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Critical care medicine is the discipline of caring for patients with acute life-threatening conditions or conditions likely to cause serious harm if not rapidly addressed. This work requires a detailed understanding of human physiology, the pathophysiology of severe illness and injury, the intricate interactions between organ systems and therapies, as well as an understanding of and experience with the rapidly changing technologies available in a modern pediatric intensive care unit (PICU). The science of caring for the critically ill patient continues to advance rapidly as the molecular mediators of illness have become better defined and new therapies are brought into clinical use. Critical care, then, is a highly complex, multidisciplinary field in which optimal patient outcomes require a team-oriented approach, including critical care physicians and nurses; respiratory therapists; pharmacists; consulting specialists; physical, occupational, and recreational therapists; and social services specialists.
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MONITORING & TECHNOLOGY
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Respiratory Monitoring
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Pulse oximetry measures arterial oxygen saturation (SaO2) continuously and noninvasively. However, pulse oximetry readings can be much less accurate in patients with saturations below 80%, poor skin perfusion, or significant movement and can overestimate the SaO2 in patients with darker skin. In addition, pulse oximetry can be dangerously inaccurate in certain clinical settings such as carbon monoxide poisoning or methemoglobinemia. To directly measure arterial oxygen content, direct arterial blood gas sampling must be performed.
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Knowing the partial pressure of oxygen (PaO2) and inspired oxygen content (FIO2), gas exchange impairment may be estimated through measures such as the PaO2/FIO2 ratio (used in diagnosing acute respiratory distress syndrome [ARDS]) or the alveolar-arterial oxygen difference (A–aDO2, or A–a gradient). The A–a gradient is less than 15 mm Hg under normal conditions, widening with diffusion impairment, shunts, and ventilation-perfusion mismatch V/Q mismatch; gradients over 400 mm Hg are strongly associated with mortality. Although calculation of the intrapulmonary shunt fraction (the percentage of pulmonary blood flow that passes through nonventilated areas of the lung) is possible with a pulmonary arterial catheter, the use of the latter has declined significantly over the past decade.
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End-tidal CO2 (ETCO2) monitoring measures exhaled carbon dioxide (CO2) noninvasively, allowing for continuous assessment of ventilation. Normally, the ETCO2 level closely approximates the alveolar CO2 level (PACO2), which should equal arterial CO2 levels (PaCO2) because carbon dioxide diffuses freely across the alveolar-capillary barrier. However, ETCO2 may not accurately reflect the PaCO2 in patients with increased dead space ventilation (tidal volume that does not participate in gas exchange) or severe airway obstruction. PaCO2 is directly measured through blood gas analysis. Although venous or capillary samples may be used, these values can be misleading ...