Event Type Ambulatory Cassette Recording

In the early 1970s, it was clear that obtaining weeks of continuous, paper-recorded EEG from inpatients undergoing depth electrode evaluations was unmanageable and overwhelming. Because the objective was to record the EEG during habitual seizures in these patients, the first step toward a rational solution was to develop a telemetry system that allowed patients to have some degree of freedom of movement during their hospitalization while recording their EEG rhythms. A system including on-the-head mounted amplifiers of the EEG-derived signal permitted this freedom. Artifact was significantly reduced using this "close to the source" amplification concept due to the lack of movement-related artifacts that are seen when recording with long electrode leads. The EEG signals were multiplexed, so 16 channels of EEG could be transmitted via a small wire to a DEC PDP-12 computer. A two-minute delay in the 16 channels of EEG was created by programming the digital computer to store EEG from the previous two minutes in a memory loop (i.e., buffering). This allowed the clinician to capture the onset of a clinical event even soon after it was in progress. [24] This started the era of EEG data manipulation and reduction.

Eventually, this loop concept was closed by including output software that reassembled the delayed EEG into the multiplexed format for transmission to another demultiplexing unit that was coupled to a Mingograph EEG machine. If a clinical event occurred, the activation of the seizure button caused it to be permanently stored on the computer tape. It also signaled the EEG machine to write out the two minutes of delayed EEG leading up to the event. The EEG machine was also programmed to take regular samples during the night in order to capture the natural sleep of the patient. [25]

As smaller microcomputers and larger memory systems became available, a stand-alone system was developed that could be moved to the patient's bedside. [26] This system consisted of an Intel 8085 computer with 1Mbyte of RAM, an A/D-D/A input/output board, and a standard cassette tape deck to record the stored data. This unit emulated previous concepts that were on the larger computer, i.e., a delay loop with multiple multiplexed channels. A timecode generator was later added and this allowed independently recorded video to be synchronized to the EEG. [27] An EEG sample control unit automatically saved timed EEG samples at preset intervals, which allowed the clinician to see pieces of the patient's routinely generated EEG.

In the early 1980s, further electronic miniaturization enabled the functions of this bedside system to fit into an ambulatory cassette recorder. This "Walkman" audiocassette recorder stored the multiplexed EEG signals and the time-of-day signal. [28]. Initially, a number of technological constraints— power consumption, limited by the availability of static RAM—allowed for only a 5-second delay on the EEG. However, advancements in the field of SRAM memory circuits allowed two or more minutes of delay to be archived on ambulatory units. It also became possible to increase the channel capacity from 16 to 24 channels. [29] Thus, all major head regions could be covered simultaneously, including the use of sphenoidal electrodes. [30-32]

During a seizure, high-frequency muscle artifact can completely obscure the underlying EEG. A variety of filtering techniques have been developed in both digital [33] and analog [34] formats. Technical developments in the field of charge-coupled capacitive filters provided a simple and inexpensive means of replaying events that have been contaminated by high-frequency muscle artifact using a 6-pole filter with variable frequency settings ranging from 9 to 70 Hz. [35] This allows the clinician to replay events with a relatively open setting and compare that recording to playbacks with more extreme filtering and to note rhythms that are suspicious for seizure events.

Finally a "time-scribe" digital clock capable of both displaying replay time and writing it out in a readable fashion on the EEG paper was developed to aid the time-locking of EEG with video, computer, or observation. [36, 37]

There were numerous advantages to the early periodic/event system. A greater number of recording channels was possible, which resulted in greater spatial resolution on recorded events. Patients and observers aided the EEG reader by selecting times of greater interest by pushing an event button when symptoms were experienced or clinical signs were observed. This also meant that there was less data to review. These systems were considerably less expensive than continuous recorders. The computer used for the automatic detection of relevant electrographic events could also be used to aid in the off-line analysis of a study.

There were also several disadvantages to the early intermittent ambulatory EEG recorders. There was dependence on an independent observer or the patient to know when a seizure was coming so the event button could be pushed. Also, the clinician could not go back and look at data prior to the delay time in cases where the event had a longer lead into it.

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Healthy Sleep

Healthy Sleep

A Guide to Natural Sleep Remedies. Many of us experience the occasional night of sleeplessness without any consequences. It is when the occasional night here and there becomes a pattern of several nights in arow that you are faced with a sleeping problem. Repeated loss of sleep affects all areas of your life The physical, the mental, and theemotional. Sleep deprivation can affect your overall daily performance and may even havean effecton your personality.

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