Hypertension is most often diagnosed in small animal practice when a cat is presented with blindness. On examination, these cats typically have retinal detachment, retinal haemorrhage, hyphema, or in most instances some combination of these findings. If fundic examinations are performed in the absence of owner-reported clinical signs, then the lesions tend to be more subtle; small retinal haemorrhages, focal retinal detachments, and retinal oedema are most often observed.

Cats may also be presented for neurological signs: ataxia, weakness, disorientation, amaurosis, vestibular signs, paraparesis, decorticate posturing, stupor, seizures and sudden death have all been reported. This is considerably less frequent than presentation for ocular abnormalities.

Hypertensive cats are rarely presented with signs of congestive heart failure. More often a murmur is noted in an older cat, and this increases the clinical suspicion of hypertension. However, heart murmurs are found in many normotensive older cats, particularly those with chronic kidney disease (CKD), so this finding alone is not enough to establish a diagnosis.

Causes of Hypertension

The first and most obvious reason for suspecting that a patient is hypertensive, and therefore measuring its blood pressure, is that it is exhibiting consistent clinical signs. Increasingly, however, we are attempting to measure blood pressure in a proactive manner, prior to the development of owner-recognised abnormalities. This requires an understanding that cats are most at risk for developing hypertension.

CKD is the problem most commonly associated with severe hypertension in the cat. In a large survey of cats presenting with ocular manifestations of hypertension, 44/69 (64%) had elevated serum creatinine concentrations. Correspondingly, when cats with CKD are evaluated, the prevalence of hypertension is high. Estimates of the prevalence have varied considerably between studies, however (from 19–65%), which may be due to different methods of blood pressure measurement, different cutoffs taken to signify hypertension, and differences in the population studied. In particular, hypertension may be more commonly diagnosed in cats with relatively mild azotaemia; most have creatinine concentrations < 300 μmol/l at diagnosis. The reason for this is undocumented but may be because many cats with severe azotaemia are hypovolaemic, resulting in reduced blood pressure.

Hyperthyroidism is commonly cited as a major cause of hypertension in the cat. Recent studies have suggested that only about 10% of hyperthyroid cats are hypertensive at the time of diagnosis and the concurrent diagnosis of hyperthyroidism and hypertensive ocular disease is infrequent. However, a significant proportion (about 20%) of cats actually develop hypertension with treatment for hyperthyroidism. Therefore, it is not only necessary to measure blood pressure in hyperthyroid cats at the time of initial diagnosis, but also to follow their blood pressure during treatment. Another potential (but less common) endocrine cause of hypertension is hyperaldosteronism due to an adrenal tumour.

Currently about 20% of the cats diagnosed with hypertension have no apparent underlying cause (so-called idiopathic hypertensives). These are cats that are euthyroid and nonazotaemic, although it is possible that many of these cats have subclinical renal disease. The average age of cats diagnosed with idiopathic hypertension is 15 years, with almost all being > 12 years. This observation is a good indication of the importance of measuring blood pressure in elderly cats, even if no predisposing cause for development of hypertension is identified.

Practicalities of Blood Pressure Measurement

Noninvasive methods are most applicable to measurement of blood pressure in conscious cats. It has been demonstrated that both oscillometric and Doppler methods correlate reasonably with direct measurement of blood pressure in anaesthetised patients. However, the oscillometric technique correlates poorly with direct measurements in awake cats and does not reliably predict which cats will have retinal lesions. For this reason the Doppler method is preferred for diagnosing hypertension in this species.

Wherever possible, we measure the blood pressure with the owners present and before admitting the patient to the hospital. Ideally measurements should be made having allowed the patient to acclimatise to its environment for 5–10 minutes and before any stressful procedures are performed. The forelimb, hind limb, or tail can be used for measurement, but it is important to be consistent since different measurements may be obtained from the various sites. The width of the cuff should be approximately 40% of the limb circumference. The first reading is disregarded and then an average of the next 4 or 5 measurements is recorded.

The decision to treat a cat that has definitive evidence of end-organ damage (most often ocular changes) as a result of systemic hypertension is usually quite straightforward. The decision to treat an asymptomatic patient for hypertension on the basis of blood pressure measurements is essentially a compromise between the probabilities of unnecessarily treating for 'white-coat' hypertension and of not treating a patient that subsequently develops end-organ damage. The balance of these possibilities is, in part, dependent on the severity of the blood pressure elevation which is reflected in the ACVIM consensus statement guidelines for the risk of end-organ damage as outlined in the table below. It is important, though, to also consider the likelihood that the patient in question is truly hypertensive; does it have an underlying predisposition? For this reason, it is not possible to give definitive cutoff points at which therapy is always indicated, because this will depend on the clinical status of the individual patient. However, in the author's practice, antihypertensive therapy is instituted in any cat with systolic blood pressure measurements over 170 mm Hg and compatible ocular lesions. If no ocular lesions are present, then the cat is not started on therapy on the basis of measurements made at a single visit (irrespective of how high these are), but the owner is asked to return the cat to the clinic in 1–2 weeks for blood pressure measurement to be repeated. It is only if blood pressure is also high on this second visit that therapy is instituted. Screening for hypertension in asymptomatic cats is only performed if they are over 9 years of age, unless they are also azotaemic or have some other identifiable risk factor for development of hypertension.

ACVIM consensus statement: Risk of target-organ damage according to blood pressure

Treatment and Followup

Amlodipine (Istin®, a calcium-channel blocker) is the currently recommended treatment for hypertension in cats. This is because other treatments (β-blockers, ACE-inhibitors) do not reduce the blood pressure sufficiently to prevent the development of ocular lesions. The initial dose of amlodipine is 0.625 mg (1/8 tablet) per cat, per day. If this dose does not reduce the blood pressure sufficiently, then the dose is doubled to 1.25 mg (1/4 tablet). This dose is adequate in almost all cats. Where a poor response is noted, it is generally due to poor owner or cat compliance. It is important to continue to monitor blood pressure even when the initial response to treatment is good, since in the long term the requirements for antihypertensive therapy may increase.

A common question is what constitutes optimal blood pressure for a cat receiving treatment for hypertension? In humans, hypertension can result in the progression or even the development of CKD. We have recently completed a study where we assessed the effect of blood pressure (after introducing therapy) on survival time of cats with hypertension. There was no difference in survival time between the cats with well-controlled or poorly controlled blood pressure once other factors (primarily proteinuria) had been taken into account. This suggests that antihypertensive therapy should primarily be aimed at prevention of ocular lesions and other signs that occur with severe systemic hypertension; maintaining systolic blood pressure below 165 mm Hg is probably adequate for this.

References

1.  Brown S, Atkins C, et al. Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats. Journal of Veterinary Internal Medicine. 2007;21(3):542–558.

2.  Jepson RE, Hartley V, et al. A comparison of CAT Doppler and oscillometric Memoprint machines for non-invasive blood pressure measurement in conscious cats. Journal of Feline Medicine and Surgery. 2005;7:147–152.

3.  Jepson RE, Elliott J, et al. Survival of cats with naturally occurring hypertension is not determined by adequacy of blood pressure control. Journal Veterinary Internal Medicine. 2007;21:402–409.

4.  Maggio F, DeFrancesco TC, et al. Ocular lesions associated with systemic hypertension in cats: 69 cases (1985–1998). Journal of the American Veterinary Medical Association. 2000;217(5):695–702.