Electrical Vectors

Generated when there is movement of electrical ions throughout the heart – ECG represents the ‘average’ direction of these electrical impulses.

Two (2) ECG electrodes are used to gather a vector by comparing the action potential - Impulse is either positive (impulse to positive) or negative (impulse to negative)

  • Positive/Exploring Lead (mostly the one looking at the heart)
  • Negative/Reference Lead

Electrical Condution System


Electrical Waves & ECG
1st Vector: Atria

From Atrial depolarisation (start in SA node) spreading from right to left atrium.

  • V1 can detect initial positive then small negative deflection (P-wave)
  • V5 shows vector leading towards it (purple line) hence showing positive (P-Wave)
2nd Vector: Ventricular (Interventricular) Septum

Septum receives PKF from left bundle branch and depolarises from left to right side – The vector is directed forward the right

  • V1 has small positive wave (R-wave) due to small septum
  • V5 show small negative wave (Q-wave) – Same vector size as V1
3rd Vector: Ventricular Free Wall

Vector from ventricular wall is directed to left then downwards; vector from the right is override by larger vectors from left ventricles and therefore, these vectors originate from there. The

  • V1 displays large negative (S-wave)
  • V5 displays large positive (R-wave)
4th Vector: Basal part of Ventricles

Activation from basal parts of ventricles which is directed backwards and upwards (away from V5)

  • V1 does not detect this vector
  • V5 detects a small negative wave (S-Wave)
T-Wave Vector:

T-Wave is rapid repolarisation (Phase 2), therefore should be in-line with QRS complex (same direction as net QRS direction)

  • Net negative QRS = Negative T-wave
  • Net positive QRS = Positive T-wave
  • Travels opposite direction = Discordant T-wave

Children (till around puberty) may have T-wave vector from left & backwards creating negative T-waves from V1 to V4 (right side of chest).

  • V1 negative T-wave is common along with negative QRS complex.


ECG Components & Waves
P Wave:

1st positive (upward) reflecting on atrial depolarisation; has one wave per QRS – Smooth wave due to low frequency impulses as atria as smaller muscle mass.

  • Abnormal P-waves indicate impulse has travelled abnormally via atrial pathways (not SA node)
  • P-wave with notches indicate left or right (or both) atria abnormality (usually atria dilation)
  • V1 & V2 may show end portion of P-wave slightly negative
  • Small & rounded
  • Amplitude 0.5mm – 2.5mm
  • Duration of < 0.10s
  • Before QRS complex
  • Positive in Lead II for sinus rhythm
Check lead activity:
  • Positive: aVL + aVF + -aVR + Lead I + V4 – V6
  • Negative: -aVR
PR Interval & PR Segment:
Baseline for comparing other waves & segments as it is isoelectric.

Distance between P-wave and QRS onset as result of AV conduction from atria to ventricle; PR is longer in elderly and short PR indicates fast heart rate.

  • PR segment is from end of P-wave to start of QRS complex; denotes AV node conduction delay
  • Normal limits indicate impulse has travelled through atrial pathways in a timely manner
  • PR interval is longer when AV conduction is poor or blocked
  • Shorter PR interval suggest pre-exciting of the impulse; usually by accessory pathways.
  • 0.12 – 0.20s
QRS (Ventricular) Complex:

Ventricular depolarisation made up of 3 waves; the larger wave reflects on larger muscle mass in the ventricles than atria. QRS waves longer/broad (> 0.10s) suggest slow ventricular contractions.

  • 1st downward deflection after PR interval is ‘Q-wave’
  • 1st  deflection after PR interval is ‘R-wave’
  • 2nd downward deflection is called the ‘S-wave’
  • QRS comes in many configurations:
    • o Large waves use capital letters (Q, R, S)
    • o Smaller waves use lower-case (q, r, s)
  • 0.08 – 0.10s
ST Segment:

Brief resting state after ventricular contraction – usually flat and isoelectric. Abnormalities results in depression or elevation as result of cardiac emergencies or conditions.

  • Baseline of ST segment acting as a junction between QRS & ST segment
  • Medical conditions can cause J-point does not meet with isoelectric lines.
  • Any elevation / depressions > 1mm
T Wave:

Ventricular repolarisation – smooth waves that end when isoelectric; can be inverted, peaked or flat based on medical conditions such as drug toxicity. V1 & V3 has highest amplitude

  • Tachycardia has T-wave often and no P-wave potentially as it represents ventricles (dominant)
  • U-wave may be present (2nd hump before P-wave); indicate PKF recovery (show in bradycardia)
  • Small U-wave show ventricle repolarisation whilst larger waves show electrolyte / drug conditions
  • Peak / Flat / Peak structure
  • Rounded & slightly asymmetric
  • Positive in Lead II
  • Amplitude < 5mm
  • Duration 0.10 – 0.25s (or higher)
Check lead activity:
  • Positive: Lead I, II + -aVR + V5 – V9
  • Negative: aVR
  • Lead III + aVL can show isolated T-wave inversion
  • aVF positive but flat
  • V1 invert or flat waves common & women
QT Interval:

Time between beginning of ventricular depolarisation to end of ventricular repolarisation (ventricular activity duration) from start of QRS to end of T-wave. Bazzett’s formula shows inverted relationship of QT duration & HR as ‘QTc'.

  • Long QT interval suggest long repolarisation; can cause lethal arrythmias
  • Duration 0.35 – 0.43s
ECG Artefact

External or internal causes which leads to poor ECG readings that produces excessive waves or frequency.

  • Check loose leads or dried electrodes
  • Ensure adequate skin preparation by cleaning, shaving &/or drying
  • Ensure ECG set to 0.05 – 40Hz for good diagnostic quality or 0.05 – 150Hz (Corpuls)
  • Advise patient to remain as relaxed and still as possible
Loose Leads:
  • Usually in sweaty/wet or hairy patients – requires proper skin preparation by shaving &/or drying skin accordingly.
Wandering Baseline:
  • Slow undulating baseline on ECG caused by patient or ambulance movement including breathing resulting in an ECG that drifts off.
Muscle Tremors:
  • Motion caused by shivering, tremors – Attempt to warm/support the patient, make them comfortable, provide pain relief etc.


  • Resulting from power interference hence ECG should be set to standard frequency or 0.05 – 40Hz in event of 60Hz pickups.


electrical interference

This is electromagnetic interference, usually from a nearby AC power source. The spikes are appearing 50 times per second, or 50 Hertz (Hz), which is the frequency at which our AC power network delivers 220-240 volts countrywide. Some known sources of this interference are:

  • Electric air-beds (as commonly found in care facilities and hospitals)
  • Pool pumps
  • Electric blankets
  • Poorly insulated electrical appliances
  • High-voltage overhead wires
  • Hair dryers
  • AC-DC adapters (e.g. laptops, etc.)

There are a few easy workarounds (and one hard one) to mitigate this reading if you ever come across it on scene;

  • Move the patient to another location
  • Shut-off the source of the interference (it may take some time to find it, unless you switch off mains power!).
  • Replace the electrodes (dried out electrodes increase the susceptibility to outside interference)

Fun fact; if your patient’s heart was beating to the rhythm presented here, it would sound like this (this is a 50 Hz square waveform, which is the most audible waveform at this frequency).:


Neuromodulation Artefact:
  • Implantable devices controlling many bodily functions and has a frequency – Can be turned off by a physician with a key.


Echo Distortion:
  • Associated with Transcutaneous Pacing (TCP) causes pseudo-QRS complex after pacing spike. Example B
ECG Lead Groups & Placements
Standard (left side) ECG
  • Placement on the left-side in most cases.
Proper 12-lead placement for left side of chest
V1 4th intercostal space, left of sternum
V2 4th intercostal space, right of sternum
V3 In between V2 and V4
V4 5th intercostal space, left midclavicular line
V5 In between V4 and V6
V6 5th intercostal space, mid-axillary line (directly under the midpoint of the armpit)
V4R 5th intercostal space, right mid-clavicular line

Electrocardiography - 12l placement

Right Side
  • Provides a view of the right ventricular walls which may be blocked indicated if;
    • ST Elevation in Lead II & III, and aVF
    • ST Elevation in V1
    • V2 ST Elevation is 50% greater than ST depression magnitude in aVF
  • Use in cases of dextrocardia
Proper 12-lead placement for right side of chest
V1 4th intercostal space, left of sternum
V2 4th intercostal space, right of sternum
V3R In between V2 and V4
V4R 5th intercostal space, right midclavicular line
V5R In between V4 and V6
V6R 5th intercostal space, mid-axillary line (directly under the midpoint of the armpit)


Posterior View of Left Ventricles
  • Use V1 & V2 to look for opposite myocardial infarction changes that is reflected from posterior surface of the heart.
Lead Views of the Heart

The 12-lead ECG displays 12 views of the electrical activity of the heart produced by 10 electrodes in specific positions.

  • These ECG analyse the same electrical events at a given time, however from different angles
  • Many views are needed for complete assessment and/or to confirm findings.
Anatomical Planes of the ECG:

The leads are positioned in certain angles to detect an electrical activity (vector).

  • The chest leads provide axial (horizontal / transverse) view of heart
    • Show anterior (front) and posterior (back) view
    • The exploring leads is located anteriorly and reference inside chest
  • The limb leads show the coronal (frontal) view of the heart
    • o Show superior (upper) and inferior (lower) view
Chest (Precordial) Leads

Like a camera looking into the heart, the view is generated from positive (electrode) to negative (centre of heart) – Due to the position of the left ventricles lying anteriorly, the right ventricle and posterior of left ventricles cannot be seen.

V1 – V2Septal Leads
V3 – V4Anterior Leads
V5 – V6Lateral Leads
Limb Leads

Electrodes from limbs provide many views of the heart – Looking from positive to negative – Legs are neutral (act as the ‘earth’).

  • Augmented leads (aVR, aVF, & aVL) views heart further away than other leads
  • Augmented leads are smaller due to distance

Einthoven’s triangle displays the electrical relationship (polarity) of the limb leads and how they produce these views.

  • aVR is a lead that can be converted to -aVR (switching view positions) – Converted on ECG device
  • Assists with more accurate inferior & lateral diagnosis of ischemia / infarction



Summary of the ECG Leads & Views
LeadsLead TypeView of HeartVessel/s & Feature/s Seen
I Lateral (side)
Lateral wall of left ventricleLeft Cx Artery
II Inferior (bottom)
Inferior Walls of Left Ventricles90% RCA &/or 10% Cx Artery
III Inferior (bottom)Inferior Walls of Left Ventricles90% RCA &/or 10% Cx Artery
aVF Inferior (bottom)Inferior Walls of Left Ventricles90% RCA &/or 10% Cx Artery
aVL Lateral (side)Lateral wall of left ventricleLeft Cx Artery
aVR Lateral
Basal part of SeptumRight Ventricular outflow tract
V1 Septal (front)
Ventricular Septum (interventricular)Proximal LAD Artery
V2 Septal (front)Ventricular Septum (interventricular)Proximal LAD Artery
V3 Anterior (front)
Left Ventricle Anterior WallsLAD Artery
V4 Anterior (front)Left Ventricle Anterior WallsLAD Artery
V5 Lateral (side)Lateral wall of left ventriclesLeft Cx Artery &/or Distal LAD &/or RCA
V6 Lateral (side)Lateral wall of left ventriclesCx Artery &/or Diagonal of LAD
V7* PosterolateralLeft Ventricle Posterior / InferobasalLeft Cx &/or RCA
V8* PosterolateralLeft Ventricle Posterior / InferobasalLeft Cx &/or RCA
V9* PosterolateralLeft Ventricle Posterior / InferobasalLeft Cx &/or RCA

* ECG electrodes must be modified

Visualisation of the Heart and the corresponding ECG (Cabrera System)

ECG Strips

The left side of the ECG strip displays bipolar limb leads (Lead I, II, & III) and augmented unipolar limb leads (aVR, aVL, & aVF) whereas the right side of the strip displays unipolar chest leads (V1 – V6).

  • A rhythm strip may produce bottom of ECG strip (usually Lead II) for rhythm analysis.
  • Cabrera System is used as a standardise layout of the ECG showing systematic view of the heart
  • We mainly use the 25mm/s paper on the 12-lead ECG paper.
Reading an ECG Strip


 Every Large Box Every Small Box
Description Shown by heavy lines – Contain five (5) small boxes.Shown by thin lines
Time / Rate (Horizontal line) 5mm → 0.2s (200ms)1mm → 0.04s (40ms)
Amplitude / Voltage (Vertical line) 5mm → 0.5mV1mm → 0.01mV


  • 1 seconds = five (5) large boxes (300 large boxes = 60s)
  • 1 mV = two (2) large boxes (20 large boxes = 1mV)

Finding Heart Rate (HR) on an ECG strip

Compared with the ECG produced heart rate & SpO2 oximeter – If in doubt, use manual pulse palpation.

  • Count total small boxes between two R-waves then divide by 1500 OR;
  • Count total large boxes between two R-waves then divide by 300 OR;
  • For Irregular patterns – Count total number of R-waves within 6 seconds (30 large squares) then multiply by 10

R-waves total in 30 large squares (6s) MULTIPLY BY 10


College of Pre-Hospital Care. (2015). 12-Lead ECG Analysis: Self-Directed Learning Package. Version 3. St John Ambulance Ltd.  

Curtis, K., & Ramsden, C. (2016). Emergency and trauma care for Nurses and Paramedics (2nd ed.). Elsevier Australia.

DeLaune, S. C., Ladner, P. K., McTier, L., Tollefson, J., & Lawrence, J. (2016). Australian and New Zealand fundamentals of nursing (1st ed.). Cengage Learning Australia Pty Limited.

ECG & ECHO Learning. (2020). Clinical ECG Interpretation. https://ecgwaves.com/topic/ecg-normal-p-wave-qrs-complex-st-segment-t-wave-j-point/

Life in the Fast Lane. (2020). ECG Library. https://litfl.com/ecg-library/

St John WA Ltd. (2017). Electrocardiography (ECG). Clinical Resources. https://clinical.stjohnwa.com.au/clinical-skills/assessment/vital-signs/electrocardiography-(ecg)

WikiEM. 2020. The Global Emergency Medicine Wiki. https://www.wikem.org

Page contributors:

60825Thanh Bui, AP60825
Event Medic, Emergency Medical Technician &
Volunteer Development Officer



Andrew Moffat, AP16790
Volunteer Training Manager & Volunteer Development Officer

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