Is It a Bird? Is It a Plane? The Role of Patient Monitors in Medical Decision Making

The radar operator sits hunched over the luminous screen in the darkened radar room. The pale green dot was at first so dim it was barely visible, but with each sweep of the display it grew brighter. Is it simply a bird or an enemy aircraft? Anti-aircraft defenses will reveal the position of the ship. Waiting only reduces the chances of a successful defense. A decision has to be made. In this issue of Anesthesia CVP = central venous pressure; PCWP = pulmonary capillary wedge pressure; CI = cardiac index. Used with permission from Cannesson et al.7To develop an ROC curve, one must define the threshold for making a decision and then calculate from the observed measurements the true-positive rate and false-positive rate. By plotting the true and false-positive rates at different decision thresholds using the same measurement tool, a curve is generated. A typical use of ROC analysis would be to evaluate a new diagnostic test and answer two questions: 1) Is the test any good? and 2) What is the right threshold value for decision making? ROC analysis can also be used to compare the quality of different methods for the same test. This approach has been used recently to compare the ability of different monitoring methods to predict fluid responsiveness.7 ROC analysis has the potential to compare the impact on clinical decision making of different methods of measuring CO. If the CO threshold at which a clinical decision would change is defined, it becomes possible to determine the sensitivity and specificity of the measurements from each device. ROC analysis can then be used to compare the utility of each device to guide decision making based upon that threshold. By comparing the ROC plots for each device, the impact of each device on clinical decision making can be understood. Although the technologies reviewed by Funk et al. are used to varying degrees in clinical practice, none have become widely adopted despite the stated importance of CO monitoring in the care of critically ill patients. In the concluding paragraph of their review, the authors state that “With an increasing number of clinical studies being published … their use should continue to gain popularity.” Although the Bland Altman approach is a useful tool to describe the limits of agreement between two methods of measurement, conclusions about the acceptability of the limits of agreement are a matter of opinion, not science. Analytical methods for comparing CO measurement techniques need to move beyond this approach to provide insight into the role of the technology in clinical decision making. Patient populations in which CO monitoring is important need to be studied within the range of values that would dictate the need for clinical intervention. ROC analysis can be used to gain insight into the impact of a tool on medical decision making. The impact of those decisions on patient outcome are the ultimate question and will require even more sophisticated protocols in larger patient populations.