4: Electrophysiology

Published on 02/03/2015 by admin

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Last modified 22/04/2025

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TOPIC 4 Electrophysiology

Topic Contents

Normal 12-lead ECG 25

Lead position 25
Cardiac axis 25
Normal resting values 25
Abnormalities of the 12-lead ECG 27

Atrial hypertrophy 27
Left ventricular hypertrophy (LVH) 28
Right ventricular hypertrophy 28
Left bundle branch block 28
Right bundle branch block 28
Stress ECG 28
Bruce Protocol 28

Pathological changes during exercise 28
Signal averaged ECG 28
T wave alternans 29

Positive 29
Negative 29
Intermediate 29
Intracardiac electrograms 31
Typical 4-wire EP study 31

Catheter position 31
Normal activation pattern 33
Accessory pathways and Wolff-Parkinson-White syndrome 33
Pacemakers 33

Normal 12-lead ECG

Lead position

See Figure 4.1.

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Figure 4.1 Standard ECG lead position. (A) Standard limb lead position with corresponding electrocardiographic vectors. (B) Standard praecordial chest lead position.

Cardiac axis

See Figure 4.2.

Normal axis: –30° to +90°
Leftward axis: –30° to –90°
Rightward axis: +90° to +180°
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Figure 4.2 Einthoven’s triangle demonstrating electrocardiographic vectors.

Normal resting values

Adult resting heart rate: 50–100 beats per minute
ECG R-R interval (25 mm/s paper speed): each large square is 0.2 s;
each small square is 0.04 s.
P wave (best seen lead II)

• Amplitude <2.5 mm.
• Duration <12 ms.
• Frontal plane axis 0 to +75°.

PR interval: 120–220 ms
QRS

• Duration : <110 ms.
• Normal q wave duration < 0.04 s, amplitude < 25% of R wave.
• R waves begin in V1 or V2 and progress in size to V5 (R-V6 usually smaller than R-V5).
• S waves begin in V6 or V5 and progress in size to V2 ( S-V1 usually smaller than S-V2).
• Usual transition from S>R in the right precordial leads to R>S in the left precordial leads is V3 or V4.

QT interval:

• Male: < 440 ms
• Female: < 460 ms
• QT interval corrected for heart rate (QTc) = QT/√RR

U wave

• Amplitude < 1/3 T wave amplitude in the same lead.
• Direction same as T wave direction in the same lead.
• Measurement of QT should not include U wave.
• U waves >1/3 T wave amplitude or merged into descending slope of T wave may be pathological.

Abnormalities of the 12-lead ECG

Atrial hypertrophy

Left:

• P wave in lead II > 2.5 mm
• and/or bifid and biphasic P wave in V1.

Right:

• P wave in leads II and V1 > 2.5 mm.
• QRS voltage in V1 < 5 mm and V2/V1 voltage ratio > 6.

Left ventricular hypertrophy (LVH)

• ESTES criteria for LVH (‘diagnostic’, >5 points; ‘probable’, 4 points).
• CORNELL voltage criteria for LVH (sensitivity = 22%, specificity = 95%).

S in V3 + R in aVL > 28 mm (men) S in V3 + R in aVL > 20 mm (women).

• Other voltage criteria

R wave in left precordial lead > 27 mm
S wave in right precordial lead > 30 mm
Sum of greatest R and S > 40 mm.

Right ventricular hypertrophy

Dominant R wave in V1 with normal QRS duration
Axis > +90°
ST segment depression V1–V3

Left bundle branch block

Usually abnormal
Prolonged QRS duration
Absence of q waves in leads I, aVL, V4–V6
Absence of secondary r (notch) wave in V1

Right bundle branch block

Normal finding in 10% of population
Prolonged QRS duration
Presence of secondary positive wave in V1

Stress ECG

Mortality rates <1/10 000 tests
Resting oxygen consumption = 3.5 ml/kg/min = 1 MET
Target heart rate = 85% maximum predicted heart
Maximum predicted heart rate = 220 – age (years) in males, 210 – age (years) females.

Bruce Protocol

See Table 2.1.

Pathological changes during exercise

ST segment depression

• Coronary disease likely if down-sloping ST segment depression >1.0 mm 80 ms after the J point.
• Ventricular arrhythmias (>10 premature ventricular beats per minute).
• Failure to increase systolic blood pressure with exercise.

Signal averaged ECG

This technique is used to detect low-amplitude surface electrograms which are undetectable by standard electrocardiography. These signals are associated with areas of slow conduction that occur following a myocardial infarction or in arrhythmogenic right ventricular cardiomyopathy (ARVC). Orthogonal XYZ leads are used to record signals which are averaged and filtered (X2 + Y2 + Z2) to produce a root mean square voltage (RMS). A normal QRS duration is ≤ 114 ms and normal RMS amplitude in the terminal 40 ms is ≥ 20 μV.

In post-infarction patients, negative predictive value is >95% although positive predictive value is approximately 20% (Figure 4.3).

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Figure 4.3 Example of a normal (left) and abnormal (right) signal averaged ECG.

T wave alternans

T wave alternans on the surface ECG is a beat-to-beat fluctuation in the shape or amplitude of the T wave, and reflects abnormal cardiomyocyte repolarization. A growing body of evidence suggests that it represents a risk marker for sudden cardiac death from ventricular tachyarrhythmias, particularly in patients with heart failure. T wave alternans develops at higher heart rates, particularly in a vulnerable zone above 95 bpm.

T wave alternans is measured using spectral analysis of the ECG and T wave.

A T wave alternans study provides either a positive, negative or intermediate result.

Positive

Sustained alternans with onset at HR <110 bpm.

Sustained alternans is defined as:

1. T wave alternans induced above of specific heart rate for a patient where HR <110 bpm.
2. At least one minute of electrical T wave alternans of >1.9 μV and alternans ratio >3.
3. Artefact-free data: <10% beats are ectopy, respiratory activity <0.25 cycles/beat, HR variation <30 bpm over 130 beats.

Negative

1. Does not meet the criteria of a positive test, and
2. Maximum HR achieved >105 bpm (see Figure 4.4).
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Figure 4.4 Example of normal/negative T wave alternans study (A) and abnormal/positive T wave alternans study (B).

Intermediate

1. Alternans free but maximum HR <105.
2. Alternans induced at HR >110.
3. ECG artefact, e.g. excessive ectopy, obscuring analysis.

Intracardiac electrograms

Normal conduction (Figure 4.5)

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Figure 4.5 Anatomy of the normal cardiac conduction system.

Normal intracardiac intervals

PA: 25–55 ms
AH: 55–125 ms
HV: 35–55 ms
Sinus node recovery time (SNRT): < 150 ms
Sinoatrial conduction time (SACT): < 100 ms
AH ‘jump’: 50 ms increase in AH time for 10 ms decrease in pacing cycle.

Typical 4-wire EP study

High right atrium (HRA) – Quadrapolar catheter
Right ventricular apex (RVA) – Quadrapolar catheter
Coronary sinus (CS) – Decapolar catheter (wide-spaced)
His – Decapolar catheter (narrow-spaced).

Catheter position

See Figure 4.6.

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Figure 4.6 Catheter position for a 4-wire cardiac electrophysiological study. (A) RAO view. (B) LAO view.

Normal activation pattern

See Figure 4.7.

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Figure 4.7 Intracardiac electrograms demonstrating the normal electrical activation pattern measured during a 4-wire electrophysiological study. Intracardiac electrogram nomenclature: HRA = high right atrium, HBE = His bundle electrogram, CS = coronary sinus, RVA = right ventricular apex. I, aVF and V4 are surface ECG leads.

Accessory pathways and Wolff-Parkinson-White syndrome

See Figures 4.8, 4.9, 4.10 for anatomical location of atrioventricular accessory pathways.

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Figure 4.8 Schematic showing the anatomical location of atrioventricular accessory pathways in the plane of the atrioventricular valves.

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Figure 4.9 Guide for identification of accessory pathway location based upon 12-lead ECG abnormalities.

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Figure 4.10 Identification of the anatomical origin of ventricular tachycardia based upon electrocardiographic appearance of the tachyarrhythmia on the 12-lead ECG.

Pacemakers

See Table 4.1.

Table 4.1 International pacemaker 4-letter code

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