Cardiac Electrophysiology Flashcards
Match the phase of the cardiac action potential with the following description:
Ca2+ channels close, allowing the open K+ channels to rectify the membrane potential
Phase 3
Match the phase of the cardiac action potential with the following description:
Action of Na+/K+ ATPase pumps and Na+/Ca2+ co-transporters return ions to their starting point
Phase 4
Match the phase of the cardiac action potential with the following description:
Voltage-gated Ca2+ channels open. Ca2+ flows into the cell.
Phase 2
Match the phase of the cardiac action potential with the following description:
Na+ ions flow down a concentration gradient into the cell, reversing the polarity of the membrane.
Phase 0
Match the phase of the cardiac action potential with the following description:
Na+ channels close and voltage-gated potassium channels open, allowing K+ to flow out of the cell.
Phase 1
Regarding the action potential of cardiac cells and how it relates to an ECG (true or false):
Phase 0 of the cardiac action potential occurs due to potassium influx into the cell.
False. Phase 0 occurs due to sodium ion influx, stimulated by spreading depolarization which opens voltage-gated sodium channels. Potassium efflux occurs in Phase 1 and Phase 3 and rectifies the resting membrane potential.
Regarding the action potential of cardiac cells and how it relates to an ECG (true or false):
Phase 2 occurs due to calcium ion influx and prolongs the refractory period.
True. Calcium ion influx occurs when voltage-gated calcium channels open and this prolongs the refractory period, a property unique to the cardiac action potential which prevents tetanic contraction of the myocardium.
Regarding the action potential of cardiac cells and how it relates to an ECG (true or false):
Depolarization of the myocardium in sinus rhythm is triggered by pacemaker cells in the ventricles.
False. The pacemaker cells are located in the sinoatrial node in the right atrium. Its membrane potential is innately unstable and undergoes spontaneous depolarization when slow inwards sodium ion influx depolarises the cell membrane to -50mV. This triggers further calcium ion influx to depolarise the cell to -40mV, which then triggers the action potential. The rate of spontaneous depolarization is faster at the SAN than other pacemaker cells in the myocardium, therefore it is the primary pacemaker, but should it fail then slower pacemaker cells at the AV node will take over setting the heart rate.
Regarding the action potential of cardiac cells and how it relates to an ECG (true or false):
The Na+/K+ ATPase pump and Na+/Ca2+ co-transporter pumps which return ions to their starting point during Phase 4 of the action potential are active processes, requiring energy to work.
True. The ATPase pump catalyses the dephosphorylation of ATP into ADP, releasing a free phosphate ion as well as energy which the ATPase pump uses to transport sodium ions up a concentration gradient. The Na+/Ca2+ co-transporter pump does not use ATP to provide the energy but instead relies on the potential difference of sodium ions across the cell membrane created by APTase pumps. This is known as secondary active transport.
Regarding the action potential of cardiac cells and how it relates to an ECG (true or false):
A supra-maximal stimulus during the absolute refractory period results in myocardial contraction.
False. No depolarization occurs during the absolute refractory period but supra maximal stimulus during the relative refractory period would result in an action potential of smaller amplitude than during the resting period, and a weaker myocardial contraction.
Regarding the role of an ECG in understanding different parts of the cardiac syncytium (true or false):
The cardiac myocytes are the contractile tissue of the myocardium.
True. The myocytes spread the action potential from one to another through gap junctions between each other and thus coordinate their contraction as a syncytium.
Regarding the role of an ECG in understanding different parts of the cardiac syncytium (true or false):
The action potential of each myocyte can be measured at the body surface via electrodes.
False. Each individual action potential is too small to be measured at the body’s surface, but the sum of action potentials in the myocardium can be measured via pairs of electrodes, between which a potential is measured; when a wave of depolarization moves towards the positive electrode, a positive deflection is seen. The potential between pairs of electrodes indicates a vector summed potential in that direction.
Regarding the role of an ECG in understanding different parts of the cardiac syncytium (true or false):
The action potential of the right ventricle predominates because it lies anterior in the chest and therefore closer to the leads.
False. The left ventricle has a much larger muscle mass so its action potential predominates.
Regarding the role of an ECG in understanding different parts of the cardiac syncytium (true or false):
Leads I, II and III form Einthoven’s triangle.
True. These leads are bipolar leads which measure the potential between two leads; Lead I left arm to right arm, Lead II right arm to left leg and Lead III left arm to left leg. When analysed together they observe the potential of the heart in three directions. In contrast, the unipolar leads measure the potential of one of the chest leads to a common neutral lead: aVF is left leg to common, aVR is right arm to common and aVL is left arm to common. The praecordial chest leads observe the potential of the heart in horizontal section.
Regarding the role of an ECG in understanding different parts of the cardiac syncytium (true or false):
A normal cardiac axis is -30° to +90°
True. When the bipolar and unipolar leads are analysed, they give a 360° view of the cardiac action potential.This can be used to determine the cardiac axis, which is the average direction of depolarization as it spreads through the myocardium. A normal axis is -30° to +90°. If it lies between -30° and -90° then this is left axis deviation (caused by, e.g. left ventricular hypertrophy) and +90° to +180° is right axis deviation, e.g. right ventricular hypertrophy.
