Future Strategies of Defib led Flashcards
Define defibrillation
Non-synchronised random administration of high-energy shock during a cardiac cycle
Define cardioversion
Synchronised administration of low-energy shock during the R waves or QRS complex of a cardiac cycle
When is cardioversion indicated?
- Haemodynamically unstable Supraventricular Tachycardias (including Atrial Fibrillation and Atrial Flutter).
- Atrial Fibrillation or Flutter that is resistant to pharmacological therapies.
- Pulsed Ventricular Tachycardia
Define: charge
Proportion of electrons to protons in a unit of matter (coulombs)
Define: voltage
Difference in charge between two points (volts)
Define: current
Rate at which charge is flowing (amps)
DC/AC
Define: resistance
Material’s tendency to resist the current (ohms)
Define: impedance
Combination of resistance and reactance.
Define:
AED
ICD
Automated External Defibrillator.
Internal Cardioverter Defibrillator
Monophasic and biphasic?
Shock delivered in one or two vectors
Define: polarity
Degree of positivity or negativity in relation to voltage
Anode/cathode
Anode = positive electorde -> flows to cathode (negative)
Basic principle of defibrillation
Shock: stuns cardiac tissue - unexcitable, preventing uncoordinated, and allowing coordinated electrical activity to resume control.
Theory of total extinction? Year?
1933: shock needs to stimulate all myocardium during any state (i.e. refractory, resting, repolarising etc)
Critical mass hypothesis? Year?
1940: not all myocardium needs to be stimulated - based on the idea that a min mass of tissue required to maintain fibrillation.
Vulnerable window and defibrillation thresholds - year/what?
1980s -
Shocks can induce arrthymias if shocked in vulnerable window (refractory period -> could cause depolarisation).
DFT = defibrillation threshold correlates with the ULV (upper limit of vulnerability)
What does the DFT correlate with?
ULV
ULV and LLV
Upper and lower limits of vulnerability
Progressive depolarisation - year? and what?
1991
Optical mapping to look at causes of defib failure.
Theory: Progressively stronger shocks depolarise, progressively more refractory myocardium, to progressively prevent post-shock wavefronts, and prolong and synchronise post-shock repolarisation, in a progressively larger volume of the ventricle, to progressively decrease the probability of fibrillation after the shock
What is the most complete explanation of the defib theories?
The virtual electrode hypothesis.
Shocks hyperpolarised cardiac tissues, in addition to depolarising tissues. Key message: different types of stimulation result in different virtual electrode patterns and heterogeneity of action potential duration.
Why is this theory important?
Virtual Electrode Theory: identifies v es as secondary focusses to induce fibrillation
Why are biphasic waveforms more superior?
Require less energy (therefore less damage for same effect).
Reverse polarity and distribution of virtual electordes, decreasing heterogeneity of post-shock tissue.
Two forms of biphasic waveforms?
Biphasic Truncated Exponential Waveforms. (BTE)
Rectilinear Biphasic Waveforms. (RBW)
What is the determinant of defibrillation success?
Current, not energy
Why are varying currents produced in different people with fixed energy?
Transthoracic Impedance (TTI):
- Energy level
- Electrode size
- Interelectrode size
- Skin-electrode interface
- Phase of ventilation
- Tissue conductive properties
How does each minute of delay to defib affect survival?
Reduces survival to discharge by 10-12%
Energy of first shock? and after?
150 J, then increasing
Where are the knowledge gaps in defib?
- The minimal acceptable first-shock energy level
- The characteristics of the optimal biphasic waveform
- The optimal energy levels for specific waveforms
- The best shock strategy (fixed versus escalating)
