Ballistocardiogaphic studies with acceleration and electromechanical film sensors

The purpose of this research is to demonstrate and compare the utilization of electromechanical film (EMFi) and two acceleration sensors, ADXL202 and MXA2500U, for ballistocardiographic (BCG) and pulse transit time (PTT) studies. We have constructed a mobile physiological measurement station including amplifiers and a data collection system to record the previously mentioned signals and an electrocardiogram signal. Various versions of the measuring systems used in BCG studies in the past are also presented and evaluated. We have showed the ability of the EMFi sensor to define the elastic properties of the cardiovascular system and to ensure the functionality of the proposed instrumentation in different physiological loading conditions, before and after exercise and sauna bath. The EMFi sensor provided a BCG signal of good quality in the study of the human heart and function of the cardiovascular system with different measurement configurations. EMFi BCG measurements provided accurate and repeatable results for the different components of the heart cycle. In multiple-channel EMFi measurements, the carotid and limb pulse signals acquired were detailed and distinctive, allowing accurate PTT measurements. Changes in blood pressure were clearly observed and easily determined with EMFi sensor strips in pulse wave velocity (PWV) measurements. In conclusion, the configuration of the constructed device provided reliable measurements of the electrocardiogram, BCG, heart sound, and carotid and ankle pulse wave signals. Attached EMFi sensor strips on the neck and limbs yield completely new applications of the EMFi sensors aside from the conventional seat and supine recordings. Higher sensitivity, ease of utilization, and minimum discomfort of the EMFi sensor compared with acceleration sensors strengthen the status of the EMFi sensor for accurate and reliable BCG and PWV measurements, providing novel evaluation of the elastic properties of the cardiovascular system.

A new method for measuring the ballistocardiogram using EMFi sensors in a normal chair

Ballistocardiography is a non-invasive technique for the assessment of cardiac function. We built a measurement setup to measure the ballistocardiogram from a normal chair using EMFi sensors. The ballistocardiogram is recorded from a subject sitting on the chair. The measured signal is amplified by a specially-designed charge amplifier and digitized by a circulation monitor. A PC provides a user interface for the measurement devices, records the data and displays the results. Impedance cardiography and ECG serve as reference measurements for the ballistocardiography. To test the system, one healthy 24-year-old male and one healthy 22-year-old female were measured. It is concluded that the ballistocardiogram waveforms described in the literature can be recognized from the EMFi signal measured from a normal chair.

Automatic analysis of electro-encephalogram sleep spindle frequency throughout the night

A fully automatic method to analyse electro-encephalogram (EEG) sleep spindle frequency evolution during the night was developed and tested. Twenty all-night recordings were studied from ten healthy control subjects and ten sleep apnoea patients. A total of 22,868 spindles were detected. The overall mean spindle frequency was significantly higher in the control subjects than in the apnoea patients (12.5 Hz against 11.7 Hz, respectively; p<0.004). The proposed method further identified the sleep depth cycles, and the mean spindle frequency was automatically determined inside each sleep depth cycle. In control subjects, the mean spindle frequency increased from 12.0 Hz in the first sleep depth cycle to 12.6 Hz in the fifth cycle. No such increase was observed in the sleep apnoea patients. This difference in the spindle frequency evolution was statistically significant (p<0.004). The advantage of the method is that no EEG amplitude thresholds are needed.

A computer-assisted visual sleep scoring program

The sleep analysis and scoring program WSCORE was developed under the European Neurological Network (ENN) project. The purpose was to implement an analysis and scoring system for ambulatory and standard polysomnography. In addition to visual Rechtschaffen & Kales scoring the program offers a possibility of free form visual scoring. It contains also analysis modules for Periodic Leg Movement Disorder, Hjort parameters, heart rate and EMG amplitude. A FTP client module was built into the program so that it can be used as a telemedicine application.

Optimization of sigma amplitude threshold in sleep spindle detection

Sleep spindles are transient EEG waveforms of non-rapid eye movement sleep. There is considerable intersubject variability in spindle amplitudes. The problem in automatic spindle detection has been that, despite this fact, a fixed amplitude threshold has been used. Selection of the spindle detection threshold value is critical with respect to the sensitivity of spindle detection. In this study a method was developed to estimate the optimal recording-specific threshold value for each all-night recording without any visual scorings. The performance of the proposed method was validated using four test recordings each having a very different number of visually scored spindles. The optimal threshold values for the test recordings could be estimated well. The presented method seems very promising in providing information about sleep spindle amplitudes of individual all-night recordings.

Vigilance stages and performance in OSAS patients in a monotonous reaction time task

OBJECTIVES

To develop improved methods for objective assessment of daytime vigilance. This is important in the diagnosis and therapy control of sleep disorders associated with excessive daytime sleepiness (EDS).

METHODS

Twenty-one patients with EDS due to obstructive sleep apnea were recorded during a daytime 90 min reaction time (RT) test in a monotonous situation. Two EEG, two EOG and a submental EMG channel were recorded simultaneously. The recordings were divided into short, stationary segments of variable length (0.5-2 s) and classified into 7 stages using our previously described system, which includes additional stages for drowsiness.

RESULTS

The duration of RT was linearly correlated to the vigilance state (P<0.05). The appearance of slow eye movements (SEM) was more consistently related to performance impairment than EEG changes.

CONCLUSIONS

Our system can provide a better tracking of vigilance changes than the standardized sleep stage scoring. A 1-2 h test is useful in the assessment of the performance of a subject suffering from EDS. We found that SEMs indicate more sensitive and consistent EDS-related performance impairment than changes in EEG activity.

Autoassociative MLP in sleep spindle detection

Spindles are one of the most important short-lasting waveforms in sleep EEG. They are the hallmarks of the so-called Stage 2 sleep. Visual spindle scoring is a tedious workload, since there are often a thousand spindles in one all-night recording of some 8 hr. Automated methods for spindle detection typically use some form of fixed spindle amplitude threshold, which is poor with respect to inter-subject variability. In this work a spindle detection system allowing spindle detection without an amplitude threshold was developed. This system can be used for automatic decision making of whether or not a sleep spindle is present in the EEG at a certain point of time. An Autoassociative Multilayer Perceptron (A-MLP) network was employed for the decision making. A novel training procedure was developed to remove inconsistencies from the training data, which was found to improve the system performance significantly.

Quality control of polysomnographic sleep data by histogram and entropy analysis

OBJECTIVE AND METHODS

Sixteen polysomnographic recordings from 8 European sleep laboratories were analyzed. The histogram analysis was used to introduce quality control of all-night EEG recordings.

RESULTS

It was found that the header information does not always provide the real saturation values of the recording equipment. The entropy measure was used for the quantitative analysis of the dynamic range of routinely used polysomnographic recorders. It was found that the recording equipment provides EEG data with entropy in the range of 8-11 bits.

CONCLUSION

In the all-night sleep EEG were observed non-linearities. It is recommended that the equipment provide the saturation values in order to apply automated overflow detection.

Vigilance transitions in reaction time test: a method of describing the state of alertness more objectively

Electrophysiological recordings are considered a reliable method of assessing a person's alertness. The aim of this study was to show, firstly, that changes in alertness during a Reaction Time Test (RTT) can be determined with certain adaptive scoring stages but not with R&K scoring and secondly, that the different adaptive stages can explain findings in reaction time. In 17 male patients (50.8+/-9.7 years, Body-Mass Index (BMI) 31.9+/-5.1 kg/m2) diagnosed with Obstructive Sleep Apnea Syndrome (OSAS) (Respiratory Disturbance Index (RDI) 53.3+/-24.1 /h sleep) a 90 min daytime vigilance test was performed twice, after the diagnostic polysomnographic investigation and after two nights spent with nCPAP. After a computerised adaptive segmentation analysis, a visual rule-defined classification system categorised alertness into one of 12 adaptive scoring stages. 6 of the 12 stages are described by the alertness conditions comparable to WAKE and NREM1.4 stages are nearly classified as NREM2-4, Rapid Eye Movement (REM) and Movement Time (MT), and one stage reflects the increase of alertness from drowsiness. The typical stage of an alert subject increased significantly from a median of 65.9% before therapy to 80.8% in the second investigation. The percentages of clearly drowsy stages decreased significantly. In contrast, there were no significant changes in the percentages of sleep stages according to R&K criteria for both investigations. According to R&K criteria 178 of 398 failed reactions (Reaction time >10 s) occurred in stage WAKE. According to adaptive scoring, only 12 failed reactions appeared in the alert stage. During the other failed reactions the electrophysiological recordings showed decreases in alertness. Neither the visual assessment nor the descriptive statistical results of R&K scoring were helpful to interpret the patient's alertness condition. In contrast, the patients' increases in alertness with nCPAP could be described by the adaptive scoring stages. This method could be a very useful procedure, when an expert opinion is necessary. It also has an actual context to the discussion of the effectiveness of CPAP in the treatment of OSAS.

Digital archival and exchange of events in a simple format for polygraphic recordings with application in event related potential studies

This paper describes a simple method of event encoding as an extension to a previously defined standard format, the European Data Format (EDF). The specification ensures full backward compatibility with the existing definition. By using this extension, the format can be used to store both continuous recordings and selected epochs of recordings. The encoding is performed in a channel of event-codes or in a pseudo-channel for annotations. Standardisation of event encoding is discussed. Decoding of events or annotations from the extended format is implemented at the application level. Existing programs that do not support the new encoding scheme still operate correctly and can simply ignore the new channels in processing 'extended' data files. The event encoding is also compatible with EDF's capability to encode channels of different sampling frequency.

The detection of drowsiness and sleep onset periods from ambulatory recorded polygraphic data

A 30 min sample recording at the sleep onset of 7 healthy male subjects was used to further develop a computer-scoring algorithm applied earlier for the analysis of MSLT recordings. The performance of this algorithm was tested on 7 patients with obstructive sleep apnea by using 6 h daytime recordings including drowsiness and sleep episodes. The total epoch-by-epoch agreement between visual and computer scoring was over 90% and the accurate detection rate of non-REM sleep was 64%. The hypnograms produced by the computer scoring corresponded sufficiently to those obtained by visual scoring. Our automatic scoring system can give a good estimation of the daytime vigilance profile but for clinical diagnosis the results have to be verified visually. However, by using modern digital recording, analyzing and scoring techniques the speed of analysis and thus the costs can markedly been reduced as compared to traditional visual analysis.

Nonlinear eye movement detection method for drowsiness studies

Automatic long-term vigilance analysis systems require information about the occurrence and type of eye movements, in addition to information about other physiological signals. This paper presents a method to detect different types of eye movements in ambulatory recordings. The method is based on the application of a weighted FIR-median-hybrid filter in the preprocessing of the signal and on the novel use of linear correlation between two EOG signals which are obtained using a new, improved electrode montage. The evaluation of the method showed that it performed well in detecting isolated unambiguous eye movements, but differences were observed in comparison to visual scoring in borderline cases. The method was found to be suitable for use as part of a signal analysis system for drowsiness studies.

Interactive computer program for the analysis of cardiovascular polysomnographic recordings

A computer program for the analysis of blood pressure, heart rate, oxygen saturation and some other polysomnographic signals is presented. The program analyzes the signals, digitized by using a 100 Hz sampling frequency, in one second resolution. The analysis and calibration periods can be entered to the program by pointing at the appropriate positions of the displayed signal with a mouse. As analysis results the program can produce statistical tables or graphical representations of the measured variables as a function of time. The program can be used in routine polysomnographic cardiovascular analysis and due to its flexibility also in experimental studies.

Evaluation of a computerized system for recognition of epileptic activity during long-term EEG recording

A new method of recognition of epileptic activity using adaptive segmentation in EEG during long-term intensive monitoring was developed in Tampere. The performance of the system was validated and compared to the commercially available discharge recognition system of Gotman. Twelve approximately 30 min EEG segments recorded during intensive monitoring from 6 patients were analysed. On these EEG segments two EEG specialists marked the occurrence of epileptic activity independently. Later they re-evaluated any differences in their scoring. This consensus file was used as a reference in validating the performance of the two computer programs. We found that the program developed in Tampere detected discharge activity more often than the Gotman system. Both systems performed poorly in spike recognition. In the specificity of the recognized segments, the Gotman system was better.

The relationship between EEG delta activity and autonomic activity as measured by SCSB during daytime sleep

The aim of the present work was to study the applicability of the static-charge-sensitive bed (SCSB) method to sleep studies, by comparing autonomic nervous activity measured with the SCSB and EEG delta activity (0.5-2 Hz). Simultaneous polygraphic and SCSB measurements of the first sleep cycle (110 min) were recorded from seven subjects. The amount of delta activity filtered from the EEG was compared in 3 min epochs to an autonomic activity index (AAI) based on cardiac, respiratory and body movement activity. Individual Spearman rank order correlation coefficients between delta amplitude and the AAI varied from 0.17 to -0.60. Therefore, it is concluded that the relationship between the delta activity and the AAI is not systematic enough for defining 'depth' of sleep at any point in time by using the AAI. However, on average, the SCSB differentiates high and low delta values quite well. With some further improvement of the scoring system the SCSB is a useful tool in comparative sleep studies.

Validation of computer analysed polygraphic patterns during drowsiness and sleep onset

A computer system for the automatic analysis of polygraphic records was validated. Records from 9 subjects made during routine MSLT tests were analysed both by two preliminary and one consensus scorer and by a computer system. Special attention was paid to the analysis of drowsiness periods. Therefore a classification system including 7 stages, three for wakefulness and movement, one for drowsiness and three for the sleep stages S1, S2 and SREM was used. Adaptive segmentation was used to divide the records into short segments of variable length (mean 1.6 sec, range 0.5-13.7 sec). The agreements between the computer and visual scores were relatively good for 5 subjects having a prominent occipital alpha activity during wakefulness (range 70-79%) but less promising (range 64-70%) for the other 4 subjects with "poor" occipital alpha activity. The values obtained corresponded to the inter-scorer agreements. Most of the discrepancies were between adjacent stages. At times in the presence of strongly fluctuating EEG amplitudes and especially with the "low-alpha" subjects it was very difficult to determine exactly even by visual scoring when, for instance, drowsiness became sleep. It is concluded that the reliability of the system is sufficient for practical purposes especially if critical parts of the records are visually reexamined. It was found to be difficult to define unambiguous scoring criteria for subjects with poorly defined EEG rhythms giving insufficient landmarks for stage determination.

The effect of small differences in electrode position on EOG signals: application to vigilance studies

The primary aim of the study was to determine the best electrode positions for EOG signals in vigilance studies. Two-channel recordings were conducted in analogy to the Rechtschaffen and Kales (1968) system. Twenty electrodes (10 electrode pairs) were compared. Both EOG amplitudes and amplitude asymmetries within an electrode pair were studied. The amplitude of the EOG signal is sensitive to relatively small differences in electrode position. This concerns especially distance from the eye, the direction of eye movement and the effect of the upper eye lid movement. Larger and more symmetrical EOG amplitudes were obtained for different eye movements by placing the electrodes more medially than in the conventionally used system. EOG asymmetry in different electrode positions was dependent on the eye movement direction and even on the starting and end points of a movement with equal angular degrees. Most of the data could be explained by a simple monopolar model when combined with the effects of the upper eye lid movements. The most unexpected finding was that the EOG amplitudes of the horizontal and oblique eye movements were significantly larger when the eyes were moving towards an electrode than when they were moving to the opposite direction.

A simple format for exchange of digitized polygraphic recordings

A simple digital format supporting the technical aspects of exchange and storage of polygraphic signals has been specified. Implementation of the format is simple and independent of hard- or software environments. It allows for any local montages, transducers, prefiltering, sampling frequencies, etc. At present, 7 laboratories in various countries have used the format for exchanging sleep-wake recordings. These exchanges have made it possible to create a common database of sleep records, to compare the analysis algorithms local to the various laboratories to each other by applying these algorithms to identical signals, and to set up a computer-aided interlaboratory evaluation of manual and automatic analysis methods.

Automatic identification of significant graphoelements in multichannel EEG recordings by adaptive segmentation and fuzzy clustering

A new approach to visual evaluation of long-term EEG recordings is proposed. The method is based on multichannel adaptive segmentation, subsequent feature extraction, automatic classification of the acquired segments by fuzzy cluster analysis (fuzzy c-means algorithm), and on the distinguishing of thus identified EEG segments by colour directly in the EEG record. The black and white variant of the described automatic system is presented. The method was evaluated by applying it to simulated artificial data and to real EEG recordings; some of the illustrative results are shown. In addition, the performance of this system is evaluated and the first experience with its application to routine EEG recordings is discussed.

Correlation between EEG and heart rate variation in deep enflurane anaesthesia

Variations in heart rate during deep enflurane anaesthesia were studied in 11 women admitted for elective surgery. Correlation was found between heart rate variations and bursts and suppressions in the EEG patterns. The onset of bursts coincided with accelerating heart rate, and the onset of suppressions was combined with a fall in heart rate in all patients. The correlation is not explained by ventilation arrhythmia. Both the cortical electrical activity and heart rate fluctuation are probably controlled by the same subcortical factor.