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Chapter 13: Physiologic Monitoring of the Surgical Patient

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The point of critical oxygen delivery (DO2crit)

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A. Represents the transition from supply-independent to supply-dependent oxygen uptake and is decreased in sepsis.

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B. Represents the minimal rate of oxygen delivery needed for aerobic metabolism and is decreased in sepsis.

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C. Represents the transition from supply-independent to supply-dependent oxygen uptake and is increased in sepsis.

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D. Represents the minimal rate of oxygen delivery needed for aerobic metabolism and is increased in sepsis.

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Answer: C

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Under normal conditions when the supply of oxygen is plentiful, aerobic metabolism is determined by factors other than the availability of oxygen. However, in pathologic circumstances when oxygen availability is inadequate, oxygen utilization (VO2) becomes dependent upon oxygen delivery (DO2). The relationship of VO2 to DO2 over a broad range of DO2 values is commonly represented as two intersecting straight lines. In the region of higher DO2 values, the slope of the line is approximately zero, indicating that VO2 is largely independent of DO2. In contrast, in the region of low DO2 values, the slope of the line is nonzero and positive, indicating that VO2 is supply-dependent. The region where the two lines intersect is called the point of critical oxygen delivery (DO2crit), and represents the transition from supply-independent to supply-dependent oxygen uptake. Microcirculatory derangements, such as those seen in sepsis, will shift this point higher. Below a critical threshold of oxygen delivery, increased oxygen extraction cannot compensate for the delivery deficit; hence, oxygen consumption begins to decrease. The slope of the supply-dependent region of the plot reflects the maximal oxygen extraction capability of the vascular bed being evaluated. (See Schwartz 10th ed., p. 400.)

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Of the following parameters, which is the least influenced by an underdamped or overdamped intra-arterial blood pressure monitoring system?

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A. Systolic blood pressure

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B. Mean arterial blood pressure

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C. Diastolic blood pressure

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D. Pulse pressure

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Answer: B

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If the system is underdamped, then the inertia of the system, which is a function of the mass of the fluid in the tubing and the mass of the diaphragm, causes overshoot of the points of maximum positive and negative displacement of the pressure transducer diaphragm during systole and diastole, respectively. Thus, in an underdamped system, systolic pressure will be overestimated and diastolic pressure will be underestimated. In an overdamped system, displacement of the diaphragm fails to track ...

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