2019B03 Draw and label a lead II electrocardiogram tracing for one cardiac cycle, indicating normal values.
What is the PR interval and what factors influence it?



·      Lead 2 trace

·      ECG components table

·      PR interval physiology including pacemaker potentials

·      PR interval abnormalities


Lead 2:


ECG components:


Normal values

P wave

0.12 seconds


Positive in II and aVF, biphasic in V1

PR interval

0.12 – 0.2 seconds, isoelectric

QRS complex

<0.12 seconds

Q wave

≤0.04 seconds

≤2mm, ≤25% height of QRS complex

ST segment

<2mm deviation from isoelectric in V1-2-3

<1mm deviation from isoelectric elsewhere

T wave

<5mm height

QT interval

Bazett formula: QTc = QT/(RR)

440 in men, 460 in women

> 350ms in both


PR interval:


Time between excitation of the atrium to excitation of the ventricles


Atria (0.5m/s)

AV node (mostly this, 0.05m/s)

Bundle of His (2-4m/s)

Pacemaker potential of AV node cells


PR interval abnormalities:

Too short

Physiological: (usually within normal limits)

·   ↑SNS -> ↑cAMP -> ↑slope phase 4 (more than other phases)

·   ↑Temperature -> ↑pacemaker cell metabolic rate

Aberrant conduction pathway:

·   WPW syndrome

·   LGL syndrome

Too long: types

·   First degree: ↑PR interval

·   Second degree Mobitz 1: ↑PR interval each beat until dropped QRS

·   Second degree Mobitz 2: intermittent dropped QRS

·   Third degree: no AV conduction, escape rhythm

Too long: causes

Physiological: (usually within normal limits)

·   ↑PSNS -> ↓cAMP and ↑K+ conductance -> ↓slope phase 4, ↑hyperpolarisation

o E.g. fit athlete

·   ↓Temp -> ↓pacemaker cell metabolic rate

·   ↓pO2 -> failure of Na+K+ATPase, unable to repolarize, HCN channel inactive

·   ↑K+ -> more negative membrane potential


·   Degeneration of conducting system

·   Fibrosis

·   Ischaemia, infarction

·   Infection: myocarditis, endocarditis

·   Infiltration

·   Connective tissue disease


·   Beta blockers

·   Verapamil, diltiazem

·   Digoxin




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