The point of critical oxygen delivery (DO):
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 supplydependent 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 supplydependent region of the plot reflects the maximal oxygen extraction capability of the vascular bed being evaluated.
Of the following parameters, which is the least influenced by an underdamped or overdamped intra-arterial blood pressure monitoring system?
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 the rapidly changing pressure waveform, and systolic pressure will be underestimated and diastolic pressure will be overestimated. It is important to note that even in an underdamped or overdamped system, mean pressure will be accurately recorded, provided the system has been properly calibrated. For these reasons, when using direct measurement of intra -arterial pressure to monitor patients, clinicians should make clinical decisions based primarily on the measured mean arterial blood pressure.
Regarding electrocardiographic (ECG) monitoring in the ICU:
Continuous monitoring of the 12-lead ECG is now available in many ICUs and is proving to be beneficial in certain patient populations. In a study of 185 vascular surgical patients, continuous 12-lead ECG monitoring was able to detect transient myocardial ischemic episodes in 20.5% of the patients. This study demonstrated that the precordial lead V4, which is not routinely monitored on a standard 3-lead ECG, is the most sensitive for detecting perioperative ischemia and infarction. To detect 95% of the ischemic episodes, two or more precordial leads were necessary. Thus, continuous 12-lead ECG monitoring may provide greater sensitivity than 3-lead ECG for the detection of peri operative myocardial ischemia, and may become standard for monitoring high-risk surgical patients.
Regarding preload, which of the following is true?
For the right ventricle, central venous pressure (CVP) approximates right ventricular end-diastolic pressure (EDP). For the left ventricle, pulmonary artery occlusion pressure (PAOP), which is measured by transiently inflating a balloon at the end of a pressure-monitoring catheter positioned in a small branch of the pulmonary artery, approximates left ventricular EDP. The presence of atrioventricular valvular stenosis may alter this relationship. Left ventricular EDP is the most commonly used proxy for preload.
All of the following are true EXCEPT:
Clinicians frequently use EDP as a surrogate for end-diastolic volume (EDV), but EDP is determined not only by volume but also by the diastolic compliance of the ventricular chamber. Ventricular compliance is altered by various pathologic conditions and pharmacologic agents. Furthermore, the relationship between EDP and true preload is not linear, but rather is exponential.
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