Cardiology Flashcards
What are the six properties of cardiac muscle cells
- 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)
What is the resting membrane potential of a cardiac muscle cell
-90 mV
How is the resting membrane potential maintained
The Na+ / K+ pump uses ATP to move 3 Na+ out of the cell and 2 K+ into the cell against the concentration gradient
At rest, what is the charge inside and outside of the cardiac cell, and what ions are predominantly present
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
What is the threshold potential of a cardiac muscle cell
-70 mV, energy levels below this will fail to depolarise the cell
What triggers the initial flow of Na+ into the cell
Stimulation from an action potential results in the cell membrane becoming more permeable, allowing Na+ to flow into the cell
What happens to a cardiac muscle cell during Phase 0
- 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
What is the typical charge on a depolarised cell
+20-30 mV
What happens to a cardiac muscle cell during Phase 1
- Repolarisation begins, closing Na+ channels and opening K+ channels.
- Cell membrane potential becomes positive
What happens to a cardiac muscle cell during Phase 2, and why is the length of this phase important
- 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
What happens to a cardiac muscle cell during Phase 3
- 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
What is the threshold potential of a cardiac pacemaker cell
-40 mV
What phases of the action potential do pacemaker cells experience
Phases 0, 3 and 4
What is the resting membrane potential of a cardiac pacemaker cell
-60 mV
What happens to a cardiac pacemaker cell during Phase 4
Slow influx of Na+ into the cell
What happens to a cardiac pacemaker cell during Phase 0
Rapid influx of Ca2+ into the cell
What happens to a cardiac pacemaker cell during Phase 3
Outflux of K+
What is the absolute refractory period, and where does it occur occur:
a) in the action potential
b) on an ECG
- 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
What is the relative refractory period, and where does it occur occur:
a) in the action potential
b) on an ECG
- 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
What are the four tropes that effect cardiac cell characteristics
Inotrope - Effects force of cardiac contraction
Chronotrope - Effects rate of contraction
Dromotrope - Effects speed of electrical conduction
Bathmotrope - Effects cardiac excitability
How does the Renin-Angiotensin-Aldosterone System (RAAS) work to alter blood pressure
- 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
What are the ways in which Angiotensin II works to increase BP
- 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
What three things affect preload
- Venous return
- Heart rate (tachycardia decreases filling time)
- Atrial contraction strength
What is Starling’s Law
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
What are the PSA criteria for Adequate Perfusion
Skin: Warm, pink, dry
Pulse: 60 - 100 bpm
SBP: > 100 mmHg
Conscious state: Alert & orientated
What are the PSA criteria for Borderline Perfusion
Skin: Cool, pale, clammy
Pulse: 50 - 100 bpm
SBP: 80 - 100 mmHg
Conscious state:
