ECG Monitoring in Anaesthesia

Quick Answer

Electrocardiography (ECG) monitors cardiac electrical activity through surface electrodes that detect voltage changes generated by myocardial depolarization and repolarization. The signal represents the algebraic sum of action potentials from millions of cardiac cells, producing characteristic P-QRS-T waveforms.

Critical Physics Concepts:

• Signal amplitude: Typically 0.5-2.0 mV at skin surface
• Frequency content: 0.05-150 Hz (diagnostic), 0.5-40 Hz (monitoring mode)
• CMRR (Common Mode Rejection Ratio): Must exceed 80 dB to eliminate interference
• Input impedance: >5 MΩ to prevent signal loading

Key Clinical Points:

• Lead II provides optimal P-wave visibility for rhythm monitoring
• V5 or CM5 configuration maximizes ST-segment ischaemia detection (sensitivity ~75-90%)
• Combined II + V5 monitoring detects >95% of intraoperative ischaemic episodes
• Monitoring filters (0.5-40 Hz) reduce artefact but may attenuate ST changes; use diagnostic mode (0.05-150 Hz) for accurate ST analysis
• Electrode impedance should be <5 kΩ; higher impedance increases noise susceptibility

Clinical Applications: Arrhythmia detection, ischaemia monitoring, pacemaker function, electrolyte disturbances, drug effects, and temperature monitoring via R-wave amplitude changes.

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Physics Principles

Cardiac Electrical Activity

The heart functions as an electrical generator, producing voltage fields that propagate through body tissues to the skin surface. Cardiac action potentials arise from transmembrane ion fluxes, primarily sodium, potassium, and calcium ions moving through voltage-gated channels. The resting membrane potential of ventricular myocytes is approximately -90 mV, becoming positive (+20 to +30 mV) during depolarization. [1,2]

Sequence of Cardiac Depolarization:

1. Sinoatrial (SA) node: Spontaneous depolarization at 60-100 bpm
2. Atrial myocardium: Wave spreads across both atria (P wave)
3. Atrioventricular (AV) node: Conduction delay (~120 ms) allows atrial contraction
4. Bundle of His and bundle branches: Rapid conduction to ventricles
5. Purkinje fibres: Distribute impulse throughout ventricular myocardium
6. Ventricular myocardium: Depolarization produces QRS complex (endocardium to epicardium)
7. Repolarization: Epicardium to endocardium, producing T wave

The body acts as a volume conductor, allowing electrical signals to reach the skin surface. However, signal amplitude is attenuated from millivolts at the myocardium to microvolts-millivolts at the skin (typically 0.5-2.0 mV for QRS complex). The voltage detected depends on the position and orientation of electrodes relative to the electrical axis of the heart. [3]

Dipole Theory: Cardiac electrical activity can be modelled as a moving dipole (a pair of equal and opposite charges separated by a small distance). The magnitude and direction of this dipole change continuously during the cardiac cycle. At any instant, the voltage measured between two points on the body surface is proportional to:

• The magnitude of the dipole (proportional to muscle mass being depolarized)
• The cosine of the angle between the dipole axis and the line joining the electrodes

This explains why different lead configurations produce different waveform morphologies from the same cardiac event. [4]

Lead Systems

Standard Limb Leads (Bipolar): Bipolar leads measure the potential difference between two electrodes. Willem Einthoven established the three standard limb leads in 1903:

Einthoven's Triangle: The three standard limb leads form an equilateral triangle with the heart at its centre. This geometric relationship yields Einthoven's Law:

\text{Lead II} = \text{Lead I} + \text{Lead III}

At any instant, the algebraic sum of the voltages in leads I and III equals the voltage in lead II. This relationship allows cardiac monitors to mathematically derive one lead from the other two, useful for electrode fault detection. [5]

Augmented Unipolar Limb Leads: Goldberger's augmented leads (1942) measure potential at one limb relative to the average of the other two limbs:

The term "augmented" reflects that these leads produce 50% greater amplitude than the original Wilson unipolar leads by excluding the exploring electrode from the reference. [6]

Precordial Leads (Unipolar): Wilson's precordial leads (V1-V6) use chest electrodes referenced to Wilson's central terminal (the average of RA, LA, and LL potentials):

V1-V2 view the interventricular septum and right ventricle, V3-V4 the anterior wall, and V5-V6 the lateral wall. This allows localization of ischaemia and infarction. [7]

Electrode Placement Systems

3-Lead Monitoring: The simplest configuration uses three electrodes (RA, LA, LL) plus a reference/ground electrode. This provides leads I, II, and III through electronic switching. Lead II is preferred for routine monitoring because:

• The lead axis (+60°) aligns with the normal cardiac electrical axis (0° to +90°)
• P waves are most prominent, facilitating rhythm analysis
• QRS complexes are typically upright and clearly defined [8]

5-Lead Monitoring: Adds a chest electrode (V) and right leg reference electrode to the 3-lead system. The operator can select any of leads I, II, III, aVR, aVL, aVF, or a single V lead position. The CM5 configuration (chest electrode at V5 position, negative electrode at manubrium) provides excellent sensitivity for detecting lateral wall ischaemia. [9]

12-Lead Monitoring: Full diagnostic monitoring requires 10 electrodes (4 limb, 6 precordial) producing 12 leads. Modern operating room monitors can derive continuous 12-lead ECGs from reduced electrode sets using mathematical reconstruction, though with some loss of sensitivity compared to conventional placement. [10]

Signal Amplification

ECG Signal Characteristics:

• Amplitude: 0.5-2.0 mV (QRS), 0.1-0.5 mV (P wave, T wave)
• Frequency content: DC to ~100 Hz (majority <40 Hz)
• High-frequency noise sources: Muscle artefact (>25 Hz), mains interference (50/60 Hz)
• Low-frequency noise: Baseline wander from respiration (<0.5 Hz), electrode motion

Differential Amplification: ECG amplifiers use differential amplification to reject common-mode signals (signals appearing equally at both inputs). The Common Mode Rejection Ratio (CMRR) quantifies this capability:

\text{CMRR (dB)} = 20 \log_{10}\left(\frac{A_d}{A_{cm}}\right)

Where:

• Ad = differential gain (for the desired ECG signal)
• Acm = common-mode gain (for interference)

Clinical ECG systems require CMRR >80 dB (10,000:1 rejection). Modern amplifiers achieve 100-120 dB. [11]

Input Impedance: High input impedance (>5 MΩ) is essential to prevent signal loading. If amplifier input impedance is too low relative to electrode-skin impedance, the signal is attenuated. High input impedance also reduces the effect of impedance mismatch between electrodes, which would otherwise convert common-mode interference to differential-mode (appearing as noise). [12]

Filtering

Filter Types and Purposes:

Monitoring Mode (0.5-40 Hz):

• Advantages: Reduced baseline wander and muscle artefact
• Disadvantages: May attenuate high-frequency QRS components, can distort ST segment
• Use: Routine rhythm monitoring, arrhythmia detection

Diagnostic Mode (0.05-150 Hz):

• Advantages: Preserves full waveform fidelity, accurate ST-segment analysis
• Disadvantages: More susceptible to baseline wander and artefact
• Use: Ischaemia monitoring, 12-lead interpretation, pacemaker spike detection [13,14]

ST Segment Considerations: The ST segment contains frequencies predominantly <1 Hz. The monitoring mode high-pass filter at 0.5 Hz can cause:

• ST depression to appear as ST elevation
• True ST changes to be attenuated or masked
• Phase distortion of the waveform

For accurate ST-segment monitoring during anaesthesia, diagnostic mode (0.05 Hz high-pass) should be used despite increased artefact susceptibility. Alternatively, some monitors offer an "ST mode" with intermediate filtering optimized for ischaemia detection. [15]

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Waveform Components

P Wave

Origin: Atrial depolarization, beginning at the SA node and spreading across both atria.

Normal Characteristics:

• Duration: <120 ms (3 small squares at 25 mm/s)
• Amplitude: <2.5 mm (0.25 mV) in limb leads, <2.0 mm in V1
• Morphology: Upright in leads I, II, aVF; inverted in aVR; biphasic in V1
• Axis: +45° to +60°

Abnormalities:

PR Interval

Definition: Time from onset of P wave to onset of QRS complex.

Normal Range: 120-200 ms (3-5 small squares)

Physiological Basis: Represents conduction through atria, AV node, bundle of His, bundle branches, and Purkinje system. The majority of the delay occurs at the AV node, allowing atrial contraction to complete before ventricular contraction begins.

Abnormalities:

QRS Complex

Origin: Ventricular depolarization, beginning at the interventricular septum and spreading through the ventricular myocardium.

Normal Characteristics:

• Duration: <120 ms (3 small squares)
• Amplitude: Variable by lead; sum of S in V1 + R in V5-V6 <35 mm
• Axis: -30° to +90° (normal), mean ~+60°

QRS Nomenclature:

• Q wave: First negative deflection before R wave
• R wave: First positive deflection
• S wave: Negative deflection after R wave
• QS complex: Entirely negative (no R wave)
• R' wave: Second positive deflection
• Lowercase/uppercase: Reflects relative amplitude (e.g., qRs vs QRs)

Abnormalities:

ST Segment

Definition: Isoelectric period between end of QRS (J point) and onset of T wave.

Normal Characteristics:

• Should be at baseline (isoelectric)
• Minor elevation (<1 mm) in V1-V3 may be normal in young patients
• J-point elevation with upward sloping ST segment (early repolarization) is benign

Clinical Significance: The ST segment is the primary target for intraoperative ischaemia monitoring. Myocardial ischaemia causes:

• ST depression: Subendocardial ischaemia (most common intraoperatively)
• ST elevation: Transmural ischaemia, acute MI, Prinzmetal angina

Monitoring Thresholds:

• ST depression >1 mm at 60 ms after J point: Significant
• ST elevation >2 mm: Highly significant, consider acute coronary occlusion

Causes of ST Changes (Non-Ischaemic):

T Wave

Origin: Ventricular repolarization.

Normal Characteristics:

• Direction: Same as QRS in most leads (concordant)
• Amplitude: <5 mm in limb leads, <10 mm in precordial leads
• Shape: Asymmetric (slower upstroke, faster downstroke)

Abnormalities:

QT Interval

Definition: Time from onset of QRS to end of T wave, representing total ventricular electrical activity.

Normal Range: 350-440 ms (varies with heart rate)

Rate Correction (Bazett's Formula):

QTc = \frac{QT}{\sqrt{RR}}

Normal QTc: <440 ms (males), <460 ms (females)

Clinical Significance: Prolonged QT increases risk of torsades de pointes, a polymorphic ventricular tachycardia. Many anaesthetic drugs and conditions affect QT interval.

Causes of QT Prolongation:

• Drugs: Droperidol, ondansetron, sevoflurane, halothane, methadone, antibiotics (macrolides, fluoroquinolones)
• Electrolytes: Hypokalaemia, hypomagnesaemia, hypocalcaemia
• Congenital: Long QT syndrome (LQTS)
• Other: Hypothermia, hypothyroidism, intracranial pathology [16,17]

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Artefacts and Troubleshooting

Electrical Interference

Mains Frequency Interference (50/60 Hz): Electromagnetic fields from power lines, equipment, and lighting induce currents in ECG leads acting as antennas. Appears as regular, fine oscillations superimposed on the trace.

Causes:

• Poor electrode contact (high impedance)
• Broken or poorly shielded cables
• Proximity to electrical equipment
• Patient not grounded properly

Solutions:

1. Ensure electrode impedance <5 kΩ (proper skin preparation)
2. Keep cables away from power cords
3. Engage 50/60 Hz notch filter
4. Check cable integrity and shielding
5. Verify reference electrode connection [18]

Motion Artefact

Appearance: Irregular, low-frequency baseline wander; may mimic arrhythmias.

Causes:

• Patient movement (shivering, tremor)
• Respiratory motion
• Electrode cable movement
• Surgeon or staff contact with patient/cables

Solutions:

1. Secure electrodes firmly with gel-type electrodes
2. Route cables away from surgical field
3. Use wireless ECG systems when possible
4. Position electrodes on flat, stable areas (avoiding muscles)
5. Apply electrodes to proximal locations (trunk vs limbs) [19]

Electrosurgery (Diathermy) Interference

Mechanism: Electrosurgical units generate high-frequency (300 kHz-3 MHz) alternating current. This produces:

• High-frequency artefact during activation
• Potential ECG monitor shutdown or false alarms
• Rare risk of burns at monitoring electrodes

Solutions:

1. Position return electrode close to surgical site
2. Keep ECG electrodes away from surgical field (ideally >15 cm from return electrode)
3. Use high-frequency filters on monitor
4. Accept temporary loss of trace during diathermy activation
5. Consider alternative monitoring during prolonged diathermy use [20]

Pacemaker Considerations

Pacemaker Spike Recognition:

• Bipolar pacing produces small spikes (often <2 mV) that may not be visible on standard monitoring
• Unipolar pacing produces larger spikes (>10 mV) easily visible
• Ensure monitor is set to detect pacemaker spikes (some monitors have dedicated mode)

Common Issues:

Electrocautery Interference: Electrosurgery can cause:

• Inhibition of demand pacemakers (treating diathermy as cardiac activity)
• Inappropriate mode switching in ICDs
• Reprogramming (rare with modern devices)

Management: Consider magnet application, reprogram to asynchronous mode, use bipolar diathermy, limit diathermy bursts to <1 second. [21,22]

Electrode Problems

High Electrode Impedance:

Electrode Polarization: DC potentials develop at the electrode-electrolyte interface (half-cell potential). Silver-silver chloride (Ag/AgCl) electrodes minimize this effect and are standard for ECG monitoring.

Lead Reversal:

Lead reversal should be suspected when P and QRS morphology appear unusual or inverted in unexpected leads. [23]

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Clinical Applications

Ischaemia Detection

Lead Selection for Ischaemia: Different leads detect ischaemia in different coronary territories:

CM5 Configuration: The modified chest lead CM5 places:

• Negative electrode: Manubrium sterni
• Positive electrode: V5 position (5th intercostal space, anterior axillary line)
• Ground: Right shoulder

CM5 provides ~75-80% sensitivity for detecting ischaemic ST changes, particularly in the LAD and circumflex territories. [24]

Combined II + V5 Monitoring: Studies show that simultaneous monitoring of leads II and V5 detects >95% of intraoperative ischaemic episodes:

• Lead II: Arrhythmias, inferior ischaemia, P-wave visibility
• Lead V5: Anterior and lateral ischaemia

For patients at high cardiac risk, consider continuous 12-lead ST-segment trending when available. [25,26]

Arrhythmia Recognition

Systematic Approach:

1. Rate: Count R waves in 10 large squares (×30) or measure RR interval
2. Rhythm: Regular or irregular? If irregular, pattern?
3. P waves: Present? Morphology? Relationship to QRS?
4. QRS: Width? Morphology consistent?
5. ST/T waves: Acute changes?

Common Perioperative Arrhythmias:

V5 Lead Importance

The V5 lead position is particularly valuable for several reasons:

1. Anatomical Alignment: V5 lies over the lateral left ventricular wall, perpendicular to the dominant direction of ventricular depolarization, producing high amplitude QRS complexes.

2. Ischaemia Detection: The lateral wall supplied by the left circumflex and LAD is prone to ischaemia. ST depression in V5 correlates well with regional wall motion abnormalities on echocardiography.

3. Left Ventricular Hypertrophy: LVH voltage criteria often rely on V5 amplitude (e.g., Cornell criteria: R in aVL + S in V3 >28 mm in men).

4. Practical Considerations: V5 position is easily accessible, can be monitored with 5-lead systems, and provides both rhythm and ischaemia information.

Studies by London et al. demonstrated that V5 alone detected 75% of ischaemic episodes, while addition of lead II increased sensitivity to 80%, and combinations including V4 approached 90-95% sensitivity. [27,28]

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Indigenous Health Considerations

Remote and Rural Monitoring Challenges

Aboriginal and Torres Strait Islander peoples experience cardiovascular disease rates 2-3 times higher than non-Indigenous Australians, with higher prevalence of ischaemic heart disease, rheumatic heart disease, and cardiomyopathy. Māori populations similarly demonstrate elevated cardiovascular risk. This increased disease burden makes reliable ECG monitoring particularly critical during anaesthesia for these communities.

Remote health facilities often face equipment and infrastructure challenges. Limited biomedical engineering support means equipment maintenance and calibration may be delayed or inadequate. Power supply instability in some remote communities can affect monitor performance and create electrical noise. Single-use electrodes may be in limited supply, and staff may lack training in troubleshooting ECG artefacts.

Telemedicine and Retrieval

The Royal Flying Doctor Service (RFDS) and similar aeromedical services provide critical links between remote communities and tertiary care. ECG telemetry enables specialist consultation during patient retrieval, but transmission quality varies with communication infrastructure. When ECG monitoring fails or produces artefact during retrieval, clinical assessment and alternative monitoring (pulse oximetry, arterial line) become essential.

Telemedicine consultations can assist remote practitioners with ECG interpretation, particularly for complex arrhythmias or ischaemia detection in high-risk patients. However, image quality and transmission delays may limit utility for real-time monitoring.

Cultural and Communication Considerations

Effective monitoring requires patient cooperation for electrode placement and cable management. Cultural considerations include:

• Aboriginal Hospital Liaison Officers or Indigenous Health Workers should be involved when possible to facilitate communication
• Family presence during procedures may be culturally important and should be accommodated when safe
• Explanations of monitoring equipment may require interpreters or visual aids
• Some patients may have concerns about electrical equipment; clear explanation of safety is important
• Body positioning for electrode placement should respect cultural sensitivities

Younger remote area nurses and Aboriginal Health Workers increasingly receive training in ECG interpretation and troubleshooting, supporting capacity building in remote communities.

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Equipment Standards and Regulations

Australian/New Zealand Standards

TGA Classification: ECG monitors are Class IIb medical devices under the Therapeutic Goods (Medical Devices) Regulations 2002, requiring conformity assessment and ARTG registration.

Applicable Standards:

Performance Specifications (IEC 60601-2-27):

ANZCA Standards

PS18 - Recommendations on Monitoring During Anaesthesia:

• ECG monitoring is mandatory for all anaesthetics
• Continuous display with audible signal (QRS beep or SpO2 tone) required
• Appropriate lead selection for patient risk profile
• Arrhythmia and ischaemia detection capability

PS55 - Minimum Facilities:

• ECG monitor with clear display
• Arrhythmia alarm functionality
• Defibrillator immediately available when cardiac risk

Electrical Safety

Type CF Classification: ECG monitors must meet Type CF (cardiac floating) electrical safety standards because the electrodes may contact the heart indirectly through the body:

• Maximum patient leakage current: 10 μA (normal), 50 μA (single fault)
• Defibrillation withstand capability required
• Isolation from mains electrical supply

Microshock Risk: Ventricular fibrillation can be induced by as little as 100 μA applied directly to the myocardium. While surface ECG electrodes present minimal direct cardiac contact risk, damaged cables or improper grounding could theoretically create hazardous leakage pathways. Regular equipment testing and cable inspection mitigate this risk.

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Assessment Content

SAQ Practice Question (20 marks)

Question:

You are anaesthetizing a 68-year-old man (85 kg) with known coronary artery disease for elective right hemicolectomy. During the case, you notice significant ST-segment depression on the ECG monitor.

(a) Describe the physics principles of ECG signal acquisition, including signal amplification, differential amplification, and the purpose of common mode rejection. (6 marks)

(b) Explain the different filter settings available on ECG monitors (monitoring mode vs diagnostic mode) and their clinical implications for ST-segment analysis. (6 marks)

(c) Outline the optimal ECG lead configuration for detecting intraoperative myocardial ischaemia and the rationale for lead selection. (4 marks)

(d) List four non-ischaemic causes of ST-segment depression that should be considered in this patient. (4 marks)

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Model Answer:

(a) ECG Signal Acquisition Physics (6 marks)

Signal Characteristics (2 marks):

• ECG amplitude at skin surface: 0.5-2.0 mV (QRS), 0.1-0.5 mV (P/T waves)
• Frequency content: 0.05-150 Hz (majority <40 Hz)
• Signal represents summation of cardiac action potentials detected through volume conductor (body tissues)
• Electrodes detect potential difference between two points

Differential Amplification (2 marks):

• ECG amplifiers measure voltage difference between two electrodes (differential signal)
• Desired signal (ECG) appears at different voltages at each electrode
• Noise (mains interference) appears at same voltage at both electrodes (common mode)
• Differential amplifier amplifies only the difference, rejecting common-mode signals

Common Mode Rejection (2 marks):

• CMRR (Common Mode Rejection Ratio) = 20 log10(Ad/Acm) in dB
• Ad = differential gain, Acm = common-mode gain
• Clinical systems require CMRR >80 dB (10,000:1 rejection)
• Modern monitors achieve 100-120 dB
• Higher CMRR = better noise rejection, cleaner trace
• CMRR reduced by electrode impedance mismatch between electrodes

(b) Filter Settings (6 marks)

Monitoring Mode (0.5-40 Hz) (3 marks):

• High-pass filter: 0.5 Hz (removes baseline wander from respiration/motion)
• Low-pass filter: 40 Hz (removes high-frequency noise, muscle artefact)
• Advantages: Stable baseline, reduced artefact, clear rhythm display
• Disadvantages: May distort ST segment, attenuate pacemaker spikes
• ST segment frequencies predominantly <1 Hz; 0.5 Hz filter may cause:
  • ST depression appearing as elevation
  • True changes attenuated or masked
  • Phase distortion of waveform

Diagnostic Mode (0.05-150 Hz) (3 marks):

• High-pass filter: 0.05 Hz (preserves low-frequency components including ST segment)
• Low-pass filter: 150 Hz (preserves QRS high-frequency components)
• Advantages: Accurate ST-segment analysis, full waveform fidelity
• Disadvantages: More baseline wander, increased susceptibility to artefact
• Required for: Ischaemia monitoring, 12-lead interpretation, QRS morphology analysis

Clinical Implication: For this patient with known CAD and suspected ischaemia, diagnostic mode (0.05-150 Hz) should be used for accurate ST-segment assessment.

(c) Optimal Lead Configuration (4 marks)

Lead Selection (2 marks):

• Combined Lead II + V5 monitoring detects >95% of ischaemic episodes
• Lead II: Optimal for arrhythmia detection (clear P waves), inferior ischaemia
• Lead V5 (CM5): Best single lead for ischaemia detection (~75-80% sensitivity)
  • Monitors LAD and circumflex territories (anterior/lateral walls)
  • Positioned at 5th intercostal space, anterior axillary line

Rationale (2 marks):

• V5 views lateral LV wall perpendicular to depolarization vector
• High-risk patient may benefit from continuous 12-lead ST trending if available

(d) Non-Ischaemic Causes of ST Depression (4 marks)

1. Digoxin therapy: Produces characteristic "reverse tick" ST depression (downsloping from J point)

2. Left ventricular hypertrophy (LVH): "Strain pattern" with ST depression and T-wave inversion in leads overlying hypertrophied muscle (V5-V6)

3. Hypokalaemia: ST depression with flattened T waves and prominent U waves

4. Bundle branch block: Secondary ST-T changes occur opposite to the direction of the QRS complex (appropriate discordance)

5. Tachycardia: Rate-related ST depression (even without ischaemia) due to shortened diastole and subendocardial "demand"

6. Post-prandial/glucose loading: Can cause transient ST changes

7. Hyperventilation/anxiety: May cause ST depression

8. Filter artefact: Monitoring mode (0.5 Hz high-pass) can create apparent ST changes; switch to diagnostic mode to verify

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Viva Scenario (15 marks)

Examiner: "Tell me about ECG monitoring in anaesthesia."

Candidate: "The electrocardiogram monitors cardiac electrical activity by detecting voltage changes at the skin surface produced by myocardial depolarization and repolarization. It's mandatory monitoring for all anaesthetics as specified in ANZCA PS18, providing information about heart rate, rhythm, ischaemia, electrolyte disturbances, and pacemaker function."

Examiner: "Describe the physics of how the ECG signal is acquired."

Candidate: "The heart acts as an electrical generator, with action potentials from millions of cardiac cells summing to produce a voltage field that propagates through body tissues to the skin. The signal amplitude at the skin is typically 0.5 to 2.0 millivolts for the QRS complex.

Electrodes placed on the skin detect these voltage changes. Silver-silver chloride electrodes are standard because they minimize electrode polarization artefacts. The electrodes convert ionic current in the body to electronic current in the cables.

The ECG amplifier is a differential amplifier with high input impedance, typically greater than 5 megaohms, to avoid loading the signal. It measures the voltage difference between two electrodes while rejecting signals common to both inputs."

Examiner: "What is common mode rejection?"

Candidate: "Common mode rejection is the ability of a differential amplifier to reject signals that appear equally at both inputs. Interference such as 50 Hz mains frequency appears at approximately the same voltage at both electrodes and is therefore common mode. The desired ECG signal appears at different voltages at each electrode, so it's differential mode.

The Common Mode Rejection Ratio, or CMRR, quantifies this capability. It's expressed in decibels as 20 times the log of differential gain divided by common mode gain. Clinical systems require CMRR greater than 80 decibels, meaning 10,000 to 1 rejection of common mode signals. Modern monitors achieve 100 to 120 dB.

CMRR is degraded by impedance mismatch between electrodes, which is why proper skin preparation to achieve low, matched electrode impedance is important for good signal quality."

Examiner: "What filter settings are used and why?"

Candidate: "ECG monitors typically offer two filter modes.

Monitoring mode uses a high-pass filter at 0.5 Hz and low-pass at 40 Hz. This removes baseline wander from respiration and high-frequency muscle artefact, producing a stable display ideal for rhythm monitoring. However, the ST segment contains predominantly low-frequency components below 1 Hz, so the 0.5 Hz filter can distort ST changes.

Diagnostic mode uses a high-pass filter at 0.05 Hz and low-pass at 150 Hz. This preserves full waveform fidelity including accurate ST segments, but is more susceptible to baseline wander and artefact.

For patients at risk of ischaemia, diagnostic mode should be used for accurate ST-segment analysis. A notch filter at 50 Hz can remove mains interference without significantly affecting the clinical waveform."

Examiner: "How would you configure the ECG for optimal ischaemia detection?"

Candidate: "For optimal ischaemia detection, I would use a 5-lead system with combined Lead II and V5 monitoring in diagnostic filter mode.

Lead V5 is positioned at the fifth intercostal space, anterior axillary line, which overlies the lateral left ventricular wall. This single lead detects approximately 75 to 80 percent of ischaemic episodes, particularly in LAD and circumflex territories.

Lead II monitors the cardiac rhythm with optimal P-wave visibility and detects inferior ischaemia from right coronary territory.

Combined monitoring of leads II and V5 detects greater than 95 percent of intraoperative ischaemic episodes. For high-risk patients, continuous 12-lead ST-segment trending provides even greater sensitivity if available on the monitor.

I would set the ST alarm thresholds at 1 millimetre depression or 2 millimetres elevation at 60 milliseconds after the J point."

Examiner: "What causes artefact on the ECG and how would you troubleshoot it?"

Candidate: "Common artefact sources include electrical interference, motion artefact, and electrosurgery.

Electrical interference at 50 Hz appears as regular fine oscillations and usually indicates high electrode impedance or poor shielding. I would check electrode contact, reapply electrodes with proper skin preparation, and ensure cables are routed away from power cords.

Motion artefact causes irregular baseline wander and may mimic arrhythmias. Causes include patient movement, shivering, and surgical manipulation. Securing electrodes firmly, positioning them on the trunk rather than limbs, and using gel electrodes improves stability.

Electrosurgical interference is inevitable during diathermy use. Keeping ECG electrodes at least 15 centimetres from the return pad, using high-frequency filters, and accepting temporary loss of trace during activation are practical approaches.

The square wave calibration function on the monitor helps distinguish true waveform from artefact by verifying amplifier response. If artefact persists, I would systematically check cables, replace electrodes, and if necessary, use an alternative monitor."

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ANZCA Primary Exam Focus

Written Examination Topics

ECG monitoring is a frequently examined topic in the ANZCA Primary Examination, appearing in both MCQ and SAQ formats. Key areas of focus include:

Physics and Engineering:

• Einthoven's triangle and lead derivations (mathematical relationships)
• Wheatstone bridge principles (comparison to pressure transducers)
• Differential amplification and CMRR calculations
• Filter characteristics (monitoring vs diagnostic mode)
• Electrode-skin interface physics (half-cell potential, impedance)

Waveform Analysis:

• Normal intervals and their physiological basis
• Recognition of chamber enlargement patterns
• Bundle branch block criteria
• QT interval calculation and prolongation causes
• ST segment analysis methodology

Clinical Scenarios:

• Lead selection rationale for different clinical situations
• Troubleshooting artefact and interference
• Pacemaker rhythm recognition and perioperative management
• Ischaemia detection strategies

Common MCQ Patterns

1. Lead derivation calculations: Given voltages in two leads, calculate the third using Einthoven's equation
2. Filter effects: Predict waveform changes when switching between filter modes
3. CMRR problems: Calculate rejection ratios or effects of impedance mismatch
4. Clinical scenarios: Identify appropriate lead configurations for specific patient populations

Viva Question Themes

Primary viva questions often progress through:

1. Basic physics of ECG signal generation and detection
2. Equipment specifications (amplifier requirements, filter settings)
3. Clinical application (lead selection, ischaemia monitoring)
4. Troubleshooting scenarios (artefact identification and management)
5. Safety considerations (electrical safety, pacemaker management)

Candidates should be prepared to draw Einthoven's triangle, explain differential amplification, and discuss the rationale for combined II + V5 monitoring in cardiac risk patients. [29,30]

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Anaesthetic Drug Effects on ECG

Volatile Anaesthetic Agents

Sevoflurane:

• Prolongs QT interval (dose-dependent)
• May cause bradycardia (vagal effect)
• Generally preserves AV node conduction
• Risk of torsades de pointes with pre-existing QT prolongation

Desflurane:

• Sympathetic stimulation at rapid increases (tachycardia, hypertension)
• Less QT prolongation than sevoflurane
• May cause transient arrhythmias during concentration changes

Isoflurane:

• Minimal QT effect
• May cause coronary steal (historical concern, now less relevant)
• Generally well-tolerated in patients with coronary disease

Intravenous Agents

Propofol:

• Minimal direct cardiac conduction effects
• May cause bradycardia (vagal, reduced sympathetic tone)
• Propofol infusion syndrome: T wave changes, arrhythmias (rare)

Ketamine:

• Sympathomimetic effects: tachycardia
• Preserved or enhanced cardiac conduction
• Useful in patients with bradycardia risk

Opioids:

• Bradycardia (vagal stimulation, particularly with remifentanil bolus)
• Generally do not affect conduction intervals
• May unmask sick sinus syndrome

Neuromuscular Blocking Agents

Succinylcholine:

• Transient bradycardia (especially with repeated doses or in children)
• Hyperkalaemia risk: Peaked T waves, widened QRS, sine wave pattern
• Mechanism: Muscarinic agonist effect at sinus node

Non-depolarizing agents:

• Pancuronium: Vagolytic effect, tachycardia
• Vecuronium, rocuronium: Minimal cardiovascular effects
• Atracurium: Histamine release may cause tachycardia

Other Relevant Drugs

Reversal agents:

• Neostigmine: Bradycardia (muscarinic), requires anticholinergic co-administration
• Sugammadex: May cause bradycardia (rarely), minimal direct effect

Antiemetics:

• Ondansetron: QT prolongation (dose-dependent, FDA warning)
• Droperidol: Significant QT prolongation (black box warning)
• Metoclopramide: Minimal effect

Local anaesthetics:

• Systemic toxicity: Widened QRS, bradycardia, AV block, asystole
• Bupivacaine most cardiotoxic (R-isomer effects)
• Treatment: Intralipid (lipid emulsion therapy) [31,32]

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Special Monitoring Situations

Cardiac Surgery

Bypass Considerations:

• ECG may show asystole during cardioplegia (expected)
• Temperature-related changes during hypothermic bypass
• Reperfusion arrhythmias common (AF, VT, bradycardia)
• Pacing wires provide backup; ventricular pacing distorts ECG

Post-bypass Monitoring:

• ST changes common (air emboli, graft issues, stunning)
• New conduction abnormalities suggest surgical injury
• Continuous 12-lead monitoring optimal if available

Neurosurgery

Posterior Fossa Surgery:

• Vagal reflexes common (bradycardia, asystole possible)
• Trigeminocardiac reflex during skull base surgery
• Immediate communication with surgeon if arrhythmias occur

Sitting Position:

• Risk of air embolism: may cause arrhythmias
• Precordial Doppler more sensitive than ECG for air detection

Obstetric Anaesthesia

Pregnancy-Related Changes:

• Left axis deviation (uterine displacement of heart)
• Increased heart rate (15-20 bpm above baseline)
• Q wave in lead III (positional)
• ST depression and T wave changes common (not necessarily pathological)
• Supraventricular tachycardias more common

Aortocaval Compression:

• Reflex tachycardia when supine
• Bradycardia possible with severe compression
• Left lateral tilt essential

Paediatric Considerations

Normal Paediatric Variations:

• Higher heart rates (neonates: 120-160, infants: 100-140)
• Right axis deviation (neonates and infants)
• T wave inversion in V1-V3 normal in children
• RSR' pattern in V1 common (normal variant)

Electrode Placement:

• Smaller electrodes for neonates and infants
• Modified positions may be necessary for surgical access
• Motion artefact more problematic [33,34]

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Advanced Monitoring Techniques

ST-Segment Trending

Modern monitors provide continuous ST-segment analysis with:

• Automated J-point detection and measurement
• Trend displays over time (hours to days)
• Alarm thresholds (typically 1 mm depression, 2 mm elevation)
• Multi-lead simultaneous analysis

Limitations:

• Baseline wander affects accuracy
• Bundle branch block, LVH create baseline ST abnormalities
• Electrode motion causes false alarms
• Diagnostic mode essential for accuracy

Heart Rate Variability

Definition: Beat-to-beat variation in RR intervals reflecting autonomic tone.

Parameters:

• SDNN: Standard deviation of NN intervals (overall variability)
• RMSSD: Root mean square of successive differences (vagal tone)
• LF/HF ratio: Low frequency to high frequency power ratio (sympathovagal balance)

Clinical Applications:

• Reduced HRV associated with increased perioperative cardiac risk
• May detect autonomic neuropathy (diabetics)
• Research applications in anaesthesia depth monitoring

QT Monitoring

Automated QT Measurement:

• Modern monitors calculate QT and QTc continuously
• Algorithm accuracy depends on T wave morphology
• Manual verification recommended for significant changes

Clinical Protocol:

• Baseline QTc before QT-prolonging drug administration
• Monitoring during drug infusion
• Action threshold: QTc >500 ms or increase >60 ms from baseline [35,36]

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