A typical electrocardiograph runs at a paper speed of 25 mm/s. At a paper speed of 25 mm/s, one small block of ECG paper translates into 0.04 s (or 40 ms). Five small blocks make up 1 large block, which translates into 0.20 s (or 200 ms).
NORMAL SINUS RHYTHM
R-R regular; rate of 60-100
P-P regular; rate of 60-100
P before & every QRS
PR 0.12-0.20 seconds
QRS 0.04-0.12 seconds
P wave: the sequential activation (depolarization) of the right and left atria
QRS complex: right and left ventricular depolarization (normally the ventricles are activated simultaneously)
ST-T wave: ventricular repolarization
U wave: origin for this wave is not clear - but probably represents "afterdepolarizations" in the ventricles
PR interval: time interval from onset of atrial depolarization (P wave) to onset of ventricular depolarization (QRS complex)
QRS duration: duration of ventricular muscle depolarization
QT interval: duration of ventricular depolarization and repolarization
RR interval: duration of ventricular cardiac cycle (an indicator of ventricular rate)
PP interval: duration of atrial cycle (an indicator of atrial rate)
MORE CRITERIA
A small U wave is normally visible in 50 to 75% of ECGs.
The baseline voltage of the electrocardiogram is known as the isoelectric line.
Typically the isoelectric line is measured as the portion of the tracing following the T wave and preceding the next P wave.
P WAVE
During normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node,
and spreads from the right atrium to the left atrium. This turns into the P wave on the ECG, which is upright in II, III, and aVF
(since the general electrical activity is going toward the positive electrode in those leads),
and inverted in aVR (since it is going away from the positive electrode for that lead.
A P wave must be upright in leads II and aVF and inverted in lead aVR to designate a cardiac rhythm as Sinus Rhythm.
The relationship between P waves and QRS complexes helps distinguish various cardiac arrhythmias.
The shape and duration of the P waves may indicate atrial enlargement.
QRS COMPLEX
The QRS complex is a structure on the ECG that corresponds to the depolarization of the ventricles.
Because the ventricles contain more muscle mass than the atria, the QRS complex is larger than the P wave.
In addition, because the His/Purkinje system coordinates the depolarization of the ventricles,
the QRS complex tends to look "spiked" rather than rounded due to the increase in conduction velocity.
A normal QRS complex is 0.06 to 0.10 sec (60 to 100 ms) in duration represented by three small squares or less,
but any abnormality of conduction takes longer, and causes widened QRS complexes.
Not every QRS complex contains a Q wave, an R wave, and an S wave. By convention, any combination of these waves can be referred to as a QRS complex.
However, correct interpretation of difficult ECGs requires exact labeling of the various
waves. Some authors use lowercase and capital letters, depending on the relative size of each wave.
For example, an Rs complex would be positively deflected, while a rS complex would be negatively deflected.
If both complexes were labeled RS, it would be impossible to appreciate this distinction without viewing the actual ECG.
The duration, amplitude, and morphology of the QRS complex is useful in diagnosing cardiac arrhythmias, conduction abnormalities,
ventricular hypertrophy, myocardial infarction, electrolyte derangements, and other disease states.
Q WAVES
can be normal (physiological) or pathological. Pathological Q waves refer to Q waves that have a height of 25% or
more than that of the partner R wave and/or have a width of greater than 0.04 seconds.
Normal Q waves, when present, represent depolarization of the interventricular septum.
For this reason, they are referred to as septal Q waves, and can be appreciated in the lateral leads I, aVL, V5 and V6.
Q waves greater than 1/3 the height of the R wave, greater than 0.04 sec (40 ms) in duration, or in the right precordial leads
are considered to be abnormal, and may represent myocardial infarction.
"Buried" inside the QRS wave is the atrial repolarization wave, which resembles an inverse P wave.
It is far smaller in magnitude than the QRS and is therefore obscured by it.
PR/PQ INTERVAL
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex.
It is usually 120 to 200 ms long. On an ECG tracing, this corresponds to 3 to 5 small boxes.
In case a Q wave was measured with a ECG the PR interval is also commonly named PQ interval instead.
A PR interval of over 200 ms may indicate a first degree heart block.
A short PR interval may indicate a pre-excitation syndrome via an accessory pathway that leads to early activation of the ventricles, such as seen in Wolff-Parkinson-White syndrome.
A variable PR interval may indicate other types of heart block.
PR segment depression may indicate atrial injury or pericarditis.
Variable morphologies of P waves in a single ECG lead is suggestive of an ectopic pacemaker rhythm such as wandering pacemaker or multifocal atrial tachycardia
ST SEGMENT
The ST segment connects the QRS complex and the T wave and has a duration of 0.08 to 0.12 sec (80 to 120 ms).
It starts at the J point (junction between the QRS complex and ST segment) and ends at the beginning of the T wave.
However, since it is usually difficult to determine exactly where the ST segment ends and the T wave begins,
the relationship between the RT segment and T wave should be examined together.
The typical ST segment duration is usually around 0.08 sec (80 ms). It should be essentially level with the PR and TP segment.
The normal ST segment has a slight upward concavity.
Flat, downsloping, or depressed ST segments may indicate coronary ischemia.
ST segment elevation may indicate myocardial infarction. An elevation of >1mm and longer than 80 milliseconds
following the J-point. This measure has a false positive rate of 15-20% (which is slightly higher in women than men) and a false negative rate of 20-30%.
T WAVE
The T wave represents the repolarization (or recovery) of the ventricles. The interval from the beginning of the QRS complex
to the apex of the T wave is referred to as the absolute refractory period.
The last half of the T wave is referred to as the relative refractory period (orvulnerable period).
In most leads, the T wave is positive. However, a negative T wave is normal in lead aVR.
Lead V1 may have a positive, negative, or biphasic T wave.
In addition, it is not uncommon to have an isolated negative T wave in lead III, aVL, or aVF.
Inverted (or negative) T waves can be a sign of coronary ischemia, Wellens' syndrome, left ventricular hypertrophy, or CNS disorder.
Tall or "tented" symmetrical T waves may indicate hyperkalemia. Flat T waves may indicate coronary ischemia or hypokalemia.
The earliest electrocardiographic finding of acute myocardial infarction is sometimes the hyperacute T wave,
which can be distinguished from hyperkalemia by the broad base and slight asymmetry.
When a conduction abnormality (e.g., bundle branch block, paced rhythm) is present, the T wave should be deflected
opposite the terminal deflection of the QRS complex. This is known as appropriate T wave discordance.
QT INTERVAL
The QT interval is measured from the beginning of the QRS complex to the end of the T wave.
Normal values for the QT interval are between 0.30 and 0.44 (0.45 for women) seconds.
The QT interval as well as the corrected QT interval are important in the diagnosis of long QT syndrome and short QT syndrome.
The QT interval varies based on the heart rate, and various correction factors have been developed to correct the QT interval
for the heart rate. The QT interval represents on an ECG the total time needed for the ventricles to depolarize and repolarize.
The most commonly used method for correcting the QT interval for rate is the one formulated by Bazett.
Bazett's formula is ,
where QTc is the QT interval corrected for rate, and RR is the interval from the onset of one QRS complex
to the onset of the next QRS complex, measured in seconds. However, this formula tends to be inaccurate,
and over-corrects at high heart rates and under-corrects at low heart rates.
QTc may also be found via the following formula: QTc = QT + 1.75(Ventricular Rate - 60).
U WAVE
An electrocardiogram of an 18-year-old man showing U waves, most evident in lead V3.The U wave is not always seen.
It is typically small, and, by definition, follows the T wave.
U waves are thought to represent repolarization of the papillary muscles or Purkinje fibers.
Prominent U waves are most often seen in hypokalemia, but may be present in hypercalcemia, thyrotoxicosis,
or exposure to digitalis, epinephrine, and Class 1A and 3 antiarrhythmics, as well as in congenital long QT syndrome
and in the setting of intracranial hemorrhage. An inverted U wave may represent myocardial ischemia or left ventricular
volume overload.
