30 Jan 2023
Gerardo Poli
Job Title
Systemic hypertension ultimately results in target organ damage – brain, heart, kidneys and eyes. Image: Fotolia/Victor Moussa.
Blood pressure monitoring is a standard practice as part of human medicine physical examination. In veterinary medicine, however, this is often omitted due to patient compliance issues, as well as inaccuracy as a result of transient hypertension caused by stress and fear.
Systemic hypertension ultimately results in target organ damage – brain, heart, kidneys and eyes.
Considering some of this damage can be reversed when caught early, and is terminal if left untreated, early detection of hypertension is pivotal. In this three-part series, I will discuss the pathophysiology, aetiologies, clinical signs, diagnosis and management of systemic hypertension in the dog and cat.
Systemic hypertension is defined as a persistent increase in arterial blood pressure (BP), the pressure exerted on the vascular walls by of blood. Blood pressure is determined by the cardiac output (CO) and systemic vascular resistance (SVR), as depicted in the equation BP = CO x SVR. Cardiac output is determined by the heart rate (HR) and stroke volume (SV; CO = HR x SV).
Going back to the basics:
The effect of persistent elevated systemic blood pressure has the greatest effect on organs with large arterial and arteriolar blood supply: the CNS, eyes, kidneys and heart.
Chronically increased pressure against the blood vessels promotes vasculature smooth muscle hypertrophy and the resultant persistent vasoconstriction will lead to brain ischaemia, haemorrhage, oedema, thrombosis and infarction, which can lead to collapse episodes or even severe neurological events.
In the eye the retinal and choridal vasculature can be affected. The retina can respond with either arteriolar constriction or torturosity, with the latter more likely to result in haemorrhage. With the increased pressure, blood and plasma from blood vessels escape and retinal oedema, haemorrhages and retinal detachment occurs. Haemorrhage can occur in both the anterior and posterior segment of the eye, and will be evident as hyphaema.
The kidneys receive a large portion of the cardiac output, making it vulnerable to the effects of hypertension. The afferent arteriolar system is the first to be damaged, making it less responsive to changes in systemic blood pressure. Eventually, this will progress to glomerular hypertension, proteinuria and reduced renal function.
The heart is also affected by the increase in afterload. Initially, the heart compensates by hypertrophying (compensatory cardiac hypertrophy). As a result, the cardiac myocardium remodels with an increase proportion of collagen, and eventually fibrosis, reducing myocardial muscle compliance and, subsequently, ventricular filling.
Understanding, the complications associated with systemic hypertension, it is important to work backwards to determine the underlying cause, if present, and begin management of the hypertension.
