So if 1 in 5 cats over the age of 9 have evidence of hypertension, what impact does CKD have on the prevalence of hypertension? Well, from the literature, the prevalence of hypertension in CKD varies between 20 up to 65% of cats. This wide range is dependent on the population of cats that are being studied and the definition of hypertension that's being used.

Being older and having a disease risk like CKD clearly increases the risk of identifying hypertension further. In fact, it's so important that evaluating blood pressure is part of the International Renal Interest Society's substaging for cats with CKD and measuring blood pressure is recommended not only at diagnosis of CKD but also as part of longer term monitoring. We should remember that even if a cat is not hypertensive at diagnosis of hypertension can develop in the future.

In this video, we're going to look at why measuring blood pressure and diagnosing hypertension is really important for cats with CKD. We'll look at the impact that hypertension has on the kidney and proteinuria and how treatment of hypertension can be beneficial. And we'll put it all together into a case example at the end.

So let's start by looking at the ACM hypertension consensus and the Iris substaging guidelines and how they help us to categorise cats in terms of their blood pressure. These criteria are used to help us to diagnose hypertension and also to evaluate the risk for target organ damage. This can really help us in terms of deciding when to start anti-hypertensive treatment.

So for cats with a systolic blood pressure that's greater than 160 millimetres of mercury, but less than or equal to 179 millimetres of mercury, we describe them as hypertensive. If is greater than 180 millimetres of mercury, then they are going to be considered severely hypertensive. These are the categories where we are considering when we decide if we need to start anti-hypertensive treatment.

It's also worth noting the pre-hypertensive category. If a cat with CKD has an SBP between 140 to 160 millimetres of mercury, we wouldn't be thinking about starting anti-hypertensive treatment. But these are cats where it's important to continue to monitor their blood pressure, as we know that it is likely to increase with time, and they could go on to hit that hypertensive category.

I'm going to pose you another question. What's the number one thing that you find most difficult about measuring blood pressure in cats? Well, there's probably a lot of things coming into your mind at the moment, but one that people often talk to me about is how to decide whether stress or anxiety in cats is contributing to situational or what was previously called white coat hypertension versus the measurement really indicating genuine hypertension.

How do we get around this issue? Well, we need to do everything we can to try and ensure that we have a quiet and calm environment for blood pressure measurement. And there are a couple of other things we can consider.

If we have a cat who has CKD and their blood pressure would place them in the hypertensive category, to try and establish that this is a genuine hypertension rather than situational, we can look for evidence of target organ damage. The quickest, easiest way to do this is with an eye examination. You can do this using direct or indirect ophthalmoscopy or even a retinal camera if you happen to have one.

If we find evidence of hypertensive ocular injury, then we know immediately that those measurements are genuine, and we should start treatment straight away. It becomes a little more challenging if the fundic examination is normal. Here we need to think about repeating blood pressure measurements on another occasion to document that that hypertension is persistent.

So normally we need to think about inviting the cat and the owner back somewhere between 3 to 10 days later to have their blood pressure measured a second time. In a cat with CKD, if SBP is greater than 160 millimetres of mercury on at least two occasions, then we recommend that anti-hypertensive treatment is started. But the eye certainly isn't the only organ that can be affected by hypertension.

And so as we're talking about cats with CKD, we should also think about the effect that hypertension can have on the kidney. In health, in order to maintain glomerular filtration rates and therefore renal function, the kidneys have regulatory mechanisms that control renal blood flow and specifically blood flow into the glomeruli, where the filtration occurs. This is what we call renal autoregulation.

Renal auto regulation ensures that renal blood flow and glomerulocapillary pressures are maintained at a constant level, even during the normal minute by minute fluctuations in blood pressure. However, renal auto regulation can only effectively regulate renal blood flow within a range of systemic pressures. These are typically quoted as being between 60 to 160 millimetres of mercury.

Outside of this pressure range, renal blood flow will fall or increase accordingly. So here we've got a senior cat who we know has CKD having their blood pressure measured. We can see that the systolic blood pressure is exceeding 180 millimetres of mercury.

If that's persistent, then this cat is diagnosed with systemic hypertension. Let's take this from the cat to look specifically at what's happening at the level of the kidney. Where systemic pressures exceed the range for effective renal auto regulation, there can be transfer of those high pressures to the glomerulus.

This results in two major consequences. Firstly, increased glomerulocapillary pressures can drive the passage of proteins from the glomerulus to the tubular filtrate, increasing proteinuria. Secondly, those increased glomerulocapillary pressures can lead to injury to the glomerulus and the development of glomerullo sclerosis.

Protein that is filtered at the glomerulus is usually reabsorbed via proximal tubular cells, but when there is excess protein, this can overwhelm the uptake mechanisms and we start to see proteinuria clinically. Increased protein processing by the proximal tubular cells is a recognised driver for tubular interstitial inflammation and fibrosis. And ultimately, if the damage to the glomerulus through glomerullos sclerosis is sufficient, then this can lead to progressive nephron loss.

Ultimately, glomerular hypertension, hyperfiltration, and the end changes of glomerulosclerosis, as well as the increased processing of protein by the kidney, are key pathophysiological mechanisms that drive the development and the progression of kidney disease. So that really leads us to ask clinically how can we identify hypertensive injury to the kidney in cats. As we've alluded to previously, if a cat has systemic hypertension that is overriding renal autoregulation, then the kidney is going to be at risk as a target organ for hypertensive injury.

However, renal biopsies are clearly not an everyday procedure to look for this damage. But what we can look for is evidence of proteinuria. Overall, cats with CKD due to tubular interstitial nephritis, particularly in the early stages, tend to have relatively low levels of proteinuria.

But as you can see on this graph, studies have shown that the magnitude of this proteinuria tends to be higher in cats that are hypertensive compared to norm attentive cats that are matched for the same iris stage of CKD. Although it's difficult to provide information on an individual cat basis, epidemiological studies that have looked at really large groups of cats with systemic hypertension have shown that proteinuria is the main risk factor associated with survival. So it's clear that thinking about the impact of hypertension on the kidney is really important, and management of hypertension is going to reduce the risk of glomerular injury and hypertension associated proteinuria.

So that's the underlying pathophysiology of CKD and hypertension. Let's move on to review how best to treat cats with CKD and hypertension.