Hypovolemic shock occurs when there is a substantial reduction in intravascular volume by 15% to 25% which represents a loss of approximately 750ml to 1,300 ml of blood in an average person weighing 70-kg (154-lbs).
Hypovolemic shock is the most common type of shock, is characterized by a marked decreased intravascular volume of blood in the circulatory system. Body fluid is contained in the intracellular and extracellular compartments. Intracellular fluid accounts for about two thirds of the total body water while extracellular fluid is found one of two compartments namely: intravascular (inside the blood vessels) and interstitial (surrounding the tissues). The volume of interstitial fluid is approximately three to four times that of the intravascular and fluid.
Pathophysiology of Hypovolemic Shock
Hypovolemic shock can be caused by external fluid losses, as in traumatic blood loss and profuse bleeding or hemorrhage, severe dehydration, severe edema and even ascitis. Intravascular volume can be reduced by fluid loss and by fluid shifting between the intravascular and interstitial compartments.
The sequence of events in hypovolemic shock typically begins with the decrease in the intravascular volume. This results in decreased venous return of blood volume to the heart and subsequent reduction in ventricular filling which in turn result in an inadequate volume of blood being pumped by the heart to the vascular system. Furthermore, decreased ventricular filling results in decreased stroke volume (the amount of blood ejected from the heart) and decreased in cardiac output. When cardiac output drops, blood pressure also drops which can lead to inadequate blood supply reaching the vital organs of the body for subsequent nourishment.
Compensatory Stage of Shock
At the compensatory stage of shock, blood pressure remains within the normal range. Vasoconstriction, increased heart rate and increased contractility of the heart contribute to maintaining the adequate cardiac output. This results from the stimulation of the sympathetic nervous system and subsequent release of catecholamine (adrenaline) which temporarily sustains the normal response of the body in maintaining homeostasis in a short period of time. Patients normally would experience the “fight or flight” response.
This bodily response is described as the body’s survival instinct present in all human beings. The compensatory state of shock follows a similar pathway in which, the body’s reserved boost of energy to maintain homeostasis enough to survive an a bodily threat. Therefore, individuals who are in the compensatory stage of shock although there is already a marked decrease in fluid volume in the body is temporarily maintained by the activation of the parasympathetic nervous system.
Medical management of hypovolemic shock
The overall management of shock including hypovolemic shock is
fluid replacement. The type of fluids administered and the promptness in which it is delivered may often vary but fluids are administered to augment and improve cardiac and tissue oxygenation. The major goals in the treatment of hypovolemic shock can be usually divided into a three-prong approach which include: (1) to restore intravascular volume to reverse the sequence of events leading to inadequate tissue perfusion, (2) to redistribute fluid volume, and (3) to correct the underlying cause of fluid loss as quickly as possible. Depending on the severity of the hypovolemic shock and the patient’s condition, it is likely that the efforts will be made to address all three goals simultaneously.