Blood pressure (BP) measurement is indicated in a large number of situations and facilities for BP measurement are considered essential in all practices seeing cats. Both hypotension (low blood pressure) and hypertension (high blood pressure) can be life-threatening.

Systemic hypertension is commonly recognised in feline practice, with most reported cases occurring secondarily to chronic renal failure or hyperthyroidism. For example, at least 20 per cent of cats with renal failure are thought to suffer from high blood pressure. It is essential to diagnose and treat these cats, because high blood pressure is very damaging – especially to the eyes, central nervous system, heart and kidneys.

Clinical consequences of high blood pressure include blindness, neurological signs (senility and seizures, for example), development of a heart murmur and renal damage which can cause or exacerbate kidney failure.

As with people, systemic hypertension is often referred to as ‘the silent killer’ since many cats will not show any symptoms until the condition causes significant damage. Sudden-onset blindness remains one of the most common presenting signs of cats with high blood pressure.

Although accurate assessment of BP requires access to an appropriate machine, an eye examination can be useful in identifying and monitoring patients with clinical hypertension.

Measuring blood pressure in conscious cats

It is currently believed that the most accurate and reliable way of obtaining BP measurements in conscious cats is by using a Doppler technique (Figure 1). The technique for measurement using a forelimb is described in this paper but the tail and back legs can also be used.

Figure 1: There are three Doppler machines readily available to UK veterinary practices: a. Parks Doppler; b. Thames Medical CAT Doppler; c. Vettex Uni BP

A quiet location away from barking dogs and telephones should be chosen, since BP can be dramatically affected by stress. The cat should be allowed a 10-minute period of ‘acclimatisation’ before readings are taken – this is designed to allow the blood pressure to settle to resting levels.

A cuff is placed between the elbow and carpus. The author recommends latex cuffs (such as Hokanson PC 2.5 and 3.3 cuffs) since these are flexible and fit comfortably. Once uneven inflation or ballooning on inflation is seen, the cuff must be replaced.

The cuff should have a width that is approximately 40 per cent of the limb circumference. For most cats, this means that if you are using a forelimb you need a 2.5 cm wide cuff (Figure 2). It is good practice to record the site and cuff size used and to be consistent when reassessing the same cat.

Figure 2: The cuff should be placed securely but comfortably. When using the forelimb, the cuff is placed between the elbow and the carpus

A surgical spirit-soaked swab is used to wet the area over the common digital artery, which is located on the palmar surface of the forelimb, between the carpal and metacarpal pads (Figure 3). This helps to improve contact with the probe and hence the quality of the Doppler signal. Clipping, even of long-haired cats, is not necessary.

Figure 3: The area over the common digital artery (between the foot pad and the carpal pad) is wiped down with a spirit soaked swab to facilitate good contact with the probe

A liberal quantity of ultrasound coupling gel is applied to the prepared area. Ultrasound coupling gel is applied to the Doppler probe whilst the machine is still switched off. The Doppler probe is placed over the common digital artery, in line with the limb, and the machine is switched on (Figure 4). It is preferable to wear headphones so that the cat does not hear any of the noise associated with the procedure.

Figure 4: The probe is gently placed over the prepared area and the machine is switched on

If pulsatile blood flow cannot immediately be heard it may be necessary to gently move the probe over the prepared area of skin until a signal is detected. It is important to hold the probe gently, and not to apply excessive pressure such that blood flow could be impeded.

Once a clear pulse sound is heard, the cuff should be slowly and gently inflated to around 20 mmHg above the point at which it can no longer be heard. The cuff is then deflated slowly, recording the systolic blood pressure (SBP) as the point at which the pulse first becomes audible. The cuff should be completely deflated and a subsequent three to five more readings obtained in the same way. The average SBP is calculated.

Top tips for success

•   Don’t rush

•   You can never have too much gel!

•   Don’t press too hard

•   Inflate the cuff gently in a step-wise manner

•   Practise on anaesthetised cats (NB. Their SBP will be lower)

•   Get a good sphygmomanometer that allows smooth, slow deflation

•   Use headphones so the cat cannot hear the noise of the procedure

•   Use gentle restraint – remember ‘less is more’ in cats.

Interpreting results

What follows is based on the author’s experience and published data. Individual clinics may wish to amend the following ranges according to the method they use and their own experiences – for example, a very noisy practice may find stress-induced (‘white coat’) hypertension more of an issue than a quiet, feline-only practice.

Clinical hypertension

In a typical ward or consulting room situation, cats with an SBP greater than 200 mmHg can generally be regarded as hypertensive. However, some healthy cats may transiently have SBP above 200 mmHg owing to ‘white coat’ hypertension.

It is important to take a detailed history and perform a full clinical examination (including ocular examination) to look for evidence of hypertension and/or clues of underlying disease such as hyperthyroidism. Antihypertensive treatment is warranted in cats where clinical evidence of hypertension, such as ocular haemorrhage, is found.

If no clinical signs of hypertensive disease are present then it is prudent to re-check the SBP, especially if readings are only slightly above 200 mmHg. Ideally, measurem
ents should be performed several times on separate days to ensure that the elevated SBP is a persistent finding before starting antihypertensive therapy in these cats.

Cats with SBP readings between 175 and 200 mmHg should be regarded as potentially hypertensive. If there is evidence of hypertensive disease (especially ocular signs) or if the cat has a condition known to be associated with hypertension, then anti-hypertensive therapy is justified.

In the absence of these findings, it can be difficult to establish whether the cat is truly hypertensive or whether it is suffering from ‘white coat hypertension. The acclimatisation period is helpful in distinguishing between these possibilities. If the result is unchanged having repeated this procedure, the cat should be monitored closely, both for changes in SBP and also for development of early clinical signs of hypertensive disease that would subsequently warrant therapy.

Most normal cats have SBP readings of between 130 and 175 mmHg. In those cats known to have a condition predisposing them to development of hypertension, such as renal disease, monitoring trends in SBP readings is useful and if it is clear that the SBP is rising, then therapy is warranted once this persistently exceeds 160 to 170 mmHg.

Hypotension

Reference ranges for abnormally low SBP readings are not well validated. SBP readings of less than 90 mmHg are consistent with hypotension. SBP should be maintained above 60 mmHg to ensure adequate tissue perfusion of vital organs.

Eye examination

The eye is an important end-organ for hypertensive damage and ocular examination can reveal clues of clinical hypertension such as haemorrhage, variation in the diameter of retinal vessels, retinal oedema and detachment (Figure 5). Blind cats often have bilateral mydriasis (pupil dilation), but many cats with clinical hypertension will retain some vision and ability to constrict the pupil. Indirect ophthalmoscopy, using a hand lens (2.2 dioptre Pan Retinal lens) with a focusing light source (a transilluminator head on an ophthalmoscope/otoscope body) allows the user to visualise a large area at the back of the eye in one view and hence quickly spot abnormalities (Figure 6). As long as the examination is done in a dark room, there is no need to apply eye drops to dilate the pupils.

Figure 5: Ophthalmic examination can be extremely helpful in the assessment of hypertensive cats. In this case, bilateral mydriasis (pupil dilation) associated with retinal detachment is apparent when examining the eye with a pen torch. Small areas of retinal haemorrhage can be seen in the left eye. This cat was presented with sudden onset blindness and never regained her sight

Figure 6: Examination of the eyes using indirect ophthalmoscopy should be done in a darkened room. This is a straightforward technique to learn and will quickly produce a good view of the cat's retina. Image by kind permission of Professor Danielle Gunn-Moore, University of Edinburgh

With practice, an eye examination only takes a couple of minutes to perform. Since systemic hypertension is one of the most common diagnoses in older cats, an eye examination is recommended as part of the clinical examination in all cats over the age of seven years.

Treatment of systemic hypertension

ACE inhibitors, such as benazepril (Fortekor, Novartis) are mild antihypertensives and can be used alone or in combination with amlodipine at a once daily dose rate of 0.5-1.0 mg/kg. The calcium channel blocker, amlodipine (not licensed for use in cats) is a much more potent anti-hypertensive and appears to be safe in cats when used at a dose of 0.625 – 1.25 mg per cat.

Systolic blood pressure should be rechecked seven to 14 days after starting treatment (or changing the dose) until stable and, thereafter, at least once every three months to ensure good control.

More information on blood pressure measurement and ophthalmic manifestations of systemic hypertension are available as free downloads in the publications section of the authors website: www.catprofessional.com

Acknowledgements

Suzanne Rudd VN, Bruno the cat, and the University of Bristol photography department for their assistance with producing some of the images

Author

Sarah M A Caney

CVT (USA) RVN REVN

Sarah is a 1993 graduate of the University of Bristol. She is an RCVS Specialist in Feline Medicine and loves both first opinion and referral feline medicine. In 2007 she founded Cat Professional which aims to provide cat owners and veterinary professionals with the highest quality information, advice, training and consultancy services.

Cat Professional has published three books 'Caring for a cat with kidney failure', 'Caring for a blind cat' and 'Caring for a cat with lower urinary tract disease' which are all available as 'softbacks' and electronic books via her website www.catprofessional.com.

Veterinary Nursing Journal • VOL 25 • No2 • February 2010 •