## 4 Discussion
As mentioned, noncontact monitoring of vital signs is not established at the adult ICU and is not in clinical use for legal reasons; especially in moving subjects the error rate is estimated to be high [24; 25]. However, when used as a backup monitor, programmed to measure vital signs and motions in patients who detached their conventional monitoring system and are no longer moving, its use could be justified and may exceed the accuracy of our device. With the added ability to monitor heart and respiration rates, the measurement of normal vital signs ranges could be used to block false negative alarms triggered by the motion detector feature. Even more important could be the prevention of false positives since the (false) detection of motions in an unconscious patient can suppress the alarm function of the detector and thus impede early detection of an emergency. Our study's amount of false positive results within the no motion scenes was 25% (N = 37/150). However, we identified a vibrating phone as a relevant confounder responsible for 43% (N = 16) of the false positive assignments. As most mobile phones cease the call automatically after 20-30 s, this confounder should not be of major concern in real-world situations. After excluding the vibrating mobile scene, the number of false positives was 14% (21/150). False positive assignments were more frequent in male test subjects. The reason for the sex difference is unclear but might indicate that the sensor susceptibility to triggering alarms has to be adapted individually. Alternatively, some physical characteristics, such as a broader torso or a larger head in men, may impede simulating the \"lying dead-still scene\" realistically. As the sensor was attached at shoulder height below the ICU bed, minimal breathing-dependent movements may have been transferred more easily to the sensor. In real-world situations in which patients have a true emergency, such as a cardiopulmonary arrest, sex differences are most probably less apparent. False positive assignments were also found for scene #1, i.e., the scene with the bed left empty, indicating that the sensor might detect motions differently if the bed is not occupied and that, so far, unidentified external interference factors might trigger minimal sensor activation. Continuous monitoring under real-world conditions should, however, minimize the error rates. In patients with a life-threatening emergency, the overall activity level may be lower than in the mock-up emergency scenes of our test subjects. Further, an actual emergency lasts longer than the predefined experimental period of 1 min. If the sensor could be programmed to sound alarms after detecting \"zero or near-zero lines\" for a specific period (e.g., 20-30 s) and after detecting longer episodes of minimal motion intensities, the probability of missing true emergencies may likely reduce. Other clinical situations in which motion detectors might be helpful are attempted or succeeded bed exits. In our study, the bed exit pattern was very characteristic. Thisscene was the second most often recognized of all motion scenes (after hitting the bed with fists). There are already motion-sensing mattresses with multiple implemented pressure sensors that are in use for the detection of bed exits and--depending on the added technical features--with multiple other functions such as measurements of respiration, heart rate, sleep pattern, sleep quality, etc. [38]. These mattresses are mainly used for geriatric or nursing home patients. Although smart mattresses may also have other indications at the ICU, such as preventing pressure ulcers [39; 40], feasibility and cost issues may limit their use in the ICU. As many patients are not delirious at admission but develop delirium at the ICU [41], affected individuals would need a bed or mattress change, which may be more labor-intensive in an uncooperative patient than the attachment of a portable motion detector underneath the bed or pillow.