The delivery of modern critical care is predicated on the ability to monitor a large number of physiologic variables and formulate evidenced-based therapeutic strategies to manage these variables. Technological advances in monitoring have at least a theoretical risk of exceeding our ability to understand the clinical implications of the derived information. This could result in the use of monitoring data to make inappropriate clinical decisions. Therefore, the implementation of any new monitoring technology must take into account the relevance and accuracy of the data obtained, the risks to the patient, as well as the evidence supporting any intervention directed at correcting the detected abnormality.
The routine use of invasive monitoring devices, specifically the pulmonary artery catheter, must be questioned in light of the available evidence which does not demonstrate a clear benefit to its widespread use in various populations of critically ill patients. The future of physiologic monitoring will be dominated by the application of noninvasive and highly accurate devices which guide evidenced-based therapy.
The Latin verb monere, which means “to warn, or advise” is the origin for the English word monitor. In modern medical practice, patients undergo monitoring to detect pathologic variations in physiologic parameters, providing advanced warning of impending deterioration in the status of one or more organ systems. The intended goal of this endeavor is to allow the clinician to take appropriate actions in a timely fashion to prevent or ameliorate the physiologic derangement. Furthermore, physiologic monitoring is used not only to warn, but also to titrate therapeutic interventions, such as fluid resuscitation or the infusion of vasoactive or inotropic drugs. The intensive care unit (ICU) and operating room are the two locations where the most advanced monitoring capabilities routinely are employed in the care of critically ill patients.
In the broadest sense, physiologic monitoring encompasses a spectrum of endeavors, ranging in complexity from the routine and intermittent measurement of the classic vital signs (i.e., temperature, heart rate, arterial blood pressure, and respiratory rate) to the continuous recording of the oxidation state of cytochrome oxidase, the terminal element in the mitochondrial electron transport chain. The ability to assess clinically relevant parameters of tissue and organ status and employ this knowledge to improve patient outcomes represents the “holy grail” of critical care medicine. Unfortunately, consensus often is lacking regarding the most appropriate parameters to monitor in order to achieve this goal. Furthermore, making an inappropriate therapeutic decision due to inaccurate physiologic data or misinterpretation of good data can lead to a worse outcome than having no data at all. Of the highest importance is the integration of physiologic data obtained from monitoring into a coherent and evidenced-based treatment plan. Current technologies available to assist the clinician in this endeavor are summarized in this chapter. Also presented is a brief look at emerging techniques that may soon enter into clinical practice.