ECG: electrocardiography of the heart

An electrocardiogram (ECG) records the heart's electrical activity in twelve standard leads and allows for the rapid detection of arrhythmias, myocardial ischemia, conduction disturbances, hypertrophy, and electrolyte imbalances. For chest pain, an ECG is considered the initial test in patients of any risk category, as early identification of ischemic changes influences the choice of treatment route and prognosis. [1]

An urgent ECG is indicated for symptoms suggestive of acute coronary syndrome, including chest pain, dyspnea, nausea, cold sweats, syncope, and ischemic equivalents in women and older patients. An ECG should be performed promptly, with repeat recordings occurring if pain worsens or instability develops. [2]

In non-acute situations, an ECG is ordered for irregular heartbeat, tachycardia, bradycardia, dizziness, syncope, before initiating many cardiotonic and psychotropic medications, and as part of a basic assessment of cardiovascular risk factors. In these scenarios, the ECG serves as a starting point for choosing monitoring or imaging. [3]

How to properly record a twelve-lead ECG

The key to reliability is the correct marking of the points and fixation of the electrodes. Limb electrodes are placed on the shoulders and legs distally or, if necessary, proximally on the torso, but maintaining the frontal angles. Chest leads are positioned strictly according to anatomical landmarks: V1 in the fourth intercostal space at the right edge of the sternum, V2 symmetrically on the left, V4 in the fifth intercostal space along the midclavicular line, V3 between V2 and V4, V5 along the anterior axillary line at the level of V4, V6 along the midaxillary line. A shift one rib higher or more laterally distorts the ST segment and QRS voltage. [4]

Filters and sampling frequency must meet standards: a high-pass filter of at least 150 Hz, a low-pass filter of approximately 0.05 Hz, and power-line interference suppression if necessary. Proper skin preparation, including removal of fat and the superficial layer of the epidermis with a gauze pad, reduces baseline drift and muscle activity artifacts. [5]

If right ventricular infarction is suspected, it is advisable to mirror the right chest leads, and in case of posterobasal ischemia, the posterior leads V7 V8 V9. These options should be available on most modern recorders and performed without delay. [6]

Table 1. Standard electrode layout and common errors

Abduction	Installation point	Typical mistake	Consequence
V1	The fourth intercostal space to the right of the sternum	One intercostal space higher	False ST depression or elevation
V2	The fourth intercostal space to the left of the sternum	Too lateral	Changes in QRS morphology
V4	Fifth intercostal space along the midclavicular line	Shifted more medially	ST segment distortion
V5	Level V4 along the anterior axillary line	Too high	Imaginary ischemic changes
V6	Level V4 at the midaxillary line	Too far back	Reduction in R amplitude
Based on practical standards and equipment guidelines. [7]

Recording quality and artifacts

Most often, interference is caused by poor electrode contact, muscle tremors, shuddering, and cable interference. Elimination involves re-preparing the skin, replacing the electrodes, securing the wires with a loop, relaxing the shoulders and arms, and checking the grounding of the device. Proper filter settings reduce noise without significantly distorting the teeth. [8]

Some artifacts mimic life-threatening conditions. Wire oscillation can create pseudo-tachyarrhythmia, and tremor can create pseudo-atrial fibrillation. The electrical activity is always checked for consistency with the pulse and, if necessary, repeated recordings are made, focusing on eliminating the specific interference. Personnel training reduces the frequency of false alarms and improves diagnostic quality. [9]

Table 2. Artifacts and quick solutions

Problem	Probable cause	What to do
Noise in all leads	Poor skin contact	Clean and replace electrodes
Torn baseline	Movements, breathing	Ask not to talk, stabilize the wires
High frequency "pile"	Muscle tension	Relax the muscles, warm the patient
Network interference	Poor grounding	Check the power supply and interference filter
Data from training programs on monitoring standards. [10]

Step-by-step interpretation

Interpretation begins with the rate and regularity of the rhythm, followed by assessment of the P wave and its relationship to the QRS complex, cardiac axis, PR QRS QTc intervals, ST segment, and T wave. This sequence reduces the risk of missing rare but significant findings.[11]

The PR interval is typically 120-200 milliseconds, the QRS duration is less than 120 milliseconds, and the QTc is calculated using correction formulas taking into account the heart rate. For borderline values, it is recommended to compare the automatic calculation with the manual measurement, especially in the case of tachycardia and bradycardia. [12]

Table 3. Interval guidelines for adults

Parameter	Norm	Comments
PR	120-200 ms	Lengthening during blockades and vagotonia
QRS	<120 ms	Wide complex for bundle branch blocks and ventricular rhythms
QTc of men	<450 ms	A 10ms increase in heart rate increases the risk of arrhythmia by approximately 5%
QTc of women	<460 ms	Consider medications and electrolytes
Threshold values and clinical interpretation. [13]

Acute coronary syndrome on ECG

ST-segment elevation criteria vary by gender and age, but the goal is the same: to identify coronary occlusion requiring immediate reperfusion. An ECG is repeated if pain persists, and additional leads are added if occult occlusion is suspected. Decisions are made in conjunction with clinical findings and high-sensitivity troponins, according to current guidelines. [14]

If posterior wall lesions are suspected, attention is paid to horizontal ST depression in V1-V3 with high R and positive T waves. Confirmation is ST elevation in V7, V8, V9. This expansion of registration increases diagnostic sensitivity and reduces treatment delays. [15]

In patients with left bundle branch block and posterior electrode position, modified Sgarbossa criteria are used, assessing the ratio of discordant ST elevation to the depth of the S wave. This improves the accuracy of infarction recognition against the background of atypical repolarization. [16]

Table 4. "Map" of walls and leads during infarction

Wall	Main leads	Additional
Front	V1-V4	High parasternal if necessary
Side	I aVL V5 V6	-
Lower	II III aVF	Right when right ventricle is suspected
Back	V7 V8 V9	Evaluation of mirror changes in V1-V3
Used in early reperfusion algorithms. [17]

Table 5. When to add right and posterior leads

Clinical situation	What to add	For what
Inferior infarction with hypotension	Right leads	Detect right ventricular infarction
Horizontal ST depression in V1-V3	V7 V8 V9	Confirm posterior occlusion
Unclear ST elevation in left bundle branch block	Revision with modified criteria	Improve diagnostic accuracy
Approaches from clinical reviews and practice guidelines. [18]

How to distinguish ischemia from pericarditis and early repolarization

Acute pericarditis most often presents with diffuse ST elevation and PR depression, while ischemia manifests as segmental changes across the blood supply zones. A diagnosis of pericarditis is established by the presence of at least two of four criteria: pain, pericardial friction rub, ECG changes, effusion on imaging, and supporting inflammatory markers. [19]

QTc prolongation: thresholds and risk

QTc should be measured manually when in doubt and interpreted taking into account gender, age, electrolytes, and medications. Normal values are less than 450 milliseconds in men and less than 460 milliseconds in women. Every additional 10 milliseconds is associated with an approximately 5% increase in the risk of arrhythmia. [20]

For borderline values, clinical and drug profile considerations are used. Validated databases classifying proarrhythmic effects are used to assess drug risks. Particular attention is paid to combinations of hypokalemia, hypomagnesemia, and bradycardia. [21]

Table 6. QTc thresholds and estimated risk

Category	Men	Women	Tactics
Norm	<450 ms	<460 ms	Routine observation
Borderline	451-470 ms	461-480 ms	Adjust risk factors
Elongation	>470 ms	>480 ms	Review therapy and electrolytes
Summary of clinical reviews and materials on pharmacosafety. [22]

Ambulatory monitoring and stress ECG

If symptoms are rare or short-lived, ambulatory monitoring is used: 24-hour recorders, long-term patch systems, and loop recorders. The choice depends on the frequency of events, tolerability, and the target application. Standardized recommendations help match the device to the clinical scenario and improve diagnostics. [23]

Stress ECG without imaging is useful for assessing exercise tolerance, reproducing angina symptoms, risk stratifying, and monitoring treatment effectiveness in stable conditions. The technique requires standardized staging, recording heart rate, blood pressure, ECG, and symptoms at each stage, with immediate cessation if any alarming signs are detected. [24]

Table 7. How to choose symptom-based monitoring

Symptoms	Preferred method	Expected find
Daily Episodes	Daily registrar	Frequent supraventricular arrhythmias
Weekly episodes	Patch for several days	Paroxysmal atrial fibrillation
Less than once a month	Outdoor loop recorder	Rare tachyarrhythmias
Syncope without prodrome	Implantable recorder	Rare pauses and tachyarrhythmias
According to the consensus on ambulatory ECG. [25]

Special situations: children and athletes

In children, age affects amplitudes and intervals, so age-specific standards are used. PR and QRS intervals are shorter, and rhythm variability with respiration is pronounced. QTc values in early childhood are interpreted cautiously, as mean values and limits change during the first weeks of life. [26]

For athletes, modern interpretation guidelines are used, distinguishing between physiological and pathological changes. This reduces the number of false-positive conclusions and prevents unjustified restrictions. When in doubt, a sequential algorithm with visualization and stress testing is used. [27]

Table 8. Pediatric landmarks for interpretation

Parameter	Landmark	Note
Sinus arrhythmia	Often the norm	Increases with breathing
PR	K is shorter for babies	Increases with age
QRS	Low voltage in newborns	It is gradually increasing
QTc	Estimate by age curves	Cautious interpretation in newborns
Summarized from regulatory frameworks and reviews. [28]

Limitations of ECG and integration with other methods

A normal ECG does not exclude ischemia, myocarditis, or structural pathology. In acute situations, ECG interpretation is always accompanied by an assessment of high-sensitivity troponins and echocardiography if necessary. In stable conditions, the choice of tests is based on symptoms and the probability of disease along the current pathways. [29]

Brief application algorithm

1. In case of chest pain - immediate ECG, repeat during dynamics, extended leads if right ventricular or posterior lesion is suspected. [30]
2. Always check the correctness of the electrode placement and the quality of the recording, and remove artifacts before interpretation. [31]
3. Interpret sequentially: rhythm, axis, intervals, ST and T, then compare with clinical and laboratory findings. [32]
4. If QTc is borderline, take into account medications and electrolytes, and compare the automatic calculation with the manual measurement. [33]
5. In rare cases, select the format of outpatient monitoring based on the frequency of symptoms; if necessary, a stress ECG with clear stopping criteria. [34]