Hypovolemic Shock

Shock: occurs when tissues of the body are not delivered or supplied with sufficient oxygen to meet metabolic demands. Vital organs and tissues are hypoperfused. Shock results are physiological compensatory mechanisms decompensate into certain states of physiological shock depending on the etiology.

Hypovolemic shock: is a type of shock that occurs when tissues of the body are insufficiently perfused due to diminished blood volume. Presents when there is an acute loss of 15-20% of the circulating blood volume.

Etiology:

  • due to decreased blood volume, presents when there is an acute blood volume deficit of 15-20%
  1. Hemorrhage: external or internal blood loss
  2. Severe Burns: plasma loss
  3. Severe dehydration
  4. Gastrointestinal fluid loss: emesis or diarrhea
  5. Third Spacing: extracellular fluids are lost as fluid shifts from vascular to interstitial spaces.

 

Pathophysiology:

  • Blood loss results in an overall decrease in the circulating blood volume. As less blood is returned to the heart the resulting cardiac function will be negatively impacted.
  • The stroke volume will be decreased as less blood with be returned to the heart and therefore less blood will be ejected from the ventricles as they contract.
  • Decreased stroke volume will result in a subsequent decrease in cardiac output.
  • The heart rate will increase as the body compensates for blood loss and therefore hemoglobin lost. Blood is consequently circulated at a more rapid rate to continue to adequately oxygenate the tissues with fewer hemoglobin molecules present to carry the oxygen molecules.
  • Arterial blood pressure will be maintained as the sympathetic nervous system compensates for the blood loss by vasoconstriction. However, this compensatory action can only be maintained by the body for blood volume losses of just under 20%. With blood volume losses greater than 20% the arterial blood pressure falls rapidly.
  • As blood volume is lost, a combination of vasoconstriction and increased heart rate maintain the heart rate within a narrow range. Heightened vessel muscle tone and increased vascular resistance serve to constrict vessels and retain more blood in the body core.

 

 

Two main Compensatory Mechanisms in Hypovolemic Shock

  1. Mechanisms that maintain cardiovascular function
  • Increased heart rate and cardiac contractility

  • Peripheral vasoconstriction in skin and nonvital organs

2. Mechanisms that maintain blood volume

  • The hypothalamus triggers the thirst response to increase fluid intake and subsequently increase overall blood fluid volume

  • The posterior pituitary stimulates ADH release to retain water

  • The adrenal cortex is triggered by the decreased blood flow to activate the renin-angiotensin-aldosterone mechanism which acts to retain water in the body

  • Aldosterone and ADH affect the kidneys allowing sodium and water to be retained and not excreted from the body as urine

  • The liver plays a vital role in increasing blood volume by constricting hepatic veins and sinusoids within the organ to release additional blood stores to the general circulation.

Manifestations:

  1. increased heart rate
  2. decreased blood pressure
  3. pulse becomes thready and weak due to vascocontriction and decreased cardiac input
  4. Respirations deepen and rate increases in effort to increase CO2 expiration (acidosis) & increased O2 inspiration (oxygenation)
  5. cool, mottled and clammy skin: peripheral veins collapse, vasoconstriction
  6. variation in mentation: agitation, restlessness, apprehension resulting from increased sympathetic outflow and increased epinephrine levels

 

 

Martin, Glenn and Porth, Carol, Mattson. 2009. Pathophysiology Concepts of Altered Health States. 8th ed. Lippincott Williams and Wilkins. Philadelphia

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