ECG Generation Flashcards
Outline the generation of action potentials within the cardiomyocytes
Depolarisation: Pacemaker cells undergo spontaneous depolarisation during diastole until the threshold potential is reached causing Na+ and Ca++ channels to open. Membrane become more permeable to Na+ which diffuse into the cell down the electrochemical gradient making inside the cell less negative
Repolarisionation: Na+ ion channels close and K+ channels open. The membrane is more permeable to k+ so these diffuse out down the concentration gradient
Hyperpolarisation: Too many K+ ions have left the cell as K+ channels are slow to close making the membrane potential more negative that resting potential
Resting potential: Sodium potassium pumps restore membrane to normal resting potential. This is -90mV in myocardium cells
Describe the bipolar lead placement in dogs and cats
In lead I, the right foreleg (RA) is negative and the left foreleg (LA) is positive.
In lead II the right foreleg (RA) is negative and the left hind leg (LL) is positive.
In lead III the left foreleg (LA) is negative and the left hindleg (LL) is positive.
Describe the bipolar lead placement in horses and cows
Typically just use a base-apex lead arrangement
Describe the generation of the normal ECG
Cardiomyocytes are electrically-excitable which can generate action potentials
Waves of depolarisation and repolarisation which are responsible for rhythmic pumping, produce an electrical current in opposite directions (Dipole). which are conducted to the surface and recorded as an ECG
Many cells involved acting in functional syncytium
Describe dipoles
Exist within the myocardium
Negative and positive charge separated by a short distance
Current flows between poles Consequence of ion movement
Explain how the different stages of an action potential result in a ECG recording
Resting Cardiac Muscle Fibre
Surface is positively charged and inside the cell is negatively charged. The cell is polarised (no electrical activity).
Electrogram shows show deflection and remains at baseline
Cardiac Muscle Fibre Stimulated
With stimulation depolarisation starts from left to right. Surface charged of the depolarised area becomes negative and the same area within the cell becomes positively charged.
Electrogram will show an upward deflection if the wave flows towards a postive electrode
Cardiac Muscle Fibre Depolarised
When the cardiac muscle fibre gets completely depolarised, cell surface charge become negative and inside cell becomes postive. At this stage there is no potential difference in various portions of the cardiac muscle cell
Electrogram recording returns to baseline
Repolarisation Begins
Begins at same point where depolarisation started with the cell surface started assuming postive charge and interior of cell assuming a negative charge leading to a difference in electric potential and current flows
Electrogram will show a downwards deflection if the wave flows away from the positive electrode
Cardiac Muscle Fibre is completely Repolarised and Retains Resting Potential
With complete repolarisation the exterior of cardiac muscle cell assumes positive charge and interior assumes negative charge. The cell is again polarised (no potential difference)
Electrogram recording returns to baseline
Explain the origin of the P wave
Depolarisation of the atria triggered by the SA node
Explain the origin of the PQ segment wave
PQ segment is when the AV node conducts the electrical signal to the ventricles.
It is noticeable on the ECG because the mass of the AV node is small thus conduction will be slower
Explain the origin of the Q wave
Ventricular depolarisation which begins in the septum and is the first negative deflection after P wave. Can be variable
Explain the origin of the R wave
Ventricular depolarisation from the endocardium to epicardium. It is the first postive deflection after P wave. Tends to have the largest deflection, mostly prominent in lead II
Explain the origin of the S wave
Ventricular depolarisation of the base. It is the first negative depolarisation after the R wave
Explain the origin of the ST segment
Represents plateau in myocardial action potential when the ventricles contract to pump the blood
Explain the origin of the T wave
Ventricular repolarisation (variable - may be postive, negative or biphasic)
Identify a normal ECG and describe it in terms of the rate, rhythm and profile of the ECG
BPM can be calculated by counting individual squares between R waves /1500 (if 25mm/second) or /3000 (if 50mm/s)
Profile can be described by R-R (regularly irregular), regular or irregular
Why are rate and rhythm assessed
May provide evidence of chamber enlargement/hypertrophy
May suggest altered myocardial metabolism
Routine – many cases
