ECG Generation Flashcards

1
Q

Outline the generation of action potentials within the cardiomyocytes

A

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

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2
Q

Describe the bipolar lead placement in dogs and cats

A

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.

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3
Q

Describe the bipolar lead placement in horses and cows

A

Typically just use a base-apex lead arrangement

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4
Q

Describe the generation of the normal ECG

A

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

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5
Q

Describe dipoles

A

Exist within the myocardium

Negative and positive charge separated by a short distance

Current flows between poles

Consequence of ion movement
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6
Q

Explain how the different stages of an action potential result in a ECG recording

A

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

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7
Q

Explain the origin of the P wave

A

Depolarisation of the atria triggered by the SA node

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8
Q

Explain the origin of the PQ segment wave

A

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

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9
Q

Explain the origin of the Q wave

A

Ventricular depolarisation which begins in the septum and is the first negative deflection after P wave. Can be variable

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10
Q

Explain the origin of the R wave

A

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

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11
Q

Explain the origin of the S wave

A

Ventricular depolarisation of the base. It is the first negative depolarisation after the R wave

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12
Q

Explain the origin of the ST segment

A

Represents plateau in myocardial action potential when the ventricles contract to pump the blood

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13
Q

Explain the origin of the T wave

A

Ventricular repolarisation (variable - may be postive, negative or biphasic)

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14
Q

Identify a normal ECG and describe it in terms of the rate, rhythm and profile of the ECG

A

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

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15
Q

Why are rate and rhythm assessed

A

May provide evidence of chamber enlargement/hypertrophy

May suggest altered myocardial metabolism

Routine – many cases

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16
Q

What does a taller P wave indicate in relation to heart size?

A

Right atrial enlargement

17
Q

What does a wider P wave indicate in relation to heart size?

A

Left atrial enlargement

18
Q

What does a taller R wave indicate in relation to heart size?

A

Left ventricular enlargement

19
Q

What does a deeper S wave indicate in relation to heart size?

A

Right ventricular enlargement

20
Q

Draw label and scale a typical ECG

A
21
Q

Define ‘wide and bizarre’ in relation to QRS complex on an ECG and how it might occur

A

Often appears as passive spread of waves. The origin is from ventricles which can often indicate ventricular tachycardia

Can happen if conducting system is damaged

Damage to one bundle branch of conduction system

P wave initiated but impulse not conducted normally

22
Q

Describe the purpose of augmented leads in an ECG

A

Provide additional information about cardiac vector.

An augmented lead compares the electrical potential at the reference limb to the sum of electrical activity at the other two limbs.

The word “a” stands for augmented, “V” stands for vector and words R, L, and F stand for right forelimb, left forelimb, and frontal limb (represents left hind limb), respectively

23
Q

List the three augmented leads commonly used

A

aVR

aVL

aVF