cardiovascular system - cardiac conduction system

Question 1

what is the cardiac conduction system

Answer 1

Network of nodes, cells and signals

Controls heartbeat

Inherent & rhythmical electrical activity
- Heart creates its own impulses

Assures that cardiac chambers contract in a co-ordinated manner
- making the heart an effective pump

Question 2

what are autorhythmic cells

Answer 2

Network of specialised cardiac muscle fibres

• In SA node, AV node & Purkinje fibres
• Self-excitable
• Pacemaker activity

Question 3

what action potentials do autorhythmic cells generate

Answer 3

spontaneous action potentials

Question 4

Nerve impulses from ANS and endocrine hormones modify heartbeat, but do not establish what

Answer 4

the fundamental rhythm

• A living heart removed from the body will continue to beat despite cutting nerves

Question 5

name the parts to a conduction system pathway

Answer 5

1. Sinoatrial (SA) node
2. Atrioventricular (AV) node
3. Bundle of His
4. Left & right bundle branches
5. Purkinje fibres

Question 6

explain the electrical conduction in myocardial cells

Answer 6

membrane potential of autorhythmic cell

this causes

membrane potential of contractile cell

(depolarization of autorhythmic cells rapidly spread to adjacent contractile cells through gap junctions)

Question 7

explain the process in the propagation of action potentials

Answer 7

1. an action potential is initiated in SA node (near top of heart)
2. action potentials are conducted from SA node to atrial muscle
3. in 50 msecs (milliseconds), action potentials spread through the atria to the AV node where conduction slows.
   100 msec delay at AV node due to smaller diameter fibres. This allows atria to fully contract, filling ventricles, before ventricles contract
4. action potentials travel rapidly through the conduction system to the apex of heart
5. in 50 msecs, action potentials spread upward through the ventricular muscle
6. eventually the entire heart returns to the resting state, remaining there untill another action potential is generated in SA node

Question 8

explain ventricular action potential

Answer 8

1. rapid depolarisation (as na enters)
2. plateau (maintained depolarisation; some k leaves)
   - slow as ca enters
   - then, binds to troponin
   - actin and myosin then slide
3. repolarisation (as k exits and ca leaves)

all of this is in 300msec

steps 1-3 are refectory period
(time interval during which a 2nd contraction cannot be triggered. Without this the chambers would not relax enough to refill)

step 2 and its steps are the cardiac muscle contraction part

Question 9

describe action potential shapes

Answer 9

they differ depending upon the node, fibre, or muscle

they all react at different times . SA node is fastest and ventricular muscle is slowest

Question 10

explain electrocardiogram (ECG)

Answer 10

Recording of electrical changes that accompany AP propagation that can be detected at surface of body

Can be resting, stress & ambulatory.

Place electrodes on 4 limbs & 6 positions on chest

Question 11

explain the use of ECGs

Answer 11

Amplifies heart’s electrical activity

Used to determine whether:
- Conduction pathway is abnormal
- Heart enlarged
- Certain regions are damaged

Question 12

explain normal sinus rhythm

Answer 12

rhythm: regular

rate: 60-100 pbm

P wave: upright, uniform and round in a 1:1 ratio

PRI: 0.12-0.20 sec

QRS I: less than 0.12 sec

Question 13

name the 3 recognisable waves that accompany each heartbeat

Answer 13

p wave
QRS complex
T wave

Question 14

explain the P wave which accompanies each heartbeat

Answer 14

atrial depolarisation (contraction)

Question 15

explain QRS complex waves which accompany each heartbeat

Answer 15

onset of ventricular depolarisation (contraction)

Question 16

explain T wave which accompany each heartbeat

Answer 16

ventricular repolarisation (just before ventricles start to relax)

Question 17

the 3 recognisable waves waves that accompany each heartbeat give clues to what

Answer 17

abnormalities

Question 18

explain how P wave in heart beat gives clues to abnormities

Answer 18

if larger wave: enlarged atrium

Question 19

explain how QRS wave in heart beat gives clues to abnormities

Answer 19

if enlarged Q: MI

if enlarged R:enlarged ventricles

Question 20

explain how T wave in heart beat gives clues to abnormities

Answer 20

if flatter: insufficient O2 e.g., coronary artery disease

Question 21

explain how P-Q interval in heart beat gives clues to abnormities

Answer 21

if lengthened: scar tissue e.g., coronary artery disease or rheumatic fever

Question 22

explain how S-T segment in heart beat gives clues to abnormities

Answer 22

if elevated, acute MI; if depressed: insufficient O2

Question 23

explain how Q-T interval in heart beat gives clues to abnormities

Answer 23

if lengthened: myocardial damage or coronary ischaemia or conduction abnormalities

Question 24

what is the conduction system

Answer 24

ECG wave production

Question 25

describe healthy adult pulse

Answer 25

At rest: average 75 bpm (range 60-100)

At sleep: 40 bpm

With strenuous exercise: 200-220 bpm

Question 26

what can modify the heartbeat

Answer 26

hormones
neurotransmitters
ectopic pacemaker

Question 27

explain hormones and neurotransmitters in terms of modifying heartbeat

Answer 27

Speed or slow pacing of heart by SA node fibres

e.g., acetylcholine (from parasympathetic division of ANS) decreases from 90-100 to 75 AP/min at rest

Question 28

explain ectopic pacemaker in terms of modifying heartbeat

Answer 28

Out-with SA node

Develops abnormal excitability
- Can produce extra beats
or abnormally pace heart

Can be triggered by stimulants
- e.g., nicotine, caffeine