Vascular ultrasound

#MedicalPhysicistOnTourPart2

Today in Medical Physics on Tour Part 2, I spent the morning in Vascular Ultrasound. Vascular scientists perform routine and urgent ultrasounds on the vascular (blood vessel) system all over the body to diagnose and rule out a range of conditions including Deep Vein Thrombosis, aneurysms and checks on fistulas used for kidney dialysis.

Vascular Scientists

Vascular Scientists are Clinical Scientists that can train through the STP like me! Although they have a lot more patient contact in one week than I will have in a year as a radiotherapy physicist! It’s really interesting to see in person how the Scientist Training Programme is used to prepare people for roles which utilise a massively diverse range of skills within different realms of healthcare. I guess the one thing that ties us all together is the ability to use our science know-how to make logical conclusions that will directly benefit the patient.

Thrombi

Today’s patients included someone with a randomly lumpy brachial artery, someone whose feet go numb when walking, a suspected calf DVT and a diabetic lady referred by the community podiatry team due to worries about the blood flow in her feet. Spoiler alert: no one had any thrombi and all were sent happily on their way back to their referrers which is a good outcome for all involved.

Thrombi are blood clots that form in vessels and then stay there. If the thrombus moves from the place it has formed then it is known as an embolus. To check that a vein is clear of thrombi, ultrasound can be used. Veins are depressible when pressure is applied with an ultrasound probe due to their lack of elasticity. Arteries do not depress unless under a large amount of pressure.

If a thrombus is present, the vein will not depress as the clot prevents the vein from being depressible. Patients will then be treated with blood thinning drugs to prevent the clot getting bigger or any new clots forming.

Systolic blood pressure

One example where arteries do depress is when systolic blood pressure is taken using a cuff. The cuff inflates to a pressure greater than the artery. At this pressure, the artery is squished to an extent that blood flow doesn’t occur hence why blood flow is not audible using a stethoscope. The cuff is gently deflated and blood flow will gradually return once the cuff and artery pressure equalise. The pressure at which the first sound is heard is your systolic blood pressure. Your diastolic blood pressure is then the pressure at which your blood flow then becomes laminar (and so no pulsatile sounds are heard) because the pressure has reached the same as the pressure as your arteries between heartbeats. [NB: This is a super unscientific explanation to help me vaguely remember in the future how the measurement is made and what it means, please do not use me as a reference in your scientific paper. ]

Ankle Brachial Pressure Index

Systolic blood pressure can be measured using ultrasound. Rather than using a cuff and a stethoscope, a cuff and an ultrasound probe in Doppler mode looking at the blood flow of an artery can be used. This technique is used in the diagnosis of Peripheral Artery Disease (PAD). [See this NHS webpage for more details on the condition: https://www.nhs.uk/conditions/peripheral-arterial-disease-pad/diagnosis/]. The ankle brachial pressure index (ABPI) is a ratio of ankle systolic blood pressure to brachial blood pressure. A ratio of between 0.9 and 1.2 is considered normal. There is slight variation between blood pressures in arms and legs even when lying down due to the length of the arteries, their diameters and stiffness. After a diseased site, pressure will decrease. This is because diseased sites cause vessels to narrow. This narrowing increases the blood velocity and therefore more kinetic losses occur due to this turbulent flow. This in turn reduces the blood pressure. An ABPI of less than 0.9 is therefore indicative of disease somewhere along the artery tree above the ankle.

Tune in on Monday for some Christmassy musings on the delights of General Ultrasound!