2.3.3 MIMIC II waveform records

All MIMIC II waveforms are stored in WFDB format. Table 2.3 summarizes the types of files you will find for the MIMIC II waveform database.

The records vary in length; some are several weeks in duration. It is common for the signal sources to be interrupted or changed occasionally during recordings of such a long duration. In a typical waveform database, you will find a directory layout including several record names or ``cases''. All files associated with each record are gathered in a sub-directory named after the record. For example, the files associated with record a40001 are all located within the directory named a40001.

Table 2.4: WFDB file types

File type	Extension	Description
Header	.hea	Contain signal file names and attributes in plain text format.
Signal	.dat	Contain signals binary data.
Annotations	-dependent, e.g. ``.al'' for alarms, ``.wqrs'' for ECG beat annotations	Contain signal custom annotations in binary format
Calibration	.cal	Contain signal calibration specifications.

Table 2.4 presents some useful WFDB commands to navigate through the waveform records. In a typical WFDB database record, a header file specifies the names of the associated signal files and their attributes, briefly:

Figure 2.17, shows the output of the ``wfdbdesc'' command, which outputs a human-readable description of a waveform record.

Table 2.5: Useful WFDB commands

Command	Description
wfdbdesc	Reads specifications for the signals described in the header file for record.
rdsamp	Reads signal files for the specified record and writes the samples as decimal numbers on the standard output. Each line of output contains the sample number and samples from each signal, beginning with channel 0, separated by tabs.
wave	Can be used to view the specified WFDB record or records on any display controlled by an X11 server. It includes facilities for interactive annotation editing. The keyboard and mouse are used to control the display interactively

**Figure 2.17:** Sample output for wfdbdesc
$\begin{figure}\begin{center} \tiny \begin{verbatim}Starting time: [16:24:28.8... ...ro: 0 Baseline: -100 Checksum: -4501\end{verbatim} \end{center}\end{figure}$

If you want to display the signal contents of a particular record, you can use the ``rdsamp'' command. Figure 2.18 shows the output of this program.

**Figure 2.18:** Sample output of rdsamp
$\begin{figure}\begin{center} {\small \begin{verbatim}time II V ABP PAP (sec) ... ...00 0.176 -0.073 0.000 60.800 90.000\end{verbatim} } \end{center}\end{figure}$

Another option is to use a visualization tool like WAVE, or ATM (http://www.physionet.org/cgi-bin/ATM), to display the contents of a particular waveform record. Figure 2.19 shows the display of a particular waveform record. Note that the signal processing algorithms can be run from this viewer.