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Wednesday, June 18, 2008

Basic ECG waveform & ECG vitals

basic ecg wave consists of waves :

P wave:represents atrial depolarisation

QRS complex:ventricular depolarisation

ST segment,T wave & U wave:ventricular repolarisation.

now you may have a doubt of where the atrial repolarisation has gone?answer is the atrial ST segment(STa) & atrial T wave(Ta) are generally not observed on normal ecg because of their low amplitudes.

exception: acute pericarditis causes PR segment deviation.

A segment is a straight line connecting two waves, whereas an interval encompasses at least one wave plus the connecting straight line.

segments

seg

WE will discuss about "Vital signs" of the ECG.

As soon as you see an ECG,scan for these 4 things in the first look itself .they are:

  • heart rate
  • PR interval
  • QRS width
  • QT interval.


Heart rate:

normal heart rate :60-100 beats/min.

if more than 100 beats/min-tachycardia

if less than 60 beats/min-bradycardia

we will discuss the conditions which cause them in future posts.now we will learn to measure it at one glance.

2 methods:

Box counting methods-if heart rate is regular

Count the number of large boxes(0.2 sec boxes) b/w 2 successive QRS complexes & divide 300 by it.{300 * 0.2=60 & heart rate is measured for 60 sec}

example-in the below ecg no. of large boxes b/w 2 successive QRScomplexes is 3.

therefore heart rate=300/3=100 beats/min

if u want to measure even more accurately ,count the no. of small(0.0 sec) boxes b/w 2 successive QRScomplexes and divide 1500 with it.{1500*0.04=60}

QRS counting methods-if heart rate is irregular.

Count the no. of QRS complexes in 6 second intervals & then multiply it by 10.

The PR Interval:

  • The PR interval is the time from the beginning of the P wave until the beginning of the QRS complex.
  • it is normally between 0.12 and 0.2 seconds (three to five small boxes) in length.
  • The PR interval may be prolonged when conduction of the electrical wave through the AV node is slow. This may be seen with degenerative disease of the node, or with digoxin, hyperkalemia, hypercalcemia, or hypothermia.
  • The PR interval may be unusually short when conduction is rapid. A mildly short PR interval may be seen with hypokalemia or hypocalcemia. An artificially-short PR interval occurs when the QRS complex begins early, as happens with an extra conducting bundle — Wolff-Parkinson-White Syndrome (WPW).
  • Following the P wave is the PR segment. (NOTE: the PR segment and the PR interval are NOT the same thing.) The PR segment is not routinely measured, but may be commented on if it is depressed or elevated. During the PR segment, the electrical wave moves slowly through the atrioventricular (AV) node. This activity is not seen on the ECG.

if PR interval > 0.2 sec -called "First degree heart block"

The QRS width:

  • If the first deflection of the QRS is downward, it’s called a Q wave. The Q wave represents activation of the ventricular septum.
  • The first upward deflection of the QRS is called the R wave. Most of the ventricle is activated during the R wave.
  • The S wave is any downward deflection following the R wave.
  • If a second upward deflection is seen, it’s called an R-prime wave(R' wave).
  • QRS complexes may be described by naming the waves that form them. For example, a complex with an R, an S, and an R’ is called an RSR’ complex.

clinical aspects:

  1. Q waves may be normal. For example in lead I, a Q less than 1/4 of the R height, and less than one box wide, is considered normal. This is the early activation of the septum. This activation goes left — away from lead I — and is therefore negative on the ECG. “Septal Qs” are normal in I, F, V5 and V6. Qs are also generally innocent in lead III and lead V1 if no other abnormality is seen.
  2. Q waves are “significant” if they are greater than 1 box in width (longer than 0.04 msec) OR are larger than 1/4 of the R wave. Significant Q waves indicate either myocardial infarction or obstructive septal hypertrophy (IHSS).
  3. The R wave may be prolonged if the ventricle is enlarged, and may be abnormally high (indicating strong voltage) if the ventricular muscle tissue is hypertrophied.
  4. Like the R wave, an abnormally large S wave may indicate hypertrophy of the ventricle
  5. R-prime waves are never normal, but indicate a problem in the ventricular conduction system.

QT interval:

The QT interval is the time from the beginning of the QRS complex until the end of the T wave. The “normal” QT length varies with heart rate. Very fast rates shorten the QT length.

QT interval should be measured in the ECG lead that shows the longest interval.

as it changes with heart rate ,a rate corrected QT interval(QTc) has been devised.

  1. Bazett equation=QTc=QT/square root of RR interval.
  2. RR=b/w 2 successive R waves.
  3. normally QTc is less than or equal to about 0.44 sec.

clinical aspects:

QT interval shortened-

  1. digitalis(in therapeutic doses)
  2. hypercalcemia.

QT prolongation predisposes to Torsade de Pointes(distinctive polymorphic ventricular tachycardia in which the QRS amplitude varies and the QRS complexes appear to twist around the baseline)

QT prologation:

1.congenital(hereditary) long QT syndromes:

  1. Romano-Ward syndrome(autosomal dominant)
  2. Jervell & Lange-Nielson syndrome(autosomal recessive with congenital deafness)

2.Acquired long Qt syndromes:

a.electrolyte abnormalities-

  1. hypocalcemia
  2. hypokalemia
  3. hypomagnesemia

b.drugs:

  1. class 1A or 3 antiarrhythmic agents(amiodarone,disopyramide,dofetilide,ibutilide,procainamide,quinidine,sotalol)
  2. antihistamines(astemizole,terfenadine)
  3. antianginal(bepridil)
  4. antimalarial(chloroquine,halofantrine)
  5. psychotropics(phenothiazines,tricyclic antidepressants,haloperidol)
  6. Gastrointestinal stimulant(cisapride)
  7. antinausea(domperidone,droperidol)
  8. antibiotics(clarithromycin,erythromycin,pentamidine,sparfloxacin)
  9. antilipemic(probucol)
  10. opiate agonist(methadone.levomethadyl)
  11. diuretic(indapamide)

c.myocardial ischemia or infarction.

d.cerebrovascular injury

e.systemic hypothermia.

f.bradyarrhythmias.

g.others:

  1. liquid protein diets
  2. starvation
  3. myocarditis
  4. arsenic poisoning

IN the next post we will discuss about P wave,ST segment,T wave,U wave & also about ECG leads.


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