Electrical and mechanical events of the heart

Question 1

Why does the heart contain the SAN?

Answer 1

Initiates aps due to the unstable membrane potential that is continuously drifting towards threshold; can initiate cardiac contraction in absence of external (hormonal/neural) control

Question 2

What is the intrinsic rate of the SAN?

Answer 2

~100bpm

Question 3

What is the effect of sympathetic stimulation on the SAN?

Answer 3

Increases in sodium funny channel current, causing further depolarisation and increase in slope drifts towards threshold

Question 4

What is the effect of parasympathetic stimulation on the SAN?

Answer 4

Increase K+ permeability, causing repolarisation/hyperpolarisation (overall decrease of the inwards current) and decrease in slope towards threshold

Question 5

What does the rate of firing of SAN depend on?

Answer 5

Initial value of membrane potential, and slope of drift towards threshold

Question 6

How is stimulation conducted through the cardiac muscle?

Answer 6

SAN originates stimulus; stimulus spreads through muscle fibres through gap junctions; stimulates atria almost simultaneously; signal delayed at AVN to allow atria to empty before ventricular contraction begins; AVN and bundle of His are only pathway of the electric stimulus to travel from atria to ventricles; ventricular stimulation spreads via network of conduits from AVN to Purkinje fibres; signal travels down interventricular septum via the two bundle branches; ventricles begin to contract at apex, depolarisation spreading upwards via Purkinje fibres

Question 7

How is the recording of an ECG influenced?

Answer 7

By size and direction of current

Question 8

What type of inflection would a large current from -ve to +ve pole create?

Answer 8

Large, positive inflection

Question 9

What type of current would a small, negative inflection indicate?

Answer 9

Small current, from +ve pole to -ve pole

Question 10

What might no inflection indicate?

Answer 10

No electrical activity, or a perpendicular current

Question 11

What type of inflection would an angled current create?

Answer 11

Smaller, positive inflection

Question 12

What does the ECG represent?

Answer 12

Composite of all change in electrical potentials of all cells at any moment in time

Question 13

What does the size and shape of ECG waves depend on?

Answer 13

Position of heart relative to placement of electrode; direction of excitement; mass of tissue being excited at each given time

Question 14

What is the Einthoven triangle?

Answer 14

Most common way to measure HR and rhythm

Question 15

What is the most complex way to measure heart electrical activity?

Answer 15

12 lead ECG; provides a 3D perspective of what happens in heart at any given time, and can indicate if a specific part of the heart isn’t operating as it should be

Question 16

What is the typical way to measure heart electrical activity?

Answer 16

3 lead ECG; can seen normal rhythm ad rate

Question 17

What does the P wave indicate?

Answer 17

Atrial contraction; initiation and completion of atrial excitation

Question 18

What does the Q wave indicate?

Answer 18

Initiation of ventricular excitation

Question 19

What does the RS complex indicate?

Answer 19

Completion of ventricular excitation

Question 20

What does the T wave indicate?

Answer 20

Ventricular relaxation

Question 21

What are the two phases of the mechanical events of cardiac cycle?

Answer 21

Systolic phase and diastolic phase

Question 22

What happens during the systolic phase?

Answer 22

Isovolumetric ventricular contraction: pressure generated by ventricles as start to contract; pressure large enough to close atrioventricular valves, but not large enough to open semilunar valves; no blood leaves the heart. Ventricular ejection: pressure inside ventricular > pressure outsie heart; pressure sufficient to open semilunar valves and blood leaves heart

Question 23

What happens during the diastolic phase?

Answer 23

Isovolumetric ventricular relaxation: pressure falls as ventricles start relaxing; pressure low enough to shut semilunar, but not low enough to open atrioventricular valves; no blood entering/leaving. Ventricular filling: pressure in atria > pressure in ventricles; atrioventricular valves open; blood into ventricles (passive initially as by gravity). Atrial contraction: boosts filling of ventricles; normally is not that important, but becomes to fill ventricles with correct amount of blood during exercise etc

Question 24

What does the pressure-volume curve show?

Answer 24

Shows what happens throughout heart cycle as far as pressure changes and what changes in pressure does to volume

Question 25

What order does the pressure-volume curve go in?

Answer 25

Starts at A (bottom left) and goes around in a circle

Question 26

What occurs at A of the pressure-volume curve?

Answer 26

Atria and ventricles are relaxed; atrioventricular valves open

Question 27

What occurs at B of the pressure-volume curve?

Answer 27

Ventricles start to contract; atrioventricular valves close

Question 28

What occurs at C of the pressure-volume curve?

Answer 28

Ventricle pressure > atrial pressure; semilunar valves open

Question 29

What occurs at D of the pressure-volume curve?

Answer 29

Ventricles relax; pressure in ventricles < arterial pressure; semilunar valves shut

Question 30

How can the SV be determined from a pressure-volume curve?

Answer 30

Difference in volume betwen C and D (ventricular ejection and relaxation)