintained when ventilation/perfusion (V/Q) matching occurs:
  • The cardiovascular system supports normal blood flow and pressure.
  • The respiratory system supports normal air delivery to the alveoli.

Pathophysiology of Hypoperfusion (Shock)

• Hypoperfusion, otherwise known as shock, occurs when the body’s cells do not receive an adequate supply of oxygenated blood.
• The 4 primary causes of shock are : hypovolemia, cardiogenic shock, distributive shock and obstructive shock.
  1. Hypovolemia occurs when blood volume drops below normal levels.
     • Absolute hypovolemia occurs with a loss of red blood cells or blood volume, as with bleeding.
     • Relative hypovolemia occurs when the liquid portion of the blood is lost without actual bleeding, as in dehydration.
       • Relative hypovolemia often is caused by increased blood vessel permeability.
     • Typical causes of hypovolemia are:
       • blood loss
       • dehydration
       • sepsis and anaphylaxis (where hypovolemia results from increased blood vessel permeability)

2. Cardiogenic shock occurs as a result of pump failure. In this case, a failed pump leads to a disruption in the flow of blood and thus to decreased perfusion.
   • Typical causes of cardiogenic shock are:
     • acute myocardial infarction
     • congestive heart failure
     • cardiac injury

3. Distributive shock occurs as a result of poor blood vessel tone and leads to decreased pressure within the cardiovascular system. Disorders such as sepsis and anaphylaxis can cause blood vessels to dilate.
   • Decreased pressure causes an inability of the pump to move blood and leads to hypoperfusion.
   • Typical causes of distributive shock are:
     • sepsis
     • anaphylaxis
     • spinal injury

4. Obstructive shock occurs when the flow of blood is blocked.
   • Typical causes are conditions that impede blood flow through large blood vessels and impair perfusion, such as:
     • pulmonary embolism
     • tension pneumothorax
     • pericardial tamponade

Cardiovascular Compensation

• The brain’s chemoreceptors sense chemical changes in the blood and cerebral spinal fluid.
  • Chemoreceptors are sensitive to changes in oxygen and carbon dioxide levels and to changes in pH.
• The brain also senses changes in pressure through stretch receptors in the large blood vessels.
• When chemoreceptor and stretch receptor feedback indicate the possibility of hypoperfusion, the brain initiates the fight-or-flight response via the sympathetic nervous system. This response improves cardiovascular performance.
  • Predictable changes occur during the fight-or-flight response:
    • The release of norepinephrine and epinephrine cause the heart to pump faster and stronger, leading to increased cardiac output.
    • Blood vessels constrict, decreasing the cardiovascular container size.
    • The brain signals the kidneys to stop eliminating water, helping to support cardiovascular volume.
    • The respiratory system increases minute volume in an effort to increase the amount of oxygen available.
  • Compensation has a cost:
    • The muscles of compensation’s increased use of O₂ and energy means that compensation is a limited capability that can fail.

Signs and Symptoms of Compensation and their Causes (Compensated Shock)

• Altered mental status
  • Cause—decreased perfusion of the brain cells
    • Remember that altered mental status in hypoperfusion can indicate a broad spectrum of events ranging from anxiety to frank unconsciousness. The level of altered mental status decreases as perfusion drops.
  • Tachycardia
    • Cause—cardiac compensation
  • Tachypnea
    • Cause—respiratory compensation
  • Narrowing pulse pressures (the difference between systolic BP and diastolic BP)
    • Cause—vasoconstriction/vascular compensation
  • Pale skin/delayed capillary refill time
    • Cause—vasoconstriction/vascular compensation
  • Hypotension
    • Cause—vascular decompensation
  • NOTE: During compensated shock, pressure within the cardiovascular system is reasonably supported, so you may not observe dramatic drops in blood pressure or seriously altered mental status.

Signs and Symptoms of Failed Compensation (Decompensated/Hypovolemic Shock)

• Altered mental status secondary to poor perfusion of brain cells
• Increasing tachycardia and bradycardia as compensation fails
• Increasing tachypnea and eventual slowing of respiration as compensation fails
• Dropping blood pressure (hypotension)
• Increasing pallor and delayed capillary refill
  • NOTE: Not all shock states will be associated with pale skin. Some conditions, such as sepsis or neurogenic shock, may cause mottled or even flushed skin.