Cardiovascular Flashcards
T/F - The normal resting membrane potential of a ventricular myocyte is -70mV
FALSE
The normal RMP of a ventricular myocyte is -90mV
The maximum diastolic potential for a pacemaker myocyte is -65mV
T/F - At the resting membrane potential, the ventricular myocyte is more permeable to K+ than is it to either Na+ or Ca2+
TRUE
The increased permeability to K+ through the inwardly rectifying K+ current maintains the RMP close to the Nernst potential for K+ (-94mV)
It has low permeability to Ca2+ and Na+ through the Na+/Ca2+ ATPase exchanger and the Na+/K+ ATPase pumps, which allow some movement of these ions and thus the RMP does not quite reach the Nernst potential for K+.
T/F - Initial depolarisation of the ventricular myocyte (phase 0) is mostly due to the opening of ligand gated Na+ channels
FALSE
Phase 0 is due to opening of VG Fast Na+ channels.
These channels open at MP -70mV, which is triggered by local currents from spread via gap junctions
T/F - In fast-response action potentials, L- type calcium channels open as the myocyte membrane potential becomes less negative in phase 0
TRUE
The L-type Ca2+ channels start to open at -40mV however they are much slower than the VG Na+ channels.
Therefore, the effects of the ionic flux that occurs with opening of L-type Ca2+ channels is not seen until phase 2
T/F - Voltage gated calcium channels maintain the relatively positive membrane potential during the plateau phase of the action potential (phase 2)
TRUE
Phase 2 of the FRAP is the plateau phase. There is a balance between positive Ca2+ ion influx through VG L-type Ca2+ channels, and positive K+ efflux through the delayed rectified current
T/F - There are multiple different types of potassium channels in the ventricular myocyte
TRUE
Delayed rectifier current - slow VG K+ channels
Inwardly rectifying current - slow VG K+ channels
Transient outward K+ current - fast VG K+ channels
T/F - Conductance for K+ ions leaving the myocyte is reduced during phase 2
TRUE
K+ conductance is greatest during phases 3 and 4
T/F - Ventricular myocytes have an absolute (effective) refractory period (ERP), which prevents the development of cardiac tetany
TRUE
The absolute refractory period is during phase 0, 1, 2, and the first two thirds of phase 3. The Fast VG Na+ channels have an inner h gate that has time dependent closure and will not open until MP reaches -60mV. This facilitates time for myocyte relaxation and prevents tetany.
Between -60mV and -90mV, if a supramaximal stimulus is applied to the myocyte, a slow-response action potential is generated (relative refractory period)
T/F - The absolute (effective) refractory period (ERP) terminates when the cardiac sodium channel h gates move from the closed to open position
TRUE
The absolute refractory period is during phase 0, 1, 2, and the first two thirds of phase 3. The Fast VG Na+ channels have an inner h gate that has time dependent closure (~0.1secs after activation) and will not re-open until MP reaches -60mV. This facilitates time for myocyte relaxation and prevents tetany.
T/F - The upstroke of the slow response action potential (phase 0) is caused by the activation of voltage gated sodium channels
FALSE
Phase 0 of the SRAP is due to activation of slow VG L-type Ca2+ channels and subsequent Ca2+ influx
T/F - The resting membrane potential (RMP) in pacemaker cells is less negative that that of cells which normal exhibit a fast response AP
TRUE
Pacemaker cells do not have a true “RMP”, but instead have a maximum diastolic potential of -65mV.
The RMP of FRAP myocytes is -90mV
T/F - Calcium channels play an important role in phase 0 of the slow response AP
TRUE
VG L-type Ca2+ channels are responsible for phase 0 of the slow response AP
T/F - In phase 4 of the slow response AP, the RMP slowly becomes more positive due to the the movement of both sodium and calcium into the cell
TRUE
The pacemaker current is determined by HCN channels which permit Na+ leak into the cell.
When MP reaches -50mV, fast BG T-type Ca2+ channels open, which permit Ca2+ leak into the cell.
T/F - Sodium flux into the cell via the funny current is more pronounced when the cell is most depolarised
FALSE
HCN channels open when MP -60mV and close when MP -40mV.
When the cell is most depolarised (e.g. MP +20mV) the funny current is inactive due to closure of the HCN channels
T/F - The ionic basis of automaticity has been found to be increasingly complex as the ability to sequence various genes has become more advanced
TRUE
T/F - Electrical activity moving toward an ECG electrode will been seen as positive deflection
TRUE
The ECG isoelectic line occurs when there is no net electrical activity. A deflection represents the magnitude and direction of the electrical.
Positive deflection = electrical activity towards the electrode (relative to the indifferent electrode)
Negative deflection = electrical activity away from the electrode (relative to the indifferent electrode)
T/F - The peak of the R wave corresponds to phase 0 in the ventricular muscle
FALSE
The QRS complex represents ventricular depolarisation.
Q wave = left to right septal depolarisation
R wave = septal and left ventricular depolarisation
S wave = late depolarisation of the ventricular walls closer to the AV junction
The peak of the R wave corresponds to phase 1 in the ventricular muscle
T/F - Increased ventricular mass results in a larger R wave as there is a larger amount of current flow
TRUE
Ventricular hypertrophy can be seen as increased QRS amplitude due to greater net electrical activity
T/F - The ST segment occurs during the plateau phase of the venticular action potential
TRUE
The QRS complex represents phases 0-1 of the ventricular AP. The ST segment represents phase 2 (plateau), The T wave represents phase 3 (repolarisation) of the ventricular myocytes.
T/F - During the ST segment, the ventricle is depolarised so there is no potential difference between to enable current flow
TRUE
Although there is ionic movement during phase 2 (plateau of the cardiac AP, it is balanced Ca2+ influx and K+ efflux, resulting in no net potential difference. This is represented with an isoelectric line on the ST segment
T/F - Ischaemic cardiac tissue is more permeable to K+ than healthy tissue, resulting in earlier repolarisation of that tissue compared to the surrounding tissue, which may contribute to the elevation of the ST segment
TRUE
During cardiac ischaemia:
1) Accelerated opening of K+ channels –> Abnormally rapid repolarisation –> ST elevation
2) K+ efflux –> Loss if intracellular [K+] –> Decreased RMP –> Infarcted fibres depolarise more slowly –> TQ segment depression (seen as ST elevation)
T/F - Hyperkalaemia is associated with peaked T waves. This could be explained by a larger gradient for inward flux of potassium into the cell during repolarisation
TRUE
Increased plasma [K+] –> Increased K+ flux into the cell during repolarisation ==> Tall, peaked T waves
T/F - Low potassium levels, may result in a more negative resting membrane potential, making normal initiation of both fast and slow action potentials more difficult
FALSE
Hypokalaemia –> RMP is closer to threshold potential –> Increases myocyte excitability
T/F - With severe hyperkalaemia, the cardiac membrane becomes un-excitable the heart arrests in diastole
TRUE
Hyperkalaemia –> Hyperpolarisation of the myocyte membrane –> Increased stimulus required to generate and propagate AP
T/F - Severe hypocalcaemia will reduce the ability of the heart to generate SRAPs, as calcium is essential for phase 0
FALSE
Severe hypocalcaemia prolongs phase 2 (plateau) of the fast response action potential.
T/F - Hypocalcaemia prolongs the ST segment as the FRAP plateau is lengthened
TRUE
Hypocalcaemia causes QTc prolongation due to prolonged phase 2 (plateau)
T/F - Lengthening of the cardiac action potential plateau will tend to prolong the QT interval
TRUE
The QT interval is the time from ventricular depolarisation (phase 0) to repolarisation (phase 3) of the fast AP. Therefore, if phase 2 (the plateau) is increased, the QT interval will be prolonged
T/F - Sodium channel blockade by local anaesthetics will have more effect on the FRAP than SRAP
TRUE
VG Na+ channels are not involved in slow response action potentials, whereas they are vitally important for phase 0 of the fast response action potential.
