Chapter 5 MI Introduction to Therapeutic Devices

Question 1

What is fibrillation?

Answer 1

Fibrillation is the uncoordinated beating of the heart

Question 2

What is atrial fibrillation?

Answer 2

The electrical pattern is abnormal prior to the QRS complex, and the frequency between the QRS complexes has increased.

Question 3

What is ventricular fibrillation?

Answer 3

There is no normal electrical activity

Question 4

Although atrial fibrillation is a serious condition, the patient’s life may not be in immediate danger as long as _____________________

Answer 4

Atrial fibrillation is a serious condition, but as long as the ventricles continue to pump blood, the patient’s life may not be in immediate danger.

Question 5

Why is ventricular fibrillation a medical emergency that requires life support?

Answer 5

Ventricular fibrillation is a medical emergency that requires life support, because the ventricles are not effectively pumping blood

Question 6

What is the most common treatment for atrial or ventricular fibrillation?

Answer 6

Defibrillation

Question 7

How does a defibrillator work?

Answer 7

A defibrillator uses special paddles to apply a charge to the heart from an external electrical source in an attempt to establish a normal sinus rhythm.

It effectively stops the heart so that the SA node can trigger a normal conduction cycle.

Question 8

How is a defibrillator pulse delivered to the patient?

Answer 8

The defibrillator pulse is delivered by placing paddles against the skin of the patient

Question 9

What is the diameter of the metal surface on the defibrillator paddles?

Answer 9

The paddles have a metal surface 8-10 cm in diameter

Question 10

Are there discharge switches mounted on the paddles?

Answer 10

Yes

Question 11

Based on the defibrillator circuit from figure 5.1.5, when the switch is at position 1 (connected to capacitor), the defibrillator is (charging/not charging)

Answer 11

The defibrillator is charging

Question 12

Based on the defibrillator circuit from figure 5.1.5, when the switch is at position 2 (connected to inductor), the defibrillator is (charging/discharging)

Answer 12

The defibrillator is discharging

Question 13

Based on the defibrillator charging circuit, when the switch is at position 1, the capacitor is being (charged/discharged)

Answer 13

Charged

Question 14

Draw the equivalent circuit for defibrillator charging circuit

Answer 14

1 Resistor (Rs)
1 Diode
1 Capacitor (C)
1 AC supply

Question 15

What is the formula for the energy stored in the capacitor when the defibrillator is fully charged?

Answer 15

W = 1/2 x C x (Vp)^2
Power = 1/2 x (Capacitance x (Vpeak)^2))

Question 16

What are the ranges of Vp and W?

Answer 16

Vp = 1000 - 6000 V
W = 50 - 400 J (joules)

Question 17

If the capacitance is C = 16microF and Vp is 5000V, calculate the energy stored when fully charged.

Answer 17

W = 1/2 x C(Vp)^2
= 1/2 x 16 x 10^-6 x (5000)^2
= 200 J (Answer)

Question 18

Based on the defibrillator charging circuit, when the switch is at position 2, the defibrillator is being (charged/discharged)

Answer 18

The defibrillator is discharging

Question 19

Why is the inductance L introduced in the discharging circuit?

Answer 19

To obtain the desired discharging shock waveform.

Question 20

What is the waveform WITHOUT the inductance L in the discharging circuit?

Answer 20

The discharging waveform without inductance (L) will look like a slope going down.

Question 21

What is the waveforms WITH the inductance L in the discharging circuit?

Answer 21

The discharging waveform with inductance (L) will move up from the origin and then move down like a slope.

Question 22

What is the effect of changes in the inductance value?

Answer 22

RL changes the shape of the output waveform.

Question 23

What is an automatic external defibrillator (AED)?

Answer 23

An AED visually and vocally instructs the operator to place the conductive adhesive electrodes on the upper and lower chest. It measures the ECG and if it detects ventricular fibrillation, it instructs the operator to press the shock button. The high voltage pulse depolarizes the cardiac cells to (hopefully) establish normal rhythm.

Question 24

What is a heart-block?

Answer 24

A heart block refers to an interruption in the normal conduction pathway. This can involve the SA node, the AV node or the bundle of His. Clinically, the most common types are the AV nodal and infra-Hisian blocks.

Question 25

What is an AV-block?

Answer 25

Atrioventricular (AV) block is partial or complete interruption of impulse transmission from the atria to the ventricles.

AV blocks are often described by degrees. A first-degree or partial block indicates a delay in conduction between the SA and AV nodes. This can be recognized on the ECG as an abnormally long PR interval.

Question 26

What is a second-degree or incomplete block?

Answer 26

A second-degree or incomplete block occurs when some impulses from the SA node reach the AV node and continue, while others do not. In this instance, the ECG* would reveal some P waves not followed by a QRS complex, while others would appear normal.

Question 27

What is a third-degree or complete block?

Answer 27

In a complete heart block, the electrical signals from the atria don’t reach the ventricles, so the heart beats in two different and independent patterns. This can make the heart beat very slowly and irregularly, which can be dangerous and cause other health problems.

In the third-degree or complete block, there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex). Even in the event of a total SA block, the AV node will assume the role of pacemaker and continue initiating contractions at 40–60 contractions per minute, which is adequate to maintain consciousness.

Question 28

What happens when arrhythmias become a chronic problem?

Answer 28

When arrhythmias become a chronic problem, the heart maintains a junctional rhythm, which originates in the AV node.

Question 29

What is used to speed up the heart rate and restore full sinus rhythm when arrhythmias become a chronic problem?

Answer 29

A cardiologist can implant an artificial pacemaker

Question 30

What does an artificial pacemaker do?

Answer 30

An artificial pacemaker delivers electrical impulses to the heart muscle to ensure that the heart continues to contract and pump blood effectively. These artificial pacemakers are programmable by the cardiologists and can either provide stimulation temporarily upon demand or on a continuous basis. Some devices also contain built-in defibrillators.

Question 31

How is a pacemaker implanted?

Answer 31

Pacemaker is surgically implanted beneath the skin

Question 32

Where are the pacemaker output leads connected to?

Answer 32

Pacemaker’s output leads are connected directly to the heart muscles.

Question 33

What are the various types of pacemakers? Name 2

Answer 33

1. Fixed rate pacemaker (asynchronous)
2. Synchronous pacemaker

Question 34

What is the difference between a fixed rate pacemaker and a synchronous pacemaker?

Answer 34

A fixed rate pacemaker functions regardless of the patients’ natural heart rhythm while a synchronous pacemaker discharges in the absence of intrinsic electrical activity. For example, a demand ventricular pacemaker (VVI) will only pace when it does not sense R waves from the ventricular electrode.

The block diagram of a fixed rate pacemaker has no feedback loop for changing pulse rate but a synchronous pacemaker has a feedback loop. The electrodes serve as a means of both applying stimulus pulse and detecting ECG signals to change the pulse rate.

Question 35

Describe the block diagram for a fixed rate pacemaker (asynchronous)
(OPLE)

Answer 35

Oscillator -> Pulse output circuit -> Lead wires -> Electrodes
(OPLE)

Question 36

Describe the block diagram for a synchronous pacemaker.
(ARTOE)

Answer 36

Amplifier -> Reset circuit -> Timing circuit -> Output circuit ->#<- Electrodes

The # indicates the feedback to the op amp.

Question 37

What does the lifetime of a battery depend on?

Answer 37

The lifetime of a battery depends on ampere-hour (A-H) rating.

Question 38

What type of batteries are used in implantable pacemakers and how long can they last?

Answer 38

Lithium iodide cells (battery) is used in implantable pacemakers.

They can last as long as 15 years

Question 39

Compute the energy in joules stored in a lithium battery having a rating 1 A-H and a terminal voltage of 1.8 V.

Answer 39

Battery energy = Rating x Voltage

1 (A-H) x 1.8 (V) x 3600 (s/h) = 6480 J

Question 40

An asynchronous cardiac pacemaker operates at a rate of 70 pulses/min. These pulses are of 2-ms duration and have an amplitude of 5 V when driving a 500-Ω load.
What is the total energy supplied to this load over a 10-year period?

Answer 40

The energy in a single pulse is E1 = V^2/R x time

Hence, energy in a single pulse is
5^2/500 x 2 x 10^-3 = 10^-4 Joules (J)

In 10 years’ time at a pulse rate of 70 pulses per minute, the total energy becomes:

E total = E1 x 70/min x 60min/hr x 24 hr/da x 365days/yr x 10yr =
3.679 x 10^4 Joules (J)

Question 41

What happens when waste products in the blood accumulate in the body?

Answer 41

They cause various metabolic disturbances

Question 42

What is used when kidney fails?

Answer 42

Artificial kidneys (haemodialysis machine)

Question 43

What are artificial kidneys (haemodialysis machine) used for?

Answer 43

Artificial kidneys or hemodialysis machines are used to remove the metabolic waste products from the blood of uremic patients.

Question 44

What are the basic physical processes involved in a hemodialysis machine? (3 types)

Answer 44

• Osmosis
• Diffusion
• Ultra-filtration

Question 45

What is osmosis?

Answer 45

Movement of water through a semi-permeable membrane to the more concentrated side

Question 46

What is diffusion?

Answer 46

Movement of solutes through a semi-permeable membrane to the more dilute side

Question 47

What is ultra-filtration?

Answer 47

Bulk flow of solution (i.e., both the solvent and the solute) by the action of (hydrostatic) pressure through a semi-permeable membrane (similar to filter paper, but on a smaller pore scale).

Question 48

What does a membrane with blood on one side and dialysate on the other side allow?

Answer 48

Allows movement of molecules between the two liquids

Question 49

How is healthy blood maintained using a hemodialysis machine?

Answer 49

The continuously incoming fresh dialysate has the concentration exactly identical to normal healthy blood plasma (except the larger molecules such as proteins), then after prolonged exchange across the membrane, the blood plasma would also reach the same concentration as the dialysate – hence becoming like healthy blood.

Question 50

How to speed up the exchange of some ions in hemodialysis machine?

Answer 50

The concentrations of the various substances in the dialysate is carefully selected so that the required speed of exchange is achieved.

Question 51

What type of electronics does a hemodialysis machine need to function?

Answer 51

Haemodialysis machine does not require electronics to function

Question 52

What are the protocols to ensure that the hemodialysis machine is useful?

Answer 52

1. Detecting leaks in membrane.
2. Ensuring the correct concentration of electrolytes in the dialysate.
3. Preventing air emboli that may be life-threatening

Question 53

How to detect leaks in membrane?

Answer 53

If there is a leak in the membrane, the patient’s blood can mix with the dialysis fluid. To detect this, the dialysis fluid is checked for any change in color or how clear it is. If the fluid becomes cloudy or changes color, it is a sign that there is a leak in the membrane and the blood is mixing with the dialysis fluid.

Can also monitor pressure in blood compartment as major leaks or clotting will change the pressure.

Question 54

How to ensure the correct concentration of electrolytes in the dialysate?

Answer 54

Measure the conductivity of the dialysate before entering the chamber (in the mixing chamber).

Question 55

How to prevent air emboli that may be life-threatening?

Answer 55

Incorporate air bubble detector. (photoelectric method, capacitance method, and ultrasound method)

Question 56

What temperature is a dialyzer typically kept at?

Answer 56

40 degrees

Question 57

What is the typical range of values for RL (Patient)?

Answer 57

50-200Ω

Question 58

Any practical inductor in a circuit will have its own resistance. Assume that the inductor in the circuit on page 9 has a resistance of 20 ohms, and the RL of patient is 50 ohms. Estimate the amount of energy that will be delivered to the patient.

Answer 58

Amount of energy delivered to the patient= [50/(50+20)] = 71.43%

Question 59

State the 3 differences between mono-phasic and biphasic waveforms

Answer 59

Mono-phasic:
* The shock is given in only one direction from one electrode to the other
* Less effective and can damage the heart muscle by the high voltage shock as it uses high energy (J)
* Outdated

Biphasic:
* Initial direction of shock is reversed by changing the polarity of the electrodes in the latter part of the shock (Both direction)
* More effective and safer as it requires low energy (J)
* Modern

Question 60

Which waveform (biphasic or mono-phasic) is used in most defibrillators and why?

Answer 60

Biphasic. They are smaller and lighter than monophasic defibrillators. They have also been demonstrated to be more effective in preventing heart and skin burns because they provide a lot less current unlike monophasic defibrillators.

Question 61

Can you defibrillate a patient implanted with a pacemaker? Why or why not?

Answer 61

Yes. The upper left side of the chest is where most pacemakers are put. Hence, it is safe as defibrillator pads are often put on the upper right side of the chest and the side of the rib cage under the left arm.

Question 62

Assume that you are a product designer for a pacemaker. List as many desirable features as possible.

Answer 62

*Long-lasting
*Small
*Bluetooth
*Properly programmed
*Light-weight