Bioengineering in Urology > Filling & Voiding Cystometry > Flashcards

Filling & Voiding Cystometry Flashcards

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1
Q

Define cystometry

A

Measurement of bladder behaviour

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2
Q

Why is measuring flow rate on its own not a diagnostic parameter?

A

Some people may use abdominal contraction (increasing abdominal pressure) to strain the flow of urine past the obstruction out of the bladder.
Therefore this masks their obstruction as their won’t be a significant change in flow rate.

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3
Q

How do we measure the pressure the bladder?

A
  • insert a catheter into the bladder
  • catheter is filled with saline
  • connect catheter to external pressure transducer
  • pressure gets transferred through saline to transducer which measures it and transfers it to a PC
  • catheter doesn’t fill whole of urethra so person can still void around it
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4
Q

What does the catheter look like?

A
  • actually made up of 2 tubes
  • 1 filled with saline for measuring pressure
  • second lumen is filling lumen which goes to a connector where patient can add saline to fill the bladder to control its volume and measure the pressure as a result
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5
Q

What is the first step?

A

1) Drain the bladder by inserting a catheter to empty all the urine
2) Insert double lumen catheter in to fill bladder with saline and then measure pressure using external transducer

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6
Q

What is the equation for pressure?

A 
Pressure = pgh
p = density
g = grabity
h = height
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7
Q

Why do we have a standardised position of the transducer?

A

P = pgh

Therefore we need to height to be constant between the bladder and the transducer otherwise it will affect pressure

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8
Q

What is the standard position of the transducer?

A
  • transducer should be level with the pubic symphysis
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9
Q

What is the vesicle pressure dependent on?

A
  • the contraction of the detrusor muscle
  • abdominal contraction
  • gravity
  • density
  • height
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10
Q

How do we overcome the effects of abdominal contraction?

A
  • add a second external transducer
  • links to another catheter fluid filled with saline which goes into the rectum
  • this measures abdominal pressure
  • not open at the end, has a balloon at the end
  • then do rectal cancellation
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11
Q

Rectal Cancellation

A

Detrusor Pressure = vesicle pressure - abdominal pressure

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12
Q

What is the cough test?

A
  • when measuring vesicle and abdominal pressure
  • get the patient to cough
  • should see a small peak in both vesicle and adbominal pressure
  • rectal cancellation carried out to remove this peak
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13
Q

What does rectal cancellation allow you to do?

A

Diagnose a neurological/muscular problem with bladder vs. obstruction

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14
Q

What is the advantage of this type of equipment?

A
  • transducer is the expensive part and because this does not come in contact with patient’s body fluids, it doesn’t have to be replaced
  • also catheter is saline filled and long enough so will not contaminate transducer
  • only need to replace catheters each time
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15
Q

What are the disadvantages of the this equipment?

A
  • The catheters are filed with saline and have a stiff wall so may get air bubbles
  • air bubbles are compressible
  • air bubbles can block the lumen of the tube and stop the pressure from the vesicle going to the transducer
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16
Q

What practical things need to be controlled when using this equipment?

A
  • height of transducer

- air bubbles in catheter

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17
Q

What are pressure tip transducers?

A
  • rather than having an external transducer
  • put transducer on the tip of the catheter
  • no problem of air bubbles or transducer position
  • but expensive as need to replace transducer every time
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18
Q

What determines the height difference?

A
  • height of the urine in the bladder (h1)
  • height between transducer and catheter tip (h2)

= height of urine in the bladder + height between the transducer and catheter tip

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19
Q

What determines the pressure in the vesicle?

A

= Pressure of the detrusor muscle + pressure in the abdomen + pg(h1 + h2)
- rectal cancellation will allow you to just get the detrusor pressure

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20
Q

What will the contribution of the height of the catheter to the pressure by?

A

Not huge = will be less than 10cm of water

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21
Q

What are advantages of the pressure tip transducer?

A
  • get over height 1 and height 2

- can measure faster frequency as don’t depend on saline column of water but this isn’t really clinically useful

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22
Q

What are air filled pressure catheters?

A
  • instead of saline they contain air

- air is compressible so there is concern whether pressure is correctly transferred from bladder to transducer

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23
Q

What parameters can we measure?

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A 
Pvesicle
Pabdominal
Pdetrusor
Volume of bladder (as can fill the bladder)
Flow Rate during void

Can then plot bladder volume against detrusor pressure and see there is not that much increase in pressure which is necessary for a highly compliant bladder (healthy)

24
Q

Where does the catheter measuring the vesicle pressure go to?

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A

Peristaltic roller hump

  • set of rollers with tubing around
  • tubing compresses down on the rollers driving the saline into the bladder
  • measure how fast pump is turning = rate at which bladder is being filled
  • should be at a constant speed
25
How do we measure compliance?
= change in bladder volume/change in detrusor pressure - 1/gradient of graph - should be high
26
What does a graph of Bladder Volume vs. Detrusor Pressure look like for a patient with obstruction?
- higher gradient - lower compliance - greater rise in pressure as bladder fills - bad as can cause incontinence as pressure overcomes sphincter pressure or get reflux into kidneys = kidney damage
27
What does overactivity look like?
- starts off compliant - then when get to higher bladder volumes, pressure increases steeply then drops down again, then increases again etc. - reflexes where stretch receptors detect extension of bladder are normally suppressed but not here = incontinence or if no co-insiding with sphincter contraction = reflux
28
What do we ask about a patient's desire to void and why?
- 1st sensation - Desire to void - strong need to void To diagnose urge incontinence Can see if this correlates with pressure increase on graph (contraction)
29
Retrograde Filling of the Bladder Investigation
- need to use saline as if use water may trigger reflux contraction - ideally at body temperature - think about flow rate at which bladder is being filled (physiological as possible due to viscoelastic behaviour) - if get these wrong can cause artefact contractions as bladder is not used to it
30
What does viscoelastic behaviour mean?
- time dependent - if pull very quickly, will act different than if you pull it slowly as requires less force - so if rapidly fill bladder will appear to have higher pressures than if gradually fill it
31
At what rate do we typically fill the bladder at and why?
10-100mL/minute - want it to be physiological which is very slow - compromise as in labs have limited time - can be adjusted based on patient size/age
32
What is the significance of stress relaxation to viscoelastic behaviour?
- when pressure increases as you fill the bladder at a faster rate you will then see it reduces and level out as it reaches a physiological rate - so either fill at a slow rate - if you have to fill faster, allow a period of stress relaxation - better not to fill faster as may set off contractions
33
What is the analogy for the viscoelastic model?
Springs - force is directly proportional to extension Vs. Dashpot - plunger in pot (movement of plunger in pot, time dependent)
34
What is voiding cystometry also known as?
Pressure Flow Study
35
What is the gold standard for measuring a bladder outflow obstruction?
- pressure flow study
36
What is the graph for voiding cystometry?
- Flow rate of urine being voided - against detrusor pressure - detrusor pressure starts off greater to start the flow then does a loop (backwards C) and lowers - peak of C is Qmax
37
Why is the detrusor pressure greater at the beginning of voiding?
Needs to be greater than the intrinsic urethral pressure (pressure which holds urethra shut)
38
What is the Qmax normally?
Greater than 10 and Less than 50cm of water | - 20 is good for example
39
What would an obstructed patient's voiding cystometry graph look like?
- wider bigger C shape - starts off at higher pressure as need greater pressure to initiate flow - flow rate is reduced so Qmax is reduced (fatter C)
40
What can we record from a pressure flow graph?
- Qmax | - detrusor pressure at Qmax
41
isometric muscle Contractions
-muscle is developing a force without changing its length (or bladder volume in this case)
42
What is the strain on the muscle?
The length at which the muscle is held at when it is holding that force
43
What is the graph of strain vs. tension of isometric muscle contraction?
- passive and active parts - passive = stretching of smooth muscle bundles, no contraction, collagen matrix starting to stretch at the end as line tension increases - active = when contraction takes place, there is a maximum point where max force is generated at particular length, relatively flat with SM
44
Isotonic Muscle Contraction
Muscle shortens whilst exerting a tension | happens in the bladder as it expels urine
45
What is the graph of isotonic muscle contraction?
- force-velocity relationship - force plotted against speed of shortening - line is negative exponential - when speed of shortening is 0 = isometric contraction
46
What is the speed of shortening analogous to?
``` Flow Rate (Q) - as faster detrusor muscle shortens, faster the reduction in volume, so greater flow rate ```
47
What is the force analogous to?
- detrusor pressure
48
What can we change the force vs. speed of shortening graph to?
- Q (flow rate) vs. Detrusor pressure - looks similar = negative exponential - this is called the BLADDER OUTPUT RELATIONSHIP -
49
What does the nature of the bladder output relationship curve depend on?
- urethral resistance (extent of obstruction) = PASSIVE URETHRAL RESISTANCE - if you have a lot of resistance = graph moves up and out (increasing power of the detrusor so gives a higher pressure at any given flow rate)
50
What does the passive urethral resistance determine?
Resistance to flow - can describe relationship between pressure + flow P = P(initial pressure required to open urethra) + 1/2 (Q^2/A) A = CSA of urethra
51
What is PURR?
Passive urethral resistance relationship | - put on Q vs Pdetrusor graph = get a perpendicular line to BOR lines (bladder output relationship)
52
Bladder Outlet Obstruction Index
BOOI = detrusor pressure at Qmax - (2 x Qmax) Qmax = ml/s
53
Normal BOOI values
Men = <20 means unobstructed, >40 means obstructed | - can use this to redraw as a nomogram
54
Bladder Contractility Index
BCI | = detrusor pressure at Qmax + (5 X Qmax)
55
BCI Normal values
``` >150 = strong contraction <100 = weak contraction ``` Can use this to determine if someone has a powerful contraction that is overcoming obstruction, but their obstruction still needs treating Can also use to create a nomogram
Bioengineering in Urology
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