Taking the stress out of blood pressure measurement

It is widely recognised that blood pressure (BP) measurement is something we should all be performing in older cats (typically older than nine years of age) as part of a routine health check, yet a study by Conroy et al suggested only approximately 4.5% of cats older than nine years of age, from a selection of UK-based practices, had systolic blood pressure (SBP) measured on presentation to their vet¹.

This raises questions about what the barriers are that prevent routine SBP measurement being performed in practice, and how we might best try to overcome these to ensure systemic hypertension is recognised and appropriately treated. A major concern relating to BP measurement can be the risk that stress and activation of the sympathetic nervous system in the clinic environment results in increased BP – otherwise known as “white coat” hypertension or what has more recently been termed situational hypertension. This can give the perception that BP measurements are unreliable and that it can be challenging to obtain accurate results.

However, implementing a number of key steps can optimise the measurement of blood pressure and diagnosis of hypertension to help avoid situational hypertension. These key steps include:

• choosing a blood pressure machine that is going to give accurate, repeatable and reliable readings
• identifying the “at-risk” population of cats for systemic hypertension – and, therefore, BP measurement
• creating the right environment for BP measurement including acclimatisation in the clinic
• being confident in the readings you take and recognising what constitutes hypertension
• checking for target organ damage (TOD) to support the diagnosis of hypertension
• when no TOD is found, repeating measurements to document persistence

Choosing the right BP machine

To have confidence in a diagnosis of hypertension, the machines used to measure BP need to provide accurate, reliable and repeatable results. Three main types of BP machine are available – standard oscillometric, high definition oscillometric (HDO) and Doppler sphygmomanometry.

In 2007, the American College of Veterinary Internal Medicine (ACVIM) hypertension consensus group developed a set of guidelines for the validation of BP measuring devices for use in cats and dogs². These were similar in design to those currently used in human medicine, giving clear guidance on performance requirements³. However, to date, no BP measuring device has reached these validation criteria for use in conscious cats and, therefore, no single “preferred” BP machine exists.

While this does raise concerns about the accuracy of BP measuring devices in veterinary medicine, clinical associations have been drawn between SBP measurements obtained with these machines and TOD giving us some level of confidence that the available BP machines provide clinically relevant and useful data for our patients⁴.

The most commonly used machines in practice today are the Doppler device and the HDO machine, which is now preferred over standard oscillometric devices for use in cats⁵. While the Doppler machine gives the opportunity to measure only SBP, the HDO device will provide systolic, mean and diastolic readings. However, to date in feline medicine, clinical associations between hypertension and TOD have only been identified with SBP, such that the advantage of having mean and diastolic readings remains to be determined.

It is important the HDO device is always used with simultaneous evaluation of the pulse wave form to indicate a reliable reading has been obtained⁶. The “best” BP machine to use is the one that you have available in your practice and with which you have the most experience.

Identifying the at-risk population of cats

Selecting the right population of cats for BP screening is important. When measuring SBP, it is important to focus on those cats that are deemed to have a clinical risk for hypertension.

Hypertension can be classified as either idiopathic – that is, no predisposing cause is identified – or secondary, where either an underlying disease condition or administration of a particular medication may predispose to the development of hypertension. At-risk features that should prompt screening of BP include:

Age:

• The median age at diagnosis of hypertension in cats is approximately 14 years. Measuring SBP in cats older than nine years of age as part of a health check is, therefore, recommended as a greater chance exists of identifying hypertension in this age of cat^5,7.

Having a condition that may be associated with hypertension:

• Chronic kidney disease (CKD): approximately 30% of cats will be diagnosed with hypertension⁸.
• Hyperthyroidism: approximately 10% of cats will be diagnosed with hypertension at diagnosis of hyperthyroidism and approximately 20% after treatment⁹.
• Hyperaldosteronism: uncommon condition, but approximately 50% of cats will be hypertensive.

Drugs associated with hypertension:

• Erythrocyte stimulating drugs – for example, darbepoetin (used under the cascade), which is used in cats with CKD.

Cats presenting with clinical signs that could be associated with hypertension:

• Ocular – for example, hyphaema, retinal detachment, sudden onset blindness, hypertensive retinopathy/choroidopathy and retinal haemorrhage.
• CNS – for example, behavioural changes, seizures, ataxia, depression, stupor and lethargy.
• Evidence of structural kidney disease or proteinuria.
• Cardiovascular changes – for example, left ventricular hypertrophy.

Do potential disadvantages to measuring SBP in younger cats exist?

Measuring BP in younger cats can be recommended as a way to “establish a baseline” or habituate cats to the BP measuring process. The International Society of Feline Medicine guidelines recommend routine BP measurement for cats older than seven years of age⁷.

If measuring SBP in cats younger than nine years of age, assuming they are healthy and do not have any of the conditions that predispose to the development of hypertension, it is important to remember the risk of hypertension is very low. Any high reading must be carefully substantiated before a diagnosis of hypertension is made.

The possibility of situational hypertension must be considered very carefully and antihypertensive therapy only started if a confident diagnosis of persistent systemic hypertension can be made.

What is acclimatisation and why is it important?

All cats can become anxious and stressed during hospital visits. The risk of situational hypertension can be reduced by providing a period of acclimatisation.

During a routine consultation, this may include allowing the cat out of its basket to explore its environment while you are obtaining a history or reviewing clinical notes with the clients. Ideally, a period of 5 to 10 minutes should be allowed as acclimatisation to help relax the cat prior to starting BP measurement^10,11.

If this is challenging to achieve in your practice, consider the use of nurse-led clinics for geriatric cats, so SBP measurements could already have been performed in a pre-consultation before the appointment with a vet. It can be tempting to admit cats or to take them to a triage area for BP measurement. However, having the owner present while measuring BP helps to reduce stress and anxiety in most cats.

Other steps to reduce the risk of situational hypertension are:

• Position of the cat – allow cats to assume their preferred position (for example, sitting, sternal, standing, curled in their carrier and sitting on an owner’s knee). If the cat is relaxed, readings are more likely to be reliable.
• Quiet environment – make sure the room where SBP is measured is away from disturbing noises; for example, other cats and dogs.
• Make use of feline pheromone diffusers in the room.
• Allow the owner to hold his or her cat for SBP measurement.
• Discard the first reading and then average five readings – however, it is important to use clinical judgement. Some cats will become more relaxed as you continue taking readings and in this situation you may need to take more than five to obtain your five consistent readings that fall within 10mmHg of each other. Conversely, some cats become more anxious during BP measurement. If something happens to scare the cat during BP measurement (for example, a door slams or dog barks) and the SBP increases dramatically, be prepared to consider using less than five readings on that occasion so you feel happy the readings are representative.

What level of SBP is too high?

The ACVIM hypertension consensus guidelines published in 2018 updated the categories for BP and the diagnosis of hypertension (Table 1).

How to decide it’s not “white coat” or situational hypertension

Deciding whether a cat has situational or genuine hypertension, and whether to start treatment, can be challenging. To make this decision, we need to use not only the SBP measurements, but also clinical information about the cat.If a cat presents with evidence of hypertensive TOD to the eye or hypertensive encephalopathy then a diagnosis of systemic hypertension can be made using measurement of SBP on a single occasion.

However, if obvious TOD is not present (that is, on fundic examination) then the situation is a little more complex. This is where repeating SBP measurements and documenting persistence of hypertension, together with an understanding of the individual cat “risks” for hypertension become important together with a clinical impression of the cat’s tolerance of the SBP measurement process:

• If the cat is deemed at increased risk – that is, older than nine years of age and has a diagnosis of a condition that can be associated with hypertension – SBP greater than 160mmHg on at least two occasions is sufficient to make a diagnosis of hypertension and start antihypertensive therapy.
• If the cat is older than nine years of age, but does not have any predisposing conditions, review for TOD – that is, fundic examination – and repeat SBP measurement should ideally be performed on a minimum of three occasions documenting persistent hypertension (SBP greater than 160mmHg) before antihypertensive therapy is considered. This is to try to prevent a “misdiagnosis” of hypertension and long-term medication when, in fact ,the cat has evidence of situational hypertension.

If the cat is younger than nine years of age and has no conditions that may predispose to systemic hypertension, always consider the most likely reason for an SBP greater than 160mmHg is going to be situational hypertension.

Standardised recording of SBP measurement technique and measurements becomes very important when serial measurements are required to try to pinpoint a diagnosis of hypertension. This helps to ensure conditions for measurement can be replicated and that measurements are as comparable as possible. Panels 1 and 2 detail case examples.