Critical care clinics
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Critical care clinics · Oct 1996
ReviewHemodynamics and positive end-expiratory pressure in critically ill patients.
This article examines the factors that affect the transmission of airway pressures to intrathoracic structures. The effects of positive end-expiratory pressure on central venous pressures, cardiac filling pressures, and right and left ventricular function are discussed. Various techniques for estimating intrathoracic pressures and their limitations are reviewed.
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Arterial blood gas (ABG) measurements are one of the most frequently requested laboratory examinations in critically ill patients. ABGs include measurement of pHa, PaCO2, PaO2, and oxyhemoglobin saturation. These measurements allow for assessment of the nature, progression, and severity of metabolic and respiratory disturbances.
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In summary, newer imaging technologies yield three-dimensional pictures of the left ventricle. Detailed information is provided on regional wall motion, wall thickening, and ventricular volumes, which can be helpful in managing patients with cardiac disease. MR imaging gives the highest resolution images, and MR angiography can be invaluable in assessing the anatomy of an aortic dissection. ⋯ First pass RNA is probably the most accurate method for measuring RVEF and can be performed at the bedside using a portable multicrystal camera. Serial measurements of RVEF may be helpful in managing patients with pulmonary hypertension of various causes. For patients with documented acute myocardial infarction or chest pain and no acute myocardial necrosis or for patients undergoing preoperative risk assessment, combined perfusion and function using nuclear techniques yields both stress-induced ischemia and resting ventricular function in a single procedure and is timely and cost effective.
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In the quest for a better monitor of end-organ injury in the critically ill patient, gastrointestinal tonometry to measure intramucosal pH (pHi) and PCO2 has been advocated by several investigators. The authors discuss the principles and practice of gastric tonometry. ⋯ Although prognostic capability is an important role for tonometry, the principal benefit of the technology is its efficacy as an index of tissue resuscitation and as a guide to medical intervention. Given recent technical advances, gastrointestinal tonometry may gain increasing use in the assessment of tissue perfusion and oxygen metabolism.
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Oxygen consumption is physiologically dependent on DO2 below the critical DO2. Thus, patients in overt shock have physiologic dependence of VO2 on DO2. The first priority of prevention and reversal of tissue hypoxia is to balance oxygen demand and oxygen supply. ⋯ Finally, we suggest that intensivists continue to assess DO2 and VO2 carefully. Global assessment of VO2 and DO2 appears inadequate to detect occult tissue hypoxia in most critically ill patients. However, research focused on regional assessment such as gastric tonometer measurement of gastric mucosal PCO2 and pH provides opportunity for safe, convenient detection of occult tissue hypoxia in critically ill patients.