How to Implement SCP - Part I: SCP Format Overview
4 SCP Description
4.1 SCP format overview
The data record that is to be interchanged shall be divided into different sections. The contents and format of each of these sections are defined in the SCP document.
A global overview of the SCP-ECG data structure is presented in the Table below:
| CRC CHECKSUM |
| SIZE OF THE ENTIRE SCP-ECG RECORD |
| POINTERS TO DATA AREAS IN THE RECORD ("Table of content") |
| HEADER e.g. PatID, Device ID, Recording ID (Time stamp etc.) |
| ECG Data in optional formats without/with (selectable) compression methods |
| Various types of processing and overreading results |
In a series of experiments with sending of SCP-ECG data via Telephone from a (moving) Emergency Car to an Emergency Department already in 1994 (see Computers in Cardiology 1994, pp. 341-4) transmission efficiency in time and error correction performance could be convincingly demonstrated!
Please Note: A specific and most attractive feature of an SCP record is the built in self-identification mechanism: as the pointer section gives already an overview what is within the whole record contained mostly already from the header of each section it can be determined which of the possible options for the information content of that section have been chosen. Therefore, a format and content checker "pre-processing" the SCP record can derive all necessary information to control a specific viewer or, respectively, select a viewer for the desired application profile (e.g. display of the ECG waveforms with or without beat annotation, processing results like global measurements, interpretation or over-reading results up to detailed lead measurements, etc.).
4.2 Detailed structure of a SCP-ECG record
The SCP standard specifies that the information described globally above has to be structured in sections as shown in the table below.
| Mandatory | 2 BYTES - CHECKSUM - CRC - CCITT OVER THE ENTIRE RECORD (EXCLUDING THIS WORD) |
| Mandatory | 4 BYTES - (UNSIGNED) SIZE OF THE ENTIRE ECG RECORD (IN BYTES) |
| Mandatory | (Section 0) POINTERS TO DATA AREAS IN THE RECORD |
| Mandatory | (Section 1) HEADER INFORMATION - PATIENT DATA/ECG ACQUISITION DATA |
| Optional | (Section 2) HUFFMAN TABLES USED IN ENCODING OF ECG DATA (IF USED) |
| Optional | (Section 3) ECG LEAD DEFINITION |
| Optional | (Section 4) QRS LOCATIONS (IF REFERENCE BEATS ARE ENCODED) |
| Optional | (Section 5) ENCODED REFERENCE BEAT DATA IF REFERENCE BEATS ARE STORED |
| Optional | (Section 6) "RESIDUAL SIGNAL" AFTER REFERENCE BEAT SUBTRACTION IF REFERENCE BEATS ARE STORED, OTHERWISE ENCODED RHYTHM DATA |
| Optional | (Section 7) GLOBAL MEASUREMENTS |
| Optional | (Section 8) TEXTUAL DIAGNOSIS FROM THE "INTERPRETIVE" DEVICE |
| Optional | (Section 9) MANUFACTURER SPECIFIC DIAGNOSTIC AND OVERREADING DATA FROM THE "INTERPRETIVE" DEVICE |
| Optional | (Section 10) LEAD MEASUREMENT RESULTS |
| Optional | (Section 11) UNIVERSAL STATEMENT CODES RESULTING FROM THE INTERPRETATION |
Each section is divided into two parts:
- The section ID Header.
- The section Data Part.
While the section Id. Header always has a length of 16 bytes, the section data part is variable. Note that the complete section length (relevant for the section length information) includes the length of the ID Header.
The SCP standard allows for a rather large number of options to store and format the ECG data. ECG data may be acquired at different sampling rates, with different quantization levels, they may be not compressed or be compressed by selectable methods and an SCP-ECG record may or may not contain analysis and overreading results. Also, the number of leads, the length of the recording interval and even the simultaneity of leads is left open to the manufacturers.
Note: Common praxis in computer assisted resting electrocardiography is that 12/8 (Einthoven, Goldberger, Wilson) leads are acquired simultaneously with a sampling rate of 500 S/s and amplitude quantisation 1.25 microV, .., 5 microV/LSB. Typically the recording interval has a length of 10 s.Therefore, many of the following explanations might assume that ECGs are acquired according to these specifications.
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