A motor nerve conduction study (MNCV.edf)

Description

We are going to describe the file MNCV.edf. It contains a motor nerve conduction study of the Left Median Nerve with two data records: the first one was obtained with stimulus at wrist, the second one with stimulus at elbow. We need an EDF Annotations signal to describe the stimulus, measurements, temperature, etc.

The header

This is the content of the global header record

Version	0
Patient	X M 29-JAN-1956 X
Recording	Startdate 01-JAN-2002 emg1 jop N1 MNC_L_Median_Nerve
Startdate	01.01.02
Starttime	10.00.00
Bytes in header	768
Reserved	EDF+D
Data records	2
Duration	0.05
Signals	2

We can see that the code as well as the name of the patient are unknown and have been replaced by the character X. Each window has a width of 50 ms (0.05 s). Since the second data record is not contiguous with the first one, the reserved field begins with EDF+D.

This is the content of the signal header record

	First signal	Second signal
Label	NC L_APB	EDF Annotations
Transducer	Ag-AgCl electrodes
Phys. dimension	mV
Phys. minimum	-25	0
Phys. maximum	25	1
Dig. minimum	-8985	-32768
Dig. maximum	10265	32767
Prefiltering	HP:10Hz LP:10kHz
Samples	500	500
Reserved

Notice that the sampling rate has been adjusted to 500 in the EDF Annotations signal. It implies the availability of a space of 1000 ASCII characters in each data record to include annotations.

The traces

The content of the first data record can be seen in figure 3.1

Figure 3.1: Plot of the record 0

The content of the second data record can be seen in figure 3.2

Figure 3.2: Plot of the record 1

Notice that amplitudes have been converted to real values based on the information contained in the header. The plot has been made following the negativity upward rule.

The annotations

Let's see the beginning of the annotations of record 0

+0[20][20]StimA[20]<EDF_XMLnote><!--Co...

This record includes a TAL (Time-stamped Annotation List) containing three annotations. The first empty annotation indicates that the first data record is coincident with the time marked in the header. The second one indicates that the stimulus is synchronous with the beginning of the data record. The third one describes different properties of the data record. Notice that [20] represents the decimal character 20.

The annotation of the second record is:

 
+30[20][20]StimB[20]<EDF_XMLnote><re...

It is similar to the previous one except for the fact that the empty annotation indicates that the record (and the stimulus) begins 30 seconds after the starttime marked in the header.

Let's see the content of the first data record

 
<EDF_XMLnote>
<!--Coded as example -->
  <temperature unit="degree C" position="left hand">33.2</temperature>
  <recording>Left Abductor Pollicis Brevis</recording>
  <distance mode="stimulus to recording" unit="cm">6.5</distance>
  <stimulus label="StimA" mode="constant intensity" 
  duration="0.2 ms" intensity="9.2 mA">wrist</stimulus>
  <measurements>
    <latency unit="ms">3.3</latency>
    <amplitude mode="baseline to peak" unit="mV">17.4</amplitude>
  </measurements>
</EDF_XMLnote>

And this is the content of the second data record

<EDF_XMLnote>
  <recording>Left Abductor Pollicis Brevis</recording>
  <distance mode="stimulus to recording" unit="cm">30.5</distance>
  <stimulus label="StimB" mode="constant intensity" 
  duration="0.2 ms" intensity="27.6 mA">elbow</stimulus>
  <measurements>
    <latency unit="ms">7.4</latency>
    <amplitude mode="baseline to peak" unit="mV">16.8</amplitude>
    <velocity mode="segmental" unit="m/s">58.5</velocity>
  </measurements>
</EDF_XMLnote>

Some comments

In this case the whole file occupies less that 5 kB. So, in a floppy we could store about 300 files like this one. Typically,files containing nerve conduction studies and evoked potentials are very small. The description about measurements, distances, etc. is located at the beginning of each data record. Some information (e.g., temperature) is only included in one data record. It would not be difficult to include additional parameters (e.g., patient's height, area of the curves...)

Notice that EDF+ does not specify the internal structure of the annotation and many other different alternatives (both in XML or non-XML format) would be possible. In this file, all the parameters considered (latencies, temperature...) include units (ms, degrees C...) as attributes.

The information contained in the annotations can be extracted for further processing (e.g., to create a report).