4.3 - ECG & The Cardiac Cycle

Question 1

What does ECG mean? What does it measure? How many leads are used?

Answer 1

ECG = electrocardiogram

• ECG records the electrical activity of the entire heart at a single point in time from the surface of the skin (external) using electrodes
• 12 leads are used clinically nowadays to give info about different regions of the heart

Question 2

Name and define the components of an ECG:

Answer 2

• waves: deflections above or below the base line, electrical events
• segments: sections of baseline between 2 waves,
  mechanical events

Question 3

List and explain the 3 major waves of an ECG:

Answer 3

• P wave: atrial depolarization (also repolarize)
• QRS complex: ventricular depolarization
• T wave: ventricular repolarization

Question 4

List and explain the 2 major segments of an ECG:

Answer 4

P-R segment: atrial contraction

S-T segment: ventricular contraction

Question 5

What is the cardiac cycle and its phases?

Answer 5

• period of time from one heart beat to the next
• systole: contraction
• diastole: relaxation

Question 6

What are the steps to the cardiac cycle?

Answer 6

1. late diastole:
   - both atria and ventricles are relaxed
   - semi lunar valves closed, AV valves open
   - blood enters ventricles passively
2. Atrial systole: “top up a little more blood”
   - atria contract, ventricles relaxed
   - semi lunar valves closed, AV valves open
   - small amount of blood enters the ventricles
3. Isovolumic ventricular contraction: ventricular systole
   - ventricles contract
   - both AV valves and semi lunar valves close
4. Ventricular ejection:
   - AV valves close, semi lunar valves open
   - blood ejected (BP high enough to open semi lunar valves)
5. Isovolumic ventricular relaxation: ventricular diastole
   - semi lunar valves closed, AV valves closed
6. Late diastole

Question 7

When are the “lub-dub” sounds heard during the cardiac cycle?

Answer 7

lub: closing of AV valves during ventricular systole
dub: closing of semi lunar valves during ventricular diastole

Question 8

Liquids and gases flow from areas of ____ pressure to ____ pressure

Answer 8

high pressure to low pressure

Question 9

When the heart contracts, blood pressure ______

Blood flows out of the heart to areas of ____ pressure

Answer 9

increases, lower

Question 10

Explain the end-diastolic volume (EDV)

Answer 10

the maximum volume of blood in ventricle at the end of ventricular filling

Question 11

Explain the end-systolic volume (ESV)

Answer 11

the maximum volume of blood in ventricles at the end of ventricular contraction