Blood pressure (BP) monitoring is a useful but often underutilised diagnostic tool. Measurement of BP can be delegated to a trained veterinary nurse, who should ensure that results are standardised and recorded, reliable and repeatable. The nurse’s role can be crucial in helping the veterinary surgeon diagnose and treat systemic hypertension. BP measurement is recommended as part of an annual screen for senior but otherwise healthy dogs and cats and for any patient that has a disease linked to hypertension or if there is any suspicion of “target organ damage” (TOD), such as retinal detachment.
What is blood pressure?
BP is the force exerted upon the vessel wall and is measured in millimetres of mercury (mmHg). It is affected by two factors: cardiac output, or the amount of blood ejected from the left ventricle during systole, and systemic vascular resistance, the diameter and elasticity of arterial walls.
It is regulated by the central nervous system, which responds to messages sent from sensory receptors in the peripheral vasculature, allowing for quick and short-term adjustments. BP also responds to a neurohormonal system – the renin–angiotensin–aldosterone system – because angiotensin II stimulates vasoconstriction, increases heart rate and improves cardiac contractility. The kidneys can autoregulate themselves, but hypertension can override this process and cause glomerular hypertension and glomerulosclerosis (Jepson, 2011). The aim of BP regulation is to maintain adequate blood perfusion to the body’s tissues and protect against hyper- or hypoperfusion, which can lead to TOD.
The kidneys can autoregulate themselves, but hypertension can override this process and cause glomerular hypertension and glomerulosclerosis
What is hypertension?
Systemic hypertension is relatively common in small animal medicine and is categorised into three types:
- Situational (or “white coat”) hypertension
- Idiopathic hypertension
- Secondary hypertension
Secondary hypertension occurs when a sustained increase in BP is associated with a disease process, toxin or medication that is known to cause hypertension (Table 1). Even if the underlying disease is treated, hypertension may persist.
| Disease | Prevalence (%) of secondary hypertension |
|---|---|
| Chronic kidney disease | Cats: 19 to 61 |
| Dogs: 9 to 93 | |
| Acute kidney disease | Cats: n/a |
| Dogs: 15 to 87 | |
| Hyperadrenocorticism | Cats: 19 |
| Dogs: 20 to 80 | |
| Diabetes mellitus | Cats: 15 |
| Dogs: 24 to 67 | |
| Hyperthyroidism | Cats: 12 to 87 |
| Dogs: n/a |
If systemic hypertension is left untreated, it can cause small blood vessels to leak or even rupture, which, in turn, can cause serious tissue injury. Evidence of sustained hypertension can often be seen in four main areas – the brain, kidneys, heart and eyes – known as “target organs”.
Pathologies related to high blood pressure in companion animals
High blood pressure can cause numerous pathologies in companion animals, many of which involve TOD (Table 2).
| Systolic blood pressure measurement | Category | Risk of target organ damage |
|---|---|---|
| Under 140mmHg | Normotensive | Minimal |
| 140 to 159mmHg | Prehypertensive | Low |
| 160 to 179mmHg | Hypertensive | Moderate |
| Over 180mmHg | Severely hypertensive | High |
Neurological signs have been reported in 29 and 46 percent of cats with hypertension and occur as a result of hypertensive encephalopathy, where white matter becomes oedematous and vascular lesions form (Littman, 1994; Maggio et al., 2000). Clinical signs vary from lethargy, altered mentation and vestibular signs to seizures and are usually sudden in onset (Jepson, 2011).
Injury to renal tissue arises from chronically high blood pressure. Recent evidence shows that somewhere between 19 and 65 percent of cats with chronic kidney disease (CKD) are hypertensive (Bijsmans et al., 2015). One comparative study investigated blood pressure measurements in healthy cats and those with CKD. The author found that 17 percent of cats diagnosed with CKD developed hypertension, compared to only 7 percent in the healthy cat group (Bijsmans et al., 2015).
Cardiac changes can occur because of sustained pressure in the left ventricle, leading to left ventricular hypertrophy. If hypertension is left untreated, it can result in cardiac failure or arrhythmias in some cases (Acierno et al., 2018).
Ocular changes are a relatively common finding in cases of feline hypertension, with a reported prevalence of between 60 and 80 percent (Jepson, 2011), and even up to 100 percent in one study (Acierno et al., 2018). Changes can include acute onset blindness, hyphaema and retinal haemorrhage or detachment and can be either unilateral or bilateral.
Recent evidence shows that somewhere between 19 and 65 percent of cats with chronic kidney disease are hypertensive
Blood pressure monitoring
The American College of Veterinary Internal Medicine (ACVIM) recommends annual screening of dogs and cats from the age of nine (Acierno et al., 2018). The International Society of Feline Medicine (ISFM) recommends annual screening during the mature stages of a cat’s life span (7 to 10 years) and to increase to a biannual assessment at the senior life phase (11 years onwards) (Taylor et al., 2017). Some conditions, such as CKD, hyperadrenocorticism and heart disease, warrant screening as soon as a diagnosis has been made, before treatment is initiated and then at regular check-ups.
Two excellent resources are available to help standardise blood pressure measurements, and both provide assistance with the diagnosis and treatment of hypertension. These are the ACVIM consensus statement on the guidelines for the identification, evaluation and management of systemic hypertension in dogs and cats (Acierno et al., 2018) and the ISFM consensus guidelines on the diagnosis and management of hypertension in cats (Taylor et al., 2017).
Indirect BP measurement methods for conscious patients include Doppler, high-definition oscillometry (HDO) and oscillometry. Doppler (Table 3) used to be considered the most accurate technique, but the introduction of HDO units now means that oscillometry can be just as accurate (Martel et al., 2013).
| A standardised protocol for blood pressure measurement using Doppler | |
|---|---|
| 1 | The procedure must be standardised and planned before attempting to take measurements. All equipment should be ready for use. Attach headphones to the unit and select an appropriate cuff before starting. Test cuff to ensure it holds inflation. The cuff should be approximately 30 to 40 percent of the circumference of the cuff site. Cuff size should be noted in the record for future reference |
| 2 | The environment should be isolated, quiet, away from other animals and generally with the owner present. The patient should not be sedated and should be allowed to remain quietly in the measurement room for 5 to 10 minutes before attempting blood pressure measurement |
| 3 | The patient should be restrained gently in a comfortable position, ideally in ventral or lateral recumbency, to limit the distance from the heart base to the cuff |
| 4 | The cuff should be placed gently but securely on a limb or the tail. The site will vary according to animal conformation and user preference. The site for cuff placement should be recorded in the medical record |
| 5 | The same individual should perform all blood pressure measurements following this standard protocol. Training of the individual is essential |
| 6 | If clippers need to be used, clip the site in one movement. If clippers are not used, wipe the area with a surgical spirit-soaked swab |
| 7 | The patient should be calm and motionless |
| 8 | If headphones are used, apply and test sound. Use as little sound as possible |
| 9 | Apply ultrasound gel to the Doppler probe and confidently but gently place on the artery to be tested |
| 10 | Increase the volume and inflate the cuff to approximately 20mmHg above where the Doppler signal is lost. The Doppler probe should be at the approximate level of the right atrium |
| 11 | Deflate the cuff slowly but completely and note when the systolic “whoosh whoosh” sound is clearly audible. Repeat up to six times in total, discarding the first reading |
| 12 | Once the measurements have been taken, turn off the Doppler machine, wipe gel from the patient and allow it to return to its kennel, owner or basket to relax |
| 13 | The first measurement should be discarded. At least three, but preferably five to seven, consecutive and consistent results should be recorded |
| 14 | Average all values to obtain mean blood pressure measurement |
| 15 | If in doubt, repeat measurements |
| 16 | Written measurements should be kept on a standard form and include cuff size and site, the values obtained and the final (mean) result |
