Ecg monitoring and lead systems

Clinical approach in theatre/ICU

Confirm ECG is providing useful clinical information (rate, rhythm, conduction, ischaemia) and is safe (electrical safety, defib compatibility).
- Choose lead(s) based on the clinical question: arrhythmia detection vs ischaemia monitoring vs pacing/defib readiness.
Set up: correct electrode placement, good skin contact, appropriate filter settings, correct gain/sweep speed, alarms on and appropriate.
- If artefact: check patient movement/shivering/diathermy, lead integrity, electrode gel drying, poor contact, mains interference.
Interpretation: always correlate ECG with pulse/arterial trace/pleth and patient condition.
- If ECG shows VF/VT/asystole: confirm with pulse/arterial line; treat patient not monitor.

Why monitor ECG?

Continuous assessment of heart rate, rhythm, conduction abnormalities, and indirect evidence of myocardial ischaemia.
Part of AAGBI/Association standards for basic monitoring during anaesthesia (continuous ECG).

Basic principles: what the ECG measures

ECG records potential differences between electrodes caused by cardiac depolarisation/repolarisation (a vector quantity).
A lead is a specific viewpoint of cardiac electrical activity defined by a pair (or combination) of electrodes; it is not the same as an electrode.
Standard calibration: 25 mm/s paper speed; 10 mm = 1 mV (monitor equivalents: sweep speed and gain).

Electrodes and skin interface

Disposable Ag/AgCl electrodes reduce polarisation artefact and provide stable half-cell potential.
Skin preparation improves signal: clean/dry, shave hair, abrade lightly if needed; place on bony/low-motion areas where possible.
Common electrode issues: dried gel (high impedance), sweat (bridging), poor adhesion, incorrect placement, broken lead wires.

Lead systems: limb leads, augmented leads, chest leads

Bipolar limb leads (Einthoven): Lead I = LA(+) − RA(−); Lead II = LL(+) − RA(−); Lead III = LL(+) − LA(−).
- Einthoven’s law: Lead II = Lead I + Lead III (instantaneous potentials).
Augmented unipolar limb leads (Goldberger): aVR, aVL, aVF use a single positive limb electrode referenced to the average of the other two limbs.
- aVR looks from right shoulder; aVL from left shoulder; aVF from foot (inferior).
Chest (precordial) leads V1–V6 are unipolar leads referenced to the Wilson central terminal (average of RA, LA, LL via resistors).
- V1: 4th ICS right sternal edge; V2: 4th ICS left sternal edge; V4: 5th ICS mid-clavicular; V3 between V2–V4; V5 anterior axillary line; V6 mid-axillary line (same horizontal level as V4).

3-lead vs 5-lead monitoring (theatre monitors)

3-lead system typically provides Leads I, II, III (derived) for rhythm monitoring; fewer electrodes, less diagnostic for ischaemia.
5-lead system adds a chest electrode (V) enabling a selected precordial lead (commonly V1 or V5) plus limb leads; improves arrhythmia/ischaemia detection.
Typical 5-lead placement: RA (white) right infraclavicular; LA (black) left infraclavicular; RL (green) right lower costal margin; LL (red) left lower costal margin; V (brown) at chosen chest position (e.g., V1 or V5).
- Colour conventions vary by manufacturer/region; always check the monitor/lead set labelling.

Which lead to choose and why (anaesthetic practice)

Arrhythmia detection: Lead II often best for P waves (axis aligns with atrial depolarisation), useful for SVT/AF/heart block recognition.
Ventricular arrhythmias and bundle branch block patterns: V1 is useful (right ventricular/anterior septal view; helps distinguish VT vs SVT with aberrancy in some patterns).
Ischaemia monitoring: V5 is most sensitive single lead for detecting ST depression due to LV ischaemia; combining Lead II + V5 improves sensitivity further.
- Inferior ischaemia: leads II, III, aVF; anterior/septal: V1–V4; lateral: I, aVL, V5–V6.
Pacemaker patients: choose a lead where pacing spikes are visible (often Lead II or V1); ensure monitor has appropriate filtering to avoid spike attenuation.

ST-segment monitoring: practicalities and limitations

ST monitoring requires stable baseline: correct electrode placement, minimal artefact, appropriate filter settings, and a defined isoelectric reference (typically PR segment).
High-pass filtering can distort ST segments (baseline wander removal may alter ST level); muscle/EMG filtering can also alter morphology.
Anaesthetic factors affecting ST: tachycardia, hypotension, anaemia, hypoxia, hyper/hypocapnia, electrolyte disturbance, hypothermia, LVH, bundle branch block, digoxin effect.

Artefact and interference (common FRCA topic)

Diathermy: produces high-frequency interference; modern monitors use filters and may blank the trace; ensure ECG leads and diathermy plate positioned to minimise current path across heart.
Mains interference (50 Hz UK): poor earthing, broken shielding, high electrode impedance; appears as regular sinusoidal noise.
Movement/shivering: irregular baseline and muscle artefact; treat cause (warming, analgesia) and improve electrode contact.
Electrode reversal: produces abnormal axis/inverted complexes; recognise patterns (e.g., RA/LA reversal flips Lead I and swaps aVR/aVL appearances).

Electrical safety and defibrillation

ECG patient connections are isolated (floating) to reduce microshock risk; leakage currents are limited by design and standards.
Defibrillation: use defib-proof ECG leads; ensure pads/paddles not placed over ECG electrodes; transient saturation/trace loss is expected.
Diathermy and invasive lines: avoid creating a pathway through the heart; place return electrode so current path is away from ECG electrodes and implanted devices.

Special situations

Dextrocardia: precordial lead placement may need mirroring to right chest for diagnostic interpretation; limb leads may show right-axis patterns.
Prone/lateral positioning: limb electrodes can be moved proximally (torso placement) to reduce artefact; note this may alter axis/amplitude slightly.
Paediatrics/neonates: higher heart rates; smaller signal amplitude; ensure appropriate electrode size and alarm limits.

Describe the difference between an electrode and a lead. How can a 3-electrode system display 3 leads?

A lead is a viewpoint (a voltage difference or derived combination); an electrode is the physical contact with the patient.

Electrode: conductive interface (usually Ag/AgCl) attached to skin; converts ionic currents in tissue to electronic current in the wire.
Lead: a defined measurement of potential difference between two points (bipolar) or one point vs a reference (unipolar/augmented).
With RA, LA, LL electrodes you can form Leads I (LA-RA), II (LL-RA), III (LL-LA). The right leg electrode (if present) is usually a driven reference/earth to reduce noise, not part of the lead calculation.

Define Einthoven’s triangle and Einthoven’s law. How is this relevant to ECG monitoring?

Einthoven’s triangle conceptualises limb leads around the heart; Einthoven’s law relates the voltages.

Einthoven’s triangle: RA, LA, LL electrodes form an approximate equilateral triangle with the heart at the centre; limb leads view cardiac vectors in the frontal plane.
Einthoven’s law: Lead II = Lead I + Lead III (instantaneous).
Relevance: helps check lead integrity/placement and understand axis changes with electrode misplacement or limb lead reversal.

Explain how augmented limb leads (aVR, aVL, aVF) are generated and what they represent.

Augmented leads are unipolar limb leads referenced to the average of the other two limb electrodes.

aVR: positive electrode on RA; reference is average of LA and LL.
aVL: positive on LA; reference is average of RA and LL.
aVF: positive on LL (foot); reference is average of RA and LA; provides an inferior view.
“Augmented” because the signal amplitude is increased compared with the original unipolar limb leads by altering the reference.

What is the Wilson central terminal? How does it relate to chest leads (V1–V6)?

The Wilson central terminal is an averaged reference used for unipolar leads.

Formed by connecting RA, LA, LL through equal resistors and taking the average potential as a reference point.
Each chest lead V1–V6 measures the potential at the chest electrode relative to the Wilson central terminal, giving transverse plane views.

Where do you place V1 and V5 for intra-operative monitoring, and what are they useful for?

Correct placement matters for morphology and ST monitoring.

V1 placement: 4th intercostal space, right sternal edge.
V5 placement: 5th intercostal space, left anterior axillary line (same horizontal level as V4).
V1: helpful for ventricular arrhythmias and bundle branch block patterns; V5: sensitive for LV ischaemia (ST depression).

Which ECG lead is best for detecting P waves and why? How does this help in theatre?

Lead selection is a common viva area.

Lead II often shows prominent P waves because the atrial depolarisation vector is roughly aligned with Lead II axis (right arm to left leg).
Helps distinguish sinus rhythm from AF/flutter, identify AV block, and interpret narrow-complex tachycardias.

You are asked to set up ECG monitoring for ischaemia detection in a high-risk patient. What lead(s) would you choose and what else would you do to improve sensitivity/specificity?

Single-lead ST monitoring can miss ischaemia; optimise both lead choice and signal quality.

Choose V5 for highest sensitivity as a single lead; consider monitoring Lead II + V5 if available.
Ensure correct electrode placement and stable baseline; minimise artefact (warming to reduce shivering, secure leads).
Use appropriate filter settings for ST analysis (avoid excessive high-pass filtering that distorts ST).
Set appropriate alarms and trend ST changes; correlate with haemodynamics, symptoms (if awake), and consider troponin/12-lead ECG if concern persists.

What are the common causes of ECG artefact in theatre and how would you systematically troubleshoot a noisy trace?

Troubleshooting is often examined as a structured approach.

Patient factors: shivering, movement, tremor, poor perfusion/sweat causing poor adhesion or bridging.
Equipment factors: dried gel/high impedance, loose/broken lead wires, poor cable shielding, incorrect filter settings, mains interference (50 Hz).
Surgical factors: diathermy interference; ensure proper return plate placement and cable routing away from diathermy leads.
Systematic steps: confirm pulse/pleth; check electrode adhesion/position; replace electrodes; change lead; reduce movement/shivering; inspect cables; adjust filters cautiously; consider moving electrodes to torso.

Describe the appearance and consequences of right arm–left arm (RA/LA) lead reversal. How might you recognise it quickly?

Lead reversal is a classic exam scenario.

RA/LA reversal inverts Lead I (because its polarity is reversed).
aVR and aVL effectively swap appearances; axis may appear markedly abnormal (apparent right axis deviation).
Quick recognition: unexpected negative P/QRS/T in Lead I with otherwise plausible rhythm; check electrode labels/colours and patient-side placement.

How can ECG filter settings affect the displayed waveform, particularly the ST segment? Why does this matter?

Filters improve readability but can distort clinically important features.

High-pass filters reduce baseline wander but can shift the ST segment and distort low-frequency components.
Low-pass (EMG) filters reduce muscle noise but can attenuate sharp features (e.g., pacing spikes) and alter QRS morphology.
Clinical impact: false ST elevation/depression or missed ischaemia; misinterpretation of conduction abnormalities or pacemaker function.

During diathermy use, the ECG trace becomes unreadable. What are the risks and what practical steps can you take?

Differentiate artefact from true arrhythmia and reduce risk of current pathways.

Risks: missing true arrhythmia/ischaemia; inappropriate treatment based on artefact; potential for current pathway across heart (especially with poorly positioned return plate).
Steps: confirm pulse via pleth/arterial line; ensure diathermy return electrode correctly placed with good contact; route ECG cables away from diathermy cables; use short bursts/lowest effective power; consider bipolar diathermy where appropriate.

Explain how a 5-lead system can provide a ‘V’ lead. What are the limitations compared with a full 12-lead ECG?

Theatre monitoring is a compromise between practicality and diagnostic completeness.

The chest electrode provides a selected precordial lead (e.g., V1 or V5) referenced to the monitor’s central terminal (derived from limb electrodes).
Limitations: only one precordial viewpoint at a time; fewer territories monitored; less reliable for localisation of infarction; baseline/filters/positioning may reduce diagnostic accuracy.
If concern: obtain a diagnostic 12-lead ECG (ideally with standard electrode positions) and correlate clinically.