Cardiology Flashcards
Membrane potential is measured on which side of the cell membrane?
the stated value (e.g. -70 mV) is the intracellular space relative to the extracellular space
The magnitude of an ion’s chemical force depends on wha t two things? What equation describes this relationship?
- it is dependent on the ratio of extracellular and intracellular ion concentrations as well as the valence of the ion
- this is described by the Nernst potential
- E = (60/z)log(extracellular/intracellular concentration)
The magnitude of an ion’s current depends on what two things?
- forces driving ion movement (i.e. electrochemical gradient)
- and the conductance of that ion
What is conductance? What factors contribute to it’s value?
- it is a measure of how easily the ion can move across the membrane in response to the electrochemical gradient driving it
- it is related to the number of open ion channels, the number of leak channels, and to the ion concentration
How does the conductance of potassium relate to the concentration of potassium?
potassium conductance is proportional to the extracellular concentration of potassium
What equation is used to describe the current of an ion?
I = g x (V - E)
How is fractional conductance of potassium and sodium calculated?
- fractional sodium conductance = sodium conductance/(sodium conductance + potassium conductance)
- fractional sodium conductance + fraction potassium conductance = 1 if we assume the cell is only permeable to sodium and potassium
How can we determine the membrane potential for a cell mathematically, based on the assumption it is permeable only to sodium and potassium?
V = (E x fractional potassium conductance) + (E x fractional sodium conductance)
What is the typical membrane potential of a resting neuron?
-70 mV
What cardiac tissue is least excitable? Why is this important for cardiac function?
since the AV node is the least excitable cardiac tissue, it delays ventricular contraction until atrial contraction has finished and complete filling of the ventricle has occurred
Describe the path or sequence of excitation through the heart.
- SA node exhibits pacemaker function
- atrial muscle
- AV node
- common bundle
- bundle branches
- purkinje fibers
- ventricular muscle
What cardiac event do the p wave, pq interval, qrs complex, st segment and t wave each represent?
- p wave: atrial contraction
- pq interval: AV node depolarization
- qrs complex: phase 0/1 of ventricular contraction
- st segment: phase 2 of ventricular contraction
- t wave: depolarization of the ventricle
Which pieces of the cardiac conduction pathway utilize calcium-dependent action potentials in which calcium is responsible for the upstroke of the AP?
the SA and AV nodes
Which ions provide the depolarizing current or upstroke of the AP in each of the various segments of the cardiac conduction pathway?
- most utilize a sodium current
- the SA and AV nodes rely on a calcium current
The magnitude of the depolarizing current during the upstroke of an action potential will determine what four things about that action potential?
- the threshold potential
- amplitude of the AP
- the rate of rise of the AP
- the conduction velocity of the AP
What determines the conduction velocity of an action potential?
the magnitude of the depolarizing current during the upstroke of the AP
Describe the phases of the SA node action potential.
- in phase 4, the funny sodium current (pacemaker potential) is greater than the repolarizing potassium current
- the difference between these two currents determines the steepness of phase 4 (how quickly threshold is met)
- when threshold is met, phase 0 begins as voltage-gated calcium channels open and depolarization occurs
- depolarization opens voltage-gated potassium channels, which open and begin to repolarize the cell
- during phase 3, the potassium current predominates over the funny sodium current, and there is a net polarization
- slowly, the voltage-gated potassium channels close again, and the funny sodium current predominates (phase 4)
What is the maximum diastolic potential?
the most negative potential achieved in the SA node (typically -50 mV), determined by the balance of the repolarizing potassium current and the depolarizing funny sodium current
What determines heart rate within the SA node?
the balance between the funny sodium current and the repolarizing potassium current
Describe the phases of atrial and ventricular action potentials.
- during phase 4, there is a stable resting potential
- during phase 0, depolarization occurs due to a sodium current
- during phase 1, there is a transient repolarization attributed to potassium current
- during phase 2, there is a plateau in which the repolarizing potassium current is balanced by the voltage-gated calcium current
- during phase 3: the repolarizing potassium current outweighs the voltage-gated calcium current as L-type calcium channels close and repolarization occurs
What role does the voltage-gated calcium current play in the ventricular action potential? What segment of the ECG describes this potential?
- it is needed to induce myocyte contraction
- it is represented by the length of phase 2 of the action potential, and those the QT interval
What is the effect of sympathetic, noradrenergic activity in the heart on each of the various currents.
- it increases funny sodium current (increasing heart rate)
- it increases the current through L-type calcium channels
- it increases the potassium current
What effects do sympathetic activity have on the various currents mediating the cardiac cycle? Through what mechanism is this? How do these molecular changes manifest?
- sympathetic nerves release NE on B1 receptors in the SA and AV nodes
- this serves to increase the funny sodium, L-type calcium, and potassium currents
- the net result is an increase in heart rate and an increase in conduction velocity through the AV node, which is seen as a smaller PR interval
- sympathetic firing does not change the magnitude of the sodium current in the atria or ventricles, so there is no change in the amplitude or width of the QRS complex or P wave
- the increase in potassium current, however, increases the rate of repolarization, so you see a spiked T wave and shorter QT interval
- the increase in L-type calcium current increases inotropic state and makes phase 2 potential more positive
What effects do parasympathetic activity have on the various currents mediating the cardiac cycle? Through what mechanism does this occur? How do these molecular changes manifest?
- acetylcholine activates muscarinic receptors in the SA and AV nodes
- the result is to decrease the funny sodium and L-type calcium currents
- at low-to-moderate vagal activity, there is a decrease in potassium current; at high vagal activity, there is activation of the K(ACh) channel, which increases potassium current
- the net result is a decrease in heart rate and a decrease in conduction velocity through the AV node, which is seen as a prolonged PR interval