No one will deny that the logical aphorism, "If you don't know, you don't know," is true. Yet, in medicine we traditionally make assumptions and act accordingly when we do not know the underlying problem with certainty. If the patient's condition improves, our "clinical judgment" was correct; if not, we alter the therapy. In many clinical situations this type of deductive therapy is effective, since time exists to allow for trial and error. In these cases, studies involving invasive methods are not necessary. However, in critically ill patients, reliance on symptoms and signs to identify abnormal cardiorespiratory patterns may lead to wrong conclusions and do the patient a real disservice. In them, time is not available to alter therapy after failure becomes fully apparent. Critically ill patients with incipient or actual multiple organ-systems failure have little tolerance to anything but prompt, proper therapy.
Wrong assumptions that can lead to incorrect