Pain is a feeling triggered in the nervous system that can range from mild, localized discomfort to agony. Pain has both physical and emotional components. Its physical part results from nerve stimulation. Pain may be contained to a discrete area, as in an injury, or it can be more diffuse, as in disorders like fibromyalgia. It is mediated by specific nerve fibers that carry the pain impulses to the brain where their conscious appreciation may be modified by many factors. Stress is the feeling of being under too much mental or emotional pressure. Common signs of stress include sleeping problems, sweating, loss of appetite and difficulty of concentrating. The identification of pain and stress periods may be helpful in diagnosing health issues. Pain in unconscious patients is different, since patient feedback cannot be retrieved.
According the definition by the International society for the study of pain the term “pain” may relate to “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage”. According to the definition of The American Institute of stress the expression “stress” may describe a highly subjective phenomenon which can be thought of “the non-specific response of the body to any demand for change”. “Discomfort” is also a subjective phenomenon and may relate “to the opposite of feeling comfortable”.
Due to the very similar nature of the terms “pain”, “discomfort” and “stress”, the term “pain” as used in this document is for simplicity reasons used as a generic term which shall also encompass discomfort and stress.
Due to the subjective nature of feeling or experiencing pain, monitoring is usually based on self-reporting and observational/behavioral data. It is clear that such subjective measures are difficult to evaluate and to generalize. Accordingly, there are several efforts to measuring pain using physiological data which are related to the sympathetic/parasympathetic balance, and which are comparatively easy to collect and to quantify.
Physiological data indicative of pain may be obtained when a person/patient is subjected to pain. Accordingly, the effect of pain (including discomfort and/or stress) may cause the physiological parameters, such as blood pressure, pulse rate, respiration rate, respiration effort, or other parameters, alone or in combination, to change and often to increase. These changes of physiological parameters may be used as a basis for determining to which degree a patient/person experiences pain.
Pain monitoring and also stress monitoring may be performed by employing a visual analog scale that requires the patient to estimate the level of pain, discomfort and/or stress. The required attention of the patient towards feeling pain may bias this estimation, making the assessment unreliable. Reliability can be increased by unobtrusively measuring a parameter that reflects the level of pain, discomfort and/or stress and does not require the attention of the patient.
The quantification of pain and stress has been a clinical need for a long time. Different approaches have been used for this purpose including, for example, pulse transit time (PTT) which has been employed for measuring arousal and stress. Electrocardiogram (ECG) electrodes and photoplethysmography (PPG) have been used to estimate PTT from heart to hand. Increasing in PTT hereby indicates stress and pain. Alternative methods rely on a combination of PPG with galvanic skin response (GSR) for pain monitoring. Frequency features of heart rate variability obtained from ECG may be also employed. All these approaches are a function of sympathetic or parasympathetic activity.
US Publication No. 2011/0112420 A1 to Nagata (patented) discloses a device for objectively judging pain. The device comprises an electrocardiographic information acquiring unit which acquires a peak-relevant value such as the peak value of an R-wave for every cycle from an electrocardiogram. Pain is judged based on fluctuation related to the peak-relevant value and output.
WO 2007/052108 A2 discloses a method and system for monitoring vital signs for the prediction and treatment of physiological ailments. The system may detect changes in respiration rate, heart rate, and body motion indicating that a patient is suffering from pain. A motion sensor may be provided from which signals resulting from respiratory motion and heart beat may be derived.
US Publication No. 2014/0123912 A1 to Menkes (patented) discloses a system for monitoring vital signs of a pet animal. A suspicion of seizure may be determined based on sound, pulse, and respiration. Accelerometers may be used for measuring bioparameters of the pet animal including resting patterns, activity patterns, moving patterns, and position patterns.
Although the above mentioned approaches take use of vital signs, none of them are actually unobtrusive as they are all based on a use of current modalities which are usually derived from an ECG with a functional heart, i.e. a heart producing output upon contraction. For instance sticky ECG electrodes, a SpO2 sensor at the finger, and heavy blood pressure cuffs are used. Alternatively, GSR patches at the hand/wrist are employed. There is a need for an unobtrusive means of pain, stress and arousal monitoring which may have a wide range of applications, on post-operative recovery to chronic pain monitoring at home. Other problems of the known systems for measuring pain/stress reside in the usage of a large number of sensors which is burdensome to the patient, cumbersome as well as inconvenient during normal daily activity.