Heart Block- Dr. Chintan

Sinoatrial Block• the impulse from the sinus node is blocked before

it enters the atrial muscle

• sudden cessation of P waves, with resultantstandstill of the atria.

• the ventricles pick up a new rhythm, the impulseusually originating spontaneously in theatrioventricular (A-V) node

• the rate of the ventricular QRS-T complex is slowed

Atrioventricular Block

• The only means by which impulses ordinarily can

pass from the atria into the ventricles is through

the A-V bundle, also known as the bundle of His

• 1. Ischemia of the A-V node or A-V bundle fibers

• 2. Compression of the A-V bundle

• 3. Inflammation of the A-V node or A-V bundle

• 4. Extreme stimulation of the heart by the vagus

nerves

Incomplete AtrioventricularHeart Block

• Prolonged P-R Interval - First Degree Block

• time between beginning of the P wave andbeginning of the QRS complex is about 0.16second

• P-R interval usually decreases in length with fasterheartbeat and increases with slower heartbeat

• when the P-R interval increases to greater than0.20 second, the P-R interval is said to be prolonged

Incomplete A-V Heart Block

• First degree block is defined as a delay of

conduction from the atria to the ventricles but not

actual blockage of conduction

Incomplete A-V Heart Block• Second Degree Block

• increase the P-R interval to 0.25 to 0.45 second,

• the action potential sometimes is strong enough topass through the bundle into the ventricles andsometimes is not strong enough

• atrial P wave but no QRS-T wave, and it is said thatthere are “dropped beats” of the ventricles

• “2:1 rhythm”, 3:2 or 3:1

Complete A-V Block• Third Degree Block

• ventricles spontaneously establish their own signal,usually originating in the A-V node or A-V bundle

• P waves become dissociated from the QRS-Tcomplexes

• rate of rhythm of the atria in ECG is about 100 beatsper minute, whereas the rate of ventricular beat is lessthan 40 per minute

• Ventricles have “escaped” from control by the atria

Stokes-Adams Syndrome

Ventricular Escape

Incomplete Intraventricular Block

• electrical alternans

• Tachycardia

• when the rate of the heart is rapid, it is impossible

for some portions of the Purkinje system to

recover from the previous refractory period quickly

enough to respond

• ischemia, myocarditis or digitalis toxicity

Grading of exercise

WHO grading of muscular exercise:

VO2 Max: Maximum amount of O2 that can beconsumed during a severe exercise.

Resting VO2 ( O2 consumption ) = 250 ml/min.

Grade HR (bpm) O2 consumption (L/min)

RLI MET

i. Light (mild) < 100 0.4 – 0.8 < 25 < 3

ii. Moderate 100 - 125 0.8 – 1.6 25 - 50 3.1 – 4.5

iii. Heavy 125 - 150 1.6 – 2.4 51 - 75 4.6 - 7

iv. Severe > 150 > 2.4 > 75 > 7

Causes of ↑ HR during exercise(1) Increased sympathetic activity – anticipatory

tachycardia.

(2) Decrease in Vagal tone.

(3) Increased activity of the limbic system and motorcortex due to psychic stimuli directly acting onmedulla – anticipatory tachycardia.

(4) Increased Peripheral reflexes originating from(i) Muscle spindle(ii) Muscle tendon receptors(iii) Joint receptors(iv) Organ of Corti

Causes of ↑ HR during exercise(5) Liberation of hormones into circulation (Centrallyacting)

(i) Catecholamines from Adrenal Medulla

(ii) Thyroxin (T4) from thyroid gland in response to stress

(6) Increase in body temperature

(7) Chemical changes occurring in the blood

Hypoxia - arterial PO2

Hypercapnia - arterial PCO2

Lactic Acidosis - pH

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