Feline systemic hypertension is a clinically significant disease in cats. It can be idiopathic in origin, occur secondary to a concurrent disease or to toxin ingestion, or seen as a reaction to situational stress. Chronic hypertension can lead to severe tissue injury which will often manifest at so called ‘target organs’, and can cause renal, cardiovascular, ocular, or neurological changes. An important part of diagnosis and evaluating anti-hypertensive therapy is obtaining accurate, reliable and repeatable blood pressure measurements. However, measuring blood pressure to this standard is not as easy as it sounds. Readings will vary with patient temperament, patient position, operator experience, and measurement method, which makes it difficult to know what the expected blood pressure ranges should be for the individual patient. Trained veterinary nurses, working to and maintaining a standard protocol can maximise the reliability of results.
What is blood pressure?
Blood pressure is the force exerted on the vessel wall which is measured in millimetres of mercury (mmHg). Blood pressure is affected by two factors; cardiac output, or the amount of blood ejected from the left ventricle in systole, and by the diameter and elasticity of the arterial walls, known as systemic vascular resistance. This can be simplified by the equation: Blood pressure = cardiac output x systemic vascular resistance
Blood pressure is regulated by the central nervous system, which responds to messages sent from sensory receptors within the peripheral vasculature. This allows for quick and short-term adjustments. Blood pressure also responds to the neurohormonal system (the renin-angiotensin-aldosterone-system) because angiotensin II causes vasoconstriction and maximises cardiac output by increasing heart rate and improving cardiac contractility. The kidney has the ability to control its own blood pressure, a process known as autoregulation, in which it maintains renal blood flow and glomerular filtration. Hypertension can, however, override this process and cause glomerular hypertension and glomerulosclerosis (Jepson, 2011). The aim of blood pressure regulation is to maintain adequate blood perfusion to the body's tissues, and to protect against hyper or hypo perfusion, which could cause target organ damage (TOD).
Normal blood pressure ranges and influencing factors
There is no magic definitive systolic blood pressure number in feline or canine medicine, as there are many variables, not least temperament, individual patient differences, and measurement technique. Payne et al (2017) used a demeanour and assessment guide for measuring blood pressure, which could prove to be very useful in practice (Table 1). Furthermore, different studies have reported different systolic blood pressure numbers (Table 2), so the American College of Veterinary Internal Medicine (ACVIM) Consensus Statement (Acierno et al, 2018) uses a scheme of measurements taken alongside a risk assessment of TOD (Table 3).
Table 1. Demeanour and assessment score chart for feline blood pressure measurements
| Demeanour | Assessment |
|---|---|
| Calm | Relaxed, looking around, relaxed body position, ears forward, tail held in vertical position, purring, trilling, and/or kneading |
| Cooperative but anxious | Generally calm and still looking around, but showing some signs of nervousness (e.g. tail between legs or slightly crouched body position) |
| Nervous | Hiding (either hiding head into the person restraining or whole body under bedding), or crouched, shivering, avoiding eye contact, ears folded sideways and downwards, tail between legs, may be purring/kneading |
| Aggressive | Hissing, growling, or swiping with claws |
| Excited | Attempting to play, difficult to keep still as more interested in interacting with people and/or environment |
Table 2. Feline systolic blood pressure average measurements taken from recent studies
| Study | Mean systolic blood pressure measurement (mmHg) — Doppler method |
|---|---|
| Kobayashi et al (1990) | 118 |
| Sparkes et al (1999) | 162 |
| Lin et al (2006) | 134 |
| Dos Reis et al (2014) | 125 |
| Taffin et al (2016) | 133 |
| Quimby et al (2011) | 137 |
| Paige et al (2009) | 131 |
| Chetboul et al (2014) | 151 |
| Payne et al (2017) | 122 |
Table 3. Blood pressure ranges with assessment risk of target organ damage
| Category | Risk of TOD | Systolic blood pressure measurement |
|---|---|---|
| Normotensive | Minimal | <140 mmHg |
| Pre-hypertensive | Low | 140–159 mmHg |
| Hypertensive | Moderate | 160–179 mmHg |
| Severely hypertensive | High | >180 mmHg |
TOD = target organ damage;
Human blood pressure is known to increase with age, but such a link is debatable in cats (Jepson, 2011). A 1998 study said there was a correlation (Bodey and Sansom, 1998), but two other studies published about the same time disagreed (Kobayashi et al, 1990; Sparkes et al, 1999). However, a more recent and wideranging study of 780 apparently healthy cats, did report an association between age and an increase in blood pressure, although the cats were not screened for concurrent disease (Payne et al, 2017). It has been reported in dogs and humans that being male predisposes to higher blood pressure (Bodey and Michell, 1996; Smulyan et al, 2001), and Payne et al's study is the first to show that this is also true of cats (Payne et al, 2017). It is also well known that blood pressure ranges in dogs vary between breeds and cross breeds, for example, Greyhounds and Deerhounds, but no such link has been found in cats (Acierno et al, 2018).
In humans there is a well-established link between obesity and increased blood pressure (NHS, 2019), but this has not been replicated in cats. In contrast, Payne reported a link between cats with lower bodyweight and a lower blood pressure (Payne et al, 2017). A link has also been reported that muscle condition score and sarcopenia can affect radial artery blood pressure measurements (Whittemore et al, 2017).
What is systemic hypertension?
Put simply, systemic hypertension is an increase in blood pressure supplying the body's organs. The 2018 ACVIM Consensus Statement (Acierno et al, 2018) suggests three definitions for hypertension, depending on the cause:
- Situational hypertension — more commonly known as ‘white coat hypertension’ in humans, is stress induced. Measurements are difficult to assess because the effects of anxiety are not predictable. For example, it was reported that blood pressure decreased in response to stress in one cat study (Belew et al, 1999).
- Secondary hypertension — persistent increased blood pressure concurrent with another disease process, such as chronic kidney disease (CKD), hyperthyroidism, or diabetes mellitus (DM). It can also occur secondary to medication or to toxin ingestion. Hypertension may persist even if the primary disease is treated (Acierno et al, 2018). The ACVIM Consensus Statement reports that hypertension due to concurrent disease is the most common type found in cats (Acierno et al, 2018).
- Idiopathic hypertension — is said to be present when blood pressure measurements are consistently increased, and all other causes have been ruled out or minimised. Approximately 13–20% of cats are diagnosed with idiopathic hypertension (Acierno et al, 2018).
Target organ damage
If systemic hypertension is left untreated, it can cause small blood vessels to leak or even rupture, and this can cause serious tissue injury. Evidence of sustained hypertension can often be seen in four main areas, the brain, kidney, heart and eyes, known as the target organs. Table 4 shows the specific damage seen to the target organs.
Table 4. Summary of target organ damage and clinical findings
| Tissue | Hypertensive injury | Clinical findings |
|---|---|---|
| Kidney | Progression of CKD | Increases in serum creatinine concentrations (SCR), symmetric dimethylarginine (SDMA) or decrease in glomerular filtration rate (GFR) |
| Eye | Retinopathy/choroidopathy | Acute onset blindness |
| Exudative retinal detachment | ||
| Retinal haemorrhage/oedema | ||
| Retinal vessel tortuosity or perivascular oedema | ||
| Papilloedema | ||
| Vitreal haemorrhage | ||
| Hyphaema | ||
| Secondary glaucoma | ||
| Retinal degeneration | ||
| Brain | Encephalopathy | Centrally localising neurological signs (brain or spinal cord) |
| Stroke | ||
| Heart and blood vessels | Left ventricular hypertrophy | Left ventricular hypertrophy |
| Systolic heart murmur | ||
| Gallop sound | ||
| Left-sided congestive heart failure (L-CHF) | Arrhythmias | |
| Evidence of L-CHF |
CKD = chronic kidney disease;
Renal
Injury to renal tissue arises from chronically high blood pressure. Histopathology from experimental feline renal hypertension models, showed evidence of interstitial fibrosis, glomerulosclerosis and tubular atrophy (Jepson, 2011). Current thinking is that renal disease causes hypertension (Bijsmans, 2015). However, there is another theory that suggests hypertension could cause the initial renal injury (Brown et al, 2007). Recent evidence shows that somewhere between 19–65% of cats with CKD are hypertensive (Bijsmans, 2015). One comparative study investigated blood pressure measurements in healthy cats and those with CKD, and found that 17% of cats diagnosed with CKD developed hypertension, compared with only 7% in the healthy cat group (Bijsmans, 2015).
Ocular
Ocular changes are a relatively common finding with feline hypertension, with prevalence reported between 60–80% (Jepson, 2011), and even up to 100% in one study (Acierno et al, 2018). Changes can include acute onset blindness, hyphaema, retinal haemorrhage or detachment and can be either unilateral or bilateral. Figure 1 shows an example of focal bullous detachments as a result of hypertension.
Cardiovascular
When blood pressure is measured on a limb (for example the radial artery) or from the tail (coccygeal artery), the measurement read there has to at least match that of the left ventricle before it pumps blood out through the aorta and to the rest of the body. This sustained increase in pressure can lead to left ventricular hypertrophy, and if the hypertension is left untreated, may in some cases, cause cardiac failure or arrhythmias (Acierno et al, 2018).
Neurological
Neurological signs have been reported in 29% and 46% of cats with hypertension, and occurs as a result of hypertensive encephalopathy, where white matter becomes oedematous and vascular lesions occur (Maggio et al, 2000; Littman, 1994). Clinical signs vary from lethargy, altered mentation, vestibular signs or seizures and are usually sudden in onset (Jepson, 2011).
What is the best method to measure blood pressure?
Blood pressure varies from individual to individual, and so it is important not only to understand the ranges for normal or abnormal, but just as importantly to be able to obtain repeatable and reliable results for that individual. For example, it may be that the cat is more relaxed with its owners, and therefore all subsequent measures should be taken under similar conditions. Standardisation of the process can be achieved by using a form, such as that seen in Figure 2.
Radiotelemetry
A radiotelemetric catheter is placed in a systemic artery and the transmitter body secured either to the abdominal wall or in a subcutaneous pocket in the flank (Wang et al, 2017). This allows direct blood pressure to be measured non-invasively and remotely. This method, though still in the developmental stage, could one day allow for assessment of blood pressure in patients when they are in their home environment, allowing for an accurate and reliable assessment (Martel et al, 2013).
Direct blood pressure measurement
Direct blood pressure measurement is considered the gold standard of blood pressure measurement. A catheter is placed into a peripheral or femoral artery which is then attached to an electronic transducer. Requiring sedation, anaesthetic or a very sick patient, it is a technically challenging skill and not widely available outside referral centres. Furthermore, the size of feline patients also makes it difficult to place arterial catheters. Direct blood pressure measurement therefore is not indicated for screening of hypertension.
Indirect blood pressure measuring — Doppler and oscillometric
Oscillometric blood pressure measures the variance of oscillations when the arterial walls contract. The Doppler method approximates arterial pressure from the changes to blood flow, caused by the inflation of a cuff. In a perfect world, the machines used for either method would be validated for use, but at the time of writing, neither have been. Until such time as validation comes, ACVIM has issued a set of guidelines to assist professionals to get reliable and repeatable results (Box 1) (Acierno et al, 2018). The ACVIM guidelines refer to both Doppler and oscillometric methods for blood pressure measurement, as some practices may not have both methods available. Newer models of oscillometric machines (high definition oscillometric or HDO) are now seen to be as reliable as the Doppler method, although it does need to be used in conjunction with pulse wave analysis (Sparkes et al, 2019). A large study published in 2017 studying 780 cats used the Doppler method, and this does seem to be the preferred method of veterinary surgeons, due to either user experience or availability of HDO machines (Payne et al, 2017). It is recommended that whatever method is chosen, that a standardised technique is used, so that results can be reliably and repeatedly recorded (Acierno et al, 2018). This is especially important when screening for disease and assessing antihypertensive treatment.
Box 1.Guidelines for taking blood pressure (BP)
- 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 5–10 minutes before attempting BP measurement.
- The International Society of Feline Medicine (ISFM) recommends the use of feline pheromones (such as Feliway®, Ceva) in the room to help cats relax and acclimatise (Sparkes, 2019).
- The cat should be gently restrained in a comfortable position ideally in ventral or lateral recumbency to limit the distance from the heart base to the cuff (see Figure 3).
- The cuff should be approximately 30– 40% of the circumference of the cuff site. The cuff size should be noted in the medical record for future reference.
- The cuff can be placed on a limb or the tail. Site will vary with animal conformation and user preference. The site for cuff placement should be recorded in the medical record.
- The same individual ideally should perform all blood pressure measurements following this standard protocol. Training of the individual is essential.
- The patient should be calm and motionless.
- The first measurement should be discarded. At least 3, and preferably 5–7, consecutive, consistent (<20% variability in systolic values) measurements should be recorded. Average all values to obtain the mean BP measurement.
- If in doubt, repeat the measurements.
- Written measurements should be kept on a standard form and include cuff size and site, values obtained, rationale for excluding any values, the final (mean) result.
Figure 3. An example of gold standard blood pressure measurement.
Conclusion
Feline systemic hypertension is seen regularly in general practice, either as a disease process in itself, as a result of stress, or secondary to a concurrent disease. Consideration should be made to the potential of TOD, and if at-risk, cats should have blood pressure measured regularly. It is important that nurses follow a structured protocol for taking blood pressure measurements, so that results can be interpreted as reliable, accurate and repeatable.
KEY POINTS
- Systemic hypertension can cause target organ damage (TOD).
- Target organs are the kidneys, eyes, heart and blood vessels, and brain.
- Blood pressure measurements should be repeatable and reliable.
- Current recommendations are to regularly monitor systolic blood pressure in senior cats, and those cats diagnosed with chronic kidney disease.