The Cardiovascular System, The normal Electrocardiogram, ECG

Remember the waves precede the contraction. The PR interval has a normal value of 0.16 seconds and is the delay between the atrial contraction and the beginning of the ventricular contraction.

The QT interval has a normal value of 0.35 seconds - the time of ventricular contraction.
The heart rate can be determined by the reciprocal of the time interval between each heart beat.

During depolarisation process, the average electrical current flows from the base of the heart toward the apex.

ECG leads - there are three bipolar limb leads. Lead I: negative lead is connected to the Right arm and the positive terminal is connected to the left arm.
Lead II: the negative terminal of the ECG is connected to the right arm and the positive terminal is connected to the left leg.
Lead III: the negative terminal is connected to the left arm and the positive terminal is connected to the left leg.

Einthoven's Law states that the electrical potential of any limb lead equals the sum of the potentials of the other two limb leads.

Einthoven's law

the potential differences between the bipolar leads measured simultaneously will, at any given moment, have the values II = I + III.

Einthoven triangle

an equilateral triangle used as a model of the standard limb leads used in electrocardiography.

Limb leads

In both the 5- and 12-lead configuration, leads I, II and III are called limb leads. The electrodes that form these signals are located on the limbs—one on each arm and one on the left leg.^[16]^[17]^[18] The limb leads form the points of what is known as Einthoven's triangle.^[19]

Lead I is the voltage between the (positive) left arm (LA) electrode and right arm (RA) electrode:

I = L A - R A .

Lead II is the voltage between the (positive) left leg (LL) electrode and the right arm (RA) electrode:

I I = L L - R A .

Lead III is the voltage between the (positive) left leg (LL) electrode and the left arm (LA) electrode:

I I I = L L - L A .

Simplified electrocardiograph sensors designed for teaching purposes at e.g. high school level are generally limited to three arm electrodes serving similar purposes. ^[20]

^{So if I= - 0.2mV, II=0.3mV, III=1.0mV.}

^{I = LA-RA=(0.3- -0.2) = 0.5}

^{II = LL-RA=(1.0- -0.2)= 1.2}

^{III= LL-LA =(1.0 - 0.3)= 0.7}

Chest leads (precordial leads) can be used to detect minor electrical abnormalities in the ventricles.
Augmented unipolar leads are also used to record the ECG: aVR, aVL, aVF

ECG Interpretation of Cardiac Muscle and Coronary Abnormalities.
Vector analysis:
Vectors can be used to represent electrical potentials.

Current flows from the area of depolarisation to polarised areas, and the potential generated can be represented by a vector, with the arrowhead pointing towards the positive direction.
The length of the vector is proportional to the voltage of the potential.
The generated potential can be represented by the instantaneous mean vector.
When a vector is horizontal and points towards the subjects left side, the apex is said to be at zero degrees.
The scale of the vector rotates clockwise from the zero reference point.
The vector points directly downward, it has a direction of +90 degrees.
If the vector points horizontally to the subjects right side it is said to have a direction of +180 degrees.
If the vector points directly upward, it is said to have a direction of -90 degrees or +270 degrees.
The axis of lead one is zero degrees, the axis of lead II is +60 degrees
The axis of lead III is 120 degrees
When the vector representing the mean direct current flow is perpendicular to the axis of one of the bipolar limb leads, the voltage recorded in the ECG is very low.
When the vector has approximately the same direction as the axis of one of the bipolar limb leads, nearly the entire voltage will be recorded in this lead.

The normal ECG represents vectors that occur during electrical potential changes in the cardiac cycle.

QRS complex - ventricular depolarisation, from the ventricular septum to the apex of the heart with an average direction of 59 degrees.

The ventricular T wave represents repolarisation of the ventricle that begins at the apex of the heart and proceeds to the base. The apex cardiac muscle become electropositive after it repolarises and the muscle near the base is electronegative.

The atrial P wave represents depolarisation of the atria, direction from the SA node to the AV node.

Shift in the Mean Axis of the Heart:

Changes in position of the heart - as occur in expiration, when a person is recumbant and the abdominal contents pressing upwards against the diaphragm.

Accumulation of abdominal fat, presses upward against the heart

Left Bundle branch block, R ventricular spread > L ventricular spread, Right Axis deviation

Hypertrophy of the left ventricle, secondary to HT, aortic valvular stenosis, or aortic valvular regurgitation (AS, AI).
LV hypertrophy secondary to HT

Left ventricular hypertrophy
Classification and external resources
Heart left ventricular hypertrophy short axis view
ICD-10	I 51.7
ICD-9	429.3
DiseasesDB	7659
MeSH	D017379

Echocardiography

Two dimensional echocardiography can produce images of the left ventricle. The thickness of the left ventricle as visualized on echocardiography correlates with its actual mass. Normal thickness of the left ventricular myocardium is from 0.6 to 1.1 cm (as measured at the very end of diastole. If the myocardium is more than 1.1 cm thick, the diagnosis of LVH can be made.

ECG criteria for LVH

There are several sets of criteria used to diagnose LVH via electrocardiography.^[3]

The Sokolow-Lyon index^[4]^[5]:

S in V₁ + R in V₅ or V₆ (whichever is larger) ≥ 35 mm
R in aVL ≥ 11 mm

Factors that shift the mean axis of the ventricles to the right (clockwise):

Inspiration
Standing up
Lack of abdominal fat - axis has a clockwise rotation
RBBB
RVH

Conditions that cause abnormal Voltages at the QRS Complex
Hypertrophy - LVH
The following decrease the voltage of the QRS:
Hearts with old MI's - decrease in cardiac muscle mass and conduction slows - therefore a prolonged QRS complex.
Pericardial Effusion - effectively "short circuit" the cardiac electrical potential. Both conducts electrical current from the heart and prevents much of the voltage from reaching the body surface.
Pulmonary emphysemia - excess air volume insulates the heart.

Conditions that cause a prolonged QRS complex:

prolonged conduction in the ventricles - both in hypertrophied and dilated hearts and increase the duration of the QRS by 0.02 to 0.05 seconds.
Blockade of the impulses in the Perkinje system

CURRENT OF INJURY
Abnormalities that cause the heart to remain depolarised all the time, and the current flows from the depolarised area to the polarised area is called the current of injury. Causes:
Mechanical trauma
Infection process that damages the cardiac muscle
Coronary Ischemia

The axis of current of injury can be determined with ECG. Seen at the end of the QRS complex when the entire heart has depolarised.

The axis is determined as follows:

determine the J point - which is the point of zero potential at the end of the QRS complex.
Determine the level of the TP segment wrt the J Point on the three standard leads.
Plot the voltages on the coordinates of the three leads and note that the NEGATIVE end of the vector points to the injured area of the ventricles.

Acute anterior and posterior wall infarctions:

Recent anterior infarction shows Q waves, inversion of the T wave.

Old anterior infarct shows normalization of the ST segment and T waves, but loss of R wave in V2-V3, Q in V4.

Acute anterior infarction

“Poor R wave progression”

Abnormalities in the T waves -

During Bundle Branch Block - one ventricle depolarises after the other. The first to depolarise is also the first to repolarise, and this causes deviation in the T wave. Therefore a LBBB causes a rightward axis deviation of the T wave.

During prolongation in depolarisation of the apex, the base of the heart repolarises before the apex, this inverts the T wave - mild ischemia of cardiac muscle in the apex.