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Practice gadgets - what they do: the blood pressure monitor

Until relatively recently, this was considered a “toy” - nice to have but not essential. However, we now know a lot more about the importance of a healthy circulatory system, and blood pressure monitoring can be vital to looking after your pet!

Informative image: blood pressure monitor

What is blood pressure?

Just what it sounds like - it’s the pressure of the animal’s blood within the vessels (arteries and veins). There are two main components to a blood pressure reading:

Systolic pressure, which is the highest, and represents the pressure wave sent out when the heart contracts.

Diastolic pressure, the lower number, which represents the “background” pressure in the vascular system.

These are normally measured in mmHg (“millimeters of mercury”), and represented as systolic/diastolic. In humans, for example, the normal pressure is 120/80, or a systolic pressure of 120mmHg and a diastolic pressure of 80mmHg.

The two components are also, loosely, linked to the two key factors that influence blood pressure. The first is the strength of each heartbeat, which mainly influences the systolic pressure; while the second is the systemic resistance (i.e. how much fluid there is in the system, and how tightly the arterial walls are constricting). This has slightly more influence on the diastolic pressure, although of course both are interrelated.

What’s normal?

For a human, it’s pretty well established that a normal blood pressure (BP) is 120/80. However, this does vary a lot - even humans get stressed when the doctor is taking a blood pressure reading, and stress pushes it up! Since we can’t explain to our pets why we’re wrapping a tight band round their leg, this effect may be more marked in all but the most laid back (or anaesthetised!) of patients. In addition, the wide range of healthy body sizes (from the 500g kitten up to the 100kg Newfoundland) means there’s a lot more variation in natural blood pressure than in humans.

In general, we assume normal for a dog is between 90 and 120/75, with an Mean BP of 100. Normal for a cat ranges from about 100-180/100 - the greater range indicating the greater scope to stress out a cat compared with a dog! - with a Mean of 135.

Why do you need to measure it?

Because blood pressure is a measure of how well the body is supplying blood - and therefore oxygen - to the tissues. If the Mean blood pressure is too low (hypotension), once it drops below 60mmHg oxygen stops being supplied to the kidneys, and kidney failure will follow.

On the other hand, if the blood pressure is too high (hypertension), the pressure can damage the small blood vessels, causing bleeding into the eyes, strokes, heart and kidney failure.

When do you monitor blood pressure?

There are a number of possibilities, but there are two situations when it’s really important:

Under anaesthesia:

Anaesthesia, keeping a patient asleep for surgery, is quite hard on the body. In particular, it tends to dramatically decrease blood pressure (because anaesthetic drugs usually slow down the heart, and also cause the blood vessels to relax). This is why kidney failure is a known risk, especially in older animals. By constantly monitoring their BP while asleep, we can act before it drops to critical values, minimising the risk of complications.

In cats with Feline Secondary Hypertension:

Excessively high blood pressure is a common symptom of a number of feline ailments (although cats can get Primary Hypertension too), in particular chronic kidney disease and hyperthyroidism. As a result, we strongly advise regular blood pressure monitoring of all old cats, especially those with kidney problems or an overactive thyroid.

How does the machine work?

There are two different machines that we use:

Doppler Blood Pressure Machine:

This is the simpler version, and contains a very sensitive microphone and a cuff that we inflate around the pet’s leg. The microphone is connected to earphones for the vet or nurse to use, so they know when they have manually inflated the cuff just enough to stop the pulse - that’s the systolic pressure. This system is pretty accurate, and most cats and dogs don’t mind it (much!), but it can’t measure the diastolic pressure. As a result, we tend to use it for outpatients and general BP checks.

Oscillometric Blood Pressure Machine:

This is an automated system that contains a cuff, a vibration sensor, and a computer and a pump. Essentially, we put the cuff on and leave the machine to do its job! It can measure systolic, diastolic and Mean blood pressure, but it’s noisy and uncomfortable for the dog or cat, so we tend to use it mainly for anaesthetised patients.

If you’re at all concerned about your pet’s health, make an appointment to see one of our vets to get them checked out!