Critical care clinics
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Critical care clinics · Oct 1996
ReviewPulmonary artery catheters in the critically ill. An overview using the methodology of evidence-based medicine.
Evidence-based approaches to assessing the clinical literature are used increasingly in issues relating to critical care medicine. As we discussed previously, this approach attempts to provide a logical and convenient framework from which the quality and relevance of clinical studies may be assessed in an unbiased manner. An evidence-based approach also allows the reader to differentiate between solid evidence and evidence that is based on a presumed mechanism, standard practice, or conventional wisdom. ⋯ However, what this exercise has taught us is that there is little objective evidence to support this conclusion. The challenge to critical care practitioners is not only to apply the evidence-based processes more frequently to our environment but also to use the information to separate out clearly what is fact versus opinion. Where there is little evidence to support a particular clinical practice, as we have demonstrated with the PAC review earlier, the challenge to the clinician should be the design and conduct of clinical trials clarifying debate between opinion and evidence.
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Critical care clinics · Oct 1996
ReviewOxygen transport and oxygen metabolism in shock and critical illness. Invasive and noninvasive monitoring of circulatory dysfunction and shock.
The common underlying physiologic problem in shock is low flow from hypovolemia or maldistributed microcirculatory flow from uneven vasoconstriction, leading to inadequate tissue perfusion (hypoxia), often in the face of increased metabolic demands. Noninvasive monitoring which was found to provide similar information to that of invasive monitoring, was used in the earliest period of time shortly after admission to the emergency department to provide objective physiologic criteria as therapeutic goals for each of the three major circulatory components: cardiac, pulmonary, and tissue perfusion functions. A clinical algorithm or branch-chain decision tree for high-risk surgical patients was developed from decision rules based on survivor and nonsurvivor patterns, outcome predictors, prospective controlled clinical trials of the oxygen delivery/oxygen consumption (DO2/VO2) concept, and the DO2/VO2 responses of a wide variety of therapeutic agents.
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NIRS is an attractive monitoring technology because it is a noninvasive, real-time, repeatable method that allows for regional assessment of the adequacy of tissue oxygenation. It is able to evaluate the oxygenation state of hemoglobin in tissue and redox state of cyt a1,a3, which reflects the overall activity of oxidative metabolism in the cells. The primary technical limitation of current technology is the inability to measure accurately the optical pathlength online, limiting the ability to quantify precisely concentrations of oxygen-dependent chromophores. Even with this limitation, NIRS can provide unique and valuable in vivo metabolic information without invasive intervention.
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Current monitoring of critically ill patients uses measurement of global parameters such as oxygen consumption and lactate levels. With development of new monitoring technologies, it may be possible to monitor patients on an organ or tissue level, allowing manipulation of specific organ or tissue perfusion. Potentially useful techniques for monitoring tissue energetics in the future include NIR and NMR spectroscopy. ⋯ Both of these techniques may be useful for identification of dysoxia or oxygen-limited mitochondrial turnover. Experimental evidence suggests that organs in the septic state are more sensitive to dysoxia. Implications for the care of the patient with sepsis include possible decreased tolerance to factors leading to dysoxia, such as hypoxemia, hemodilution, or ischemia.
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Critical care clinics · Oct 1996
ReviewDetermination of oxygen delivery and consumption versus cardiac index and oxygen extraction ratio.
Measuring cardiac output without knowledge of the oxygen extraction by the tissues is of limited value. Instead of calculating oxygen consumption and oxygen delivery, a diagram relating cardiac index to the oxygen extraction ratio is proposed to interpret hemodynamic data at the bedside. This diagram is particularly helpful in interpreting cardiac index in the presence of changes in hemoglobin or oxygen demands and in evaluating the effects of therapy.