2020B02 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

Parameter	Normal values
P wave	· ≤0.12 seconds · <2.5mm · 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

Significance

· Time between excitation of the atrium to excitation of the ventricles

· Hence atrial kick

· Hence higher cardiac output

Components

· Atria (0.5m/s)

· AV node (mostly this, 0.05m/s)

· Bundle of His (2-4m/s)

Pacemaker potential of AV node cells

Short PR

Physiological

Usually within normal limits

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

· ↑Temperature -> ↑pacemaker cell metabolic rate

Pathological

Accessory pathway

· WPW syndrome

· LGL syndrome

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
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⁺ -> ↑membrane potential -> ↓excitability of conduction system
Pathological	· Degeneration of conducting system · Fibrosis · Ischaemia, infarction · Infection: myocarditis, endocarditis · Infiltration · Connective tissue disease
Medication	· Beta blockers · Verapamil, diltiazem · Digoxin