3.4 Annotation Structures

The annot argument of getann and putann is an object of type WFDB_Annotation containing these fields:

long time

time of the annotation, in samples from the beginning of the record. The times of beat annotations in the ‘atr’ files for the MIT DB generally coincide with the R-wave peak in signal 0; for the AHA DB, they generally coincide with the PQ-junction.

char anntyp

annotation code; an integer between 1 and ACMAX. See section Annotation Codes, for a list of legal annotation codes. ACMAX is defined in ‘<wfdb/ecgcodes.h>’.

signed char subtyp

unsigned char chan

signed char num

numbers between -128 and 127. In MIT DB ‘atr’ files, the subtyp field is used with noise and artifact annotations to indicate which signals are affected (see section Annotation Codes). The chan field is intended to indicate the signal to which the annotation is attached. More than one annotation may be written with the same time if the num or chan fields are distinct and in ascending order. The semantics of the chan field are unspecified, however; users may assign any desired meaning, which need not have anything to do with signal numbers. In user-created annotation files, these fields can be used to store arbitrary small integers. The subtyp field requires no space in a standard annotation file unless it is non-zero; the chan and num fields require no space unless they have changed since the previous annotation.

char *aux

a free text string. The first byte is interpreted as an unsigned char that specifies the number of bytes that follow (up to 255). In MIT DB ‘atr’ files, the aux field is used with rhythm change annotations to specify the new rhythm, and with comment annotations to store the text of the comment (see section Annotation Codes). The string can contain arbitrary binary data, including embedded nulls. It is unwise to store anything but ASCII strings, however, if the annotation file may be transported to a system with a different architecture (e.g., on which multiple-byte quantities may have different sizes or byte layouts). The aux field requires no space in a standard annotation file if it is NULL. Note that conversion of annotation files to other formats may entail truncation or loss of the aux string. Note also that the aux pointer returned by getann points to a small static buffer (separately allocated for each input annotator beginning with WFDB library version 9.4) that may be overwritten by subsequent calls.

See section Example 3: An Annotation Printer, for a short program that examines the contents of a WFDB_Annotation.