P-waves represent atrial depolarisation, this moves from right to left, with the right atrium being activated before the left 

Structure:
- The first 1/3 of the P wave corresponds to right atrial activation

- The final 1/3 corresponds to left atrial activation

- The middle 1/3 is a combination of the two


In most leads (e.g. lead II), the right and left atrial waveforms move in the same direction, forming a monophasic P wave

However, in lead V1 the right and left atrial waveforms move in opposite directions:
- This produces a biphasic P wave with the initial positive deflection
corresponding to right atrial activation and the subsequent negative deflection denoting left atrial activation.

In sinus rhythm, there should be a P-wave preceding
each QRS complex

The p wave duration is < 0.12 seconds (three small squares)

The maximal height of the P wave is 2.5 mm in leads II and / or III
and <1.5mm in the precordial leads

Tall or Wide p waves in lead II or V1 may
indicate atrial hypertrophy (due to increased electrical activity) 

The p wave is positive in II and AVF, and biphasic in V1

Should be inverted in aVR

The PR-interval is from the start of the P-wave to the start of the Q wave

It represents the time taken for electrical activity to move between the atria and ventricles

The PR-segment is the AV nodal conduction delay

P-R interval is measured from start of P wave to start of QRS complex

Should be less than 0.2 seconds (5 small squares)

If longer than 0.2s : presence of heart block is highly likely

Determine if the P-R interval is constant (ie. The same for each set of complexes)

The P-R interval will help diagnosethe presence of 1st, 2nd and 
3rd degree heart blocks

PR segment depression is seen in Pericarditis

The QRS-complex represents depolarisation of the ventricles

It is seen as 3 closely related waves on the ECG  (Q,R & S wave)

The QRS complex should be 0.12 seconds (3 small squares)

It consists of a Q wave, R wave and a S wave

A Q wave is any negative deflection that precedes an R wave

The Q wave represents the normal left-to-right depolarisation of the
interventricular septum

Small Q waves are normal in most leads

•Small ‘septal’ Q waves are typically seen in the left-sided leads (I, aVL, V5 and V6)

Deeper Q waves (>2 mm) may be seen in leads III and aVR as a normal variant

Under normal circumstances, Q waves are not seen in the
right-sided leads (V1-3)

Q waves are considered pathological if:

• > 40 ms (1 mm) wide
• > 2 mm deep
• > 25% of depth of QRS complex
• Seen in leads V1-3

Pathological Q waves usually indicate current or prior myocardial infarction

The absence of small septal Q waves in leads V5-6 should be considered abnormal

Absent Q waves in V5-6 is most commonly due to LBBB

The ST-segment starts at the end of the S-wave & 
finishes at the start of the T-wave

It represents the time during which the ventricles are contracting and emptying

The T-wave represents ventricular repolarisation

It is seen as a small wave after the QRS complex

A T-wave is considered tall when it is greater than;

• 5mm in the standard leads
• 10mm in the precordial leads (V1-V6) 
• Upright in all leads except aVR and V1
• Amplitude < 5mm in limb leads, < 15mm in precordial leads

TP segment is the isoelectric interval (baseline for an ECG) 
on the electrocardiogram (ECG)

It is the region between the end of the T wave (ventricular repolarization) and the next P wave (atrial depolarization)

It represents the time when the myocardial cells are electrically silent

TP interval shortens when the heart rate increases and vice versa

Ideally, elevation and depression of ST segment and PR segment are measured with reference to the TP segment (Isoelectric line)

But when the heart rhythm is fast, a good TP segment may not be seen as the T wave of once
cycle merges with the P wave of the succeeding cycle

This often occurs in the setting of an exercise test and in such situation,
the E point which is at the beginning of the QRS complex can be taken as the reference instead