EKGs and the Cardiac Cycle

Question 1

Action Potentials of Contractile Heart Cells

Answer 1

• Most of the heart is made of contractile cells
• contractile cells are responsible for the heart’s pumping function
• like skeletal muscle, depolarization precedes contraction
• unlike skeletal muscle, action potentials in cardiac muscle cells have a characteristic hump/plateau

Question 2

non contractile cells

Answer 2

pacemaker cells

Question 3

3 steps of cardiac action potential

Answer 3

1. depolarization
2. voltage change opens Ca2+ channels, influx of extracellular Ca2+
3. repolarization

Question 4

depolarization

Answer 4

opens fast sodium (na+) channels, extracellular Na+ enters
- rising phase of action potential (-90mV to +30mV)
- influx of Na+ will stop quickly

Question 5

voltage change opens ca2+ channels, influx of extracellular ca2+

Answer 5

• ca2+ influx prolongs depolarization - the plateau
• cells will contract as long as ca2+ is entering

Question 6

repolarization

Answer 6

results from inactivated ca2+ channels; the opening of potassium (k+) channels - an efflux of K+
- resting potential (-70mV) is restored
- ca2+ is either pumped out of the cell or into the sarcoplasmic reticulum

Question 7

function of the plateau

Answer 7

• both the action potential and the contraction phase are longer in cardiac muscle than skeletal muscle
• sustained contraction ensures efficient ejection of blood from the ventricles
• longer absolute refractory period avoids tetany

Question 8

electrocardiography

Answer 8

detects the electrical currents generated in and transmitted through the body

Question 9

ECG/EKG

Answer 9

graphic recording of the heart’s activity
- composite of all action potentials generated by nodal and contractile cells
- typically has 3 distinguishable waves or deflections

Question 10

P wave

Answer 10

EKG wave
- lasts 0.08 seconds
- results from movement of the depolarization wave from SA to AV node
- the atria contract 0.1s after the P wave begins

Question 11

QRS complex

Answer 11

EKG
- lasts 0.08s
- results from ventricular depolarization

Question 12

T wave

Answer 12

EKG
- lasts 0.16s
- results from ventricular repolarization

Question 13

atrial repolarization

Answer 13

occurred during ventricular depolarization - the resultant wave was obscured by the qrs complex

Question 14

intverval

Answer 14

duration of time that includes 1 segment and 1+ wave

Question 15

segment

Answer 15

a region between 2 waves

Question 16

P-R interval

Answer 16

0.16s - the beginning of atrial depolarization to the beginning of ventricular depolarization

Question 17

S-T segment

Answer 17

• action potentials of ventricular myocytes are in plateau, the entire ventricular myocardium is depolarized
• an elevated or depressed ST segment can indicate cardiac ischemia

Question 18

depolarization and repolarization

Answer 18

always precede mechanical events of the heart

Question 19

Intrinsic conduction with EKG

Answer 19

1. Atrial Depolarization
   - Completed by SA Node, causes P Wave
2. Atrial Depolarization Complete
   - Impulse delayed at AV Node
3. Ventricular Depolarization
   - Begins at apex, causes QRS complex
   - Atrial repolarization occurs, but is obscured
4. Ventricular Depolarization Complete
5. Ventricular Repolarization
   - Begins at apex, causes T Wave
6. Ventricular Repolarization Complete

Question 20

cardiac cycle

Answer 20

mechanical events of the heart

Question 21

systole

Answer 21

contraction
- blood is forced out of the hearts chambers

Question 22

diastole

Answer 22

relaxation
- blood refills the heart’s chambers

Question 23

1 cardiac cycle

Answer 23

all events associated with blood flow through the heart in 1 complete beat
(atrial systole + atrial diastole + ventricular systole + ventricular diastole)

Question 24

steps of the cardiac cyclel

Answer 24

1. ventricular filling
2. isovolumetric contraction
3. ventricular ejection
4. isovolumetric relaxation

Question 25

ventricular filling

Answer 25

- pressure is low, blood flows from atria to ventricles - av valves open, sl valves are closed - responsible for 80% of ventricular filling - following atrial depolarization (p wave), atrial systole occurs, and blood is compressed into the ventricles - EDV - atrial diastole and the start of ventricular depolarization

Question 26

End diastolic volume (edv)

Answer 26

the maximum volume of blood that the ventricles will contain in the cardiac cycle

Question 27

isovulmetric contraction

Answer 27

- atria relax, the ventricles start contracting, pressure in the ventricles rises - AS valves close - for a moment, the ventricles are closed chambers with a constant blood volume - pressure continues to rise until it exceeds rhe pressure in the great vessels - sl valves open

Question 28

ventricular ejection

Answer 28

- blood moves from the ventricles to the great vessels (pulmonary trunk vs aorta) - pressure in the aorta is typically 120mmHg

Question 29

isovolumetric relaxation

Answer 29

- follow the t wave, the ventricles relax - ESV - ventricular pressure drops, blood slides from the great vessels back towards the ventricles - sl valves close - ventricles are again closed chambers - dicrotic notch

Question 30

End systolic volume (ESV)

Answer 30

the un-ejected blood remaining in the ventricular chambers

Question 31

dicrotic notch

Answer 31

a brief rise in aortic pressure caused by blood rebounding off the newly closed aortic valve

Question 32

restarting the cardiac cycle

Answer 32

- flow of blood through the heart is governed by pressure changes - blood always follows down a pressure gradient (high-low) - while the ventricles are contraction in systole, the atria are in diastole - when the pressure in the filling atria exceeds the pressure in the. ventricles, the av valves open, and the ventricular filling begins again

Question 33

1 cardiac cycle

Answer 33

0.8s - atrial systole = .1s, ventricular systole = .3s, quiescent period = .4s

Question 34

pulmonary circulation

Answer 34

low pressure circuit - pressure in the pulmonary arteries = ~24/10 mmHg

Question 35

systemic circulation

Answer 35

higher pressure circuit - pressure in the aorta + ~120/80 mmHg - despite difference in pressure, both sides of the heart eject the same volume of blood with each beat

Question 36

heart sounds

Answer 36

- 2 sounds can be hear with each heart beat "lub-dup" - sounds heard are the sounds of the valves closing - the pause between sounds is the quiescent period

Question 37

1st sound

Answer 37

av valves closing - start of ventricular systole - longer, louder sound

Question 38

2nd sound

Answer 38

sl valves closing - start of ventricular diastole - shorter, sharper sound

Question 39

valve closure

Answer 39

typically, the mitral valve closes slightly before the tricuspid valve, and the aortic valve closes slightly before the pulmonary valve

Question 40

murmur

Answer 40

an abnormal heart sound secondary to turbulent blood flow - more common in children and elderley people; thin-walled hearts allow more vibration - can be indicative of a valve problem - insufficient or incompetent valves allowing regurgitation or backflow

Question 41

stenotic valves

Answer 41

fail to open all the way - the narrowed opening restricts blood flow, and a high-pitched sound (or click) can be hear