Page 46 - Basic Monitoring in Canine and Feline Emergency Patients

veterinary patients develop hypertension secondary to angiotensin II production. Common ACEi include
another disease process (see Table 2.2). Therefore, if a both enalapril and benazepril, while the most studied
VetBooks.ir patient is assessed as hypertensive, further evaluation ARB in veterinary medicine is currently telmisartan.
Aldosterone blockage (e.g. spironolactone) can have
to deduce the underlying condition should be pursued
and, if warranted, appropriate therapy for that condi-
The effect is typically mild and this class of drugs is
tion should be implemented. The classification of the antihypertensive effects by reducing blood volume.
degree of hypertension is based on the risk of TOD infrequently used in the treatment of hypertension.
and is outlined in Table 2.3. One effect of angiotensin II is efferent arteriole
Dogs and cats measuring in the range of pre- constriction in the kidney glomerulus; therefore,
hypertension are not usually treated with antihy- angiotensin II blockade via ACEi or ARB induces
pertensive medications. However, monitoring the efferent arteriolar dilation. This can have the effect
patient’s blood pressure more frequently (e.g. every of decreasing glomerular filtration rate (GFR).
6 months) may be warranted to ensure the animal Therefore, care should be taken before starting
does not develop true hypertension. either of these medications in dehydrated patients
If the patient is diagnosed with hypertension or which may already have a decreased GFR.
severe hypertension, anti-hypertensive therapy is Whenever ACEi or ARB are prescribed, renal val-
indicated. Efforts should be directed at diagnosing ues should be monitored.
and treating any underlying disease while anti- In addition, blockade of the RAAS, and specifi-
hypertensive therapy is initiated. The goal for anti- cally the aldosterone regulation of sodium and
hypertensive therapy in most cases is a gradual potassium, can lead to hyperkalemia; electrolytes
reduction in blood pressure over the course of must be periodically monitored in patients receiv-
several days with repeated blood pressure evalua- ing ACEi or ARB, and require repeated monitoring
tion performed 7–10 days later. This slower adjust- as the drug dose is increased. Although hypona-
ment is preferable because autoregulatory vascular tremia is a theoretical concern, the degree of aldos-
function within the target organs, especially within terone blockade is usually insufficient to cause
the brain and kidneys, may have become relatively drastic changes in sodium; water movement with
well-adapted to chronic hypertension. When that sodium typically maintains normal sodium levels.
has occurred, sudden normalization or decrease in In cases of severe hypertension, multimodal ther-
blood pressure may potentially lead to hypoperfu- apy is likely indicated and is achieved through a
sion within those organs. Therefore, treatment is combination of RAAS inhibition and calcium chan-
initially started at the lower end of the dose range nel blockade. Calcium channel blockers (CCB) func-
with a gradual increase in dose or frequency as tion by preventing calcium entry into excitable cells.
indicated by the patient’s clinical response. As calcium is critical to smooth muscle contraction,
Occasionally dogs suffer from a disease process CCB cause smooth muscle dilation; this mechanism
that mandates specific therapy for hypertension (e.g. leads to a decrease in SVR and (presuming the car-
aldosterone secreting tumor requiring aldosterone diac output stays constant) a decrease in blood pres-
blockade). However, in most cases, dogs are initially sure (Fig. 2.3). Dogs should not receive CCB
treated with drugs which inhibit RAAS (see Fig. 2.4). treatment as their lone treatment for hypertension as
Angiotensin converting enzyme inhibitors (ACEi) CCB have the effect of afferent arteriole dilation
are currently most widely used in veterinary medi- within the kidney glomerulus. This can blunt the
cine, but alternative options include angiotensin protective effects of autoregulation in the kidney and
receptor blockers (ARB) and aldosterone receptor subject the glomerulus to a greater degree of hyper-
blockers. Both ACEi and ARB serve to block tension and potential damage. In some cases, CCB

Table 2.3. Categories of hypertension in dogs and cats.
Pre-hypertensive (low Hypertensive (moderate Severely hypertensive
risk of TOD) risk of TOD) (high risk of TOD)
Systolic blood pressure 140–159 160–179 ≥180
(mmHg)
TOD, target organ damage.

38 D.S. Foy