How I approach… The hypertensive patient

Introduction

Arterial Hypertension (AH) is defined as a persistent increase in systolic blood pressure (BP). The normal values for both dogs and cats vary between published studies, and different factors – such as age, breed, temperament, or the technique used – can influence the results, and for this reason the real prevalence of AH in dogs and cats is unknown. However, a recent study found that cats over nine years of age are more likely to develop AH (1), especially if chronic kidney disease (CKD) is present, hence blood pressure measurement (BP) should form part of all feline geriatric check-ups. The effect of age on BP in dogs is currently less evident. There is no known relationship between BP and breed in cats, but there is some variability with respect to dogs; for example, Greyhounds can have a BP around 10-20 mmHg higher than other breeds (2). It is difficult to conclude if gender influences BP, as some studies show no association, while in others the difference between neutered and unneutered male/female dogs is < 10 mmHg (2). In cats, which are usually neutered, any difference is not clear, but where it does exist, it is minor (2). This article offers a brief overview of AH in pets, with a focus on diagnosis and treatment. Note the values given in this article all refer to systemic arterial pressure (SAP).

Types of hypertension

There are three categories of AH: 

1. Circumstantial hypertension (“white coat” effect) is the result of excitement and nervousness from going to the veterinary clinic. To combat this, certain measures can be taken, e.g., measuring BP outside the clinical environment. It is important to rule out white coat AH to avoid treating animals unnecessarily. 

2. Secondary hypertension is the most frequent cause and, as the name suggests, is secondary to a disease or medication/drug. It should be pointed out that hypertension can persist in some cases despite treatment of the primary condition, hence ongoing BP monitoring is always to be recommended. Some of the causes of secondary hypertension are:

a) Kidney disease. This is the most frequent cause of AH in both dogs and cats. 20-65% of cats with CKD have AH, although their IRIS staging is not related to the seriousness or prevalence of the hypertension (3). One of the factors that affects both the CKD stage and BP of animals is proteinuria; dogs with glomerular disease or leishmaniasis often have AH (3), and in this species AH may also contribute to the appearance of proteinuria.

b) Hyperthyroidism. The prevalence of AH in cats with hyperthyroidism is variable, possibly affecting 25% of cases, although severe hypertension is infrequent (4). Nevertheless, around 20% of cats develop AH despite treatment, which is why it is important to monitor BP during check-ups (5).

c) Cushing’s syndrome (pituitary or adrenal origin). In dogs with Cushing’s syndrome, 59-86% are hypertensive, with a higher prevalence of AH in animals with a unilateral adrenal tumor (6). The raised BP of these animals is often quite significant, so it should be assessed in all animals with thrombocytosis, proteinuria or hypokalemia. Dogs with thrombocytosis have a sensitivity of 61% and specificity of 100% of having AH (7); hypertension in both cats and dogs may persist or develop despite treatment (6,7).

d) Primary hyperaldosteronism. More than 90% of cats with primary hyperaldosteronism have AH that will resolve with the removal of the tumor (3,6). This type of tumor is rare in dogs.

e) Pheochromocytoma. In this type of adrenal tumor, which is uncommon in dogs and rare in cats, around 50% of affected animals can have AH, which may be persistent or episodic (6).

f) Diabetes mellitus. Around 35-46% of diabetic dogs have hypertension, but usually at a relatively low level, around < 160 mmHg (3). In diabetic cats the prevalence of AH is lower.

g) Obesity. Whilst this is a known cause of hypertension in humans, the presence of AH in obese dogs may be mostly related to a concomitant disease. In cats, no relationship has been found between obesity and the prevalence of AH (2). 

Unlike humans, heart disease does not lead to AH, but hypertensive dogs and cats can develop signs of cardiovascular disease from conditions such as left ventricular hypertrophy, aortic dilation or diastolic dysfunction; in cats, left atrial dilation and congestive heart failure can also occur (3). Note that AH can also be a side effect of some medications (Table 1) (2). 

 

Table 1. Medication associated with secondary hypertension in dogs and cats (2).

 

3. Idiopathic hypertension is seen when there is a persistent increase in BP alongside normal blood and urine tests, and up to 20% of hypertensive cats can fall into this category (3). Importantly however, AH can lead to polyuria (“pressure diuresis”) which, if present, could lead to an erroneous diagnosis of renal disease – for example, if a urine specific gravity (USG) test is < 1.030, renal disease may be suspected as the underlying cause of AH, whereas a normal USG (> 1.030) would make a diagnosis of kidney disease improbable. In this situation, if clinically justifiable, further tests are recommended; measurement of SDMA and urine protein/creatinine ratio, renal ultrasound, and assessment of total T4 (in cats) and basal cortisol (in dogs) to rule out other potential causes of AH (2).

Measurement of blood pressure

To avoid errors, it is vital that an experienced person performs the BP measurement. Two points are extremely important: knowing how to make the animal comfortable in the environment, and having a good knowledge of the preferred device. Most clinics will employ indirect blood pressure monitoring, using either Doppler ultrasonography or oscillometric measurement. The Doppler method is relatively easy to use, does not require a great deal of expertise, and is both affordable and readily available in most clinical settings. The values obtained are generally regarded to be systolic arterial pressure (SAP). An oscillometric method gives more information (systolic, diastolic and mean blood pressure values) but it can be less accurate in very small patients, or animals with cardiac arrhythmias, significant tachycardia or bradycardia, vasoconstriction or hypothermia. 

Both methods employ a cuff applied to a limb or tail; the width of the cuff should be 30-40% of the circumference of the limb or tail the cuff is to be placed on. This diameter can be calculated using a tape measure (Figure 1) or estimated using the cuff itself (Figure 2). If using a Doppler method, the probe is placed distally to the cuff, on the caudal aspect of the forelimb, ideally after removing a small amount of hair and applying gel to the area. The use of a stethoscope facilitates the procedure.

In order to standardize BP measurement, the following points are to be recommended (2): 

• Calibrate the device twice a year as per the manufacturer’s instructions.
• Choose an environment that is calm, isolated, and away from other animals, preferably with the owner present. The animal must not be sedated and should be given 5-10 minutes to acclimatize.
• The animal must be in lateral or sternal recumbency (Figure 3) with the cuff placed on one of its limbs or tail.
• The width of the cuff must be selected with care; a cuff that is too small will result in falsely high values, while a cuff that is too large will give results that are falsely low. An arrow can usually also be found on the cuff next to the word “artery”; this part must point to the vessel being used to measure the BP.
• The same person should always perform BP measurements to ensure a standardized procedure.
• Measurements must only be taken when the animal is calm and still, with the first measurement ignored. Five to seven consecutive measurements must be taken with consistent values, and an average taken to obtain the final result. If the BP value goes down each time, keep taking measurements until they are consistent.
• A record must be kept of the limb/tail used to measure BP, the cuff number used, and the BP value obtained for the animal.

Clinical manifestations of arterial hypertension

An animal with AH may appear healthy, or it can show clinical signs of an underlying disease. In addition, chronic AH can give rise to changes in certain target organs (e.g., the eyes, brain, kidney and heart), known as “target organ damage” (TOD) (Figure 4). Animals with BP > 160 mmHg are at moderate risk of developing TOD, while animals with a value > 180 mmHg are at high risk for TOD. Above all, the objective of treating AH is to minimize or prevent this TOD (Table 2).

 

Table 2. Target organs and the main clinical findings in animals with hypertension (2).

 

The ISFM recommends considering BP measurement in adult cats (three to six years of age) once a year, as AH is rare in this age range (8). In healthy older cats (7-10 years) BP must be measured at least once a year, whilst geriatric cats (> 11 years) should be checked every 6 months. Cats with recognized risk factors (e.g., CKD) should have their BP assessed every 3-6 months. Guidelines also recommend that all dogs nine years of age and older have their BP measured (2). 

Diagnosis of arterial hypertension 

Following the steps listed above, once a final BP value is obtained, the risk of TOD can be determined to decide if intervention is required, and how urgent treatment may be. Animals can be classified as follows:

• BP < 140 mmHg; normotensive (minimal risk of TOD) 
• BP = 140-159 mmHg; prehypertensive (low risk of TOD) 
• BP = 160-179 mmHg; hypertensive (moderate risk of TOD) 
• BP ≥ 180 mmHg; severe hypertension (high risk of TOD) 

If BP is above 160 mmHg, it is essential to immediately assess if there are signs of TOD. If so, the recommendation is to start treatment and reassess after 7-10 days; if the BP remains > 160 mmHg and/or there is TOD, the animal must remain on treatment. However, if reassessment shows BP to be less than 160 mmHg and there is no TOD, it is sufficient to monitor the BP regularly (e.g., monthly initially, then every three months if BP remains within the normal range). When the value is < 160 mmHg, monitoring just needs to be every 6-12 months, unless there is a clinical reason to do this sooner.

Treating arterial hypertension

As mentioned previously, most animals with AH have an underlying cause, and this must be treated alongside the chosen hypertensive therapy. In some cases, treating the primary disease can result in at least partial resolution of the AH. Progressive and gradual reduction of BP is preferable; however, more than one antihypertensive drug may be necessary, especially in dogs. The goal will be to keep the animal’s BP below 160 mmHg, with the ideal being < 140 mmHg in order to minimize the risk of TOD. If BP drops to < 120 mmHg, the medication must be adjusted to avoid the risk of weakness or syncope. Note that restricting sodium in an animal’s diet is controversial, and there is little evidence to show that this in itself is effective in reducing BP (2).

Treating canine hypertension

Antihypertensives must be chosen in accordance with the animal’s clinical situation – for example, animals with pheochromocytoma should preferably be treated with alpha- and beta-adrenergic blockers (2). However, the most frequently used antihypertensives are renin-angiotensin-aldosterone system (RAAS) inhibitors and calcium channel blockers (CCBs) (2). RAAS are the first choice in dogs (due to their antiproteinuric effect), the most notable being angiotensin-converting enzyme (ACE) inhibitors, angiotensin II receptor blockers (ARB), and aldosterone antagonists (AA). ACE inhibitors are generally the first choice, although the ARB telmisartan is an equally effective alternative. When the BP is > 200 mmHg, a CCB (i.e., amlodipine) should be added to the RAAS inhibitor. If the AH still persists and the animal is already receiving high doses of RAAS and CCB inhibitors, another antihypertensive can also be used (Table 3). 

 

Table 3. Antihypertensive medication in dogs (D) and cats (C) (from 2).

 

Treating feline hypertension

CCBs are the first choice of antihypertensive in cats, and can produce a reduction in BP of 28-55 mmHg in hypertensive animals (2). The initial dose of amlodipine is 0.625 mg/cat/day for BP < 200 mmHg and 1.25 mg/cat/day if above this value (2). It is rarely necessary to increase the dose, and it is a good idea to confirm that the treatment is working before doing so. Telmisartan can be used to control proteinuria in cats with CKD (9) and has also shown efficacy in the treatment of feline AH (for values of 160-200 mmHg), at a dose of 2 mg/kg/day (10). The combination of a CCB with an ARB was studied in a small group of cats and reported to be well tolerated (9). These antihypertensives must not be used in dehydrated animals, as their glomerular filtration rate may fall rapidly. Finally, ACE inhibitors are not indicated as a first choice in cats due to their limited effectiveness in significantly lowering blood pressure (unlike dogs), but they can be used as an “add-on” when CCBs are not sufficient to control hypertension (Table 3). In cats with primary hyperaldosteronism, when surgery is not an option, AAs may not be sufficient to manage AH, so CCBs also have a role in these patients.

Emergency treatment

In cases of severe hypertension with acute TOD, it is necessary to start aggressive treatment aimed at progressively reducing BP. A human antihypertensive agent which is apparently safe in both dogs and cats is fenoldopam, a dopamine D1 receptor agonist which causes renal artery dilation and sodium excretion (2). Other parenteral medications include labetalol, hydralazine and nitroprusside, but none cause renal vasodilation. Where there is severe hypertension but no TOD, oral treatment can be initiated, with hydralazine recommended for both dogs and cats (2); amlodipine can also be used in cats as a first-line approach (Table 3). 

Monitoring of hypertensive animals

After a diagnosis of hypertension, reassessment is essential; the higher the BP, the sooner the reassessment, and animals with TOD should always be reassessed within 48 hours after starting antihypertensive treatment. In addition to measuring BP, there must be monitoring of the primary condition if this has been diagnosed – so for example this may involve evaluation of serum creatinine and the urine protein/creatinine ratio, and appropriate assessment of the target organs to determine if there has been any damage to them. In most cases AH is not a medical emergency, and a reasonable interval is permissible before deciding if antihypertensive therapy needs altered; however, an animal with CKD must be reassessed 5-10 days after any medication adjustment. 

Conclusion

Arterial hypertension is common in dogs and very common in cats, especially in animals of a geriatric age, but also in many patients that have an underlying disease. It is fundamental to identify all animals that are at risk of developing arterial hypertension in order to take medical action before severe complications occur, and to reassess them regularly to monitor progress. Various medications are available which are often successful in reducing blood pressure to a normal or near normal level, but it is vital that the underlying cause of the hypertension is identified wherever possible, and treated appropriately.

References

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9. Sent U, Gossl R, Elliott J, et al. Comparison of efficacy of long-term oral treatment with telmisartan and benazepril in cats with chronic kidney disease. J. Vet. Intern. Med. 2015;29:1479-1487.
10. Glaus TM, Elliott J, Herberich E. et al. Efficacy of long-term oral telmisartan treatment in cats with hypertension: Results of a prospective European clinical trial. J. Vet. Intern. Med. 2019;33:413-422.