Causes of Systemic Hypertension
Elevation in blood pressure occurs most often as a secondary phenomenon to pre-existing systemic disease. Renal insufficiency including glomerular as well as interstitial disease is the most common cause of secondary hypertension in dogs and cats (through activation of the renin-angiotensin system and sodium retention). Conversely however, hypertension can also cause renal insufficiency through progressive ischemia. Thus, in many chronic cases, it may be difficult to determine which disease came first. In one survey (Cowgill, UCD), 73% of dogs with renal disease were found to have elevated blood pressure.
Ocular disease associated with systemic hypertension occurs as a result of rupture of small vessels and vascular effusion. There may also be a secondary anterior- or pan-uveitis. Often the ocular signs (acute vision loss and pupillary dilation due to retinal detachment) may be the first symptoms noted by the owner.
Diagnosis of Systemic Hypertension
Measurement of systemic blood pressure may be done directly (through arterial cannulation) or indirectly by measuring the amount of external pressure required to occlude blood flow in an extremity. The Doppler sphygmomanometer is the indirect method of blood pressure measurement used in our practice. The pressure cuff is applied to the tail or a forelimb or hindlimb. The procedure is easily performed with minimal restraint; several repetitive readings are obtained to minimize artifact. Normal systolic pressure is 120 mm Hg; animals exhibiting hypertensive retinopathy often have pressure in excess of 200 mm Hg.
Treatment of Systemic Hypertension
Successful therapy requires an overall assessment of the patient’s general health and identification of the underlying condition. If renal function is normal, systemic diuretics are often successful in causing resorption of the subretinal fluid with retinal re-attachment. As the treatment for renal failure (rehydration, increased salt intake) and hypertension (diuretics, reduced salt intake) are in direct opposition to each other, treatment of renal-induced hypertension is most challenging. Therapy usually involves a combination of drugs, carefully titrated for each patient.
Drugs Used to Treat Systemic Hypertension
|Drug||Dosage||Mechanism of Action|
|Amlodipine||0.625 mg SID||calcium channel blocker|
|Hydrochlorothiazide||1.0-2.0 mg/kg||diuretic, decreases extracellular fluid volume|
|Propranolol||0.5-2.0 mg/kg||b-adrenergic blocker, decreases renin, HR, CO|
|Captopril||0.5-1.0 mg/kg||inhibits angiotensin converting enzyme|
|Prazosin||1.0-2.0 mg/kg||a-adrenergic blocker, vasodilator, decreases TPR|
|Hydralazine||0.5-1.0 mg/kg||calcium channel blocker, vasodilator, decreases TPR, increases HR|
|Prednisone||0.5-1.0 mg/kg||decreases vascular effusion|