ECG etc.

ECG – Investigating PE
- Small/medium pulmonary embolism
· Usually normal, except for sinus tachycardia
· Sometimes atrial fibrillation another tachy-arrhythmia occurs
· May be evidence of right ventricular strain
- massive pulmonary emboli
· Show right atrial dilation with tall peaked P waves in lead II
· Right ventricular strain and dilation give rise to right axis deviation, some degree of right bundle branch block, and T wave inversion in the right precordial leads
- multiple recurrent pulmonary emboli
· Can be normal or show signs of pulmonary hypertension
- acute pulmonary embolism
· S wave in lead I
· A Q wave in lead III
· An inverted T wave in lead III
· Sinus tachycardia
· Incomplete right bundle branch block pattern (R wave in aVR and V1 and an S wave in V6)

P Wave
- An impulse that begins in the SA node is not recorded on the ECG, however –
- The spread of depolarisation through the atria (atrial depolarisation) is observed
- First phase of P wave – electrical stimulation from the SA node stimulates the right atrium and reaches the AV node
- Downslope of P wave – stimulation of the left atrium
- Atria begin to repolarise at the same time the ventricles depolarise
- Wave of atrial repolarisation is lost in the QRS complex
- Normal Characteristics of a P Wave
o Smooth and rounded
o No more than 2.5mm in height
o No more than 0.11 sec in duration
o Positive in leads I, II, aVF and V2-V6

QRS Complex
- Spread of electrical impulse through the ventricles (ventricular depolarisation)
- Depolarisation = contraction, thus, ventricles contract shortly after QRS complex begins
- Much larger as there is a considerably larger muscle mass being depolarised
- QRS duration – measurement of the time required for ventricular depolarisation
- Adult – normal duration – 0.06 – 0.10 seconds
- If conduction is delayed, QRS complex produced by the ventricle is wider
- Normal characteristics of the QRS complex
- Normal duration of the QRS complex in an adult varies between 0.06 and 0.10 sec
- A normal Q wave is less than 0.04 sec in duration and less than 1/3 the amplitude of the R wave in that lead

T Wave
- Ventricular repolarisation
- Slightly asymmetric – peak is slightly closer to the end than the beginning
- Normal characteristics of T wave
- Slightly asymmetric
- T waves are not normally more than 5mm in height in any limb lead or 10mm in any chest lead; T waves are not normally less than 0.5mm in height in leads I and II

U Wave
- Not always seen, cause not known
- Normal characteristics
- Rounded and symmetric
- Usually less than 1.5mm in height and smaller than in the preceding T wave
- In general, a U wave more than 1.5mm in height in any lead is considered abnormal

Segments
PR Segment
- Horizontal line between the end of the P wave and the start of the QRS complex
- Activation of the AV node, the bundle of His, the bundle branches and the Purkinje fibres

ST segment
- ST segment is between the QRS complex and the T wave
- Early part of repolarisation of the right and left ventricles
- ST segment deviation
- Myocardial Ischaemia
- Injury
- Infarction
- Normal characteristics of an ST segment
- Begins with the QRS complex and ends with the onset of the T wave
- In the limb leads, the normal ST segment is isoelectric (flat) but may normally be slightly elevated or depressed
- In the chest leads, ST segment deviation may vary from -0.5 to +2mm

TP Segment
- Between T wave and P wave
- Isoelectric when the heart rate is within normal limits
- With rapid heart rates, the TP segment is often unrecognisable because the P wave encroaches the preceding T wave

Intervals

An interval is a waveform and a segment

PR Intervals
- P wave + PR segment = PR interval
- PRI changes with HR but normally measures 0.12 to 0.20 sec in adults
- As HR increases, the duration of the PR interval shortens
- A conduction problem above the level of the bundle branches will largely affect the P wave and PR interval
- Normal characteristics of the PR interval
- Normally measures 0.12 to 0.20 sec in adults; may be shorter in children and longer in old people
- Normally shortens as heart rate increases

QT Interval
- This represents total ventricular activity à the time from ventricular depolarisation (activation) to repolarisation (recovery)
- Start of QRS Complex + End of T Wave = QT Interval
- Varies according to age, gender and heart rate

Analysing a Rhythm Strip

Assess the rate
- 6 Second Method
o To determine ventricular rate
o Count the number of QRS Complexes within a 6 second period
o Multiply this by 10 to find the number of complexes in a minute
o Can be used for regular and irregular rhythms
- Large Boxes
o To determine ventricular rate
o Count number of large boxes between two consecutive R waves (R-R interval)
o Divide this number into 300
o To determine atrial rate
o Count number of large boxes between two consecutive P waves (P-P interval)
o Divide this number into 300

Assess rhythm
- Rhythm indicates the site of origin of an electrical impulse (eg sinus rhythm, junctional rhythm)
- Also used to describe the regularity or irregularity of waveforms
- We assess regularity by measuring the distance between the QRS waves

Identify and examine P waves
- To locate P waves, look to the left of the QRS complex
- Normally, P waves looking similar in size, shape and position
- If no P wave is present, the rhythm originated in the AV junction or the ventricles
- If one P wave is present before each QRS and the QRS is narrow:
o Is the P wave positive? If so, the rhythm probably began in the SA node
o Is the P wave negative or absent? If so, and the QRS complexes occur regularly, the rhythm probably started in the AV junction

Assess Intervals (Evaluate Conduction)

PR Interval
- Measured from where the P waves leaves the baseline to the beginning of the QRS Complex
- Normal – 0.12 to 0.20 seconds
- If they are the same – ‘constant’
- ‘lengthening’ – dysrhythmia
QRS Duration
- Narrow (normal) if less than 0.10 seconds
- Wide – more than 0.10 seconds
QT Interval
- Measure the QT interval in the leads that show the largest amplitude T waves
- Measured from beginning of QRS complex to end of T wave
- Duration varies according to age, gender and heart rate
R-R and P-P Intervals
To determine the rate and regularity of a cardiac rhythm, the intervals are used
o R-R interval – measure and compare distance – ventricular rhythm
o P-P – atrial rhythm


· T wave
o Negative – MI
o Tall, pointed – hyperkalemia