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.
- 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)
- Hypovolemia occurs when blood volume drops below normal levels.
- 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
- Typical causes of cardiogenic shock are:
- 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
- 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
- Typical causes are conditions that impede blood flow through large blood vessels and impair perfusion, such as:
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.
- Predictable changes occur during the fight-or-flight response:
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.
- Cause—decreased perfusion of the brain cells
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.