SvO2 = SaO2 - VO2/DO2 DO2 = Volume of O2 delivered per minute = CO x CaO2 x 10
VO2 = Oxygen consumption per minute
= C(a-v)O2 x CO x 10 SaO2 = Arterial O2 saturation (1.0)
CaO2 = 1.39 x Hgb x SpO2 + 0.003 x PaO2 where VO2 is oxygen consumption, CaO2 is arterial oxygen content, CvO2 is venous oxygen content, CO is cardiac output, DO2 is oxygen delivery, SaO2 is arterial oxygen saturation, Hgb is hemoglobin, SpO2 is oxygen saturation, and PaO2 is partial pressure of arterial oxygen.
Accurate measurement of SvO2 requires that vasoregulation be intact (40), and there must be a continuous flow of blood past the tip of the catheter. SvO2 values may be incorrect if the tip of the pulmonary artery catheter migrates into the distal pulmonary artery or comes in contact with the arterial wall. Other causes of incorrect SvO2 values include miscalibration of the microprocessor or light intensity that is too low. Note that the tip of the catheter must be in the pulmonary artery to have true mixed venous oxygen.
Mixed venous oxygen saturation (SvO2) monitoring provides information about the balance between total body oxygen consumption and delivery. SvO2 (0.65-0.75) measures the amount of oxygen not taken up by organs and tissues. Therefore, the lower the SvO2 is, the higher the fraction extraction of oxygen by the tissues is, and a possible imbalance between oxygen consumption and oxygen delivery may exist. SvO2 is dependent on arterial oxygen saturation, oxygen consumption, concentration of hemoglobin, and cardiac output. A significant change in SvO2 may be caused by decreased oxygen delivery (decreased cardiac output and hemoglobin), increased oxygen consumption, or decreased arterial oxygen saturation.
Continuous SvO2 monitoring is useful in conditions for which there is significant oxygen transport imbalance, including severe cardiac and respiratory disease, sepsis, or dysfunctional oxygen transport (41). During stable arterial oxygen content and consumption, SvO2 reflects cardiac output (42,43). Furthermore, monitoring of SvO2 may provide vital information in the medical management of either critically ill (44,45) or cardiac surgery patients (46). If SvO2 drops during a period of increased oxygen demand, it may indicate inadequate tissue perfusion and oxygen delivery; this information would not be available with the monitoring of cardiac output alone.
It is considered that SvO2 may be a better measurement of myocardial performance than cardiac output itself. Acute decrease in SvO2 below 0.65 indicates a disparity between oxygen delivery and oxygen consumption. A change in SvO2 greater than ±0.1 is considered significant. Medical management of critically ill patients by implementing measures to keep SvO2 normal may decrease morbidity and mortality (30,47). Conditions for which SvO2 is greater than 0.75 include increased oxygen delivery and low oxygen consumption. SvO2 may be elevated during septic shock, hyperoxygenation, and cyanide toxicity and in patients with arterial venous shunts (40,41). The increase in SvO2 during sepsis is caused by the loss of vasoregulation and does not mean that organ tissues are adequately oxygenated. Under general anesthesia, the SvO2 value is increased because of the decreased metabolic requirement for oxygen by tissues.
In clinical settings for which a pulmonary artery SvO2 catheter is not possible (i.e., for pediatric patients), a central venous oxygen saturation (ScvO2) monitor may be used. The advantage of ScvO2 is that a pulmonary artery catheter is not required; only central venous access is needed. The ScvO2 obtains venous oxygen saturation readings from the superior vena cava or right atrium. During normal physiological and hemodynamic conditions, ScvO2 correlates well with SvO2 (48-50). However, in critical illness and shock, the ScvO2 does not accurately reflect the true SvO2 (51-53); therefore, true SvO2 can only be measured in the pulmonary artery in such cases (54). Resuscitation and medical management of critically ill patients with a
ScvO2 monitor may provide benefits over conventional monitors, such as provision of vital signs and central venous pressure (51).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.