The measurement of blood pressure (23/09/13 prac)

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The measure of blood pressure (23/09/13 prac)

Arterial blood pressure

Sphygmomanometry is the

indirect measurement of blood pressure. To do it directly would involve

inserting a cannula attached to a diaphragm and transducer into the radial or

dorsalis pedis arteries. However, this is invasive. Sphygmomanometry uses the

brachial artery because it branches directly off the aorta and can be measured

at the level of the heart, so it is considered equal to aortic pressure. When

measuring the blood pressure, one hears:

·

The systolic

pressure, which is the highest pressure produced when the ventricles contract.

This is typically ~120 mmHg and heard as turbulent/noisy intermittent squirting

of blood as the high pressure of the cuff is released. It reflects the amount

of work the heart is being asked to perform with each beat to generate

pressure.

·

The diastolic

pressure, which is the lowest pressure produced when the ventricles fill. This

is typically ~80 mmHg and it is a continuous, smooth (laminar) flow of blood

only allowed when the cuff pressure is lower than diastolic pressure. It is

heard as the pulse sounds disappearing – previously the cuff had been inflated

above diastolic pressure and so the brachial artery was still closing and

opening and creating pulse sounds as blood squirts through. This reflects the

state of the blood vessels: both their stretchiness and how much resistance to

blood flow there is (to maintain blood pressure).

Auscultatory sphygmomanometry – relies on the sounds of blood through the artery.

Includes both using a stethoscope, and automatic blood pressure devices

containing a microphone, such as the DGS (PC programme in Phys lab which

produces graphs).

Oscillometric

sphygmomanometry – relies on shock

waves from the opening and closing of the artery changing the pressure in the inflated

cuff as they reach it. Is the more common technique in automatic blood pressure

monitoring; includes the Omron and Omega automated blood pressure monitors.

We first looked at blood pressure changes by attaching

a cuff around the brachial artery and analysing the pulse changes on the DGS.

The way the cuff pressure oscillates as the cuff is deflated can be used to

estimate systolic and diastolic pressures.

We then measured blood pressure manually (using an

Aneroid cuff and stethoscope kit). I was unable to find my partner’s pulse, or

mine.

Next, we used the Omega oscillometric automatic blood

pressure monitor. We compared this with an Omron oscillometric automatic blood

pressure monitor, but were unable to compare either of the automated methods

with the manual.

Class data:

·

Using a paired T-test, there was no

significant difference between the manual and automated, and between the

different automated, methods of measuring blood pressure – EXCEPT in the

diastolic manual measurement vs. both the Omron and Omega measurement. One

explanation could be slow reaction to sounds disappearing in the manual method.

·

Using an unpaired T-test, there was strong

evidence of a difference between male and female systolic pressure, evidence of

a difference between male and female diastolic pressure, and no evidence of a

difference between male and female heart rates.

·

Using a paired T-test, there was no evidence

of a difference in systolic pressure, diastolic pressure, or heart rate when

the class held their breath while these measurements were made, than when

breathing was normal. This could have been a result of a lack of controlled

experimental conditions.

Venous pressure

Venous pressure is very

low (only a few mmHg) because of dissipation of ventricular pressure energy

throughout the vascular tree from pressure to heat energy, and the large

diameter of the vessels providing little resistance. The normal range is 0-6 mmHg.

We measured it by holding our arms at our sides (so the veins in the backs of

our hands would fill with blood) then raising and lowering our arms to find the

height at which the vessels collapse and refill. We then measured this vertical

distance (in mm) from the level of the hand to the heart (4th rib),

and converted these mm into mmHg (mm x 12.9, as Hg is 12.9x heavier than

blood).

Class data:

Not available.

My results were inaccurate (an average of

7.75 mmHg) as I did not mark the two heights and work out the difference, or

ask somebody to help me measure, but rather estimated the difference using a

ruler.

Pathologies

High arterial/venous

blood pressure: high intrathoracic pressure, arterial stenosis, hypervolaemia,

obstruction, heart failure. Low arterial/venous blood pressure: hypovolaemia,

low intrathoracic pressure.

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