06. Clinical Measurement

Question 1

Which of the following is used in measurement of pressure?

1. Change in electrical resistance in a wire
2. Variable inductance
3. Piezoelectric effect
4. Change in flow through a narrow tube
5. Torricellian vacuum

Answer 1

Everything except flow through a narrow tube - this is Venturi and you can’t really measure the pressure even if it drops.

Piezoelectric -Invasive blood pressure transducer make use of the piezoelectric properties of certain materials through a change in resistance in response to a force acting on it.

Question 2

How does non-invasive BP measurement relate to reading of systolic and diastolic compared to direct means?

Answer 2

UNDER reads systolic

OVER reads diastolic

Question 3

Diastolic is recorded at what sound during non-invasive BP monitoring?

Answer 3

The second muffing sound - silence (fourth phase - 5th phase)

The systolic pressure is taken as the first time the sounds are heard as the cuff is deflated from a supra-systolic pressure.

There may then be muffling and resurgence of the sounds (Korotkoff’s second and third phase) before the sudden muffling of the sound (fourth phase) and then silence (fifth phase).

Question 4

Invasive BP measurement:

1. Target resonance

Answer 4

Resonant frequencies should be as high as possible to prevent resonance and ideally above the 10th harmonic of the highest frequency in the signal being transduced.

Question 5

Invasive BP measurement?

• Transducer should be lax or rigid?

Answer 5

The transducer should be RIGID (low displacement)

• so as not to take energy out of the system.

Question 6

Underdamped

Answer 6

D < 0.5

Over shoots + resonates

Question 7

Optimally damped

Answer 7

D - 0.64
Small overshoot (7%)
no resonance

Question 8

Critically damped

Answer 8

D = 1.0
accurate measurement
no overshoot

Question 9

Over damped

Answer 9

D > 1

slow response as damping increases

Question 10

Causes of distortion in the invasive BP trace?

Answer 10

Bubbles, clots and connections in the tubing.

Question 11

Thermodilution measure of CO:

1. Over how many seconds?
2. problems with too little inject ate?

Answer 11

Over 4 seconds

Too little injectate = OVER estimation

Question 12

Oesphageal doppler monitoring:

What needs to be known?

Answer 12

1. The angle of insonation between the ultrasound waves and blood stream must be known - 45 degrees, posteriorly
2. The cross-sectional area of the aorta must be known.

Question 13

How much cardiac output is measured in descending aorta?

Answer 13

70%

Question 14

Nerve stimulators:

1. how much voltage needed?
2. how much current?

Answer 14

1. May reach 150V or more
2. Pulse currents are commonly 20 - 80 mA.

At a constant voltage an increase in resistance will reduce the current flow and typically constant current stimulation is used to produce reproducible twitch strengths

Question 15

What does TOF deliver?

Answer 15

It consists of four supramaximal twitches at 0.5 sec intervals (2 Hz)