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Exam prep tutorials (Physics and Equipment) > Invasive Monitoring > Flashcards

Invasive Monitoring Flashcards

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1
Q

What formula describes the relationship between pressure, flow through a cannula and the viscosity of the fluid?

A

The Hagen-Poiseuille formula

Flow = change in pressure x pi x r4

8 x fluid viscosity x length

  • the pressure difference is maintained by a pressure bag set to 300mmHg
  • the radius of the cannula is fixed but may be reduced by clot formation
  • length is fixed
  • viscosity of fluid is assumed to be constant but may be altered by using 5% dextrose by accident
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2
Q

What is the dynamic response?

A

Refers to the speed at which a system is able to settle on a new value, following a stimulus.

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3
Q

What factors interact to affect the dynamic response of an arterial line?

A
  • natural (resonant) frequency
  • input frequency
  • damping
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4
Q

What is the natural (resonant) frequency?

A

The frequency which a system oscillates when set in motion.

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5
Q

What is resonance?

A

When the energy is input at the same frequency as the natural frequency, resonance occurs.

Resonance increases the amplitude of an oscillating body exponentially.

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6
Q

What is damping?

A

Damping is the loss of energy of a swinging/oscillating body through friction (air resistance).

Damping also affects the dynamic response of a system. For instance, if there is under-damping, it takes a long time for a system to settle in a new value.

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7
Q

What curve represents critical damping?

A

Damping coefficient of 1.0.

It is characterized by no overshoot and a long time for the amplitude to settle at zero.

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8
Q

What curve represents under-damping (D = 0.2)?

A

It takes a long time to settle at zero. It has a large degree of overshoot.

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9
Q

What curve represents optimal damping?

A

D = 0.64.

It takes the shortest time for the amplitude to settle at zero.

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10
Q

What happens when the input frequency (If) approaches the natural frequency (Nf)?

A

Maxiumum resonance occurs and the response amplitude increases exponentially.

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11
Q

What happens as the input frequency increases towards the natural frequency, and then increases beyond the natural frequency? How does damping affect this?

A

Max resonance will occur when the input frequency is equal to the natural frequency. If : Nf ratio equals 1.

As the input frequency continues to increase, the If:Nf ratio changes and the response amplitude decreases.

As increasing levels of damping are applied, the amplitude of the response is decreased.

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12
Q

When does resonance occur in an arterial line?

A

When the input frequency (HR) equals the natural frequency of the system (15Hz).

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13
Q

How can you avoid resonance?

A

The natural frequency of a system should be at least 8 times greater than the anticipated input frequency.

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14
Q

What does a flush test do?

A

Can be used to determine the natural frequency and the level of damping in an arterial line system.

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15
Q

What is the minimum natural frequency that should be used clinically?

A

20 Hz

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16
Q

How is the natural frequency calculated?

A

natural frequency = speed / wavelength

= 25 (mm/s paper speed) / 1 mm

= 25 Hz

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17
Q

How do you calculate the damping coefficient? What is the optimum damping for a system?

A

Optimum damping = 0.64

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18
Q

What is a transducer?

A

A device that converts one form of energy into another

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19
Q

What do arterial pressure transducers do?

A

They convert pressure energy into electrical energy so it can be conveniently displayed and recorded

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20
Q

What type of transducer are commonly used in arterial pressure measurement?

A
  • wire strain gauge
  • bonded strain gauge
  • capacitive transducer
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21
Q

What is resistivity?

A

The degree to which a material opposes the flow of electrical current. It is constant for a given material at a given temperature.

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22
Q

What is the formula for resistivity?

A
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23
Q

As wire temperature increases, how does this affect semiconductor and metal resistivity?

A

Semiconductor resistance decreases.

Metal resistance increases.

24
Q

What are most transducers made of?

A

Copper-nickel alloy constantan.

This does not change resistivity with temperature.

25
Q

How does a wire strain gauge work?

A

Increased arterial pressure decreases the tension of the resistance wire which in turn increases it’s cross-sectional area and reduces it’s length.

Both of these factors reduce the resistance.

The change in resistance can be plotted against time and is calibrated to a known pressure (atmospheric) when the transducer is zeroed. A pressure-time arterial trace can then be displayed.

26
Q

How does the bonded strain gauge work?

A

Uses a coil of resistance wire bonded to the diagphragm.

As increased arterial pressure moves the diaphragm, the coil gets stretched, coil tension increases, and therefore, resistance also increases.

27
Q

How do capacitive transducers work?

A

The diaphragm forms one plate of the capacitor. An increase in arterial pressure reduces the distance between the 2 plates. As the distance decreases, the capacitance increases and therefore reduces it’s resistance.

The change in resistance of the circuit produces the arterial waveform.

28
Q

What is the formula for capacitance?

A
29
Q

What is reactance?

A

It is the resistance of a capacitor. Reactance is inversely related to capacitance.

30
Q

What is a Wheatstone bridge?

A
  • contains 4 resistors which have different properties
  • the unknown resistor (R4) represents the resistance wire of the strain gauge and fluctuates with changes in arterial pressure
  • it’s value can be calculated by adjusting the variable resistor (R3) until the volmeter (V) reads zero:

R1 = R3

R2R4

31
Q

Why do most modern arterial pressure transducers use full-bridge circuits containing 4 strain gauges?

A

This makes it much more sensitive and allows it to compensate for changes in temperature.

32
Q

What is Fourier analysis?

A

It brakes down the arterial waveform into it’s consituent sine waves. This allows mathematical processing of the wave, such as integration to find the area under the curve - this is called pulse contour analysis and allows derivation of stroke volume and cardiac output.

33
Q

What information does the arterial waveform provide?

A
  1. Systolic BP
  2. Diastolic BP
  3. Contractility (dP/dtmax)
  4. MAP = shaded area/(t2-t1)
  5. HR = no. of peaks/time
  6. Dicrotic notch - represents the end of systole and aortic valve closure, shifts to the right in hypovolaemia
34
Q

How does respiration cause changes in the arterial waveform?

A

Early inspiration

  • increased intrathoracic pressure compresses pulmonary vessels
  • increases LV preload and therefore stroke volume

Late inspiration

  • prolonged increase in intrathoracic pressure compresses the now “empty” pulmonary vessels
  • reduces LV preload and therefore stroke volume
35
Q

How does pulse contour analysis use respiratory swing?

A

It uses stroke volume variation and pulse pressure variation. These both increase in hypovolaemia and can be used to guide fluid management.

36
Q

When is the only time that pulse contour analysis calculations are accurate?

A

In ventilated patients in sinus rhythm.

37
Q

What kind of amplification do arterial transducers use?

A

Simple amplification. This increases the amplitude of the signal by adjusting gain.

Gain is the ratio of the input signal to the output signal. Can be manipulated during system calibration.

38
Q

What is differential amplification?

A

Occasionally employed by arterial transducers.

Reduces electrical interference by using

  • common-mode rejection
  • bandwidth frequency
39
Q

What is common-mode rejection?

A

High frequency interference may occur by capacitance or inductance in the electrical circuit connected to the arterial transducer.

2 electrodes - 1 for refernce and 1 for measurement, are incorporated into the circuit so that:

  • interference occurs equally in both measurement and reference electrodes
  • the arterial transducer signal is only present in the measuring electrode
  • selectively amplifying the difference between both electrodes reduces the effect of interference
    • this is differential amplification or common mode rejection
40
Q

What is bandwidth frequency?

A
  • the response of an amplifier to different input frequencies is not uniform
  • the range of frequencies at which it has the same response is known as it’s bandwidth
  • bandwidth is very useful as it allows the amplifier to reduce high frequency interference
41
Q

What is zero offset (bias)?

A

This is calibration when an actual pressure reading of zero does not correspond to a reading of zero on the display.

This is corrected when the pressure transducer is zeroed to atmospheric pressure.

42
Q

What effect does an undamped arterial line have on the readings for systolic and diastolic BPs?

A

It overestimates both. The MAP is unaffected

43
Q

When does maximum resonance occur?

A

When the natural (resonant) frequency of the system is equal to the input frequency.

44
Q

Does damping alter the resonant frequency?

A

No, frequency at which resonance occurs is constant for a given system.

Damping does reduce the amplitude of the resonance though.

45
Q

What is resistance of a semi-conducting wire directly proportional to?

A

Resistance is directly proportional to it’s length

46
Q

What happens to a semi-conductor as it’s temperature decreases?

A

It’s ability to conduct decreases because electrons are more tightly bound to it’s lattice structure. Resistivity therefore increases.

47
Q

What are the SI units of resistivity?

A

Ωm

This can be worked out from the formula:

resistivity = resistance of wire (Ohms) x CSA (m2)

length (m)

48
Q

What is constantan?

A

It’s a metal alloy of nickel and copper.

It is NOT a semi-conductor

49
Q

Give examples of semi-conductors.

A

Carbon

Silicon

Germanium

50
Q

What is the Stewart Hamilton equation?

A

It calculates CO during thermodilution. It finds the area under the temperature change-time curve.

51
Q

Which direction is the dicrotic notch shifted to in hypovolaemia?

A

The right, because there’s delay in aortic valve closure

52
Q

Why does AF increase the SV variation?

A

Because of the uncoordinated atrial activity. So SVV is a poor indicator of the patient’s volume status in this setting.

53
Q

How does common mode rejection reduce interference?

A

By ignoring electrical interference present in both reference and measurement electrodes.

54
Q

What is used to filter high frequency interference?

A

The bandwidth of an amplifier is used to filter high frequency interference.

55
Q

How is gradient error corrected?

A

By adjusting the gain of the amplifier

56
Q
A

Exam prep tutorials (Physics and Equipment) (8 decks)

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