Beat-to-beat heart rate variability analysis is a powerful tool for the diagnosis of neuropathy. Respiration-related heart rate variability (respiratory sinus arrhythmia, RSA) reflects the function of parasympathetic nervous system during spontaneous ventilation while awake. RSA is also claimed to monitor the depth of anaesthesia. Power spectrum analysis or various averaging techniques of the heart rate variability are usually applied. The current literature, however, does not usually interpret the ground rules and limitations of the method used, and this may sometimes lead to erroneous conclusions on the data. The aim of our study was to compare and analyse critically the performance of different methods of evaluating RSA during anaesthesia and positive pressure ventilation. Power spectrum analysis, the root mean square of the successive RR-interval difference (RMSSD), and two respiration related methods, RSA index and average phase RSA, were included in the comparison. To test these methods, 11 patients were anaesthetised with isoflurane and their lungs were ventilated mechanically with a frequency of 6 cycles min-1. Each patient received a bolus dose of atropine (20 micrograms kg-1) during the trial. Electrocardiogram, electroencephalogram and tracheal pressure signal from respirator were recorded and analyses were performed off-line. We demonstrated that general indices, such as RMSSD, may be strongly affected by heart rate level and other non-respiration related variations in heart rate. We also showed that the effect of unwanted fluctuations on RSA can be reduced with respiration dependent beat-to-beat methods. Furthermore we confirmed that in addition to the amplitude, also the pattern of respiratory sinus arrhythmia is of interest: the pattern is reversed in phase compared to spontaneous breathing while awake, as we have shown earlier. To analyse RSA during anaesthesia, we recommend the use of an average phase RSA method based on beat-to-beat variability that shows both the amplitude and pattern of RSA. Finally, no measure of RSA should be used without a presentation of the actual beat-to-beat heart rate curve.
We examined the functionality and reliability of a wearable physiological monitoring system (LifeShirt) during normal daily activities and in a hospital operating room (OR) environment. The garment collects physiological data such as oxygen saturation, and stores them in a recorder from which it can be read afterwards. Ten normal subjects wore the shirt continuously for 8 h per day. Feedback from the testers was quite positive, although the collected data varied in quality. Ten hospital patients also wore the shirt during endoscopy. The data collected during the hospital stay were qualitatively adequate. Measuring respiratory function caused the biggest problems. The study showed that intelligent garment technology could be used in an OR environment for patient monitoring, albeit not in realtime. It may also be useful in home follow-up.
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