Cardiology

Question 1

What are the six properties of cardiac muscle cells

Answer 1

• Contractility (ability of fibres to shorten when stimulated)
• Conductivity (ability of fibres to easily transmit action potentials)
• Excitability (capacity to respond to a stimulus)
• Automaticity (ability of the cell to spontaneously depolarise without neurohumoral control)
• Refractoriness (the time that the cell will not respond to a stimulus)
• Expansibility (the ability of the heart to stretch as it fills)

Question 2

What is the resting membrane potential of a cardiac muscle cell

Answer 2

-90 mV

Question 3

How is the resting membrane potential maintained

Answer 3

The Na+ / K+ pump uses ATP to move 3 Na+ out of the cell and 2 K+ into the cell against the concentration gradient

Question 4

At rest, what is the charge inside and outside of the cardiac cell, and what ions are predominantly present

Answer 4

Outside the cell has a high concentration of cations (Na+) and is positively charged

Inside the cell has a high presence of anions and is negatively charged, but some K+ cations are present

Question 5

What is the threshold potential of a cardiac muscle cell

Answer 5

-70 mV, energy levels below this will fail to depolarise the cell

Question 6

What triggers the initial flow of Na+ into the cell

Answer 6

Stimulation from an action potential results in the cell membrane becoming more permeable, allowing Na+ to flow into the cell

Question 7

What happens to a cardiac muscle cell during Phase 0

Answer 7

• Rapid depolarisation occurs
• The interior of the cell becomes less negative
• At about -60 mV voltage-gated ion channels open, allowing more Na+ to flow into the cell

Question 8

What is the typical charge on a depolarised cell

Answer 8

+20-30 mV

Question 9

What happens to a cardiac muscle cell during Phase 1

Answer 9

• Repolarisation begins, closing Na+ channels and opening K+ channels.
• Cell membrane potential becomes positive

Question 10

What happens to a cardiac muscle cell during Phase 2, and why is the length of this phase important

Answer 10

• K+ moves out of the cell
• Slow Ca+ channels are opened, moving Ca+ into the cell
• Ca+ acts on actin filaments causing contraction of myocyte (power stroke)
• Cl- channels open in response to Ca+ entering cell
• Long plateau phase is important for allowing myocytes time to contract and avoid cardiac arrythmias

Question 11

What happens to a cardiac muscle cell during Phase 3

Answer 11

• K+ channels remain open and K+ leaves the cell
• Na+/Ca+ exchanger activates, moving Na+ into the cell and Ca+ out
• Na+/K+ pump activated, moving 3 Na+ out of the cell and 2 K+ into the cell, using ATP
• Net outward current flow, creating negative membrane potential

Question 12

What is the threshold potential of a cardiac pacemaker cell

Answer 12

-40 mV

Question 13

What phases of the action potential do pacemaker cells experience

Answer 13

Phases 0, 3 and 4

Question 14

What is the resting membrane potential of a cardiac pacemaker cell

Answer 14

-60 mV

Question 15

What happens to a cardiac pacemaker cell during Phase 4

Answer 15

Slow influx of Na+ into the cell

Question 16

What happens to a cardiac pacemaker cell during Phase 0

Answer 16

Rapid influx of Ca2+ into the cell

Question 17

What happens to a cardiac pacemaker cell during Phase 3

Answer 17

Outflux of K+

Question 18

What is the absolute refractory period, and where does it occur occur:

a) in the action potential
b) on an ECG

Answer 18

• Period where cardiac cells cannot contract again or conduct another action potential as they have not depolarised to their threshold potential

a) Phase 0 to halfway through Phase 3
b) Q wave to about peak of T wave

Question 19

What is the relative refractory period, and where does it occur occur:

a) in the action potential
b) on an ECG

Answer 19

• Cardiac cells have repolarised to their threshold potential and can be stimulated by another action potential to depolarise again
  a) Halfway through Phase 3 to the end of Phase 3
  b) Peak of T wave to the end of the downstroke of the T wave

Question 20

What are the four tropes that effect cardiac cell characteristics

Answer 20

Inotrope - Effects force of cardiac contraction
Chronotrope - Effects rate of contraction
Dromotrope - Effects speed of electrical conduction
Bathmotrope - Effects cardiac excitability

Question 21

How does the Renin-Angiotensin-Aldosterone System (RAAS) work to alter blood pressure

Answer 21

• Renin is released by the kidneys in response to a decrease in renal perfusion (decreased BP)
• Angiotensinogen is a plasma protein released by the liver
• Renin converts Angiotensinogen to Angiotensin I
• Angiotensin Converting Enzyme (ACE) is secreted in the capillaries of lungs and kidneys
• ACE converts Angiotensin I to Angiotensin II, which is a powerful vasoconstrictor
• Angiotensin II works on multiple systems to increase BP

Question 22

What are the ways in which Angiotensin II works to increase BP

Answer 22

• Increases sympathetic activity
• Triggers Aldosterone secretion from the adrenal gland, which increases Na+, Cl- reabsorption, K+ excretion and H2O retention, increasing the effective circulating volume
• Increases arterial vasoconstriction
• Triggers pituitary gland to release Antidiuretic Hormone (ADH), which promotes H2O retention

Question 23

What three things affect preload

Answer 23

• Venous return
• Heart rate (tachycardia decreases filling time)
• Atrial contraction strength

Question 24

What is Starling’s Law

Answer 24

Increased venous return increases the end diastolic volume, which stretches the ventricular wall. This stretching increases the force of cardiac contraction, which increases stroke volume

Question 25

What are the PSA criteria for Adequate Perfusion

Answer 25

Skin: Warm, pink, dry
Pulse: 60 - 100 bpm
SBP: > 100 mmHg
Conscious state: Alert & orientated

Question 26

What are the PSA criteria for Borderline Perfusion

Answer 26

Skin: Cool, pale, clammy
Pulse: 50 - 100 bpm
SBP: 80 - 100 mmHg
Conscious state: