Print version of Knowledge bytes used in this lesson.

The rate of paper (i.e. of recording of the EKG) is 25 mV/s which results in:

• 1 mm = 0.04 sec (or each individual block)
• 5 mm = 0.2 sec (or between 2 dark vertical lines)
• Distance between  Tick marks = 3 seconds (in the rhythm strip)

The voltage recorded from the leads is also standardized on the paper where 1 mm = 1 mV (or between each individual block vertically) This results in:

• 1 mm = 0.1 mV
• 5 mm = 0.5 mV (or between 2 dark horizontal lines)
• 10 mm = 1.0 mV

Heart rate calculation:

Normal range at rest is between 60-100 beats per minute (bpm).

The basic way to calculate the rate is quite simple. You take the duration between two identical points of consecutive EKG waveforms such as the R-R duration. Take this duration and divide it into 60. The resulting equation would be:

Rate = 60/(R-R interval)

A quicker way to obtain an approximate rate is

1. to go by  RR or PP interval. If it is 1 big box (0.2 secs) then the rate is 60/0.2 = 300 bpm. The rest of the sequence would be as follows.
• 1 big box = 300 beats/min (duration = 0.2 sec)
• 2 big boxes = 150 beats/min (duration = 0.4 sec)
• 3 big boxes = 100 beats/min (duration = 0.6 sec)
• 4 big boxes = 75 beats/min (duration = 0.8 sec)
• 5 big boxes = 60 beats/min (duration = 1.0 sec)
2. Count the number of RR intervals between two Tick marks (6 seconds) in the rhythm strip and multiply by 10 to get the bpm. This method is more effective when the rhythm is irregular.

Rhythm can be quite variable. It could be

• Regular : RR interval constant
• Basically regular
• Premature ectopic beat
• Escape ectopic beat
• Regularly irregular : RR interval variable but with a pattern. Normal and ectopic beats grouped together and repeating over and over.
• Irregularly irregular. RR interval variable with no pattern, totally irregular

Normal:

Normal sinus rhythm (NSR): indicates that the rate is between 60 and 100, inclusive, and that the P waves are identifiable and are of the same morphology throughout. The RR interval or PP intervals between beats are same.

Sinus arrhythmia: There is a cyclical acceleration of heart rate with inspiration and slowing  with expiration. The beat to beat interval is slightly different.The rhythm is regularly irregular, in the sense that there is a pattern to irregularity.  This is termed sinus arrhythmia.

P wave

Represents discharge of SA node and depolarization of both atria

Normal:

• The best lead to look at the P wave is V1.
• Normal P wave is upright and rounded
• The P wave in general should not be more than 1 box wide
• The P wave in general should not be more than  1 box tall.
• The p wave is biphasic in
• The P wave contour is constant

Abnormal:

• If  P wave exceeds the normal range for duration or voltage, it generally means that either or both atria is enlarged (hypertrophied)
• If P wave contour
• Peaking of P wave (Voltage increase) suggests Right atrial hypertrophy
• Broad slurred (increased duration)  suggests Left atrial hypertrophy
• When biphasic the initial positive wave is prominent with RA hypertrophy and the negative deflection is prominent wit LA hypertrophy
• If the P wave contour changes between beats it could mean that there is an ectopic atrial focus

QRS

QRS complex is a series of wave forms following P wave.

Naming convention:

• Q wave: first downstroke of the QRS complex. Usually very small or absent.
• R wave: first upward deflection of the QRS complex. Upward deflections occurring after an S wave are noted by a "prime mark" such as R'
• S wave: the first downward deflection occurring after the R wave.
• A monophasic negative QRS complex is called QS.

Normal

Duration: 0.08-0.12 seconds (2-3 horizontal boxes)

Contour is same between beats

Abnormal

Duration:

Delay in conduction through the ventricles leads to prolongation of QRS complex

• Prolonged: Bundle branch blocks, drug toxicity, electrolyte imbalance
• Shortened:  WPW

Contour

Change of contour between beats suggests ectopic foci

Abnormal  but constant contour suggests

• Bundle branch blocks
• Drug toxicity
• Electrolyte imbalance

Q wave

Normal:

Usually very small or absent

Normal in III and AVR .

Abnormal:

A Q wave is significant if it is greater than 1 box wide (0.04 secs) in leads other than III and AVR

Greater than 1/3 the amplitude of the QRS complex.

Greater than 1/4th of R wave

Abnormal Q waves: indicate presence of infarct

T wave

First upward deflection after QRS complex. Represents: ventricular repolarization

Normal:

In general, T waves are in the same direction as the largest deflection of the QRS (normally the R wave).

Negative in AVR

Inverted T waves in precordial leads V1, V2, V3 can be seen in normal, young athletes

Low T voltage changes may occur in the absence of any heart disease at all.

Abnormal:

T wave changes can be primary or secondary.

Primary T wave change refer to abnormal repolarization

Secondary T wave changes are caused by QRS changes.  T wave changes caused by bundle branch block or ventricular hypertrophy are secondary.

Tall peaked T waves

Electrolyte imbalance =  Hyperkalemia causes tall peaked T waves.  overall maximum of 15 mV but this is not sensitive.  T wave looks like an isosceles triangle.

Low voltage T waves

• Hypokalemia causes low voltage T waves and prominent U waves.  T waves less than 1mV in the limb leads and less than 2mV in the precordial leads.

• low T voltage and sagging or flattened ST segments.  these changes may occur in the absence of any heart disease at all.

Inverted T waves

• Inverted T waves that are symmetrical, "round-shouldered" can be caused by coronary ischemia. especially when it occurs in a pattern as previously described for ST segment changes. .
• Inverted T waves in precordial leads V1, V2, V3 can be seen in normal, young athletes, as well as CNS diseases.

U wave

What it represents is not certain.

This upright wave, when present, follows the T wave.

Abnormal:

The presence of  U waves may indicate Hypokalemia.

Hypokalemia is associated with flat T waves, U waves. U waves taller than T waves.

PR interval

Represents: atria to ventricular conduction time (through His bundle)  It includes P wave and PR segment.

Normal duration: 0.12-2.0 seconds (3-5 horizontal boxes). This is measured from the onset of the P wave to the onset of the QRS complex regardless if the initial wave is a Q or R wave.

Abnormal duration:

Prolonged:

If the PR interval is greater than 0.2 sec, then an AV block is present. There are several types of AV blocks:

• 1st degree AV Block: PR>0.20 sec.
• 2nd degree AV Block: 2 types:
1. Type I (Mobitz I or Wenckeback): increasing PR interval until a QRS complex is dropped. It is usually benign.
2. Type 2 (Mobitz II): QRS dropped without any progressive increase in PR interval (i.e., PR interval is constant but still >0.20 sec).
• 3rd degree AV Block: atria and ventricles are electrically dissociated. Therefore, P waves and QRS complexes will occur independent of each other. As always, use the QRS complexes to determine heart rate.

Shortened:

A PR interval that is <0.12 sec (when associated with a prolonged QRS) should prompt evaluation for Wolff-Parkinson-White Syndrome (WPW).
<0.12 sec when associated with prolonged QRS should prompt evaluation for Wolff-Parkinson syndrome (WPW).

ST segment

Represents early phase of repolarization of ventricles.

Begins at the end of S wave and ends at the beginning of T wave.

In normal situations, it serves as the isoelectric line from which to measure the amplitudes of the other waveforms.

ST segments are usually isoelectric and normal.

When examining the ST segment, evaluate elevations or depressions 0.06 seconds after the J point (since the ST segment can at times be sloping).

Abnormal:

This segment is important in identifying pathology such as myocardial infarctions (elevations) and Ischemia (depressions).

ST segment elevation

• In general, an ST segment elevation indicates infarction.

• ST segment elevation is a current of injury - can be seen in pericarditis as well as Prinzmetal's angina.

• Early repolarization causes ST segment elevation in the lead of normal EKG's.

The location of the ST elevations on the EKG can help to identify a location of the infarct:

• Anterior Wall Infarct (corresponding to Left Anterior Descending Artery): V1, V2
• Lateral Wall Infarct (Circumflex Artery): V3, V4
• Inferior Wall Infarct (can be combination of Circumflex or Right Coronary Artery): V5, V6, I, avl
• when in all leads suggests Pericarditis

ST segment depression

• In general, an ST segment depression indicates Ischemia

• ST segment depression can be Ischemia as in exercise EKG's or subendocardial injury current.

• Digitalis causes ST segment sagging and shortens the Q-T interval.

QT  and QTc (corrected QT) interval

QT represents the duration of activation and recovery of the ventricular muscle.

This duration varies inversely with the heart rate

Since the duration of QT varies inversely with the heart rate, the QT is not used, but rather the corrected QT is.

QTc interval

QTc = QT + 1.75 (ventricular rate - 60)

Normal:

The normal QTc is approximately 0.41 seconds. It tends to be slightly longer for females and increases slightly with age.

Abnormal:

Prolonged QT

• Quinidine Toxicity
• Hypocalcemia

Shortened QT

• May be shortened in hypocalcemia.