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Safety features of gas supply
Switch-on lock
A safety mechanism prevents gas from being delivered
if the machine is switched on although there is no O
delivery pressure. The ventilator indicates »Pressure
supply?« in this case. The O
and the message confirmed on the ventilator.
N
O cut off and AIR changeover
2
If the O
supply fails during operation,
2
– the O
shortage signal sounds (pneumatic whistle),
2
– delivery of N
O is stopped and
2
– »AIR« is automatically activated on the bank of
measuring tubes, although the control lever is still set
to »N
O«.
2
Resumption of the O
supply pressure is detected by the
2
machine and the former status is restored automatically.
Drive gas changeover
Approx. 2 L/min drive gas are required for the ventilator
and approx. 12 L/min for the secretion aspirator. This
gas is normally drawn from the compressed air supply.
If it is not available or has failed, the machine automatic-
ally switches over to the O
gas from there.
Principles of the processed EEG (pEEG)
The electrical activity of the cortical neurone causes
potential variations that can be measored on the surface
of the scalp by electrodes (EEG, electroencephalo-
graphy). These signals have amplitudes in the order of
10 µV to 100 µV, i.e. roughly 100 times less than ECG
signals. Another difference compared to ECG signals is
the high complexity and variability of the EEG signals.
Intensive training is therefore required in order to
interpret unprocessed EEG signals reliably.
It is not practically feasible for the anaesthetist to evaluate
the unprocessed EEG of patients under anaesthesia.
However, with pEEG measurement, special frequency
analysis methods are used to reduce the volume of data,
in order to provide the anaesthetist with EEG
characteristics relevant to anaesthetic management.
With Dräger pEEG measurement, two EEG signals
derived by electrodes from the left and right hemispheres
of the brain are processed. The frequency content and
amplitudes of these signals contain important diagnostic
information which can be determined with the aid of
Fourier transformation and displayed on the monitor.
2
supply must be connected
2
supply and draws its drive
2
The amplitudes are determined for specific time slots
(epochs). Characteristic values are then calculated from
the frequency spectrum. These calculations take place in
"real time", i.e. they are performed at the same time as
the signals are received, so that the results are available
after completion of the next epoch.
To provide a visible illustration of the trend of the EEG
data, the frequency spectra of the individual epochs are
represented in "Density Special Array" format (DSA).
Here, together with time and frequency, amplitude is
represented as the third dimension by colour shading.
Clinically significant EEG parameters include the 90%
spectral edge frequency (SEF90) and the spectral
median frequency SMF.
SEF90 is the frequency below which 90% of the
frequency spectrum output is situated.
SMF
is the frequency below which 50% of the
frequency spectrum output is situated.
Both parameters are sensitive indicators of changes in
EEG activity. The parameters correlate with the depth of
hypnosis of the patient and can therefore support the
control of anaesthesia by the anaesthetist. The informa-
tion is represented as relative band power values output
values of the four conventional EEG bands:
δ: 1 to 4 Hz,
ϑ: 4 to 8 Hz,
α: 8 to 12 Hz and
β: 12 to 30 Hz.
High delta activity is typical of adequate anaesthesia or
sleep.
Theta activity indicates light anaesthesia, sleepiness or
certain pathological conditions.
Alpha activity occurs in healthy awake adults with closed
eyes.
Beta activity predominates in healthy, mentally active
adults.
The "burst suppression ratio" (BSR) is useful when
EEG activity is heavily suppressed. This ratio gives the
percentage of EEG silence over the last 60 seconds.
The ratio of the alpha + beta band outputs to delta
(delta ratio) is another parameter that can be
represented as trend.
In addition to the EEG data, pEEG measurement deter-
mines a mean value of EMG activity from the EEG signal
of the left pair of electrodes. This parameter, which is
represented as the numerical value and trend, provides
information on the spontaneous muscular activity of the
frontalis muscles.
Description
Safety features of gas supply
183
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