Finger on the pulse – keeping on top of hypertension

Blood pressure (BP) can be defined as the force exerted on the blood vessel wall. BP is determined by cardiac output or the amount of blood ejected from the left ventricle in systole, and by the diameter and elasticity of the vessel walls, which is known as systemic vascular resistance. This is the driving force behind tissue oxygenation.

BP = cardiac output × systemic vascular resistance

Systolic and diastolic elements to BP exist. Systolic pressure is produced at the end of the cardiac cycle when the ventricles contract and the pressure in the arteries is at its highest. Diastolic pressure occurs at the beginning of each cardiac cycle when the ventricles relax and the pressure is at its lowest. The higher the resistance within the blood vessels, the higher the BP will be, and, conversely, the lower the resistance, the lower the BP (Panel 1).

Hypertension is more commonly diagnosed in cats than dogs, and is typically secondary to either renal disease or hyperthyroidism. Older cats are at an increased risk of developing systemic hypertension (Bijsmans et al, 2015), and a study found the estimated incidence risk of hypertension for cats aged nine and older to be 23.7% (Conroy et al, 2018). Secondary systemic hypertension is by far the most common form of hypertension, with 80% of cases in dogs and cats (Poli, 2018). In dogs, it is usually secondary to another underlying process, such as renal disease, hyperadrenocorticism, phaeochromocytoma or diabetes mellitus (DM).

Systemic hypertension alone is often asymptomatic until it is severe, making early detection difficult. Ideally, hypertension should be diagnosed and managed before systemic effects occur; therefore, BP measurement of at-risk patients is recommended as a part of routine examinations and as part of the ongoing management of diseases known to predispose hypertension. Clinical signs relating to hypertension should be assessed in general health checks during annual vaccination consultations, six-month nurse health checks and senior nurse clinics.

Following assessment, clients should be encouraged to book a BP check. A study by Conroy et al (2018) suggested approximately 4.5% of cats older than nine years of age, from a selection of UK-based practices, had their systolic BP (SBP) measured on presentation to their vet.

Jepson (2020) maintains one of the barriers to routinely performing BP checks in practice is the risk of stress and activation of the sympathetic nervous system in the clinic environment, resulting in increased BP. However, the implementation of some key steps can optimise the measurement of BP and diagnosis of hypertension to avoid situational hypertension. These key steps include creating the right environment, using accurate and reliable equipment, and identifying the at-risk population of patients for systemic hypertension and providing the nursing team with the opportunity to run BP clinics.

Systemic hypertension describes a persistent increase in blood pressure and can be categorised into three types: situational, secondary and idiopathic. Systemic hypertension can be caused by environmental or situational stressors, in association with other disease processes that increase BP (secondary hypertension), or it may occur in the absence of other causative disease processes (idiopathic hypertension; Acierno et al, 2018).

Situational hypertension is a transient increase in SBP associated with stress, excitement or anxiety, and activation of the autonomic nervous system (Caney, 2020). Situational hypertension is commonly referred to as “white coat hypertension” in humans.

A study reported that a BP increase is considered to be up to 30mmHg, although one study reported an increase of 75mmHg in one cat (Belew et al, 1999). It is, therefore, recommended to allow a period of acclimatisation prior to BP measurements to reduce the degree of situational hypertension. It is also important to bear in mind that, in some cats, BP decreases with stress and/or anxiety, which can result in “masked hypertension”.

Secondary hypertension is a persistent, pathological increase in SBP concurrent with another disease process, such as chronic kidney disease (CKD), hyperthyroidism or DM. It can also occur secondary to medication or following toxin ingestion. Acierno et al (2018), in the American College of Veterinary Internal Medicine (ACVIM) Consensus Statement, reported hypertension due to concurrent disease is the most common type found in cats.

Idiopathic hypertension is a persistent, pathological increase in SBP where the cause of this increase is unknown. Approximately 13% to 20% of cats are diagnosed with idiopathic hypertension (Acierno et al, 2018).

Consequences of high BP

As in human medicine, in the majority of cases, hypertension is a silent disease or it causes low-grade, non-specific signs including dullness, lethargy, depression and reduced exercise tolerance – all of which are often dismissed as the ageing process.

If left untreated, systemic hypertension can cause small blood vessels to leak or rupture, and this can cause serious tissue injury. Hypertension can affect many parts of the body before detection and is strongly associated with target organ damage (TOD; Taylor et al, 2017). The target organs are the eyes, brain and spinal cord, heart and blood vessels, and kidneys.

Increased pressure can cause retinal oedema and haemorrhage, leading to retinal detachment. If not treated, blindness can develop quickly. Patients that are reported to have a sudden blindness should have their BP measured. The kidneys receive a large amount of cardiac output, which adds vulnerability from the effects of hypertension. As well as secondary hypertension as a result of chronic renal failure, hypertension can also cause progressive renal damage if left untreated.

Bleeding into the CNS can be a consequence of hypertension. Signs include ataxia, disorientation, seizures, coma and sudden death. The heart is also affected by the increase in afterload. A persistent hypertension can cause hypertrophy of the left ventricle as a result of the excessive force required to pump blood into the high pressure aorta. Left ventricular hypertrophy secondary to increased afterload is quite a consistent finding in hypertension (Chetboul et al, 2003), and may present as a new murmur or gallop.

Cats are more likely to develop cardiac abnormalities and typically have physical findings, such as tachycardia, heart murmurs, gallop rhythm and arrhythmias (Poli, 2018; Table 1).

Measuring BP

BP measurement provides information about cardiovascular function, and is an indirect measure of cardiac output and tissue blood flow. Routine BP measurement should be done in patients diagnosed with renal disease, cardiac disease and DM, as well as those at a mature age as part of wellness programmes.

The International Society of Feline Medicine recommends healthy adult cats (3 to 6 years of age) are monitored every 12 months, healthy senior cats (7 to 10 years of age) at least every 12 months, healthy geriatric cats (more than 11 years of age) at least every 6 to 12 months, and cats with recognised risk factors are measured immediately and reassessed at least every 3 to 6 months (Taylor et al, 2017).

BP can be measured directly (a probe within an artery) or indirectly. Direct BP measurement is not indicated for screening of hypertension.

Two non-invasive methods are in general use: the Doppler method and the oscillometric. The Doppler technique is the method of choice as it is reliable and accurate in measuring SBP – particularly in conscious cats.

l Doppler method: a Doppler probe is placed over an artery to hear pulsatile blood flow. A cuff, attached to a sphygmomanometer, is placed proximal to the probe. The SBP reading is collected.

l Oscillometric methodology: a cuff, containing a special sensor, is placed over an artery. The cuff is attached to the oscillometric machine. When the machine is activated, the cuff is automatically inflated and deflated. The sensor detects pressure wave oscillations in the artery wall and uses this information to produce a reading for SBP, diastolic BP pressure, mean BP and heart rate.

A single high reading should not be used to diagnose systemic hypertension as stress-associated increases in BP can be very large in some cats (Belew et al, 1999). Measuring blood pressure in a quiet location, with the owner present, and, after a period of 5 to 10 minutes acclimatisation, are all extremely important in reducing the possibility of situational hypertension.

The ACVIM (2007) recommends a standardised protocol to help to reduce the effect of external influences on BP, and make measurements more clinically reproducible and reliable (Panel 2).

Management of hypertension

Management aims to reduce SBP readings to an “ideal” reference range of below 140mmHg, and the vet should identify and treat any underlying medical conditions, such as CKD. Treatment seeks to prevent or slow the progression of TOD.

Once the BP is within the reference range, patients should be assessed every four to six weeks, reducing the frequency to a minimum of once every three months in very stable patients. Follow-up assessments should include measurement of BP, assessment for evidence of TOD, blood and urinalysis, including creatinine levels and assessment of proteinuria (Caney, 2020).

Owner compliance is paramount in the management of hypertension. Missed medication and BP checks can risk potentially life-threatening increases in SBP.

Conclusion

BP monitoring is a useful diagnostic tool that should be used in all aspects of veterinary practice.

VNs play a vital role in facilitating the diagnosis of systemic hypertension, as well as monitoring patients and supporting owners with medication and home care.

Owners should be encouraged to bring their pets to nurse-led “wellness” clinics from an early stage to obtain baseline results, habituation, and early detection of diseases and conditions.

• Reviewed by Angharad Spence-Wilson, BVM, BVS, MRCVS