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L. A. Folkman, S. Berland and D. J. Hotchkiss <<TableOfContents(3)>> ==== Abbreviations ==== {{https://upload.wikimedia.org/wikipedia/commons/thumb/3/34/Abbrev2.png/497px-Abbrev2.png}} === Overview === Whilst the treatment of systemic hypertension in humans has become a commonplace part of 21st century medicine, the treatment of systemic hypertension in domestic animals remains a relatively new field, with research still ongoing to establish the most effective therapies for use in different species. Furthermore, a complete understanding of the modes of action of the antihypertensive drugs currently available on the market is lacking, with some drugs well understood and others less so. Whilst other domestic animals may suffer from systemic hypertension, to date cats and dogs remain the most commonly treated for the condition. The aim of this paper is to undertake a comparative analysis of the antihypertensive drug therapies currently in use for treatment of systemic hypertension in cats and dogs and assess the physiological differences between the approaches. Systemic hypertension is an increase in systemic arterial blood pressure. Hypertension can be classified as either primary (idiopathic), where no underlying disease is present, or secondary, where an underlying disease is the root cause of the hypertension (Brown et al., 2007). It is reported that 13-20% of hypertensive cats have idiopathic hypertension (Elliott et al., 2001 & Maggio et al., 2000), whilst idiopathic hypertension is almost non-existent in dogs (Kittleson, 2018). In dogs the most common cause of hypertension is renal disease/failure, whilst in cats the most common causes are renal disease, primary hyperaldosteronism and hyperthyroidism. Hyperadrenocorticism, diabetes mellitus and pheochromocytoma are other causes of systemic hypertension in dogs, but rarely seen in cats. Dogs and cats with systemic hypertension rarely show clinical signs. Acute blindness is the most common sign due to retinal lesions; blood tests may demonstrate abnormalities, but treatment should be initiated in any animals showing consistently measurable hypertension with an established underlying cause (Kittleson, 2018). ---------- === Physiological Modes of Action of Antihypertensive Drugs === The treatment of hypertension with pharmacotherapeutics takes advantage of the multifactorial nature of the physiological regulation of blood pressure, with modes of action dependent upon drug interactions with one or more of the control mechanisms the body has in place. The commonly used antihypertensive drugs can be divided mechanistically into '''vasodilators''' (typically calcium channel blockers), '''ACE inhibitors''', '''positive inotropes''', '''diuretics''' and '''β-blockers'''. We shall consider each of these subgroups in turn and look at their different mechanisms of action. Vasoactive drugs can be categorized as afterload reducers or preload reducers. Afterload is reduced by arterial dilation whilst preload is reduced by venous dilation. Arterial dilators are used in the treatment of systemic hypertension in cats and dogs and '''calcium channel blockers''' are the most widely used subgroup. These drugs block the calcium channels in the vascular smooth muscle of systemic arterioles, arresting the mechanism of contraction of the muscles and thus causing vasodilation. Consequent reduction in peripheral resistance lowers the systolic blood pressure. The most widely used drug in this class in veterinary medicine is '''amlodipine besylate''', although hydralazine has also been used in dogs. Amlodipine appears to be better tolerated by the GI tract (Gordon et al., 2018). Both drugs are administered PO; hydralazine is not suitable for cats but can be administered at 0.5-3.0 mg/kg, PO, bid in dogs; amlodipine is used at 0.1-0.2 mg/kg, PO, bid in dogs and at 0.625-1.25 mg/cat, PO, once daily. A study conducted in 2016 (Bijsmans et al., 2016) on the factors influencing the relationship between the dose of amlodipine required for blood pressure control and change in blood pressure in hypertensive cats, revealed that individual cat factors, and differences in amlodipine pharmacokinetics between individuals, did not significantly influence the antihypertensive response. Indeed, a higher dose of amlodipine was indicated only in cats where their initial systolic blood pressure was higher than 200 mmHg at diagnosis. Doses of Ca2+ channel blockers must be titrated starting with a low dose and titrating up to an effective clinical endpoint while monitoring for systemic hypotension (Gordon et al., 2018). Gingival hyperplasia has been reported as a side effect of amlodipine usage in dogs and cats and usage of hydralazine should be done with care as its incidence of toxicity is higher and its mechanism of action not fully elucidated; (it is believed that in addition to halting calcium influx, hydralazine increases local prostacyclin concentrations in order to elicit its clinical effect). The angiotensin-converting enzyme (ACE) inhibitors are widely use in the treatment of congestive heart failure in dogs and cats (Gordon et al., 2018), however they have also begun to find utility as a treatment for systemic hypertension in dogs. The proteolytic enzyme renin is released by the kidneys and acts on angiotensinogen, which is produced by the liver and distributed in the blood, to produce angiotensin I. Angiotensin I is then converted into angiotensin II by the action of ACE in the capillary endothelium of the lung, or in the circulating plasma, the kidney or other organ beds (Dukes et al., 2004). Next to vasopressin, angiotensin II is the second most potent vasoconstrictor produced in the body, directly increasing blood pressure; however, it also indirectly increases blood pressure by stimulating aldosterone release in the adrenal cortex, leading to retention of sodium ions and water in the kidney. ACE also has a degradative effect on the vasodilator bradykinin; hence by a range of pathways ACE prompts a rise in blood pressure. ACE inhibitors are thus effective antihypertensives and are balanced in that they reduce both preload and afterload. Effects during congestive heart failure (CHF) include decreased vascular resistance and cardiac filling pressures, increased cardiac output and exercise intolerance. ACE inhibitors however should not be used as monotherapy in animals with moderate to severe systemic hypertension (>160 mmHg) as they are only mild afterload reducers. '''Enalapril''' and '''benazepril''' can both be used in dogs, both being prodrugs requiring hepatic metabolism to be activated. Long term administration of benazepril has been used with dogs suffering from chronic kidney disease (CKD) and a dosage of 0.25-0.5 mg/kg PO daily was shown to reduce proteinuria. However, no benefit of benazepril administration versus placebo was shown to occur on the renal survival time of 49 dogs in a 2-year trial in 2017 (King et al., 2017). ACE inhibitors should be used with care with amlodipine or diuretics, as hypotension may result; concurrent use of NSAIDs may also increase the risk of adverse effects (Gordon et al., 2018). Monitoring of BUN and creatinine levels is warranted, as azotemia may result from ACE inhibitor treatment. From a clinical standpoint, systemic hypertension and CKD are not well treated with ACE inhibitors, but ACE inhibitor usage in dogs and cats is indicated in treatment of CHF, stemming from a wide variety of diseases. Positive inotropes increase the strength of cardiac muscle contraction by increasing the concentration of intracellular Ca2+ for binding by muscle proteins, by increasing the sensitivity of contractile proteins to calcium, or a combination of both. This, in turn, augments contractile protein interaction in myocytes. Intracellular Ca2+ can be increased by altering the Na+/Ca2+ exchange pump, by increasing production of cAMP via stimulation of adenylate cyclase, or by decreasing degradation of cAMP via inhibition of phosphodiesterases (Gordon et al., 2018). Cardiac glycosides such as digoxin find little utility in veterinary practice, however phosphodiesterase inhibitors such as '''pimobendan''' have been approved for treatment of CHF due to DCM or DMVD in dogs. Pimobendan, sildenafil and tadalafil are all phosphodiesterase inhibitors that have been used in the treatment of pulmonary hypertension in dogs. However, pimobendan is contraindicated in dogs with known outflow tract obstruction, e.g. SAS. Pimobendan is not approved for use in cats. {{https://upload.wikimedia.org/wikipedia/commons/9/9b/Mols2.png}} ''FIGURE 1 - Antihypertensive Drugs'' Diuretics are the cornerstone of CHF therapy, where illness is characterized by cardiogenic pulmonary oedema, pleural effusion and ascites. Three classes of diuretics are used in dogs and cats: loop diuretics, thiazide diuretics and potassium-sparing diuretics (Gordon et al., 2018). The relative risk of azotemia, dehydration & electrolyte imbalance is increased when a diuretic is used concurrently with an ACE inhibitor or an NSAID. The most common electrolyte & acid-base abnormalities include hypokalemia, hyponatremia, hypomagnesemia and metabolic alkalosis. '''Furosemide''' is a loop diuretic used to treat CHF in dogs and cats, available in oral and parenteral formulations. Compounded liquid formulations may be better tolerated in cats than alcohol-based syrups. All loop diuretics inhibit Na+, K+ & Cl- reabsorption in the thick portion of the ascending loop of Henle, leading to inhibition of Na+ absorption and commensurate water reabsorption by the nephron. Furosemide also acts as a mild systemic venodilator. Cats are more sensitive to furosemide than dogs when administered orally (Gordon et al., 2018). Clinical uses tend to be either (i) treatment of life-threatening cardiogenic pulmonary oedema in dogs and cats, where parenteral doses of 2-4 mg/kg every 1-6 hours, IV, IM or SC in dogs and 0.5-2.0 mg/kg every 1-8 hours, IV, IM or SC in cats is common; or (ii) long term treatment of CHF in dogs and cats, where PO doses in dogs of 2.0-5.0 mg/kg bid are common, compared to 1.0-2.0 mg/kg once to twice daily in cats. Thiazide diuretics such as hydrochlorothiazide are rarely used in dogs and cats; the thiazides act by reducing membrane permeability to Na+ and Cl- in the distal convoluted tubule. These drugs promote K+ loss and produce a large increase in urine sodium ion levels but only mild to moderate increases in urinary volume. Potassium-sparing diuretics such as spironolactone inhibit the action of aldosterone on distal tubular cells or block sodium reabsorption in the latter regions of the distal tubule and the collecting tubules. Although these drugs find some use in adjunctive therapeutic treatment of CHF, they are not used to treat systemic hypertension. The last important group of drugs to discuss are the β-adrenergic receptor blocking agents (also termed “class II antiarrhythmic drugs”) (Gordon et al., 2018). β-blockers are dose-dependent negative inotropes and chronotropes, thus decreasing cardiac output. '''Atenolol''' is a good example of the class and is a β1-selective blocking agent commonly used in the treatment of SAS, CHF, heart failure and heart disease, at 0.2-1.0 mg/kg, PO, bid in dogs or 1.0-2.5 mg/kg, PO, bid in cats. β-blockers are generally ineffective for treatment of systemic hypertension; however, they have found utility in the treatment of the hypertensive effects of feline hyperthyroidism, decreasing neuromuscular and cardiovascular effects of high thyroid hormone levels (Atkins, 2001 & Bright, 2000). There is also evidence from a 2014 study that atenolol does not significantly influence survival of dogs with severe SAS, i.e. with a pressure gradient across the stenosis of more than 80 mmHg (Eason, 2014). ---------- === Treatment of Systemic Hypertension in Cats === Blood pressure measurement in cats is most effectively performed in clinical practice by indirect measurements; Doppler sphygmomanometry or oscillometry remain the most common techniques used (Taylor et al., 2017). Doppler sphygmomanometry has been shown to be more accurate; however, neither of the mentioned techniques has been fully validated according to the ACVIM 2007 criteria for BP measurement (Haberman et al., 2004 & Jepson et al., 2005). From a clinical viewpoint, systolic hypertension is generally most important to assess (Gordon et al., 2018). Blood pressure in cats is labile and varies considerably within and between individuals. Factors often playing a role in the measurements are amount of stress, activity and level of arousal (Bewel et al., 1999). To date no evidence has been obtained that there is any significant difference between genders on blood pressure and there are also no documented breed effects on feline blood pressure (Sansom et al., 2004 & Mishina et al., 1998). However, a small but significant increase in blood pressure does occur with age. (Bijsmans et al., 2015). Current studies suggest a median age for diagnosis of hypertension is 13-15 years (Jepson et al., 2007 & Chetboul et al., 2003), but it has been reported at ages as low as 5-7 years (Syme et al., 2002). Early diagnosis gives better chances to prevent target organ diseases (TOD). Indirect systolic blood pressure measurements (mmHg) of healthy cats has been shown to fall over a significant range and thus caution must be exercised in diagnosing a hypertensive state: {{https://upload.wikimedia.org/wikipedia/commons/f/fb/Table_1fk.png}} ''TABLE 1 - Indirect Systolic Blood Pressure Measurement'' Hypertension often affects organs rich in arterioles, although not always. Organs most commonly affected are the eye, brain, kidney & myocardium (Littman, 1994). When organs are affected by the hypertensive disease state it is called a target organ disease (TOD): * Lesions of the eye: Hypertensive ocular changes have been reported in approximately 50% of hypertensive cats (Stiles et al., 1994); it is suggested that systemic blood pressure above 160 mmHg causes changes in the retinal layer * Lesions of the brain: Hypertensive encephalopathy occurs when the blood pressure has been high enough for a long period that it overcomes the auto-regulatory baroreceptor ability of the cerebral vasculature (Williams and Brust, 2004) * Lesions of the cardiovascular system: With hypertension, systemic vascular resistance often occurs. This forces the heart to pump harder which may lead to concentric left ventricular hypertrophy. This can produce auscultatory abnormalities such as gallop sounds and less commonly murmurs and arrhythmias (Maggio et al., 2000). Heart failure can occasionally occur in severe cases (Scollan and Sisson, 2014 & Wey and Atkins, 2000) * Lesions of the kidneys: In a controlled study of over 200 cats it was demonstrated that in cats with increased blood pressure, increased glomerulosclerosis and arteriosclerosis occurred (Chakrabarti et al., 2012). This supports a link between kidney function damage and feline hypertension. However, these lesions are often not solely caused by hypertension, as many cats suffering from hypertension are hypertensive as a secondary sequela to chronic kidney disease (Syme, 2011). Despite hypertension being a common disease, routine blood pressure measurements and monitoring is infrequently performed amongst cats, which likely means that there is an underdiagnosis of feline hypertension. The following thresholds may be used to perform diagnosis in most cases (Brown, 2016): {{https://upload.wikimedia.org/wikipedia/commons/2/24/Table_2fk.png}} ''TABLE 2 - Categorisation of Hypertension in Cats and Dogs'' The following diseases commonly result in secondary hypertension in cats: {{https://upload.wikimedia.org/wikipedia/commons/8/80/Table_3fk.png}} ''TABLE 3 - Prevalence of Secondary Hypertension in Cats'' ==== Treatment ==== Since most cases of feline hypertension are secondary, the underlying diseases must be diagnosed and treated. The goal of antihypertensive therapy is to decrease the risks of target organ diseases. This is achieved by keeping the blood pressure less than 160 mmHg (Gordon et al., 2018). Amlodipine besylate, a dihydropyridine calcium channel blocker, is widely regarded as the best choice for the management of hypertension in cats (Taylor et al., 2017). Systemic blood pressure is shown to be reduced by around 30-70 mmHg with 60-100% of cats responding to amlodipine besylate as a monotherapy (Henik et al., 1997). Amlodipine besylate has also been shown to reduce the magnitude of proteinuria in hypertensive cats with chronic kidney disease. The drug can thus be regarded as a first line therapy in management of hypertension in cases of CKD, contributing to a significant extension of life (Jepson et al., 2007). Other treatments include ACE inhibitors (Van Israel et al., 2009), angiotensin receptor blockers (Jenkins et al., 2015), and β-blockers (Stepien, 2011). These drugs have been shown to have a lesser effect than amlodipine besylate and should therefore only be used as a supplement if amlodipine cannot effectively control the systolic blood pressure as a monotherapy (Elliott et al., 2004). Enalapril, diltiazem, atenolol and furosemide are generally of lower effectivity in treating systemic hypertension in cats (Kittleson, 2018); however, success has been found with telmisartan in the treatment of feline proteinuria secondary to CKD (Acierno et al., 2018). Drug dosages commonly used are summarized below: {{https://upload.wikimedia.org/wikipedia/commons/2/2d/Table_4fk.png}} ''TABLE 4 - Antihypertensive Drugs used in Treatment of Cats'' ---------- === Treatment of Systemic Hypertension in Dogs === When measuring blood pressure in dogs it should be kept in mind that situational or “white coat” hypertension may often occur in response to veterinary care and lead to erroneous diagnosis (Remillard et al., 1991). Ultrasound, SDMA, GFR, cortisol measurements and urine protein measurements should be conducted to search for underlying causes. Hypertension may also not be resolved despite treating the underlying cause (Goy-Thollot et al., 2002). With the use of intra-arterial direct measurement, oscillometry and Doppler ultrasonography the results for canine blood pressures in mmHg below have been obtained as follows: {{https://upload.wikimedia.org/wikipedia/commons/c/c1/Table_5fk.png}} ''TABLE 5 - Blood Pressure Measurements in Dogs'' In contrast to humans there is less correlation between age and blood pressure in dogs; some studies found a connection (Bodey et al., 1996), while others found nothing (Meurs et al., 2000). Some studies have found gender differences (Bodey et al., 1996), and in contrast to cats, breed variations have been demonstrated, e.g. greyhounds have been shown on average to have higher blood pressures (Schneider et al., 1964). An increase in blood pressure in obese dogs has also been found, although this may be a result of underlying disease caused by obesity (Perez-Sanchez et al., 2015). Secondary systemic hypertension can result from several underlying diseases. In dogs with chronic kidney disease prevalence of hypertension has been found to be between 9% (Bodey et al., 1996) and 93% (Anderson et al., 1968). Acute kidney disease shows an 87% prevalence (Francey et al., 2004), while diabetes mellitus shows a prevalence for hypertension of up to 67% (Marynissen et al., 2016). Hyperadrenocorticism presents with hypertension in up to 80% of cases (Rapoport et al., 2001), while pheochromocytoma shows up to 86% prevalence of hypertension (Gilson et al., 1994). Lastly, leishmaniasis may also present with associated secondary hypertension (Braga et al., 2015). ==== Treatment ==== Treatment of systemic hypertension should be conducted on an individual basis in dogs with no single drug being the first drug of choice. Hypertension is usually not an emergency in dogs and should be treated gradually, as sudden drops in blood pressure may present dangers to the animal. The goal should be to prevent TOD as a result of the high blood pressure and the priority should also be to treat the underlying cause of the hypertension, although, as in cats, this may not always resolve the high blood pressure (Goy-Thollot et al., 2002). In dogs the use of two or more agents may be necessary against resistant hypertension. Treatment should be based on the underlying root cause; in CKD cases with associated hypertension in dogs, reducing urine protein concentration may be a good parameter of therapeutic progression. Renin-Angiotensin-Aldosterone System (RAAS) inhibitors and calcium channel blockers (CCB) are recommended in general treatment of high blood pressure in dogs. In cases of severe hypertension, the two agents may be combined. However, CCBs should not be used alone due to their mechanism of strongly dilating the afferent renal arterioles, leading to increased hydrostatic pressure in the glomerulus. As ARBs and ACEi’s tend to dilate the efferent arterioles, combined use with CCBs has a positive effect (Acierno et al., 2018). Benazepril, an Angiotensin Converting Enzyme inhibitor (ACEi) has been shown to significantly reduce proteinuria in dogs with chronic kidney disease (King et al., 2017). A tumor in the adrenal cortex may lead to hyperaldosteronism and administration of an Aldosterone Receptor Blocker (ARB) like telmisartan leads to increased diuresis without affecting potassium and creatinine levels, which may help lower blood pressure in such cases (Schierok et al., 2001). Pheochromocytoma (tumor in the adrenal medulla) may lead to increased levels of catecholamines. This can be treated with α- and β- adrenergic blockers. The β-blocker propranolol has been shown to be effective in reduction of blood pressure in hypertensive dogs with pheochromocytoma (Richards, 1966). In dogs, low dose amlodipine at 0.2-1.0 mg/kg/day or hydralazine (1.0-3.0 mg/kg, bid) are consistently effective drug therapies for managing idiopathic systemic hypertension, although primary hypertension is rare. Prazosin at 1.0-4.0 mg total dose, PO, 1-2 times/day or phenoxybenzamine (0.25-2.0 mg/kg, PO, bid) have also had some success in dogs with pheochromocytoma. The usage of sildenafil (1.0-3.0 mg/kg, bid-tid), a phosphodiesterase inhibitor, is probably the most effective drug to reduce clinical signs of pulmonary hypertension in dogs, commonly warranted in cases of right heart failure, reducing syncope and ascites (Gordon et al., 2018). ---------- === Comparative Analysis and Conclusions === Over the last decade significantly more information has been gathered about treatment of systemic hypertension in dogs and cats, and its treatment is becoming more and more common in practice. Nevertheless, a reliable, validated and practical method for regular indirect checks of blood pressure in dogs and cats remains unavailable. The limited techniques currently available do however permit clinicians to establish when the animals in their care enter a hypertensive state with a systolic blood pressure greater than 160 mmHg. Above this level some degree of therapeutic intervention is needed to prevent target organ damage even if the animal is currently asymptomatic. In both cats and dogs, it is critical to establish the underlying etiology and treat any underlying conditions which may be causing secondary hypertension. However, whether the hypertension be primary or secondary, a pharmacological approach has been established for both species to treat the hypertension most effectively with current drugs and knowledge. In cats, amlodipine besylate, a calcium channel blocker has been shown to be the most effective pharmacotherapy, suggesting that hypertension in cats stems physiologically more dominantly from excessive endogenous vasoconstriction; arterial dilation thus is the key goal of therapy in cats and it seems that the cat kidney is able to tolerate better the effects of arteriolar vasodilation than the dog kidney. In dogs, where (a) the etiology of hypertension is more complex, (b) hypertension is usually secondary, and (c) the kidney is more at risk from excessive glomerular pressure damage, calcium channel blockers should not be used alone, particularly in severe cases. Hypertension in dogs appears to stem most significantly from an overstimulation of the Renin-Angiotensin-Aldosterone System. Thus, in comparison to the situation in cats, the first line drug therapy in canines is often the administration of ACE inhibitors, with enalapril and benazepril being most common. Adjunctive therapy with calcium channel blockers like amlodipine or hydralazine can be used in more severe cases to get the animal back to a normotensive state. Further research in this area is required both in the field of blood pressure measurement and in better understanding the best pharmacological approach to treating systemic hypertension in dogs and cats. There remains debate about which drugs are the most effective, and practising veterinarians are advised to treat all hypertensive dogs and cats on a case-by-case basis. Some useful guidelines do exist however, and it is advisable always to determine and treat any underlying causes first, titrate gradually up to an effective management dosage with any drug or drug combination used, and always monitor carefully animals during pharmacotherapy. ---------- === References === 1. Acierno, M.J.; Brown, S.; Coleman, A.E.; Jepson, R.E.; Papich, M.; Stepien, R.L.; Syme, H.M. 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A Comparative Physiological Analysis of Antihypertensive Drug Therapies used in the Treatment of Systemic Hypertension in Cats and Dogs
The student essay has been deleted due to the authors' request.