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Patients are admitted to an intensive care unit (ICU) for two main reasons. One is for delivery of mechanical ventilation, the varied aspects of which are the subject of this book. The second is to observe a patient more closely than is possible on a hospital ward; that is, to avail of specialized devices used for the monitoring of vital functions (and of staff who have expertise in their operation).

The literature published on these two subjects constitutes the unique corpus of knowledge required for the expert practice of intensive care medicine. Management of some critically ill patients is based on knowledge outside these two areas, such as patients with acute gastrointestinal bleeding or acute renal failure. The principles for managing such conditions have been developed by physician-investigators within the relevant subspecialties rather than by intensive care physicians. Beyond the areas of mechanical ventilation and monitoring, an intensivist seeking the most authoritative writing on a subject must turn to articles and texts published by nonintensivists.

Patients receiving mechanical ventilation are exposed to the full range of monitoring devices. An in-depth discussion of each device would require as much space as the rest of this book. For such discussions the reader is referred to the companion text, Principles and Practice of Intensive Care Monitoring.1 Rather than attempting a synopsis of each individual monitoring technique, this chapter provides a bird’s-eye view of the subject. We map out the territory of monitoring through discussion of its goals, the principles of measurement, usefulness of monitoring in various settings, the forms of clinical reasoning used in interpreting generated data, evaluation of the benefit of monitors, and the problems in designing studies that attempt such evaluations. To give a sense of the detailed contours of the terrain being traversed, we offer specific examples of how the general topics relate to the everyday use of monitors.

An unsatisfactory—and embarrassing—aspect of writing about monitoring is the absence of a generally accepted definition. A definition tries to set criteria that demarcate the boundaries between concepts (or things) so as to prevent overlapping or confusion, and has the goal of providing order or a clear understanding.

The word monitor comes from the Latin monere, which means “to warn.” This meaning connotes one goal of monitoring, to provide an alert, an alarm. But monitors serve other functions. Monitors replicate many of the characteristics of diagnostic testing, and interpretation of the generated data must comply with the scientific principles developed for use of diagnostic tests. Thus, it might seem reasonable to define monitoring as the serial performance of diagnostic tests at frequent intervals. But how should we demarcate the frequency at which measurements are repeated? Certain signals have such a high-frequency content that obtaining fifty measurements in a second is insufficient to capture a critical data point.

Having struggled long and hard in trying to formulate a meaningful but precise definition of monitoring, ...

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