Electrocardiogram Annotations

WFDB Annotation Standards

The WFDB (Waveform Database) software package provides a standardized system for annotating cardiac signals, predominately surface electrocardiogram (ECG) data. Annotations are polymorphic, meaning multiple annotation sets can be applied to a single signal dataset. The limitation is that these are constrained to physiological events associated with surface ECGs.

Annotation Table Structure

All WFDB-compatible annotations in the {EGM} package are stored as annotation_table objects. The columns are type-specific, which allows them to be easily stored. These tables contain the following required columns:

annotator: Name of the annotation function or creator (e.g., “sqrs”, “ecgpuwave”, “manual”)
time: Time stamp constructed from sample number and sampling frequency (format: HH:MM:SS.mmm)
sample: Integer index of the annotation position in the signal
type: Single character symbol describing the annotation type (see annotation types below)
subtype: Single character providing additional classification
channel: Integer indicating which signal channel the annotation applies to
number: Additional numeric qualifier providing context-specific information

# Create annotations for detected R-peaks
# Example
ann <- annotation_table(
  annotator = "qrs",
  sample = c(100, 350, 600, 850),
  type = "N",
  frequency = 250,
  channel = 0
)

# Print
ann
#> <annotation_table: 4 `qrs` annotations>
#>          time sample   type subtype channel number
#>        <char>  <num> <char>  <char>   <num>  <int>
#> 1: 00:00:00.4    100      N               0      0
#> 2: 00:00:01.4    350      N               0      0
#> 3: 00:00:02.4    600      N               0      0
#> 4: 00:00:03.4    850      N               0      0

Standard ECG Annotation Types

The WFDB standard defines 41 annotation types for surface electrocardiograms. These annotations can be broadly grouped into several categories:

Beat Annotations

Beat annotations mark individual cardiac cycles and their characteristics:

# Load internal annotation data
beat_symbols <- c("N", "L", "R", "a", "V", "F", "J", "A", "S", "E", "j", "/", "Q", "~")
beat_labels <- EGM:::.surface_annotations[EGM:::.surface_annotations$symbol %in% beat_symbols, ]

knitr::kable(
  beat_labels[, c("symbol", "mnemonic", "description")],
  col.names = c("Symbol", "Mnemonic", "Description"),
  row.names = FALSE,
  caption = "Beat Annotation Types"
)

Beat Annotation Types
Symbol	Mnemonic	Description
N	NORMAL	Normal beat
L	LBBB	Left bundle branch block beat
R	RBBB	Right bundle branch block beat
a	ABERR	Aberrated atrial premature beat
V	PVC	Premature ventricular contraction
F	FUSION	Fusion of ventricular and normal beat
J	NPC	Nodal (junctional) premature beat
A	APC	Atrial premature contraction
S	SVPB	Premature or ectopic supraventricular beat
E	VESC	Ventricular escape beat
j	NESC	Nodal (junctional) escape beat
/	PACE	Paced beat
Q	UNKNOWN	Unclassifiable beat
~	NOISE	Signal quality change

Key beat types:

N (NORMAL): Normal sinus beat
V (PVC): Premature ventricular contraction
A (APC): Atrial premature contraction
L/R (LBBB/RBBB): Bundle branch block beats
F (FUSION): Fusion of ventricular and normal beat
/ (PACE): Paced beat

Waveform Boundary Annotations

These mark the beginning, peak, and end of ECG waveforms:

wave_symbols <- c("p", "t", "u", "(", ")")
wave_labels <- EGM:::.surface_annotations[EGM:::.surface_annotations$symbol %in% wave_symbols, ]

knitr::kable(
  wave_labels[, c("symbol", "mnemonic", "description")],
  col.names = c("Symbol", "Mnemonic", "Description"),
  row.names = FALSE,
  caption = "Waveform Boundary Annotations"
)

Waveform Boundary Annotations
Symbol	Mnemonic	Description
p	PWAVE	P-wave peak
t	TWAVE	T-wave peak
u	UWAVE	U-wave peak
(	WFON	Waveform onset
)	WFOFF	Waveform end

Rhythm and Signal Quality Annotations

rhythm_symbols <- c("+", "|", "s", "T", "~", "x")
rhythm_labels <- EGM:::.surface_annotations[EGM:::.surface_annotations$symbol %in% rhythm_symbols, ]

knitr::kable(
  rhythm_labels[, c("symbol", "mnemonic", "description")],
  col.names = c("Symbol", "Mnemonic", "Description"),
  row.names = FALSE,
  caption = "Rhythm and Signal Quality Annotations"
)

Rhythm and Signal Quality Annotations
Symbol	Mnemonic	Description
~	NOISE	Signal quality change
\|	ARFCT	Isolated QRS-like artifact
s	STCH	ST change
T	TCH	T-wave change
+	RHYTHM	Rhythm change
x	NAPC	Non-conducted P-wave (blocked APB)

Specialized Annotations

special_symbols <- c("*", "D", "\"", "=", "!", "[", "]", "@", "r", "^", "B", "e", "n", "f")
special_labels <- EGM:::.surface_annotations[EGM:::.surface_annotations$symbol %in% special_symbols, ]

knitr::kable(
  special_labels[, c("symbol", "mnemonic", "description")],
  col.names = c("Symbol", "Mnemonic", "Description"),
  row.names = FALSE,
  caption = "Specialized Annotations"
)

Specialized Annotations
Symbol	Mnemonic	Description
*	SYSTOLE	Systole
D	DIASTOLE	Diastole
”	NOTE	Comment annotation
=	MEASURE	Measurement annotation
B	BBB	Left or right bundle branch block
^	PACESP	Non-conducted pacer spike
!	FLWAV	Ventricular flutter wave
[	VFON	Start of ventricular flutter/fibrillation
]	VFOFF	End of ventricular flutter/fibrillation
e	AESC	Atrial escape beat
n	SVESC	Supraventricular escape beat
@	LINK	Link to external data (aux_note contains URL)
f	PFUS	Fusion of paced and normal beat
r	RONT	R-on-T premature ventricular contraction

Common Annotation File Types

When working with WFDB files, annotations are stored with specific file extensions that indicate the annotator used:

.atr: Manually reviewed and corrected reference annotations
.ann: General annotation file
.ecgpuwave: Surface ECG wave boundaries (P, QRS, T waves) generated by the ecgpuwave algorithm
.sqrs, .wqrs, .gqrs: R-peak detections from different QRS detection algorithms

Accessing All Standard Annotations

The complete list of standard WFDB annotations can be accessed using:

# View all standard ECG annotation types
wfdb_annotation_labels()

# Filter for specific symbols
wfdb_annotation_labels(symbol = c("N", "V", "A"))

# Decode annotations in an existing annotation table
ann <- annotation_table(
  annotator = "example",
  sample = c(100, 200),
  type = c("N", "V")
)

# Add human-readable descriptions
wfdb_annotation_decode(ann)

Example: Working with ECG Annotations

Here’s a practical example of reading, interpreting, and visualizing ECG annotations:

# Read an ECG with annotations
record_path <- system.file('extdata', package = 'EGM')
ecg <- read_wfdb(
  record = "muse-sinus",
  record_dir = record_path
)

# Read associated annotations (if they exist)
ann <- read_annotation(
   record = "muse-sinus",
   annotator = "ecgpuwave",
   record_dir = record_path
)

# Decode annotation types
ann_decoded <- wfdb_annotation_decode(ann)
head(ann_decoded)

Understanding Annotations