Page 328 - Clinical Application of Mechanical Ventilation

294 Chapter 10

MIxED VENOUS OxYGEN SATURATION

A special version of the pulmonary artery catheter uses fiberoptic technology to
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monitor the mixed venous oxygen saturation (SvO ). The fiberoptic central venous
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catheter measures the SvO accurately within the clinical range (between 50%
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and 80%) (Fletcher, 1988). When SvO is used with other monitoring capabilities
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of the pulmonary artery catheter, it can provide valuable information concerning
oxygen delivery and consumption.
Decrease in Mixed Venous Oxygen Saturation

For individuals with a balanced oxygen delivery (DO ) and oxygen consump-
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The normal SvO 2 is about tion (VO ), the measured SvO is between 68% and 77% with an average
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75%. SvO 2 measurements 2 # 2
from 50% to 70% indicate of 75%. SvO measurements from 50% to 70% indicate decreasing DO or
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decreasing oxygen delivery increasing VO with compensatory O extraction—a process to meet the mini-
( DO 2 ) or increasing oxygen 2 2 #
➞
consumption ( ➞ VO 2 ) with mal oxygen needs by the body. When the SvO drops to a range of 30%–50%,
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compensatory O 2 extraction. lactic acidosis becomes evident due to exhausting of extraction. From 25%
to 30%, severe lactic acidosis is common. Below 25%, cellular death is ensured
(Zaja, 2007).
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Common causes of decreased SvO due to poor oxygen delivery include low car-
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diac output, anemia, and hypoxic hypoxia. Causes of decreased SvO due to exces-
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sive oxygen consumption include fever, seizures, increased physical activity or work
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of breathing, stress, and pain. Some conditions that may lead to a decreased SvO
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are summarized in Table 10-10.
Increase in Mixed Venous Oxygen Saturation
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Increases in SvO above 75% are uncommon but may occur when the tip of the
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Increases in SvO 2 above pulmonary artery catheter is improperly wedged. Once in this abnormal position,
75% are uncommon but may
occur when the tip of the the forward mixed venous blood flow is obstructed while the catheter tip senses the
pulmonary artery catheter is blood from an area with a high ventilation/perfusion ratio, and therefore a high
improperly wedged.
oxygen saturation. Other conditions that reduce metabolic oxygen consumption
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may also lead to an increase in SvO . Some examples include use of analgesics
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or sedatives, full ventilatory support on mechanical ventilation, and hypothermia
(Zaja, 2007).
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In some uncommon conditions, an increased SvO may occur to patients with sep-
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sis or cyanide poisoning in which the tissues cannot fully utilize oxygen. The mecha-
nism of hypoxia for sepsis is due to peripheral shunting. Cyanide poisoning causes
histotoxic hypoxia that renders the tissues unable to carry out normal aerobic metabo-
lism. These patients may have normal PaO , SaO , CaO , and oxygen transport, but
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they are often hypoxic. A plasma lactate concentration of greater than 10 mEq/L in
smoke inhalation or greater than 6 mEq/L after reported or strongly suspected pure
cyanide poisoning suggests significant cyanide exposure (Leybell et al., 2011). Some
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conditions that may lead to an increased SvO are summarized in Table 10-10.
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