Flashcards in Stimulation Deck (42):
The ability of tissues to respond to a specific stimulus
A cell membrane is always negative, how many + charges leave compared to enter
3 + charges out to 2- charges in
What is the approx baseline voltage potential across cell membrane CARDIOMYOCYTE (PHASE 4)
During phase 0 what happens to sodium (NA+)
Rapid influx of sodium
What happens during Phase 1 and what is the TMP (Tranasmembranepotential)
Efflux of Potassium (K+) which returns TMP to 0mV
What happens to calcium during Phase 2
Calcium channels open and influx of calcium
What happens to calcium during Phase 3
Calcium channels close, however efflux of potassium continues and returns TMP to -90mV
Potassium is always LEAVING the cell TRUE/FALSE
TRUE - There is never an influx of potassium, it always leaves.
Which phase is considered repolarisation
Outward currents exceed inward currents
Which phase is deemed the absolute refractory period
Inward Na+ and Ca2+ currents balanced by outward slow, rapid, and ultra-rapid K+ currents
Which phase is deemed the diastolic phase
• Occurs after complete repolarisation of membrane
• Cell now fully excitable
Which cells exhibit gradual upward drift in transmembrane potential until threshold is reached.
Cells capable of spontaneous automaticity (i.e SA/AV nodal cells)
voltage of a constant voltage strength-duration curve is the lowest stimulation voltage amplitude at an infinitely long pulse duration -- Describes what
Rheobase voltage = threshold voltage - TRUE/FALSE
True. Because the curve is essentially flat at a pulse duration of 1.5-2.0ms
practical rheobase = threshold @ 1.5-2.0ms
Threshold pulse duration at twice the rheobase voltage amplitude (i.e. pulse duration at double the voltage threshold) ---- Describes what
What does the Chronaxie do
Closely approximates the point of minimum threshold stimulation energy (micro Joules/μJ)
Are modern pulse generators constant voltage or constant current generators:
What are strength duration curve differences between constant voltage and constant current generators:
Constant current = current decline is more gradual
THUS energy requirements in a constant current system are higher than in a constant voltage system
Describe the wedensky effect
Stimulation thresholds measured by decrementing the stimulus voltage until loss of capture are usually 0.1-0.2V lower than when the stimulus voltage is gradually increased from sub-threshold until capture is achieved.
stimulus voltage, current, and pulse duration to stimulus energy is provided by the equation:
E = V2 / R x t
Doubling pulse duration =
two-fold increase in stimulation energy
Doubling stimulus voltage =
four-fold increase in stimulation energy
Longer pulse durations than necessary DONT result in a wastage of stimulation energy without providing a substantial increase in output safety margin. TRUE/FALSE
In CRT systems is the RV the anode or cathode
In CRT systems is the LV the anode or cathode
Why is Anodal capture bad in CRT (3 points)
• Results in earlier activation of the RV
• Not ideal in patients who require a desirable offset between the LV and RV
• May prove deleterious to biventricular
How to automatic capture algorithms work, what do they look for?
An evoked response
Define evoked response
Evoked Response = electrical event that results from myocardial capture after an output pulse.
What could an evoked response be mistaken for
How does autocapture measure differences between evoked response and polarisation artefact
Sends paired impulses
o First pulse captures myocardium and gives evokes response
o Second pulse occurs in the absolute refractory period, thus causing only polarisation artefact
What is retry mode (Boston only)
Automatic Capture is active, and the device is operating in beat-to-beat mode and is unable to confirm capture due to reasons such as loss of capture or due to signal/noise issues.
Retry pacing amplitudes will be at 2X previous measured threshold
All automatic capture algorithms use beat to beat capture apart from....
Medtronic Capture management
What are the two benefits of automated capture management
Improve safety related to pacing by avoiding situations where loss of capture could occur due to a rise in the stimulation threshold
Improve battery longevity by reducing the programmed safety margin closer to the stimulation threshold,
Voltage drop across lead distal/proximal results in what
Heat - wasted energy
Small electrode tip size does what to the impedanec
Why can't we have even smaller electrode tip sizes than what we have now
If high impedances are good for battery longevity - why dont we install higher impedance leads?
Implantation of high pacing impedance leads for ventricular stimulation does not result in a benefit with respect to pacemaker longevity as compared with standard impedance leads.
Etsadashvili et al. 2009
Movement of positive IONs to cathode
Cathode repels negative ions
What has a greater effect on polarisation - Voltage or Time
Time - effect magnitude directly relates to the duration of stimulus
Two ways to minimize polarization
Minimised by short pulse durations
Larger surface area = reduced polarisation
If a smaller surface area is required for maximum electrode resistance, yet larger surface area required for reduced polarisation… how to get around this?
Electrodes with small geometric radii but a complex, porous surface that increases microscopic surface area may be used.
• In addition, various coatings (e.g. platinium or iridium oxide) may help minimise polarisation.