Electrocardiogram (ECG)

Rate

Normal rate is 60-100 bpm, with everything below or above termed bradycardia or tachycardia, respectively.

 

There are a few ways to count the rhythm:

 

Big blocks: 300, 150, 100, 75, 60, 50

 

60000/small boxes

Rhythm

Assess for sinus rhythm by measuring the interval between a number of R waves. Mark a paper at 3 peaks and slide the paper along to compare spacing.

Sinus rhythm is present there is a QRS after every P, P after every QRS and Narrow QRS complexes.

If sinus rhythm is not present, identify the arrhythmia.

 

P Waves

The P wave is composed of the contractions of the left and right atria. The right atrium contracts slightly faster than the left. Both RA and LA contractions are positive in lead II, while the RA is positive and the LA negative in lead V1. This can shed some light on enlargements of each.

 

 

P wave abnormalities

• Absent P wave and irregular rythm suggests atrial fibrillation
• Saw tooth pattern of P waves depicts A flutter
  
• Two different P waves suggests ectopic focus that does/does not activate ventricle. Causes including anything that excites atria, including coffee.

• P mitrale: LAH- Prolonged P wave>120msec/3 small boxes,notching/"bipeak" P
  wave,Biphasic P wave in Lead V1 with negative deflection >1 small box
• P pulmonale: RAH- Peak P wave with amplitude >1 big box

PR Interval

The normal PR interval, measured from the beginning of the P wave to the onset of QRS, is 0.12-0.20 sec, or 3-5 small boxes. It is decreased in junctional rhythms and increased in first degree AV block.

 

Second and third degree heart block..

 

If PR interval is shortened, suspect

• left atrial hypertrophy
• ectopic foci that take over pacing
• Wolf-Parkinson-White

 

QRS Complexes

The normal QRS length, measured from the beginning to the end of the QRS complex, is <0.1 sec, or less than 2.5 small boxes.

Complexes greater than 0.12 seconds are usually due to interventricular conduction delay.

 

 

Assess for R wave progression throughout V1-6, morphology, QRS duration and sometimes amplitude. It is better to do the assessment by examining them by regional grouping to identify abnormalities.

• inferior: II,III,AVF (RCA)
• anterior: V1-4 (?)
• lateral: I, V5-6,aVL (circumflex)

 

Although each lead represents a unique view, they can be grouped together according to their representative area of the heart. This is particularly important when assessing the possibility of myocardial infarction.

• I, aVL, V3, V6 - lateral
• II, III, aVF - inferior
• aVr

 

Poor R wave progression can be caused by

• previous MI
• ideopathic
• PE
• LVH
• right ventricular overload
• ?copd

 

Morpohology - (>1small box for duration + >1/3 height or Rwave), most like indicates previous infarct.

 

Patterns of Heart block if QRS widened

• M wave on V1-2 and slurred ST segment on V5-6 with right block
• M wave on V5-6 if left block
  

 

Sometimes QRS complexes are shorter than 1 small box in amplitude. Causes include:

• low voltage
• athlete
• ischaemia
• pericarditis
• electrolyte abnormality

Axis

While all information is relevant, the mean QRS axis is the most clinically important and is calaculated from the six limb leads.

 

Lead I, traveling from right to left, is defined as the 0^o axis, and from there all other leads are assigned a relative axis.

A normal axis is between -30 and +90.

 

A quick way to determine mean QRS axis is to observe which of the six limb leads has the most isoelectric-appearing complex. The mean axis is simply perpindicular to it. It could, however, be +90^o or -90^o perpendicular, so observing a perpindicular lead for whether or not it is positive will provide that answer.

 

Another quick way is to assess leads I and II. If the QRS is positive in both, there is likely no axis deviation.

 

Left axis deviation is more common. It suggests

• mechanical shift (ascites, pregnancy, tumour)
• left ventricular hypertrophy
• left anterior fascicular block
• COPD
• previous anterior/inferior MI
• Wolf-Parkinson-White

 

Right axis deviation suggests:

• RVH
• left posterior fascicular block
• pulmonary embolus
• mechanical shift (COPD)
• lateral infarction

 

 

 

ST segment

ST depression suggests myocardial ischemia, while ST elevation is representative of myocardial infarction. It is wise to assess it quickly in cases of clinical suspicion due to the potential urgency of what it could suggest.

 

The ST segment represents a period of relatively little electrical activity in the heart.

 

The QT interval, measured from the beginning of the Q wave to the end of the T wave, represents the duration of ventricular depolarization and repolarization.

 

T waves

Ischemia and Infarction

One of the major reasons for performing ECG is to diagnose acute coronary syndrome, which involves tissue ischemia and/or infarction. Ischemia commonly appears as ST depression, while ST elevation follows infarction, or tissue death. Inverted T waves may also signify ischemia.

 

Q waves suggest a previous MI.

 

ST elevation can also be seen with:

• pericarditis
• normal early repolarization (most common in young black men)

 

As described, different leads monitor different areas of the heart. At least two continuous leads are required to diagnose acute coronary syndrome.

 

For diagnosis, at least 1mm elevation is required for the limb leads, and at least 2mm elevation in the precordial leads.

 

 

I and aVL: circumflex

II, III, aVF: RCA

V1-V5: LAD

 

 

 

 

 

If leads anterior leads (V1,V2) show ST depression, ensure you perform a 15 lead to assess for the possibility of posterior MI.

Heart Blocks

 

result of conduction disturbance -slowing or blocking - at the AV node.

 

Waveform

delayed PR intervals

 

 

 

 

First degree heart block

impulse delayed; prolonged PR interval >.2 s

often idiopathic

 

rate

usually normal

 

rhythm

usually regular

 

 

 

 

 

Second Degree Heart Block (Mobitz Type I)

PR interval increases intil aQRS complex is dropped

 

rate

atrial - usually within normal limits (60-100 bpm)

ventrical - slower

 

rhythm

artial - regular

ventricular- irregular

 

waveform

normal P waves, though not all followed by QRS

PR interval progressively longer until QRS is dropped

QRS normal

 

 

 

 

Second Degree Heart Block Type II

get a pacemaker- can progress to Type III

 

waveform

P waves

normal configuration

not all followed by QRS complexes due to

 

PR interval - may be normal

 

 

 

Third Degree Heart Block

atria and ventricles beat independently

 

rate

atria - unaffected - 60-100 bpm

ventricular - slower

 

rhythm

atria and ventricular - regular

 

waveform

P waves normal

PR interval - absent

QRS complex - normal if blocked at the AV node; wide if blocked in the bundle branch

 

 

 

Atrial Flutter   rate: atria - 250-350/min; ventricles depend on AV block rhythm: regular; ventricular - variable; dependent on number of atrial beats conducted and on condition ration (ie 4:1)   P waves: not truly present; may appear with sawtooth pattern PR: undefinable QRS: less than 0.12 s

Atrial Fibrillation   very common in elderly rate: depends on ventricle response: uncontrolled >100, controlled less <100. Atria: indiscernable 400-500 bpm rhythm: irregularly irregular   waveforms P waves, and PR interval, are undefinable wavy baseline QRS complex less than 0.12s   rhythm grossly irregularly irregular

 

Accessory-mediated Supraventricular Tachycardias

Supraventricular Tachycardias (SVTs) are also called narrow rhythm tachycardias. These occur as impuses cycle through abnormal tissue. Usually sudden onset.

 

rate: > 100/minute; usually 150-250/min
rhythm: sinus
P waves: difficult to detect, as buried within T waves

QRS: narrow

 

• atrial tachycardia
• paroxysmal atrial tachycardia
• PAT/PSVT/SVT
• atrial flutter
• atrial fibrillation

SVT wave form configurations

P waves: arise from irritable foci above ventricle (not SA node)

 

significance:

• decreases heart's ability to pump effectively, decreasing perfusion
• thrombi formation can result in emboli
• can be insignificant if it is a perfusing rhythm; rarely progresses

 

Ventricular Tachycardia   ventricular rate is 100-250 bpm; rhythm is regular.   waveform no P waves, no PR interval QRS complexes normally wide, greater than 0.12s

Ventricular Fibrillation   rate: indeterminate; can vary rhythm: irregular QRS: no P, QRS, or T waves   fine: peak to trough of 2-5 mm medium: 5-10 mm coarse: >10 mm

 

Left Atrial Hypertrophy (P mitrale) widened, biphasic P wave in II, III, or AVF   Left Ventricular Hypertrophy   In adults, R in V6 and S in V1 adding to over 55 mm R in aVL >11 mm. left axis deviation left atrial hypertrophy is also often present, with M-shaped p waves resulting

Right Atrial Hypertrophy (P pulmonale) peaked P in II, III, or AVF   Right Ventricular Hypertrophy   In adults, R in V1 and S in V5/V6 above 1.10 mV right axis deviation   In kids In V1, upright T wave in days 3-6; q wave, tall R in V1 and deep S in V6

 

 

 

 

 

Bundle Branch Blocks

 

Right Bundle Branch Block

QRS widens because RV takes longer to depolarize (3 blocks or more)

An RSR' wave form is seen in lead V1.

 

 

Left Bundle Branch Block

Negative (QS wave) in V1, V2

An RSR' wave form is seen in V5 or V6.

An alternate way:

• count the S height in V1 or V2
• count the R height in V5 or V6
• >35 is LVH

Other

 

 

• pericarditis - Diffuse ST segment elevation
• right heart overload - get RSR' or rR' in V1, which suggests

Acidosis: smaller amplitude QRS complexes

Hyperkalemia

• wide-complex QRS
• T waves tall and peaked
• small P waves
• sine-wave PEA

hypokalemia

• wide-complex QRS
• T waves flatten
• U waves
• QT prolonged
• tachycardia

hypothermia

• J or Osborne waves

overdose

• predominantly QT prolongation

cardiac tamponade

• narrow complex
• tachycardia

tension pneumothorax

• narrow complex
• bradycardia