Cardiac Conduction

CARDIAC CONDUCTION

Q. 1

Choose the TRUE/FALSE statements regarding cardicac conduction functions:

a) The SA node is composed of a cluster of small fusiform cells in the sulcus terminalis at the right atrial–superior vena caval junction
b) The SA nodal artery arises from the right coronary artery in 20 % and the left circumflex artery in 80 % of persons
c) SA node dysfunction may be completely asymptomatic 
d) Almost all  patients with SA node dysfunction develop supraventricular tachycardia, usually atrial fibrillation or atrial flutter
e) Failure to increase the heart rate with exercise is referred to as chronotropic incompetence
 
 A

a,b,c,d,e-True & None-False

 B

a,e-True & b,c,d-False

 C

a,c,e-True & b,d-False

 D

b,c,d-True & a,e-False

Q. 1

Choose the TRUE/FALSE statements regarding cardicac conduction functions:

a) The SA node is composed of a cluster of small fusiform cells in the sulcus terminalis at the right atrial–superior vena caval junction
b) The SA nodal artery arises from the right coronary artery in 20 % and the left circumflex artery in 80 % of persons
c) SA node dysfunction may be completely asymptomatic 
d) Almost all  patients with SA node dysfunction develop supraventricular tachycardia, usually atrial fibrillation or atrial flutter
e) Failure to increase the heart rate with exercise is referred to as chronotropic incompetence
 
 A

a,b,c,d,e-True & None-False

 B

a,e-True & b,c,d-False

 C

a,c,e-True & b,d-False

 D

b,c,d-True & a,e-False

Ans. C

Explanation:

The SA nodal artery arises from the right coronary artery in 55–60% and the left circumflex artery in 40–45% of persons. One-third to one-half of patients with SA node dysfunction develop supraventricular tachycardia, usually atrial fibrillation or atrial flutter.

 
Ref: Harrisons Principles of Internal Medicine, 18th Edition, Page 1869

Q. 2

SA node acts as a pacemaker of the heart because of the fact that it:

 A

Is capable in generating impulses spontaneously

 B

Has rich sympathetic innervations

 C

Has poor cholinergic innervations

 D

Generates impulses at the highest rate

Q. 2

SA node acts as a pacemaker of the heart because of the fact that it:

 A

Is capable in generating impulses spontaneously

 B

Has rich sympathetic innervations

 C

Has poor cholinergic innervations

 D

Generates impulses at the highest rate

Ans. D

Explanation:

The sinoatrial (SA) node normally displays the highest intrinsic rate. All other pacemakers are referred to as subsidiary or latent pacemakers because they take over the function of initiating excitation of the heart only when the SA node is unable to generate impulses or when these impulses fail to propagate. 

There is a hierarchy of intrinsic rates of subsidiary pacemakers that have normal automaticity: atrial pacemakers have faster intrinsic rates than AV junctional pacemakers, and AV junctional pacemakers have faster rates than ventricular pacemakers.
 
Ref: Chen P., Antzelevitch C. (2011). Chapter 38. Mechanisms of Cardiac Arrhythmias and Conduction Disturbances. In V. Fuster, R.A. Walsh, R.A. Harrington (Eds), Hurst’s The Heart, 13e.

Q. 3

Slowest conduction is seen in:

 A

SA node

 B

AV node

 C

Bundle of his

 D

Pukinje fibres

Q. 3

Slowest conduction is seen in:

 A

SA node

 B

AV node

 C

Bundle of his

 D

Pukinje fibres

Ans. B

Explanation:

The Atrio Ventricular (AV) node is the slowest conducting structure with the longest refractory period in the heart.

Cardiac tissue conduction velocity (fastest –> slowest):
Purkinje system –> Atrial muscle –> Ventricular muscle –> AV node
The slowest conduction velocity is in the AV node 
  • Allows time to the atria to contract
  • Allows complete ventricular filling
  • The fastest Conduction velocity is in the Purkinje fibers 
  • Allows the ventricles to contract at the same time simultaneously
Cardiac tissue Conduction: The heartbeat originates in a specialized cardiac conduction system and spreads via this system to all parts of the myocardium. The structures that make up the conduction system are the sinoatrial node (SA node), the internodal atrial pathways, the atrioventricular node (AV node), the bundle of His and its branches, and the Purkinje system.
 
The factors related to the speed of conduction are the
  • Magnitude and rate of rise of the action potential 
  • Diameter of the fibres (AV node radius of the fibres 7 µm and purkinje fibres is close to 50 µm).
  • The conduction also depends on the tight junctions: The AV node conduction is slow due to diminished gap junctions between cells, taking ~ 0.09 sec and 0.04 sec, respectively; there is an abundance of gap junctions in intercalated discs between Purkinje fiber cells, therefore traversing entire expanse of ventricle only takes 0.03 sec after depolarization front exits the septum.
AV nodal delay: There is delay of 0.1 s before the excitation spreads to ventricles. This delay is shortened by stimulation of the sympathetic nerves to the heart and lengthened by stimulation of the vagi. 
 

Tissue

Conduction speed (m/s)

Sino atrial node

0.05

Atrial pathways

1

Atrio ventricular node

0.05

Bundle of His

1

Purkinje system

4

Ventricular muscle

1

Ref: Cardiovascular physiology, By William R Milnor, Page 88.


Q. 4

The physiological change occurs in a cardiac muscle cell when there is plateau phase of action potential is:

 A

Influx of Na

 B

Influx of Ca2+

 C

Influx of K+

 D

Closure of voltage gated K channels

Q. 4

The physiological change occurs in a cardiac muscle cell when there is plateau phase of action potential is:

 A

Influx of Na

 B

Influx of Ca2+

 C

Influx of K+

 D

Closure of voltage gated K channels

Ans. B

Explanation:

The transmembrane action potential of single cardiac muscle cells is characterized by,

  • Rapid depolarization (phase 0)
  • Initial rapid repolarization (phase 1)
  • Plateau (phase 2)
  • Slow repolarization process (phase 3) that allows return to the resting membrane potential (phase 4). 
The initial depolarization is due to Na+ influx through rapidly opening Na+ channels (the Na+ current, INa). The inactivation of Na+ channels contributes to the rapid repolarization phase. Ca2+ influx through more slowly opening Ca2+ channels (the Ca2+ current, ICa) produces the plateau phase, and repolarization is due to net K+ efflux through multiple types of K+ channels. 
 
Ref: (2012). Chapter 29. Origin of the Heartbeat & the Electrical Activity of the Heart.In Barrett K.E., Boitano S, Barman S.M., Brooks H.L. (Eds), Ganong’s Review of Medical Physiology, 24e.

Q. 5

In conduction system of heart the maximum velocity of conduction is seen in?

 A

SA node

 B

AV node

 C

Bundle of HIS

 D

Purkinje fibers

Q. 5

In conduction system of heart the maximum velocity of conduction is seen in?

 A

SA node

 B

AV node

 C

Bundle of HIS

 D

Purkinje fibers

Ans. D

Explanation:

Purkinje fibers are characterized by action potentials with a low resting membrane potential (— 90 mV), a rapid maximum upstroke velocity (500 to 700 V/s), and, therefore, a rapid conduction velocity.

Purkinje’s fibres have many gap junctions which allows the rapid flow of current and hence the conduction velocity is maximum in Purkinje’s fibres. This rapid conduction allows simultaneous contraction of both the ventricles.

Ref: Moss and Adams’ Heart Disease in Infants, Children, and Adolescents … By Arthur J. Moss, Hugh D. Allen, M.D., 2007, Page 24, Table 1.2; Guyton’s physiology, 22nd edition, Page 105.


Q. 6

The following statements are true regarding the SA node except:

 A

Is located at the right border of the ascending aorta

 B

It contains specialized nodal cardiac muscle

 C

It is supplied by the artial branches of the right coronary artery

 D

It initiates cardiac conduction

Q. 6

The following statements are true regarding the SA node except:

 A

Is located at the right border of the ascending aorta

 B

It contains specialized nodal cardiac muscle

 C

It is supplied by the artial branches of the right coronary artery

 D

It initiates cardiac conduction

Ans. A

Explanation:

A i.e. Is located at the right border of ascending aorta

S.A node is located at the junction of superior venacave with right atriumQ just deep to the epicardium, near the superior end of sulcus terminalis and AV node is located in the right postero-inferior region of interatrial septum near the opening of coronary sinus.


Q. 7

Plateau phase of ventricular muscle is d/f opening of

 A

Na+ channel

 B

K+ channel

 C

Ca++ – Na+ channel

 D

Closure of K+ channel

Q. 7

Plateau phase of ventricular muscle is d/f opening of

 A

Na+ channel

 B

K+ channel

 C

Ca++ – Na+ channel

 D

Closure of K+ channel

Ans. C

Explanation:

C i.e. Ca++-Nat Channel

In cardiac (e.g. ventricular) muscle, rapid depolarization phase (0) is d/t Nat influx through rapidly opening fast Nat channels (Nat current, INa); initial rapid repolarization phase (1) is d/t inactivation of Nat channel; platue phase (2) is d/t Cattinflux (Ca+tcurrent, Ica) through more slowly opening (slow) Ca++channels (also called calcium-sodium channels) and a slow repolarization phase (3) due to net Kt efflux through multiple types of K+ channels, allows the cell to return to resting membrane potential. Myocardial fibers (cells) have RMP of approximately -90mv.

Action Potential in Cardiac Ventricular Muscles

volts in ventricular muscle fibers instead of -55 milivolts in the nodal fibers.

– Action potential in cardiac muscle fibers averages about 105 mili volts i.e. the intracellular potential rises form a very negative -90 mV between beats to slightly positive +20 mV during each beat.

3 type of membrane ion channels in cardiac muscles responsible for causing the voltage changes include

Channel

Effect

Fast Na’

Rapidly opening of fast sodium channels for

Channel

10,000 the of a second) causes rapid influx of

positive sodium ion (Na+) to the interior of

cardiac muscle fiber (Necurrent, IN.). It is

responsible for rapid upstroke spike of action

potential or rapid depolarization phase 0.

 

Initial rapid repolarization phase lis caused by

inactivation of Na+ channels.

Slow Ca++

Slower opening of slow calcium (or sodium‑

(Na+ – Ca++)

calcium) channels for about 0.3 second (i.e

Channels

longer duration) causes large influx of both

 

Ca*+ and Na+ positive ions to interior of cardiac

muscle fibre. This (Ca++ current/influx, Ica

mainly) is responsible for maintenance of

prolonged period of depolarization or platue

phase (2) of ventricular AP.

Potassium

Rapid diffusion of large amounts of positive K+

(K+) Channels

ions in outward direction (efflux) from the

cardiac muscle fibre (Ek+, K+ current)

immediately returns the membrane potential to

its resting levels, thus ending the AP

Difference b/w AP of Cardiac of Skeletal Muscles

Cardiac muscles have prolonged AP (Action potential) and a platue phase, whereas skeletal muscles do not because

  1. AP of skeletal muscle is caused and almost entirely by sudden opening of fast Na+ channels for a very short period. At the end of this abrupt closure, repolarization occurs and AP is over in another thousandth of a second. Whereas AP in cardiac muscle is caused by same fast Na+ channels as well as slow Ca+ (Na+

channels which are slower to open and more importantly remain open for several tenths of a second (i.e prolonged duration) thereby maintaining prolonged period of depolarization, causing the platue phase.

  1. In skeletal muscles, Ca++ ions for contraction is derived from intracellular sacroplasmic reticulum. Whereas, in cardiac muscles the Ca** that enters during plate phase activate the muscle contractile process.
  2. Immediately after the onset of AP, the permeability of cardiac muscle fiber for K+ decreases about 5 folds as a result of excess Ca++ influx through Ca++ channels. Therefore, greatly decreased outflux of positively charged K+ ions during AP platue in cardiac muscles prevents early return of AP voltage to its resting levels. This greatly decreased K+ permeability is not seen in skeletal muscles.

Difference b/w AP of Cardiac Muscle & Sinus Nodal Fibers

– AT the less negative (- 55 mV) RMP of nodal fibers the fast Na* channels mainly have already become inactivated (or blocked). Therefore only the slow Na+ channels can become activated (open) and cause AP.

– So AP is slower to develop in nodal fibers than ventricular fibers. Similarly return of potential to its negative resting state occurs slowly as well rather than the abrupt return that occurs in ventricular muscles.


Q. 8

Speed of conduction is fastest in:

 A

AV node

 B

SA node

 C

Bundle of His

 D

Purkinje system

Q. 8

Speed of conduction is fastest in:

 A

AV node

 B

SA node

 C

Bundle of His

 D

Purkinje system

Ans. D

Explanation:

D i.e. Purkinje System


Q. 9

Single most important factor in control of automatic contractility of heart is:

 A

Myocordial wall thickness

 B

Right atrial volume

 C

SA node pacemaker potential

 D

Sympathetic stimulation

Q. 9

Single most important factor in control of automatic contractility of heart is:

 A

Myocordial wall thickness

 B

Right atrial volume

 C

SA node pacemaker potential

 D

Sympathetic stimulation

Ans. D

Explanation:

D i.e. Sympathetic stimulation

Intrinsic factor

Length of activations

Most important factor

Frank- starling law: “Energy of contraction is

proportional to initial length of cardiac muscle fiber.” Length of muscle fiber is proportionate to end diastolic blood volume (preload)Q.

Extrinsic factor

Adrenergc neural stimulation/ Sympathetic stimulationQ Inotropic effect is smaller, Maximum in atria

– It becomes important when the heart is increased as it permits adequate diastolic filling.



Q. 10

Highest rate of impulse generation is given in:

 A

SA node

 B

AV node

 C

Bundle of HIS

 D

Purkinje system

Q. 10

Highest rate of impulse generation is given in:

 A

SA node

 B

AV node

 C

Bundle of HIS

 D

Purkinje system

Ans. A

Explanation:

Answer is A (SA Node)

The rate of impulse generation/rate of discharge is highest in the SA node.

Note:

Highest Rate of impulse generation in seen in     : SA NodeQ

Highest Rate of conduction is seen in                  : Purkinje SystemQ


Q. 11

True about SA node are all except ‑

 A

Supplied by nodal artery

 B

Primary pacemaker

 C

Supplied by left vagus nerve

 D

Made up of nodal cells and connective tissue

Q. 11

True about SA node are all except ‑

 A

Supplied by nodal artery

 B

Primary pacemaker

 C

Supplied by left vagus nerve

 D

Made up of nodal cells and connective tissue

Ans. C

Explanation:

 

  • SA node is located in the upper part of crista terminalis at the junction of SVC and the right atrium.
  • It is the pacemaker of the heart and generates impulse at a rate of 70-100/min.
  • SA node is suplied by nodal artery, a branch of RCA in 65% cases and a branch of circumflex branch of LCA in 35% cases.
  • SA node is supplied by right vagus/parasympathetic (inhibitory) and right Sympathetic (excitatory) system as it develops from structures on the right side of embryo.
  • The SA node consists of connective tissue stroma containing an irregular whorled network of cardiac nodal cells, the SA nodal artery and numerous nerve endings (postganglionic parasympathetic and postganglionic sympathetic).

Q. 12

SA node is located in ‑

 A

Triangle of Koch’s

 B

In crista terminalis

 C

In membranous part of interventricular septum

 D

Upper part of interatrial septum

Q. 12

SA node is located in ‑

 A

Triangle of Koch’s

 B

In crista terminalis

 C

In membranous part of interventricular septum

 D

Upper part of interatrial septum

Ans. B

Explanation:

In crista terminalis


Q. 13

Vagal stimulation in heart causes decrease in heart rate by ‑

 A

Decrease in action potential spike

 B

Decrease in slope of prepotential

 C

Increase in repolarization

 D

Decrease conduction

Q. 13

Vagal stimulation in heart causes decrease in heart rate by ‑

 A

Decrease in action potential spike

 B

Decrease in slope of prepotential

 C

Increase in repolarization

 D

Decrease conduction

Ans. B

Explanation:

Ans. is ‘b’ i.e., Decrease in slope of prepotential

Effect of autonomic nervous system

The SA node develops from structures on the right side of the embryo and the AV node from structures on the left. This is why right vagus (parasympathetic) is distributed primarily to SA node and the left vagus (parasympathetic) mainly to AV node. Similarly, the sympathetic innervation on the right side is distributed primarily to the SA node and the sympathetic innervation on the left side primarily to AV node.

Effects of ANS on cardiac physiology are as follow : ‑

A) Parasympathetic (vagal) stimulation

i) Negative chronotropic (decreased heart rate) : – Vagal stimulation cause decrease in slope (flattening) of prepotential (pacemaker potential) and therefore the time taken to reach the threshold level is increased –> Heart rate is decreased.

ii) Negative dromotropic (decreased conduction).

iii) Increased refractory period of all type of cardiac cells.

B) Sympathetic stimulation

i) Positive chronotropic (Increased heart rate):- Sympathetic stimulation increases the slope of phase 4 prepotential (pacemaker potential); therefore, time taken to reach the threshold is decreased and heart rate is increased.

ii) Positive ionotropic (Increased contractility).

iii) Positive dromotropic (Increased conduction velocity in conductive tissue).

iv) Decreased in refractory period of all type of cardiac cells.

v) Positive bathmotropic (Increased automaticity).


Q. 14

AH interval on ECG is for conduction ‑

 A

Through His-Purkinje system

 B

From AV node to bundle of His

 C

From Purkinje system to ventricular fibres

 D

Through ventricular fibers

Q. 14

AH interval on ECG is for conduction ‑

 A

Through His-Purkinje system

 B

From AV node to bundle of His

 C

From Purkinje system to ventricular fibres

 D

Through ventricular fibers

Ans. B

Explanation:

Ans. is ‘b’ i.e., From AV node to bundle of His

Certain intervals are measured to provide information regarding AV block.

i) PA interval : It is time taken from onset of P-wave to onset of deflection in atrial catheter. It is an index of intra-atrial conduction. Normal vlaue is less than 55 mS.

ii) AH interval : it represents conduction through AV node, i.e., conduction from AV node to bundle of His. Normal vlaue is less than 130 mS.

iii) HV interval : It represents conduction through His-Purkinje system, i.e., conduction from His-Purkinje system to ventricular muscle fibers. Normal value is less than 50 mS.


Q. 15

The plateau phase of this graph is  due to:

 A

The movement of fewer sodium ions across the cell membrane

 B

The calcium channels remaining open longer than the sodium channels

 C

The increased membrane permeability to potassium ion

 D

A decrease in the amount of calcium diffusing across the membrane

Q. 15

The plateau phase of this graph is  due to:

 A

The movement of fewer sodium ions across the cell membrane

 B

The calcium channels remaining open longer than the sodium channels

 C

The increased membrane permeability to potassium ion

 D

A decrease in the amount of calcium diffusing across the membrane

Ans. B

Explanation:

Ans: B.)The calcium channels remaining open longer than the sodium channels.

Action potential of cardiac muscles:

 

Phase 2 / “Plateau phase”:

  • Calcium channels open and fast potassium channels close.
  • A brief initial repolarization occurs.
Why plateau-shaped?
  • Action potential then plateaus as a result of,
    • Increased calcium ion permeability
    • Decreased potassium ion permeability.

Events during phase 2:

The voltage-gated calcium ion channels open slowly during phases 1 and 0, and calcium enters the cell. Potassium channels then close, and the combination of decreased potassium ion efflux and increased calcium ion influx causes the action potential to plateau


Note on other phases:

1. Phase 0 / Depolarization:

  • Fast sodium channels open. 
  • When the cardiac cell is stimulated and depolarizes, the membrane potential becomes more positive. 
  • Voltage-gated sodium channels (fast sodium channels) open and permit sodium to rapidly flow into the cell and depolarize it.
  • The membrane potential reaches about +20 millivolts before the sodium channels close.
2. Phase 1 / “Initial Repolarization”:
  • Fast sodium channels close. 
  • Cellular repolarization starts, and potassium ions leave the cell through open potassium channels.

3. Phase 3 / “Rapid Repolarization”:

  • Calcium channels close and slow potassium channels open.
  • The closure of calcium ion channels and increased potassium ion permeability.
  • This permits potassium ions to rapidly exit the cell, ends the plateau and returns the cell membrane potential to its resting level.

4. Phase 4 / “Resting membrane potential”:

  • Averages about  “−90 millivolts”

 



Leave a Reply

%d bloggers like this:
Malcare WordPress Security