The many facets of hypertension

The cardiovascular system supplies the body with life-sustaining blood by generating adequate intravascular pressure to perfuse the tissues. If the pressure slumps, the organs may not receive suitable perfusion. But if it swells, they can suffer barotrauma.

The rate and force at which blood courses through the body combine with the diameter and elasticity of the vessels to create blood pressure (BP). Systolic BP (the maximal pressure against the walls of the arteries) is produced by cardiac contraction. Diastolic BP (the minimal pressure against the arterial walls) occurs as the heart refills.

How high is high?

Studies show systolic BP (SBP) ranges from 131 +/-20 to 154 +/-31 in clinically healthy dogs^1-5 and runs slightly lower in healthy cats.^6,7 Elevated pressures can cause target organ damage (TOD) that, according to the Veterinary Blood Pressure Society, tracks along with the severity of hypertension. Pressures above 150/95 pose risk for end-organ injury and may warrant intervention as follows⁸:

• Normotensive (minimal TOD risk): SBP < 150 mm Hg
• Prehypertensive (low TOD risk): SBP 150-159 mm Hg
• Hypertensive (moderate TOD risk): SBP 160-179 mm Hg
• Severely hypertensive (high TOD risk): SBP > 180 mm Hg

Target organs

The organs most vulnerable to hypertensive harm are the eyes, brain, heart, and kidneys.

Photo by Kelly Ehrmann

Eyes

Ocular conditions, including retinal detachment, papilledema, hyphema, and glaucoma, are the most common hypertensive injuries in cats. Prevalence rates for hypertensive retinopathies and choroidopathies in cats are as high as 100%^9-12 but are also significant in dogs.^13,14 Retinal detachment is the most frequently observed finding in both species, and acute onset of blindness may be the presenting complaint.^15-17

Brain/cerebrovascular system

Hypertensive encephalopathy can occur in dogs¹⁸ and cats.¹⁹ Clinical signs, possibly related to hemorrhage and/or infarct, include seizures, altered mentation, weakness, and vestibular problems.^20,21 Neurologic issues have been reported in up to 46% of hypertensive cats.^10,19 They are more likely to occur with a sudden jump in BP, a pressure exceeding 180 mm Hg, or both; they often resolve if hypertension is treated early.²²

Heart/cardiovascular system

Hypertension raises the load against which the heart must contract, sometimes leading to thickening of the heart muscle and cardiomegaly. In cats and dogs, this most often manifests as left ventricular concentric hypertrophy.²³ In cats, sustained hypertension causes cardiomyopathy at a rate of 60%¹⁹ but rarely progresses to heart failure.^10,20,24

Systolic heart murmurs and arrhythmia may be detectable on examination, with some patients presenting with exercise intolerance and—less frequently—epistaxis^25-27 associated with vascular hypertensive injury.

Kidneys

Elevated systemic BP can damage the delicate vascular beds in the nephrons. Renal vessels then constrict, and glomerular filtration drops, resulting in sodium retention and—in a destructive feedback loop—water retention that can further elevate BP.²⁸

Increased glomerular pressure leads to sclerotic changes in the glomeruli, enabling protein to leach out. Proteinuria likewise hastens progression of chronic kidney disease^28,29 and has been associated with shorter survival times in hypertensive cats^30,31 and dogs.^32,33 In dogs, the severity of hypertension was shown to correspond directly to the magnitude of proteinuria and shortened survival times.^34-36

Although it ravages sensitive vascular networks in key organs, hypertension usually does not announce itself with a clinical bang, such as a nosebleed or a seizure, sudden blindness, or even polyuria/polydipsia. Hypertension is alarmingly silent, and its damage to target organs occurs in increments over time.

Triad of hypertension

Hypertension is categorized into 3 types: situational, primary, and secondary.

• Situational hypertension is fleeting and brought on by environmental stressors. The psychological jolt generated by a visit to the veterinarian can prompt the autonomic nervous system to drive up BP. Anxiety-induced rises in BP can lead to erroneous diagnosis and unnecessary treatment.
• Primary (or “essential”) hypertension occurs without underlying triggers.
• Secondary hypertension, which accounts for more than 80% of cases in dogs and cats,^5,15,37 is characterized by persistently elevated BP concurrent with precipitating disease (or therapeutic agent).

Etiologies include acute³⁵ and chronic kidney disease,^31,38 hyperthyroidism,³⁸ hypothyroidism,³² hyperadrenocorticism,^39,40 hyperaldosteronism,⁴¹ pheochromocytoma,⁴² and diabetes.⁴³

Of these, chronic kidney disease has the strongest association with hypertension: both cause and effect for one another, the 2 are inextricably linked. Among other things, diseased kidneys are unable to properly couple the excretion of salt and water with changes in blood volume. And they inappropriately activate the renin-angiotensin-aldosterone system (RAAS), which further spikes blood volume.

The main iatrogenic causes of secondary hypertension are glucocorticoids,^39,40 mineralocorticoids,⁴¹ and erythropoietin.⁴⁴

Tallying tension

The American College of Veterinary Internal Medicine (ACVIM) advocates BP screenings every 6 months for animals with the underlying diseases.¹ Readings should also be performed in geriatric (over age 9 years) and/or obese cats and dogs, as well as those exhibiting clinical signs compatible with hypertensive end-organ damage, such as retinal hemorrhage, ataxia, dyspnea, and proteinuria.

Measurements should be conducted in a quiet room with the owner present, if possible. The patient should not be sedated and allowed to acclimate to the examination room for 5 to 10 minutes before testing, and minimally restrained for positioning in sternal or lateral recumbency.

Although direct BP measurement—which involves arterial catheterization—is the gold standard, it is not practical. In the clinical setting, noninvasive, indirect techniques are used: Doppler devices utilize ultrasound waves that detect blood flow; oscillometric units monitor arterial pulse waves.

Proper cuff selection is key to obtaining accurate measurements. For Doppler and oscillometric techniques, the cuff width should be 30% to 40% of the circumference of the cuff site (leg or tail).

ACVIM recommends obtaining 5 to 7 values and discarding the first. Measurements should be repeated until values are consistent. Values are then averaged into a single number.

Readings are not always straightforward, and can be altered by measurement method, operator experience, patient position, gender, age, breed, and temperament.^45-50 Aging dogs have been shown to experience a 1 to 3 mm Hg per year, and cats 0.4 +/- 0.1 mm Hg per 100 days.⁵¹ In sight hounds (ie, greyhounds), pressures run 7 to 20 mm Hg higher than in other canine populations,^47,52 attributed to situational rather than true hypertension.

Animals deemed truly hypertensive should be retested at least twice over 4 to 8 weeks. For severe hypertension, however, testing should be completed within 2 weeks to meet the risk of target organ damage. If organ injury, such as retinopathy, is already present, treatment should be initiated after the first measurement session, with results confirmed over subsequent visits.

Easing the pressure

Where secondary hypertension is present, the underlying disease should be treated first. Although it may lower BP and render the patient’s hypertension more amenable to treatment, this alone will rarely make the patient normotensive. Medications are usually required:

Angiotensin-converting enzyme (ACE) inhibitor (ie, enalapril, benazepril): RAAS inhibitors, antiproteinuric, usually initial drug of choice in dogs,⁵³ less effective in cats.^37,54

Angiotensin II receptor blocker (ie, losartan, telmisartan): RAAS inhibitors, antiproteinuric, first-line antihypertensive agents in dogs.^55,56

Aldosterone antagonist (spironolactone): RAAS inhibitor, antiproteinuric, potassium-sparing diuretic, effective in dogs and cats.⁵⁷

Calcium-channel blocker (amlodipine): Inhibits vasoconstriction; treatment of choice in cats, used as monotherapy or combined with ACE inhibitor or angiotensin-receptor blocker^9,10,19; in dogs, used in multiagent therapy only, combined with ACE inhibitors.⁵⁸

α-adrenergic blocker (prazosin, phenoxybenzamine): Produce vasodilation; adjunctive therapy for dogs when initial antihypertensive is inadequate.⁴²

β-blocker (atenolol): Slows heart rate, lowers BP; add-on treatment for dogs and cats when initial antihypertensive agent falls short of desired effect.^9,19,59

Direct vasodilator (hydralazine, acepromazine): less commonly used to manage hypertension, except for emergency situations.²⁰

Thiazide diuretic (hydrochlorothiazide): used in minority of patients in whom volume expansion is clinically apparent.¹⁰

Hypertension related to certain conditions may be best addressed using specific types of agents, such as α-adrenergic and β-adrenergic blockers in cases of pheochromocytoma or aldosterone-receptor blockers in animals with adrenal tumors.²¹

Medical management of hypertension can be augmented by weight loss in obese patients, and dietary salt restriction. In managing hypertensive patients, the goal is to achieve gradual drops in systolic pressure to less than 170 mm Hg. BP checks should be performed every 2 weeks, along with corresponding changes in dosages and/or drugs. Once regulated, SBP should be rechecked every 3 months and blood work performed annually.

Joan Capuzzi, VMD, is a small animal veterinarian and journalist based in the Philadelphia, Pennsylvania area.

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