In the investigation of sleep problems, such as suspected OSAS, periodic limb movement syndrome (PLMS) and insomnia, the effect of being attached to multiple transducers - especially EEG and sub-mental EMG electrodes is a problem for patient comfort. Recording with the minimum number of transducers in a situation closely resembling the normal sleeping environment - clinically desirable as well as economically advantageous.
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The initially attractive use of pulse oximetry alone cannot determine the presence of symptomatic sleep disruption due to upper airway obstruction and thus the recording of other parameters are necessary.
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The Visi-3 unit as a minimum uses only an oximeter probe and ECG (ECG was not needed in the original Visi-Lab) with otherwise non contact methods (infra-red video and sound) to determine several indices of sleep disruption, body movements and snoring. No overnight observation by a technician or subjective analysis of sleep EEG traces is required.
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The standard system can be set up in an otherwise non-instrumented bedroom, possibly a side room of a hospital ward. The data analyses from the signal recordings provide succinct summaries of the patient's overnight recording. The signals are reviewed for evidence of sleep-induced upper airway narrowing leading to sleep disturbance. These allow a definitive diagnosis in the sleepy patient, and can also indicate the periods for direct review on the digital video recording.
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Pulse Transit Time (in Visi-3)
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Changes in blood pressure cause changes in arterial pulse wave velocity. It is known that there are blood pressure rises associated with arousals caused by sleep related respiratory events. A simple method of monitoring the changes in pulse wave velocity is to time the interval between the 'R' wave and the pulse arriving at the finger: the pulse transit time (PTT). This has been shown to be an objective alternative to EEG measures. In essence, PTT is a continuous noninvasive monitoring analogue of blood pressure (see Smith RP., et al. Pulse Transit Time: an appraisal of potential clinical applications Thorax 1999 54: 452-458).
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