Critical illness associated with acute respiratory failure (ARF) results from direct and indirect injuries to the lungs. Patients are admitted to the intensive care unit (ICU) for organ support, principally mechanical ventilation, and invasive monitoring. It is widely believed, based on dramatic improvements in perioperative outcomes since the 1960s, that monitoring the critically ill patient, invasively and noninvasively, leads to clear therapeutic goals, detects early manifestations of disease, and reduces morbidity and mortality.
A number of important goals of ICU monitoring can be identified. One is to ensure adequacy of respiratory and circulatory function in patients who appear clinically stable. Another is to provide close surveillance for early signs of respiratory and circulatory instability, with the presumption that early detection improves outcome. In addition, measurement of the response to therapeutic interventions, including intravenous fluids and the application of supportive devices, such as endotracheal tubes (ETTs) and mechanical ventilators, are routinely performed in the ICU. Although life-saving, these devices, like all therapeutic interventions, are associated with risks that must be considered. Finally, monitoring respiratory and hemodynamic derangements over hours to days provides valuable insight about prognosis, since the trend in physiological derangements over time predicts outcomes far better than does the severity of abnormalities on admission. Consequently, failure to improve over days, despite full support and appropriate treatment, suggests the need for alternative therapeutic strategies, including patient comfort as the primary goal of care.
Physiological parameters normally vary in critically ill patients. The devices used to measure these parameters are often imprecise and, at times, inaccurate. Therefore, clinical assessment and decision making should not be based, in general, on single data points. Rather, trends in data collectively add reliability to interpretation of measurements. This chapter reviews methods currently available for monitoring hemodynamics and respiratory function in patients with ARF.
Discussed in the following sections are important aspects of hemodynamic monitoring, which in its many forms, is routinely utilized in critical care settings. Respiratory monitoring is considered later in the chapter.
Indications for Hemodynamic Monitoring
Hemodynamic monitoring is used to provide information that is not readily apparent from clinical examination, or when bedside monitoring is unreliable or equivocal.1 Such circumstances include: (1) for the patient with pulmonary edema, determining whether the edema is cardiogenic or noncardiogenic in origin; (2) assessing whether hypoperfusion is causing, or contributing to, the patient's end-organ dysfunction; (3) deciding whether the patient with ARF who presents with or develops shock be given intravenous fluids, and, if so, how much; (4) deciding for those patients with volume overload, given the risk of causing or exacerbating organ dysfunction, how much fluid should be removed by using diuretics or dialysis to reduce lung water and improve respiratory function.
The first question requires an estimate of ventricular preload, the second an estimate of cardiac output adequacy, and ...