Overview

The principal function of the cardiovascular system is to generate the appropriate amount of intravascular pressure in order to assure adequate tissue perfusion. In clinical practice, we can measure pressure within systemic arteries (commonly referred to as blood pressure or BP) or pressure within central systemic veins (central venous pressure). While the latter parameter is most well correlated with body fluid volume status, it is the arterial pressure that served as the driving force for tissue blood flow. As with most physiological parameters, control systems normally regulate BP within an appropriate range and this range is referred to as normal BP. If BP falls too low (i.e., systemic hypotension) organ perfusion may be inadequate; if it rises too high (i.e., systemic hypertension) organs may be overperfused or suffer barotrauma.

Systemic hypertension (i.e., high pressure within systemic arteries) is often observed in dogs or cats with kidney disease, hyperthyroidism, as well as other metabolic conditions. To properly manage these metabolic conditions, it has becoming increasingly important for veterinarians to measure BP. Further, a diagnosis of systemic hypertension should always be based upon determination of systemic arterial blood pressure. Indiscriminate use of antihypertensive agents without reliable measurements of BP should be avoided.

Patient Selection

While it is possible to measure BP in all clinical patients, currently there is not sufficient rationale to do so in veterinary medicine. Hypotension may be present in anesthetized animals as well as those with suspected or confirmed cardiovascular diseases, such as heart failure, arrhythmias, shock, or excessive hemorrhage. Hypertension is often suspected in dogs and cats that present with clinical diseases associated with hypertension or with clinical findings compatible with end-organ injury from high BP (Table 1). Because of the high prevalence of certain occult diseases in elderly patients, such as chronic kidney disease, the routine screening of geriatric dogs and cats is an appropriate consideration. We will focus upon the measurement of BP in conscious animals as would be done in screening for the presence of systemic hypertension. The same general principles apply to the diagnosis of hypotension in anesthetized or critical care patients as well.

Methods of Measurement

Blood pressure may be measured by either direct or indirect methods. While direct blood pressure measurement is the "gold standard", it is technically difficult in unsedated dogs and cats, may be painful, and complications, such as hematoma formation, are more likely to develop. The indirect techniques are more readily applicable to a clinical setting, as they require less restraint and are technically easier to perform.

Indirect methods of blood pressure measurement include auscultatory, ultrasonic Doppler, oscillometric, and plethysmographic devices (Table 2).

Blood Pressure Measurement Technique

You should provide an environment that is quiet, away from other animals and (generally) have the owner present. You should allow for a quiet equilibration time for the animal in this room of 5-15 minutes. The same individual (preferably a technician with calm demeanor) should perform all BP measurements following a standard, written protocol. Measurements should be obtained only in a patient that is calm, minimally restrained, and motionless. Regarding choice of equipment, it is perhaps most critical to utilize an indirect device in which the operator has experience and confidence (often developed by practice on normal animals presented for vaccination).

Select a cuff with a width that is approximately 30-40% of the carefully measured circumference of the chosen site for cuff placement. Choice of site depends on preference of the operator and patient comfort. Generally, the median artery is used for a Doppler device and the brachial (cat) or coccygeal (dog) artery for the oscillometric devices.

The operator should obtain at least 5-7 consistent measurements (<20% variation in systolic readings) from the first cuff placement. The cuff should be removed and replaced with 5-7 consistent measurements being from this second cuff placement. This cuff repositioning should be repeated, as often as necessary, until results from cuff sites agree.

The first value from each cuff positioning should be discarded and then an average of all other values calculated. An alternate approach is to average all values obtained after the first, highest and lowest pressure readings are discarded. The overall average is then taken as the final value. If in doubt, repeat measurement session on another day or later the same day. A diagnosis of systemic hypertension should never be based solely on a single BP measurement session. Measuring BP is complex interaction among technician(s), animal, owner, and device and will require at least 15 minutes in the best of circumstances (45 minutes or more in difficult cases).

What is Normal BP?

This is actually a very difficult question. The Veterinary Blood Pressure Society has suggested BP values should be interpreted in light of clinical and laboratory findings. The species, gender, and age may also be considered in evaluating a patient's BP. The Society suggests that elevation of BP in a patient produces a risk which is directly related to the severity of the hypertension (Table 3). Thus systolic/diastolic BP (mmHg) that exceeds 150/95 pose some risk for hypertensive end-organ injury and intervention should be considered; values above 180/120 pose a high risk and intervention (e.g., administration of a pharmacological antihypertensive agent) is clearly indicated. Similarly systolic/diastolic BP (mmHg) below 100/60 poses some risk for reduced organ perfusion; values below 70/40 pose a high risk that mandates intervention (e.g., IV fluid therapy and/or reduction of dosage of anesthetic agent).

Antihypertensive Therapy: General

Treatment is generally by sequential trials. Generally, dosage adjustments or changes in treatment should be instituted no more frequently than every 2 weeks, unless extreme hypertension necessitating emergency treatment is present. When using pharmacological agents, a wide range of dosages should be considered with initial dosages at the low end of the range. If an agent or combination of agents is incompletely effective, the dosage(s) may increased or additional agents added. Often, especially in dogs, multiple agents will be used concurrently.

The diagnosis of hypertension associated with chronic renal disease necessitates life-long antihypertensive treatment with periodic dosage adjustments based upon blood pressure measurements. Hypertension associated with hyperthyroidism and hyperadrenocorticism can be expected to resolve within 1-3 months following effective treatment of the underlying condition, unless chronic renal failure is also present. Occasionally, dogs with well controlled hyperadrenocorticism remain hypertensive. In other patients, the duration of treatment cannot be predicted, though it may be required life-long. Periodic dosage adjustments based upon blood pressure measurements are indicated.

It is usually not possible to restore blood pressure to normal values when treating a hypertensive animal. It should be the veterinarian's goal to lower the blood pressure to within 25-50 mmHg of the normal ranges for blood pressure, thus lowering pressure (systolic/diastolic) to < 170mmHg/100mmHg. If an oscillometric unit is employed, the systolic, mean, or diastolic blood pressure can be used to judge effectiveness of therapy. If a Doppler ultrasonic device is used, the systolic blood pressure should be used to monitor effectiveness of treatment. In general, the Doppler ultrasonic device will be most reliable in cats and either unit will provide equivalently reliable results in dogs.

Dietary Therapy

Though poorly studied, the usual recommendation is to initially institute a low sodium diet which provides <0.25% sodium on a dry weight basis. Frequently, dietary sodium restriction is employed as a first step in order to enhance the efficacy of pharmacological agents. In animals with chronic renal disease and hypertension, it may more important to maintain adequate caloric rather than to insist that a low sodium diet be fed.

Obesity can elevate systemic arterial pressure in human beings and dogs and, perhaps, in cats. Consequently, weight loss is desirable in obese, hypertensive animals.

Pharmacological Agents

Medical treatment of hypertension in dogs and cats has, until recently, been extrapolated from human protocols. Recommendations for medical therapy have included vasodilators, beta-blockers, and diuretics; these agents are generally given in concert with dietary sodium restriction. In human beings with systemic hypertension and renal disease, vasodilator therapy is the preferred initial choice because of the renoprotective effects of certain classes of these agents (angiotensin converting enzyme inhibitors and calcium channel).

An inhibitor of angiotensin converting enzyme (e.g., 0.5 mg enalapril/kg orally every 12 hours or 0.25-0.5 mg benazepril/kg every 12-24 hours) will lower blood pressure in many hypertensive dogs. In cats, the role of the renin-angiotensin system in the maintenance of systemic hypertension has been questioned and though less effective in cats, a higher dosage (1-2 mg enalapril/kg orally every 24 hours) may prove efficacious in hypertensive cats.

Some drugs classified as calcium channel antagonists reduce total peripheral resistance, leading to a decrease in blood pressure. Amlodipine besylate, a long-acting dihydropyridine calcium antagonist, has been used successfully as a single agent in hypertensive cats at a dosage of 0.625 mg/cat orally every 24 hours.14 Larger cats (>4 kg) often require 1.125 mg orally every 24 hours. Blood pressure decreases significantly during amlodipine treatment, and significant adverse effects (i.e., azotemia, hypokalemia, weight loss) are not frequently identified. Because amlodipine has a slow onset of action, adverse effects such as hypotension and loss of appetite are usually avoided. In dogs with chronic renal disease, a dosage of 0.05 mg/kg given orally once daily lowered blood pressure in initial pharmacokinetic trials. In many hypertensive dogs, though, amlodipine appears to be less effective, even at dosages as high as 1 mg/kg twice daily. However, dosages utilized in hypertensive dogs should generally be in the range of 0.05-0.25 mg/kg once daily.

Table 1. Indications for screening dogs or cats for systemic hypertension.

 Acute or chronic kidney disease

 Hyperthyroidism (especially cats)

 Hyperadrenocorticism

 Diabetes mellitus (especially dogs)

 Hyperaldosteronism

 Pheochromocytoma

 Marked obesity

 Geriatric patients (dogs and cats > 10 years of age)

 Clinical findings compatible with hypertensive end-organ injury such as:

 Blindness, retinal vascular tortuosity or hemorrhage, retinal detachment, hyphema

 Seizures, ataxia, sudden collapse

 Dyspnea, unexplained left ventricular hypertrophy or gallop rhythm

 Proteinuria or low urine specific gravity

Table 2. Examples of indirect blood pressure measurement devices in use in dogs and cats.

Table 3. ACVIM Hypertension Panel: Classification of blood pressure levels (mmHg) in dogs and cats based on risk for future target-organ damage (TOD).^1,2

1. BP measurements should always be interpreted in light of the condition of the animal. Factors to consider include those that may alter cardiovascular control mechanisms (e.g., excitement, anxiety, and/or pharmacological agents) as well as those that may affect cardiovascular function directly (e.g., dehydration and/or pharmacological agents).
2. There are breed, gender and age variations that should be considered in evaluating BP measurement. Known effects are generally small, except for the increased BP commonly observed in Sight hounds (approximately 15 mmHg).

References

1.  Egner B, Carr A, Brown S: Essential facts of blood pressure in dogs and cats, Vet Verlag, Babenhausen, Germany, 2003.

2.  Brown SA, Henik RA: Diagnosis and treatment of systemic hypertension. Vet Clin North Am Small Anim Pract 1998; 28: 1481-94.

3.  Jacob F, Polzin DJ, Osborne CA, et al: Association between initial systolic blood pressure and risk of developing a uremic crisis or of dying in dogs with chronic renal failure. J Am Vet Med Assoc 2003; 222: 322-9.

4.  Bodey AR, Michell AR: Epidemiological study of blood pressure in domestic dogs. J Small Anim Pract 1996; 37: 116-25.

5.  Littman MP: Spontaneous systemic hypertension in 24 cats. J Vet Intern Med 1994; 8: 79-86.

6.  Sansom J, Rogers K, Wood JL: Blood pressure assessment in healthy cats and cats with hypertensive retinopathy. Am J Vet Res 2004; 65: 245-52.