Dr. Rosie DVM

Hypertension in Dogs and Cats: Detection and Treatment

Adapted from the IRIS education article "Systemic hypertension" by SA Brown (Athens, USA; original author, 2016), updated by X Roura (Barcelona, Spain) in 2022.

Introduction

Systemic hypertension can be associated with environmental or situational stress, can occur secondary to other diseases or drugs (secondary hypertension), or can arise from other, unidentified causes (idiopathic hypertension). In dogs and cats, secondary hypertension has been associated with many conditions, such as kidney disease, hyperadrenocorticism (including the use of glucocorticoids), hyperthyroidism, hypothyroidism, diabetes mellitus, obesity, pheochromocytoma, and primary hyperaldosteronism. Idiopathic hypertension is more common in older cats, with a prevalence of 12.7% in apparently healthy cats aged 10 years or older (Jepson et al., 2009), but it has also been described in dogs.

Target-Organ Damage to the Kidney

Systemic hypertension is a concern because a chronically sustained increase in blood pressure injures the kidneys, eyes, brain, and cardiovascular system. The reason for treating hypertension in dogs and cats is to minimize or prevent this injury. Damage caused by sustained high blood pressure is called end-organ or target-organ damage (TOD). In the kidney, TOD generally shows up as progression of kidney disease: a faster decline in kidney function, increased mortality, more frequent uremic crises, or more severe proteinuria. Hypertension can be present at any IRIS stage of chronic kidney disease (CKD) or grade of acute kidney injury (AKI), because blood creatinine concentration is not directly related to blood pressure. As well as kidney disease progression, many dogs and cats with hypertension also have TOD in the eyes, central nervous system, or cardiovascular system, so effective treatment generally reduces the risk of further TOD in these tissues as well.

Measuring Systolic Blood Pressure

Diagnosing and managing hypertension in dogs and cats with AKI or CKD should be based on repeated measurements of systolic blood pressure (SBP). Published data suggest that better blood pressure monitoring in clinical practice may reduce the illness associated with hypertension.

The choice of measuring device depends on the operator's experience and preference, although many veterinarians prefer Doppler devices for cats. Whatever method is used, it should be applied consistently, following the same standard operating procedure, so that measurements in a given patient can be compared from one session to the next. Each practice should identify the persistent blood pressure measurements associated with eye damage in its patients, which usually fall in the IRIS categories of hypertension or severe hypertension. This helps practices define their own reference ranges for blood pressure measured with their own procedure.

In general:

The cuff width should be 30% to 40% of the circumference of the measurement site, which may be the tail, forearm (antebrachium), tarsus, or upper arm (brachium), and the cuff must be at the level of the heart.
At least five measurements should be taken. The highest and lowest SBP readings are discarded, and the result is the average of the remaining three or more values, provided all the SBP results are within 20 mmHg of each other. If the remaining readings differ by more than 20 mmHg, the measurement session should be repeated.
All SBP measurements, the reasons for excluding any values, the final (mean) result, and the veterinarian's interpretation should be recorded, along with the animal's position and demeanor, the cuff size and site, and the circumference of the cuff site in centimeters.
The animal's age, demeanor, sex, neuter status, and clinical history should be taken into account when interpreting SBP.

It is customary to hold at least two measurement sessions, normally at least one week apart, before concluding that an animal needs antihypertensive treatment, particularly when there is no evidence of TOD.

Interpreting Systolic Blood Pressure

About 20% of animals with CKD are hypertensive when CKD is first diagnosed, and a further 10% to 20% of animals that are initially normotensive become hypertensive over time. IRIS recommends that SBP be measured and that the CKD substage be assigned using the criteria below. The same SBP classification should be used in dogs and cats with AKI and in those suspected of having hypertension caused by other diseases or of unknown cause.

IRIS Blood Pressure Substages Based on the Risk of Future Target-Organ Damage

Normotensive: SBP below 140 mmHg; minimal risk of future TOD
Prehypertensive: SBP 140 to 159 mmHg; low risk
Hypertensive: SBP 160 to 179 mmHg; moderate risk
Severely hypertensive: SBP 180 mmHg or higher; high risk

Treatment

The initial assessment of an animal suspected of having systemic hypertension should include recognizing conditions that may be contributing to the high SBP, identifying and characterizing any TOD, and identifying any seemingly unrelated conditions that could complicate antihypertensive treatment. Because hypertension is often a silent, slowly progressive condition that requires vigilance and lifelong treatment, it is important to be as certain as possible of the diagnosis. A high SBP in a dog or cat with CKD usually means hypertension secondary to the kidney disease. However, it can also be artifactual, caused by anxiety ("white-coat" hypertension), or related to another disease. White-coat hypertension is not an indication for treatment. The decision to use antihypertensive drugs should be based on the SBP substage and on all the available clinical information.

Deciding to Start Treatment

In people, any reduction in SBP that does not cause overt hypotension lowers the risk of TOD. This remains to be confirmed in dogs and cats, but IRIS recommends that SBP be categorized by the risk of future TOD. Although blood pressure differs between dog breeds, only sighthounds, whose values are about 20 mmHg higher in each category, currently warrant separate categorization.

The general consensus is to start treatment in a patient with evidence of TOD when reliable SBP measurements exceed 160 mmHg (that is, in hypertensive and severely hypertensive patients), and in patients with persistent hypertension. Animals with CKD are presumed to have TOD, even when it is not clinically apparent.

Antihypertensive Therapy

Antihypertensive treatment must be tailored to the individual patient and its other conditions. Whatever the initial SBP, the ideal goal of treatment is to reduce the risk of future TOD to minimal, an SBP below 140 mmHg. At a minimum, SBP should be brought below 160 mmHg, which is the goal set in the IRIS treatment recommendations. Some dogs and cats have extremely high SBP that can be difficult to reduce. In these patients, an alternative goal is to lower the risk of TOD by at least one substage. In most circumstances, SBP should be lowered gradually and steadily over several weeks. Severe central nervous system or eye damage justifies emergency management as a hypertensive crisis, with rapid lowering of SBP over several hours. Unfortunately, severe eye damage, such as retinal detachment or marked bleeding within the eye, is usually associated with chronic retinal injury, so the animal may remain blind even if treatment is effective.

The available evidence suggests that sodium restriction alone generally does not reduce SBP, but a high salt intake may have adverse consequences in some settings. Normal dogs and cats are apparently not as salt-sensitive as people or some inbred strains of rats, but those with kidney disease, especially those with nephrotic syndrome, are likely to be salt-sensitive. Salt restriction also enhances the effect of some antihypertensive drugs, particularly those that act on the renin-angiotensin-aldosterone system (RAAS). This evidence from laboratory studies in dogs and cats is the basis of the recommendation to feed dogs and cats with CKD a diet with reduced sodium chloride, whatever their IRIS stage.

Treatment in Dogs

In dogs with systemic hypertension, the first choice is often a drug that acts on the RAAS, specifically an angiotensin-converting enzyme inhibitor (ACEi) or an angiotensin receptor blocker (ARB). Clinical experience indicates that both classes reduce SBP slightly, by about 10% to 15%, and both reduce proteinuria. The starting dose should be tailored to the individual patient within the recommended range (see the dosage list below). Caution is needed when adjusting doses in azotemic dogs.

If the drug of choice is only partly effective, the usual approach is to increase the dose before adding another drug. So if a dog with CKD receiving an ACEi or an ARB remains hypertensive or severely hypertensive, increasing the dose should be the first consideration. Many veterinarians start RAAS inhibitors at the lower end of the recommended range, and many experts increase the ACEi dose further, up to 2 mg/kg. Increasing a RAAS inhibitor beyond the suggested range is not recommended. If an ACEi or ARB is being given at the upper end of its range and the response is inadequate, the next step is to add a calcium channel blocker (CCB), typically amlodipine at 0.25 to 0.5 mg/kg once daily. The amlodipine dose may be increased to 0.75 mg/kg once daily but should not exceed this in dogs.

Although not ideal, some dogs with significant hypertension need more than two drugs. There is no consensus on which drug (ACEi, ARB, sympatholytic, or direct vasodilator) to add next. In human medicine, several publications have proposed that combining an ACEi and an ARB controls hypertension better than increasing the dose of a single RAAS inhibitor. However, combining an ACEi and an ARB has led to more adverse effects in people and in dogs (see our article on inhibition of the renin-angiotensin-aldosterone system). The IRIS treatment recommendations for dogs reserve adding an ARB or hydralazine to an ACEi and amlodipine for dogs that need more treatment, with the risks and benefits weighed for each dog and kidney function monitored carefully, because the risk of worsening kidney function is higher in IRIS Stages 3 and 4. Certain diseases may be best managed by adding specific drug classes to the ACEi/ARB/CCB combination, such as alpha- and beta-blockers, or surgical removal of the tumor, for pheochromocytoma, and an aldosterone receptor blocker, or surgical removal of the adrenal tumor, for hypertension associated with hyperaldosteronism. Diuretics are rarely used as antihypertensives in dogs and cats, but they may be useful in either species when hypertension is accompanied by chronic overhydration.

Treatment in Cats

In cats with systemic hypertension, the principles are the same, but the first choice should be a CCB, specifically amlodipine, when SBP is 180 mmHg or higher or TOD is present. An alternative first choice, particularly in proteinuric cats, is telmisartan, since clinical trials have shown that this ARB reduces SBP in hypertensive cats with an efficacy similar to that of amlodipine. Second-choice drugs are those that act on the RAAS, specifically an ACEi. These drugs can be given together (a CCB and an ACEi, or a CCB and an ARB). Triple therapy is not usually needed to control SBP in cats, and the same caution applies as in dogs when considering combining an ARB with an ACEi.

Hypertensive Emergencies

An exception to this gradual approach, which allows several weeks between dose adjustments, is animals that are severely hypertensive with evidence of severe or progressing damage to the nervous system or eyes. This is generally an emergency, in which a CCB plus a RAAS inhibitor is an appropriate first step in dogs, and a CCB alone is often used in cats. If the animal's mental state is impaired, injectable or rectally administered medications should be used at first. In this critical situation, the goal in both species is to reduce SBP within hours, adjusting doses within that time frame as necessary.

Oral Antihypertensive Drugs for Dogs and Cats

Some of these treatment recommendations are not authorized in all geographical regions, and some may not be authorized for use in dogs or cats. The recommended dose rates are therefore empirical, and it is the treating veterinarian's duty to make a risk-benefit assessment for each patient before giving any treatment. The usual dosages are:

Benazepril (ACE inhibitor): dogs 0.5 to 2 mg/kg every 12 hours; cats 0.5 to 2 mg/kg every 24 hours
Enalapril (ACE inhibitor): dogs 0.5 to 2 mg/kg every 12 hours; cats 0.5 to 2 mg/kg every 24 hours
Ramipril (ACE inhibitor): dogs and cats 0.25 to 1 mg/kg every 24 hours
Imidapril (ACE inhibitor): dogs and cats 0.25 to 1 mg/kg every 24 hours
Telmisartan (angiotensin receptor blocker): dogs 1 to 4 mg/kg every 24 hours; cats 2 mg/kg every 24 hours
Irbesartan (angiotensin receptor blocker): dogs 1 to 5 mg/kg every 12 to 24 hours
Spironolactone (aldosterone receptor blocker): dogs and cats 1 to 2 mg/kg every 12 to 24 hours
Amlodipine (calcium channel blocker): dogs and cats 0.125 to 0.5 mg/kg every 24 hours
Atenolol (beta-blocker): dogs 0.25 to 1 mg/kg every 12 hours; cats 6.25 to 12.5 mg per cat every 12 hours
Labetalol (alpha- and beta-blocker): dogs 1.1 mg/kg per hour (range 0.2 to 3.4) by constant rate infusion, not by mouth
Prazosin (alpha-blocker): dogs 0.5 to 2 mg/kg every 8 to 12 hours; cats 0.25 to 0.5 mg per cat every 24 hours
Phenoxybenzamine (alpha-blocker): dogs 0.25 mg/kg every 8 to 12 hours, or 0.5 mg/kg every 24 hours; cats 2.5 mg per cat every 8 to 12 hours, or 5 mg per cat every 24 hours
Hydralazine (direct vasodilator): dogs 0.5 to 2 mg/kg every 12 hours, starting at the low end of the range; cats 2.5 mg per cat every 12 to 24 hours
Acepromazine (direct vasodilator): dogs and cats 0.5 to 2 mg/kg every 8 hours
Hydrochlorothiazide (thiazide diuretic): dogs and cats 2 to 4 mg/kg every 12 to 24 hours
Furosemide (loop diuretic): dogs and cats 1 to 4 mg/kg every 8 to 24 hours

Proteinuria and the Kidney

In people and in cats, the benefit of lowering blood pressure on kidney damage, which shows up as a progressive decline in glomerular filtration rate, depends directly on the degree of proteinuria, and a similar relationship presumably exists in proteinuric dogs. Monitoring the urine protein-to-creatinine ratio (UPC) is therefore important, with the goal of reducing it to the non-proteinuric range, or at least by 50%. The CCB amlodipine, which is commonly used, may reduce proteinuria less than RAAS inhibitors because it preferentially dilates the afferent arteriole. However, CCBs often do reduce proteinuria in cats.

IRIS substages of persistent renal proteinuria, based on the UPC in mass units:

Non-proteinuric: UPC below 0.2 in cats and dogs
Borderline proteinuric: UPC 0.2 to 0.4 in cats; 0.2 to 0.5 in dogs
Proteinuric: UPC above 0.4 in cats; above 0.5 in dogs

Monitoring Antihypertensive Therapy

Acute damage to the eyes or nervous system is a hypertensive crisis and requires rapid lowering of SBP. In most other situations, hypertension is not an emergency, and 3 to 4 weeks should be allowed between dose adjustments. There has been some concern that RAAS inhibition could acutely worsen azotemia, but this is unusual, and modest increases in blood creatinine (less than 30%) are generally tolerable. Nonetheless, a dog in IRIS Stage 1 or 2 should be re-evaluated 7 to 14 days after any change in antihypertensive treatment, with a clinical assessment and measurement of blood creatinine, SBP, and UPC. In unstable patients and those in IRIS Stage 3 or 4, this recheck should be earlier, perhaps within 3 to 5 days. Patients hospitalized as hypertensive emergencies, particularly those receiving fluid therapy or drugs with cardiovascular effects, should be assessed at least daily, with a clinical evaluation and measurement of blood creatinine and SBP. The purpose of these short-term assessments is to identify unexpected findings, such as new or worsening TOD, or adverse effects, such as marked worsening of azotemia or low blood pressure. Weakness or fainting with an SBP below 120 mmHg indicates systemic hypotension, and treatment should be adjusted accordingly.

Re-evaluation is appropriate every 1 to 4 months, depending on stability (more often if SBP or other conditions are unstable) and on the degree of hypertension (more often if SBP remains at 180 mmHg or higher). Follow-up evaluations to assess the response and adjust treatment should include SBP, blood creatinine, a urinalysis with UPC, an examination of the back of the eye (fundic examination), and any other tests the patient's circumstances call for, such as those related to TOD, causes of secondary hypertension, or other conditions. A key predictor of the benefit of antihypertensive treatment is its effect on proteinuria: a benefit is predicted if the treatment reduces proteinuria, for example by reducing the UPC by at least 50%. The frequency and nature of re-evaluations will depend on the SBP substage, the stability of SBP, the patient's other health problems, and how often doses are being adjusted. Because signs of progressing TOD can be subtle, SBP should be monitored closely over time in patients receiving antihypertensive treatment, even when hypertension seems well controlled.