A full retinal examinationbeing carried out in adarkened room in a cat withsuspected hypertension

Feline hypertension: pathophysiology,clinical signs and treatment options

MYRA FORSTER-VAN HIJFTE

Myra Forster-vanHijfte graduatedfrom the Universityof Ghent in 1986.She currently runsa private referralservice in Surreyand also works at areferral practice forinternal medicine.She holds theRCVS certificates inradiology and smallanimal medicine andis a diplomate of theEuropean College ofVeterinary InternalMedicine. She is arecognised Europeanand RCVS Specialistin Small AnimalMedicine.

FELINE hypertension is being diagnosed with increasing frequency as more and more veterinary practicesacquire the necessary equipment to perform indirect monitoring of blood pressure in cats. Mean arterialblood pressure is determined by the cardiac output (stroke volume x heart rate) and the peripheralvascular resistance. It is known to increase with age in cats, and patients suffering from the morecommon feline diseases such as chronic renal disease, hyperthyroidism and hypertrophic cardiomyopathyare thought to be particularly susceptible to high blood pressure. While hypertension in cats usuallyoccurs secondarily to other diseases, there is emerging evidence that primary hypertension exists in thisspecies. However, the underlying mechanisms involved in the development of feline hypertension arestill not fully understood and more research is required in this area. This article presents practical adviceto assist in the early detection and management of the condition, which are key to preventing severeclinical disease.

AETIOLOGY ANJD PATHOPHYSIOLOGY

SECONDARY HYPERTENSIONMost cases ot tfclinie hypertension occur seconCdaily toothei diseases and it is importaint to ealuate the hyper-tensixe patient filly to diagnose alny underlying condition.The moost commiiiioni diseases associated xxith hypertelsionare renal disease, hyperthyroidism iand cardiac disease.Other diseases xvhich aire also thought to be associatedxith hypertension (based on intormation in the humaLnliterature) include diabetes mellitus, acromegaly, hyper-adrenocorticism, chr-onic anaemiat phaeochromocytoma,minller-alocor-ticoici excess aind arteriosclerosis.

Renal diseaseIn cases of renal disease, the reduced localised renalblood floxx leads to actixation of' the renina-nciotensi-alldosterone system (RAAS) xxwhich results in X asocon-strictioin aInd a subsequent increalse in peripher-al xvascularresistance. It ailso caluses retention ot' sodiumii and fluids,both of xxhich increase blood X olume. An increalse in

total peripheral resistance and blood xvolume in turnlle,ads to high blood pressure. The presence of endothelill-I (a potent xtasoconstrictor) andc decreased production ot'x asodilatory substanU1ces (eg. prostLagliandins) by the dis-eased kidney haxve also been implicated in the devxelop-ment of hyper-tensioni in cats xvith chronic renlal failure.

The percentage of cats xwith rcnal disease thought to

suffer friom hypertension x aries fromn study to study. In a

recenit report. olnlN 19 per- cenit ot' cats xith cx idncllc otchlrolic renal diseasexcsrCICtfnld to be hypertenlsixe(SyNme aInd other-s 2001). The same studN founld a sMo-niticant mxrse relationshlip betxeen plasmia potassiuconcenitratio) anld hyperten soll.

HyperthyroidismThe effects ot' thyroid hormonie on the heart are thoughltto be both dir-ect (ie, upregulation of cardiac f-receptors.increased proteini synthesis and a shortened actioni poten-tial durationi) and indir-ect (ie, enhainced metabolic state

of other tissues exertinc, increased demands). This resultsin an inicrcased heart rate, caidiac muscle hypertrophyand increcased caLrdiac contractility.

Cardiac diseaseHy pertrophic cardiomryopathy in parU-ticulalr, is ofteinassociated with hypertension. The reduced cardiac outputcaused by this condition leads to actixvation of the RAAS,xhich in turnl caLises asoconstriction, retention of sodiumand fluids and subsequent hyper-tension. Cardiac diseasemay also be the result of hypertension (ie, the thickniessof the left ventricular xvall increases in response to theincreased workload of the heairt xxhich is haxing to pumpa.gainst the high svstemic blood pressure).

PRIMARY HYPERTENSIONThe role of I P-hydroxy steroid dehydrogenase- .And -2( I -HSD- I atnd -2) in the reguluation of xwater homeo-

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stasis atid blood pressur-e in cats is currently under inves-tigation (Schipper and Fink-Gremmels 2001). Theseenzymiles (cspecially I l3-HSD-2) regulate the break-dowvn of cor-tisol (the active fornm) into cor-tisonie (theinmactixc 'ormi). It is knowin that, in humans, a lackof- I I -HSD-2 or inhibition of this enzyme causes appar-enit minier-alocorticoid excess synidrome (AMES) whichresults in the retenitioni of sodium land thus hypertension.Loxs sodium diets or the usc of mineralocorticoidreceptor- anitagonists (such as spironolactone) havc beenlfound to reduce the hypertension in human patientssvith AMES and this prox ides further evidence thatthese cnzymes may play a role in the decelopment ofhypertensionl.

Initial resear-chi suggests that similar enzymatic re(ac-tions OCCUI iln the cat and also that the inhibition of theenzymes is carried out by the same inhibitors as inhulimanls. If primary hypertension does exist in the cat,this poses the further question of whether or not it isactucally one of the causes, rather thaIn the result, ofchronic renial discase and hypertr-ophic cardiomyopathyin this specics.

CLINVICAL SIGNS

The most common prcsenting sign in the hypertensivepatient is sudden blindness due to hyphaemna or retinaldetachment. As clinicians become more aware of thepossibility oft hypertenision, especially in older cats orthose animi,.Als xwith diseases predisposing to hyperteni-sion, the diagnosis xNvill hopefully he made before severeclinical signs such as bllindness ocClur.

Other cliical signs are ofteni associated with theunderlying, primary disease such as polyurica/polydipsiain chronic rcnal failure or hyperthyr-oidism and poly-phag ia and weight loss in the hyperthyroid patient.Neurological signs such as strokes and seiz'ures xhichaeC commo(nly seen in humians asLI result of hyperten-sion. are less commlloni in the hypertensive cat.

PHYSICAL EXAMINATION

A full physical examination of the patient is important todiagnose hypertension at ain early stage and also to helpidentify any underlying disease in these cats. A thoroughcardiovascular examination should be carried out andshould include auscultation of the heart to determine thehear-t rate, rhythm and the presence of alny car-diac mur-

murs. In cats, it is important to listen at the level of thecaudall sternium as some murmurs are most aludible in

this region. Soft, high-pitched murmurs can be detectedusing the smalilller head of the stethoscope aind pacdiatricstethoscopes are better for this purpose. An assessmentof the quality of the femoral pulse aInd the capillary refilltime give anl indication cardiac output. Armed

with all of this information, it is often possible to estab-lish whether or not there is a cardiac problem.

On abdominal palIpation, the size and shape of the kid-neys ccan be assessed and may give aln indication of a

renal problem. Palpation of the neck region from the area

of the caudal larynx dowvn to the thoracic inlet may reveal

abnormal nodules suggestive of enlarged thyroid tissue.A physical exaiminationi is not complete without a full

retinal examination, which may give an indication ofhypertension (see box below). It should be c(arried out usinga lens and a bright light source and needs to be perfoimnedin a dark room to allow the pupils to dilate fully (mydriaticcyc drops may be necessary to achieve this). The inclusionof a. retinal examination in the physical examination willalid earlie- detection of hypertension before hyphacma or

retin(al detachment occurs. Retinal oedema xvill occur in

the early stages of hypertension. Small retinal haemor-rhages miay be visible and, if left untreated, oftenl progressto hyphaem-a and retinal detachment.

DIAGNOSIS

The meatsurement of blood pressure has been discussedin an earlier article (Stepien 2000). When measuring

Ocular signs of hypertension

Multifocal grey areas ofretinal oedema and bullousretinal detachment

Multifocal areas of retinal degeneration with associatedpatchy increases in tapetal reflectivity and pin-pointretinal haemorrhages. Note the tortuosity and irregularcalibre of the retinal vasculature

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blood pressure in cats, it is very important to be aware ofthe 'white-coat effect'; in most cases, simply bringingthe cat into a veterinary surgery will cause an increasein blood pressure. Blood pressure measurements shouldtherefore be carried out in as stress-free an environmentas possible. The measurements need to be taken by anexperienced vet or nurse, who has the time and patienceto repeat the measurements several times.A diagnosis of hypertension is only made if there is a

sustained high systolic blood pressure of more than 170mmHg. Once the diagnosis has been made, the emphasishas to be on finding the underlying cause for the hyper-tension. The first step is to assess the cat's generalmetabolic status by performing a fasting full biochemicaland haematological analysis together with a urinalysis.Thyroid hormone levels should be checked if the cat ismore than eight years old. If a cardiac problem is sus-pected on the basis of the physical examination, echocar-diographic examination will give a reliable evaluation ofcardiac function and anatomy.

UNDERLYING CAUSESAny underlying causes for the hypertension should betreated. In cases of hyperthyroidism, treatment of thedisease will often reduce the blood pressure. Initially, itmay be necessary to manage hypertension in the hyper-thyroid patient with drugs, but treatment can often bediscontinued after one to three months once the diseaseis controlled. In most other diseases, indefinite medicaltreatment of the hypertension is required in addition to

any therapy instituted for the management of the under-lying disease.

TREATMENT IN EMERGENCY CASESIn cases of hyphaema or retinal detachment, emergencymeasures are necessary to save vision. Sodium nitroprus-side (an arterial and venous dilator) given by constant rateinfusion can only be used in well equipped, well staffedclinics. The blood pressure needs to be monitored very

MI.IM0SIJ i; !a;0:

TREATMENT Drug Dose Route of administration

WHEN TO TREAT?Sustained hypertension can cause severe damage toorgans and early identification and prompt treatmentis therefore vital. Organ damage to the eyes is welldocumented, but there is also increasing evidence thatkidneys are damaged by hypertension, especially thosewith a reduced renal function. Such kidneys are notable to respond appropriately to increased arterialpressure which can lead to further damage in animalswith glomerular hypertension, for example. If the sys-tolic blood pressure is more than 200 mmHg, treatmentis necessary. If the systolic blood pressure is between170 and 200 mmHg, treatment should be considered,especially if there is evidence of hypertensive damage orthe presence of an underlying disease predisposing tohypertension.

AmlodipineAtenolol

Benazepril

Enalapril

Frusemide

Hydrochlorothiazide

Hydralazine

Phenoxybenzamine

Prazosin

Sodium nitroprusside*

Spironolactone

0 625-1-25 mg/cat sid or bid

6-25-12 5 mg/kg sid

0-25-0-5 mg/kg sid or bid

0-25-0-5 mg/kg sid or bid

1-2 mg/kg bid

1-4 mg/kg bid

2 5-10 mg/cat bid

0 5-1 mg/kg bid

0 25-1 mg/cat sid or bid

0-5 mg/kg/minute

1-2 mg/kg bid

po

po

po

po

sc, po, iv

po

po

po

iv

po

NB Not all the drugs listed in this table are licensed for use in cats. When using non-authorisedproducts, informed written consent should be obtained from clients*Dissolved in 5 per cent dextrose in water. The bag and giving set must be wrapped as the solutionis light-sensitivesid Once daily, bid Twice daily, po Oral, sc Subcutaneous, iv Intravenous

TOW _0kmniIr.tlnal heeorr-m

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Potential side effeXcs of

anttlypertensive therapy

Antihypertensive agents can induce side effects andsystemic hypotension is the most common complica-tion. Excessive water and electrolyte losses may alsooccur. It is important that the clinician is confidentof the diagnosis and is aware of any potential sideeffects of the drugs used and can monitor thepatient accordingly.

closely and infusion pumps are required to ensure thataccurate doses of the drug are administered. Severehypotension is a common side effect of this drug. Ina general practice situation, hydralazine (an arterialvasodilator) and frusemide can be used in cases withhyphaema or retinal detachment. Blood pressure and fluidand electrolyte balance need to be monitored closely.

TREATMENT REGIMENS FORLESS URGENT CASESDietLow sodium, low chloride and high potassium diets are

useful in the control of blood pressure, but usually donot control the blood pressure sufficiently on their own.

Obesity may play a role in hypertension and a weight-reducing diet is indicated if the patient is obese.

Drugs* The calcium channel blocker amlodipine has beenfound to be the most useful druc for controllingfeline hypertension. It is a vasodilator and lowers sys-

temic blood pressure by reducing the total peripheralresistance.

* ACE inhibitors (eg, enalapril, benazepril) also workvia their vasodilatory effects although they are often lesseffective than amlodipine in more severely affectedhypertensive patients.* ux-blockers (cg, prazosin, phenoxybenzamine) willreduce peripheral vascular resistance.* I-blockers (eg, atenolol) will reduce cardiac outputand reduce renin release.

VeterirRecc

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CORRECTION The Veterin

Urinary tract infections in small animals: updKtherapeutic options and management ofproblem cases (In Practice, October 2002, pp 518-527) of the cont

The dose rate given for enrofloxacin in the table on page It pi519 of the above article should have read 5 mg/kg bysubcutaneous injection daily for up to five days, or up-to-date list

5 mgi/kg given orally once daily or as a divided dose also lists recru

twice daily for three to 10 days, in accordance with the 7UK datasheet recommendations. The authors apologisefor any confusion.

* Diuretics (eg,, hydrochlorothiazide, spironolactone)wvilI reduce the extracellular fluid volume. Close moni-toring of electrolytes is necessary as serum potassium

levels can drop dramatically, especially wvith the use of a

thiazide diuretic.

MONITORING OF TREATMENTIt is important to start treatment at the lower end of thedose range and to monitor blood pressure frequently.There is the possibility of inadequate response to treat-

ment and it may take some time to stabilise patients.Several dose adjustments different drugs, or event a

combination of several drugs, may be necessairy to

control the blood pressure.

In the future, treatment options may include endothe-lin receptor antagyonists.

PROGNOSIS

The earlier hypertension is dietected, the better, so thattreattment can bc initiated before severe organ damnalgeoccurs. Once a cat suffers a retinal detachment or

hyphaema, the prognosis for regaining vision is guarded.Hoxvever, there haive been a number of cases where a

return of limited vision wtas accomplished once bloodpressure had been controlled.

AcknowledgementThe pictures which appear in the box on pages592 and 593 were supplied by Dr GillianMcLellan, Iowa State University.

ReferencesBODEY, A. & SANSOM, J. (1998) Epidemiologicalstudy of blood pressure in domestic cats. Journalof Small Animal Practice 39, 567-573SCHIPPER, L. & FINK-GREMMELS, J. (2001) Roleof 11(3-hydroxy steroid hydrogenase in theregulation of water homeostasis and bloodpressure in cats. Proceedings of the ESVIMCongress, September 2001. pp 70-71STEPIEN, R. L. (2000) Blood pressuremeasurement in dogs and cats. In Practice 22,136-145SYME, H., BARBER, P. J., MARKWELL, P. J. &ELLIOTT, J. (2001) Prevalence of systemichypertension in cats with chronic renal failureand evaluation of associated risk factors.Proceedings of the BSAVA Congress,Birmingham, April 2001. p 506

In Practice NOVEMBER/DECEMBER 2002

ary Practiceu.vetrecordEco.ukiary Record/lIn Practice website,ated weekly, gives detailsents of each of the two journals.rovides an extensive anding of forthcoming CPD events andtitment advertisements appearing in"he Veterinary Record.

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doi: 10.1136/inpract.24.10.590 2002 24: 590-594In Practice

 Myra Forster-van Hijfte clinical signs and treatment optionsFeline hypertension: pathophysiology,

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