Volume Assessment
Volume status determination is an important component of cardiovascular system support. 'Endpoints of resuscitation' are clinical and laboratory findings that indicate a patient has returned to normovolemia after an episode of hypovolemic shock.
Determining Hypovolemia
The assessment of intravascular volume is challenging. No one number or clinical finding confirms that the vascular space has adequate volume for organ perfusion. The fact that the size of the vascular space can change in response to mediators adds complexity to the equation.
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Clinical Findings
Heart Rate and Blood Pressure are poor indicators of volume status.
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Objective Measurements
Clinical studies have consistently shown poor correlation between CVP and measures of blood volume. (Marik PE, Baram M, Vahid B. Does Central Venous Pressure predict fluid repsonsiveness? Chest 2008; 134;172-178)
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Laboratory Studies
Serum Lactate
Elevated serum lactate is an accurate tool to determine volume status in the setting of acute blood loss. It is less reliable in other forms of hypovolemia.
ScVO2
Superior Vena Cava oxygen saturation is a surrogate measure of the mixed venous O2 saturation (from the pulmonary arteries). This surrogate represents the difference between oxygen delivered to the tissues (DO2) and the oxygen the tissues consumed (VO2). When hypovolemia occurs, cardiac output decreases:
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HR x SV = CO
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A normal ScvO2 range is 70 - 80%
If symptoms raise concerns for hypovolemia, an ScvO2 can be an additional data point to confirm the diagnosis. However, it should not be used in isolation to determine volume status.
Hemoglobin
Hemoglobin is a very poor indicator of volume status in the acute setting. Changes in hemoglobin and hematocrit are a result of resuscitation with asanguinous fluids (e.g. crystalloids, albumin, hespans, etc). Neither hemoglobin nor hematocrit should be ever used to assess acute volume status changes.
"A Comedy of Errors"