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

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)

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

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)

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 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.

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

• Positive: aVL + aVF + -aVR + Lead I + V4 – V6
• Negative: -aVR

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

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

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)

• 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

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

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

• Usually in sweaty/wet or hairy patients – requires proper skin preparation by shaving &/or drying skin accordingly.

• Slow undulating baseline on ECG caused by patient or ambulance movement including breathing resulting in an ECG that drifts off.
  

• 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.

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).:

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

• Associated with Transcutaneous Pacing (TCP) causes pseudo-QRS complex after pacing spike. Example B

• Use V1 & V2 to look for opposite myocardial infarction changes that is reflected from posterior surface 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.

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.

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

^* ECG electrodes must be modified

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

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.
  

Therefore:

• 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