Photoplethysmography (PPG) is used to estimate the skin blood flow using infrared light. Researchers from different domains of science have become increasingly interested in PPG because of its advantages as non-invasive, inexpensive, and convenient diagnostic tool. Traditionally, it measures the oxygen saturation, blood pressure, cardiac output, and for assessing autonomic functions. Moreover, PPG is a promising technique for early screening of various atherosclerotic pathologies and could be helpful for regular GP-assessment but a full understanding of the diagnostic value of the different features is still lacking. Recent studies emphasise the potential information embedded in the PPG waveform signal and it deserves further attention for its possible applications beyond pulse oximetry and heart-rate calculation. Therefore, this overview discusses different types of artifact added to PPG signal, characteristic features of PPG waveform, and existing indexes to evaluate for diagnoses.

On the Analysis of Fingertip Photoplethysmogram Signals

Photoplethysmography (PPG) is an uncomplicated and inexpensive optical measurement method that is often used for heart rate monitoring purposes. PPG is a non-invasive technology that uses a light source and a photodetector at the surface of skin to measure the volumetric variations of blood circulation. Recently, there has been much interest from numerous researchers around the globe to extract further valuable information from the PPG signal in addition to heart rate estimation and pulse oxymetry readings. PPG signal's second derivative wave contains important health-related information. Thus, analysis of this waveform can help researchers and clinicians to evaluate various cardiovascular-related diseases such as atherosclerosis and arterial stiffness. Moreover, investigating the second derivative wave of PPG signal can also assist in early detection and diagnosis of various cardiovascular illnesses that may possibly appear later in life. For early recognition and analysis of such illnesses, continuous and real-time monitoring is an important approach that has been enabled by the latest technological advances in sensor technology and wireless communications. The aim of this article is to briefly consider some of the current developments and challenges of wearable PPG-based monitoring technologies and then to discuss some of the potential applications of this technology in clinical settings.

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A review on wearable photoplethysmography sensors and their potential future applications in health care.

A system and method of managing therapy provided to patients in an institution. The system monitors all aspects of the medication delivery to a patient, as well as other information related to the patient, such as values of vital signs, laboratory results and patient factors such as history, diagnosis, allergies and the like. The system includes one or more databases of information, including institutionally developed rules, guidelines and protocol representing the best medical practices of the institution. The system provides alerts and/or recommendations based on the application of the rules to the information being monitored, and alerts care givers accordingly, providing for dynamic adjustment of the patient's therapy. The system also monitors the status of the alerts, and if no action is taken in a selected period of time, may escalate the priority of the alert and/or halt the delivery of medication to the patient until the alert is resolved.

System and method for dynamically adjusting patient therapy

Background Inadequate analgesia during general anaesthesia may present as undesirable haemodynamic responses. No objective measures of the adequacy of analgesia exist. We aimed at developing a simple numerical measure of the level of surgical stress in an anaesthetized patient. Methods Sixty and 12 female patients were included in the development and validation data sets, respectively. All patients had elective surgery with propofol–remifentanil target controlled anaesthesia. Finger photoplethysmography and electrocardiography waveforms were recorded throughout anaesthesia and various waveform parameters were extracted off-line. Total surgical stress (TSS) for a patient was estimated based on stimulus intensity and remifentanil concentration. The surgical stress index (SSI) was developed to correlate with the TSS estimate in the development data set. The performance of SSI was validated within the validation data set during and before surgery, especially at skin incision and during changes of the predicted remifentanil effect-site concentration. Results SSI was computed as a combination of normalized heart beat interval (HBInorm) and plethysmographic pulse wave amplitude (PPGAnorm): SSI=100–(0.7*PPGAnorm+0.3*HBInorm). SSI increased at skin incision and stayed higher during surgery than before surgery; SSI responded to remifentanil concentration changes and was higher at the lower concentrations of remifentanil. Conclusions SSI reacts to surgical nociceptive stimuli and analgesic drug concentration changes during propofol–remifentanil anaesthesia. Further validation studies of SSI are needed to elucidate its usefulness during other anaesthetic and surgical conditions.

Assessment of surgical stress during general anaesthesia

A processor-based system for analyzing physiologic data and medical care is provided wherein the patient data is analyzed to construct images that are representative of a patient's condition. The processor provides a self-modulating analysis, which is responsive to the occurrence of additional data items to increase the information contained in the images. Identifications of modes of physiologic failure by the analysis of the generated images provides for earlier recognition and intervention and improved protocolization of testing and treatment.

Patient safety processor

Background The surgical stress index (SSI) is based on a sum of the normalized pulse beat interval (PBI) and the pulse wave amplitude (PPGA) time series of the photoplethysmography As a measure of the nociception–anti-nociception balance in response to a standardized pain stimulus, SSI was compared with EEG changes in state and response entropy (SE and RE), PPGA, and heart rate (HR) during various targeted pseudo-steady-state concentrations of propofol and remifentanil Methods Forty ASA I patients were allocated to one of the four groups to receive a remifentanil step-up/-down effect-compartment target-controlled infusion (Ceremi )o f 0, 2, 6, 2, 0n g ml 21 , or 6, 2, 0, 2, 6 ng ml 21 , and an effect-compartment target-controlled propofol infusion (Ceprop) to keep the SE between 30 and 50 or 15 and 30, respectively At each steady-state Ceremi, maximum change in SSI, SE, RE, PPGA, and HR after a noxious stimulus was compared with the baseline value A correlation and prediction probability (PK) with Ceprop and Ceremi was measured Results Static and dynamic values of SSI correlated to Ceremi better than SE, RE, HR, and PPGA SSI was independent of Ceprop, in contrast to SE and RE The PK for Ceremi both before and during a noxious stimulus was better with SSI

Changes in a surgical stress index in response to standardized pain stimuli during propofol–remifentanil infusion

Background: Analgesia is a part of balanced anaesthesia, but direct indicators of nociception do not exist. We examined the relationship between motor reactions and physiological variables during skin incision in sevoflurane anaesthesia and hypothesized that nociception could be detected and graded by significant changes in these variables.



Methods:  Thirty-one women scheduled for abdominal hysterectomy participated in the study. Anaesthesia was induced with fentanyl (1 µg kg−1), propofol (1 mg kg−1) and sevoflurane. Skin incision was performed 14 min after induction during 1.6% end-tidal sevoflurane anaesthesia without neuromuscular blockade. Electrocardiography (ECG), photoplethysmography (PPG) and electroencephalography (EEG) were registered, and a range of variables was computed from these signals. The postincision values, normalized with respect to their preincision values, of movers vs. non-movers were compared. The variables showing significant differences between movers and non-movers were used to develop a logistic regression equation for the classification of patients into movers or non-movers.



Results:  Twenty-six patients were eligible for analysis, and 12 (46%) displayed a motor reaction to skin incision (movers). Many ECG, PPG and EEG-related variables showed significant differences between the pre- and postincision periods. The best classification performance, assessed by leave-one-out cross-validation, between movers and non-movers was achieved with the combination of response entropy of EEG, RR-interval and PPG notch amplitude. The corresponding equation yielded 96% correct classification with 90% sensitivity and 100% specificity. The classification performance of any single variable alone was considerably worse.



Conclusion:  Combination of information from different sources may be required for monitoring the adequacy of analgesia during anaesthesia.

EEG spectral entropy, heart rate, photoplethysmography and motor responses to skin incision during sevoflurane anaesthesia.

Background. Direct indicators for the evaluation of the nociceptive–anti-nociceptive balance during general anaesthesia do not exist. The aim of this study was to combine physiological parameters to obtain such an indicator. Methods. Fifty-five females scheduled for surgery under general anaesthesia combining targetcontrolled infusions of propofol and remifentanil were studied. Propofol was given to maintain state entropy (SE) at 50 and remifentanil was targeted at 1, 3 or 5 ng ml � 1 . The patients’ reactions and clinical signs of nociception, remifentanil levels and estimation of noxious intensity of incision were combined into a clinical score [Clinical Signs–Stimulus–Antinociception (CSSA)] to evaluate the nociceptive–anti-nociceptive balance. ECG, photoplethysmography (PPG), response entropy (RE) and SE were recorded from 60 s before to 120 s after skin incision. Differences between post- and pre-incision values of heart rate variability (HRV), PPG and pulse transition time related parameterswereanalysed off-linetoevidencethe bestpredictors ofCSSA.Thosebestpredictors of CSSA served to develop a response indexof nociception (RN), scaled from 0to 100. This index was further tested in 10 additional patients. Results. HRV, RE, RE–SE and PPG variability were the best predictors of CSSA. The prediction probability of RN at predicting CSSA was 0.78. RN response was higher after larger incision, in movers and with lower remifentanil concentrations. Conclusions. The empirically developed algorithm of RN leads to an index that seems to adequately estimate the nociceptive–anti-nociceptive balance at skin incision during general anaesthesia. In the future, CSSA may serve as a reference for studies investigating methods aimed at evaluating this pharmacodynamic component of anaesthesia. Br J Anaesth 2006; 96: 367–76

Novel multiparameter approach for measurement of nociception at skin incision during general anaesthesia

TM (SE) at a predetermined level, SSI would be higher in patients receiving a b-blocking agent (esmolol) than in those receiving an opioid (remifentanil) during laparoscopy. Methods. Thirty women undergoing gynaecological laparoscopy were assigned randomly to receive esmolol (n¼15) or remifentanil (n¼15). Anaesthesia was induced with propofol and fentanyl and maintained with desflurane and nitrous oxide 50% in oxygen to keep SE at 50(5). The infusion of esmolol or remifentanil was started before laparoscopy and adjusted to keep the systolic blood pressure at 220 to þ10% from the preoperative value. Results. During the fentanyl phase, before surgery, both groups behaved similarly, with an increase in SSI after intubation. In the patients receiving esmolol, the SSI reacted to the initial incision (P,0.05), and remained high after trocar insertion (P,0.05). In patients receiving remifentanil, it did not react to the initial incision, but increased after trocar insertion (P,0.05), and it remained lower both after incision (P,0.05) and after trocar insertion (P,0.05). Conclusion. SSI was higher in patients receiving esmolol. The index seems to reflect the level of surgical stress and may help guide the use of opioids during general anaesthesia. Br J Anaesth 2007; 98: 456‐61

Matti Huiku

Papers

Assessment of surgical stress during general anaesthesia

Changes in a surgical stress index in response to standardized pain stimuli during propofol–remifentanil infusion

EEG spectral entropy, heart rate, photoplethysmography and motor responses to skin incision during sevoflurane anaesthesia.

Novel multiparameter approach for measurement of nociception at skin incision during general anaesthesia

Surgical stress index reflects surgical stress in gynaecological laparoscopic day-case surgery †