Brain Salad Surgery and the CAT Doppler

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1994 Neuro Operating Theatres, Brook Hospital, Shooters Hill. I was working with Alan the SODA and a couple of Anaesthetists, learning about the skills and dangers of hypotensive anaesthesia for open brain surgery.

The Flat Probe Doppler

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#ThrowbackThursday: A great selection of our older probes!

The key difficulty is balancing the hypotension to facilitate surgery on the open brain (with top of skull removed) with little bleeding into the surgical site in a patient sitting upright in a chair. This had to be done whilst avoiding air entrainment through the open capillary network and the early detection of life threatening air embolus.

“What if we could detect the air bubbles as they popped round the patient?” I thought. It reminded me of lying in bed as a child, listening to the pop of the air bubbling round the radiators. On a cold winter’s night. Dopplers (both foetal and vascular) detect movement utilising the Doppler Effect. Think of a race car as it speeds towards and then past you: It emits a high frequency sound as it approaches, which appears to pitch lower as it passes and drives away from you.

Dopplers work in exactly the same way – but how to position the crystals at the precise angle required in the probe to maximise the effect on a patient sitting upright with the top of their brain exposed? As a result, the flat probe doppler was born, as a 5MHz semi deep penetrating vascular probe. We called it the AED Air Embolus Detection system, and we sold about 5 if my memory serves me well.

The CAT Doppler – A Gold Standard Product

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The CAT Doppler as it is known today, however, came later. Thames Medical had just introduced the 1st veterinary pulse oximeter and I was asked if I could build a blood pressure system that would work on conscious cats and for monitoring pulse quality and BP in anaesthetised patients.

After a failed relationship with another UK manufacturer (who told me they couldn’t build my design, but later took their version to market and became a competitor), I called my friend and colleague Nick See who had just started his own doppler ultrasound company (Ultrasound Technologies) in Chepstow, Wales.

We started production and shortly after in 1997, with some helpful input from Andy Sparkes at Bristol University, the original CAT Doppler was born. A flat probe doppler that quickly and easily picks up the smallest pulses from the smallest patients – all powered by a mini 9 volt battery all in a hand held unit.

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Carla Finzel RVN: Demonstration of Feline Blood Pressure with a CAT+Doppler

In this day and age the CAT is unique. While most of the electronics on the board are machine-made (except for the addition of certain components and the removal of others – one of the key differences between the human and veterinary versions of the CAT) but the rest is put together lovingly by hand using the best materials available. Each pair of organically grown doppler crystals are mounted into the probe, soldered onto their own controller board, joined to the heavy-duty shielded cable, and then back filled with a special epoxy resin. The probe is then cured in an oven for 48 hours, before the cable is mounted onto the main controller board and each crystal individually tuned using an oscilloscope and a musicians trained ear for both transmission and reception!

We’ve also stress-tested our current CAT Doppler model, with incredible results. We’ve tried running it over with a car, setting it on fire, freezing it in a block of ice and cracking the probe with a machete. The CAT survived all of them!

Over the last 24 years the CAT Doppler is now the longest-serving European Safety Standard Certified Doppler available and far and away the best selling product for Thames Medical in the UK with over 6500 sold in the UK alone. On the way we have found other uses too – in human medicine a similar but different version of the CAT is used in dialysis patients, particularly children, for vessel location and tracking the blood flow during dialysis, diabetic medicine for measuring the differential blood pressure between limbs APBI Index. What started as brain salad surgery is now the most widely used blood pressure system for cats (and dogs) in the UK, The CAT Doppler, made by hand in Wales.

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