FIELD REFERENCE
Full criteria & treatment guidance
A normal tracing does not rule out acute MI
- Do not delay stabilization, pacing, cardioversion, defibrillation, or transport to obtain a 12-lead.
- A normal tracing or one negative for STEMI does not rule out acute MI or serious angina.
- Machine interpretation may be unreliable with paced rhythms, bundle branch blocks, and some tachycardias; tell the base hospital when those rhythms are present.
- Base-hospital contact is required when a patient refuses BLS or ALS transport after a field 12-lead.
Recognize an indication
- Obtain a 12-lead when a cardiac event is suspected, including atypical presentations. Examples include unexplained chest discomfort; cardiac history, tobacco use, hypertension, or diabetes with chest discomfort, dyspnea, or unexplained nausea/vomiting; radiating pain; unexplained diaphoresis; syncope; significant bradycardia; or unexplained tachycardia.
- In a patient over 45, acute anxiety or generalized weakness may represent a cardiac presentation even without classic pain. For unexplained tachycardia, PR-105 says to consider a tracing at 100-130/min and requires it at 130/min or more.
- BLS personnel may place electrodes on any patient as directed by on-scene ALS personnel.
Stabilize first
- Complete the initial assessment and stabilizing treatment before acquisition. Do not delay time-critical care to obtain a 12-lead.
- If arrest, acute respiratory failure, systolic BP below 90, altered consciousness, or another severe condition makes delay unsafe, acquire the tracing at the incident location or in the vehicle immediately before transport begins.
Prepare skin & place electrodes
- Expose only what is needed, prepare skin with alcohol, and remove body hair as needed. For a female patient, use the dorsal surface of the hand to move tissue when necessary; do not place chest leads over breast tissue.
- Place V1 in the fourth intercostal space at the right sternal border and V2 in the fourth intercostal space at the left sternal border.
- Place V4 at the left fifth intercostal space on the midclavicular line, then place V3 halfway between V2 and V4.
- Place V5 level with V4 at the anterior axillary line and V6 level with V5 at the midaxillary line. Place limb leads on the upper arms and lower abdomen or legs.
Acquire, transmit & document
- Acquire the tracing according to the monitor manufacturer and relay the interpretation to the base hospital when indicated. Transmit positive or suspected acute-MI tracings before arrival at the receiving cardiovascular center.
- Document that the 12-lead was obtained and record the interpretation. Transmit the tracing from the field and attach or upload a copy to the prehospital care record.
- Leave electrodes in place unless removal is directed. If defibrillation or synchronized cardioversion is needed, position pads without covering electrodes, removing electrodes if necessary.
VISUAL FIELD REFERENCE
Chest leads: locate V4 before V3
All patients · BLS / ALS
Placement sequence only. Prepare skin, protect privacy, and follow the complete acquisition procedure.
- 4th intercostal space
V1 and V2
Place V1 in the fourth intercostal space at the right sternal border and V2 in the fourth intercostal space at the left sternal border.
- V4 → V3
V4, then V3
Place V4 at the left fifth intercostal space on the midclavicular line, then place V3 halfway between V2 and V4.
- Same horizontal level
V5 and V6
Place V5 level with V4 at the anterior axillary line and V6 level with V5 at the midaxillary line. Place limb leads on the upper arms and lower abdomen or legs.
- Do not delay stabilization, pacing, cardioversion, defibrillation, or transport to obtain a 12-lead.
- BLS personnel may place electrodes on any patient as directed by on-scene ALS personnel.
Selected checkpoints; read the complete guide for criteria, exclusions, doses, and ongoing care. PR-105 · B-030
UNDERSTAND THE CARE
Why consistent electrode placement matters
The tracing is a measurement made from specified positions. Incorrect placement, motion or poor skin contact can alter the recorded pattern and complicate comparison with a later ECG. Follow the anatomical landmarks in the guide and identify V4 before placing V3 between V2 and V4. Maintain the stated horizontal relationship for the lateral chest leads.
Applying the guidance
Acquisition should fit around time-critical care. Interpret the tracing with the patient’s symptoms and physiology and communicate limitations such as paced rhythms or conduction abnormalities. Preserve the tracing in the care record and transmit when indicated so the receiving team can inspect the actual evidence rather than relying on a verbal label.
Measuring ECG time and voltage
Putting it together
Teaching example: a repeat tracing looks different after electrodes were moved. Check and document placement before attributing every change to evolving physiology; retain and transmit relevant tracings.
Diagram text & teaching checkpoints
- Indication
- Symptoms and clinical context.
- Quality
- Landmarks, contact, calibration and artifact.
- Communication
- Interpret and transmit concerning findings.
- Continuity
- Preserve tracing and document changes.
Sources for this explanation
Original OCMedic teaching synthesis of the references below. Educational expansion updated ; this is not a new independent clinical-review date. Source-specific dates and limitations remain in the full guidance.
Preserve the context of a repeat tracing
Two ECGs are most useful when the receiving team can tell what changed between them. Identify the time of each tracing, the symptoms and patient condition at that moment, and any treatment or electrode change between recordings. Retain the earlier tracing even when the later one appears more reassuring or more concerning.
Before explaining a difference as a physiologic change, check whether the recording conditions were comparable. The anatomical chest-lead positions, skin contact and patient movement matter. If an electrode had to be replaced or moved, document that circumstance. This does not mean waiting for a perfect recording while stabilization or transport is needed.
Communicate the interpretation and any limitation, then transmit the actual tracing when indicated so the receiving clinician can inspect it. A monitor statement alone cannot show the quality or evolution of the evidence. A normal or non-STEMI tracing also does not remove the separate requirement for base contact when a patient refuses BLS or ALS transport after a field 12-lead.