Chapter 2: Basic Electrophysiology
Terms
undefined, object
copy deck
- What are the two categories of the cardiac cells?
-
*Miocardial Cells (working or mechanical cells)
*Pacemaker Cells (electrical cells) - What are myocardial cells?
- They are working or mechanical cells that contain contractile filaments.
- What are pacemaker cells?
- Specialized cells of the electrical conduction system responsible for the spontaneous generation and conduction of electrical impulses
- Major electrolytes that affect cardiac function
-
*Sodium (Na+)
*Potassium (K+)
*Calcium(Ca++) - What determines the cell's electrical charge?
- The differences of the electrolyte concentration.
- Normal cell charge
-
*Excess of + ions on outside
*Excess of - ions on inside - Membrane potential
- Difference in electrical charge across the membrane.
- Action potential
- Five-phase cycle that reflects the differences in the concentrations of the ions across the cell membrane at any given time.
- Two types of action potentials
-
*Fast-response
*Slow-response - Fast Response Action Potentials
-
*Occur in cells of atria, ventricles, and Purkinje fibers.
*Occurs b/c of the presence of may Na+ channels that allow a rapid influx of Na+ when these channels are open and prevent influx when they are closed - Slow Resonse Action Potentials
-
*In SA and AV nodes
*Have slow Ca++ and slow Na+ channels
*Can sometimes occur anywhere in the heart, usually secondary to ischemia, injury or an electrolyte imbalance. - Polarization (resting membrane potential)
-
*Resting state, no electrical activity
*Intracellular ions include K+ and some anions (neg. charged ions) - Depolarization
-
*When the cardiac cell is stimulated.
*Inside of cell becomes more positive b/c of the entry of Na+ ions into the cell through Na+ membrane channels.
*Proceeds from endocardium to epicardium - P Wave
- Atrial depolarization
- QRS Complex
- Ventricular depolarization
- PEA
-
*Pulseless Electrical Activity
*Occurs when there is electrical activity on the monitor but no palpable pulse - Repolarization
-
*The diffusion of Na+ into the cell stops
*K+ is allowed to diffuse out of the cell, leaving the anions inside the cell.
*Occurs b/c of outward diffusion of K+
*Proceeds from epicardium to endocardium - ST Segment
- Early ventricular repolarization
- T Wave
- Ventricular Repolarization
- Phases 1,2,3 of cardiac action potential
- Electrical Systole
- Phase 4 of the cardiac action potential
- Electrical Diastole
- Phase 0 of the cardiac action potential
-
*Rapid depolarization phase / upstroke / spike / overshoot
*Begins when the cell receives an impulse.
* -
Phase 1 - Early Repolarization
Cardiac Action Potential Cycle -
*Fast Na+ channels partially close, slowing the flow of Na+ into the cell.
*K+ moves out of the cell rapidly
*Positive electrical charges within the cell decrease, producing a small negative deflection in the action potential. -
Phase 2 - Repolarization (Plateau Phase)
*Plateau = a raised flat area
Cardiac Action Potential Cycle -
*Occurs b/c of slow, inward movement of Ca++ and continued movement of K+ outward.
*Allows cardiac muscle to sustain an increased period of contraction.
*ST segment
*Is a part of the absolute refractory period. -
Phase 3 - Rapid Repolarization
Cardiac Action Potential Cycle -
*The cell rapidly completes repolariztion as K+ flows quickly out of the cell and the slow channels close, stopping the influx of Ca++ and Na+.
*The rapid efflux of K+ from the cell causes it to become progressively more electrically negative.
*T Wave -
Phase 4 - Resting Membrane Potential
Cardiac Action Potential Cycle -
*Return to the resting state
*Excess of Na+ inside the cell and excess of K+ outside the cell.
*Cell will remain in this state until the cell membrane is reactivated by another stimulus. - Refractory Period
- The extent to which a cell is able to respond to a stimulus. Longer than contraction.
-
Absolute Refractory Period
*aka Effective Refractory Period -
*Corresponds with the onset of the QRS complex to the peak of the T Wave and includes phases 0, 1, 2, and part of phase 3 of the cardiac action potential.
*Myocardial cell will not respond to further stimulation -
Relative Refractory Period
*aka Vulnerable Period -
*Corresponds w/ the downsloap of the T Wave
*Some cardiac cells have repolarized to their threshold potential and can be stimulated to respond (depolarized) to a stronger than normal stimulus. - Supernormal Period
-
*A weaker than normal stimulus can cause depolarization of cardiac cells.
*Extends from the terminal portion of phase 3 of the action potential to the beginning of phase 4
*End of T-Wave
*It is possible for cardiac dysrrhythmias to develop during this period. - Four primary properties of cardiac cells
- Automacity, excitability, conductivity, and contractility
- Automacity
- Ability of cadiac pacemaker cells to spontaneously initiate an electrical impulse w/o being stimulated from another source.