Blood pressure should be monitored via direct or indirect methods. The minimum goal is to maintain organ perfusion by maintaining a mean arterial blood pressure >60 mmHg (systolic >90 mmHg); however, a normal blood pressure is considered an ideal goal. In hypotensive animals with adequate cardiac function, treatment consists of intravascular volume infusion, oxygen administration, and pain control. Hypotension unresponsive to intravascular volume replacement can be due to one or more of a variety of causes: hypoglycemia, acidosis, alkalosis, electrolyte disorders (eg, potassium, calcium, magnesium), brain-stem pathology, cardiac arrhythmias, metabolic toxins (eg, hepatic, renal), ongoing fluid loss, relative hypoadrenocorticism (eg, cortisol deficiency), heart or pericardial disease, excessive vasodilation, and excessive vasoconstriction. Patient assessment for these causes should be performed and immediately addressed. (Also see Assessment of Resuscitation Efforts in Animals.)
The need for cardiovascular support with positive inotropes or vasopressors is considered when the above listed causes are ruled out. An experienced ultrasonographer may be able to assess ventricular contractility and/or capacitance vessel size to provide an estimate of preload. Once intravascular volume (central venous pressure >8 cm H2O (although this is measured less commonly) and cardiac function are assessed as adequate, vasopressor therapy with CRI of dopamine (5–15 mcg/kg/minute), norepinephrine (0.05–2 mcg/kg/minute), epinephrine, or other pressor agents is instituted. A CRI begins at the lower end of the dosage range and increased incrementally until blood pressure goals are reached. Objective measurements of global perfusion may also include lactate monitoring; animals with a significantly increased lactate concentration may have a poorer prognosis. Studies have demonstrated that serial lactate monitoring is more useful than a single measurement. Central venous oxygen measurement is another objective measurement of global perfusion; normal values are 70–80 mm Hg, whereas lower values may indicate increased oxygen extraction.
Hypertension is not a common condition in veterinary medicine, but it can lead to catastrophic problems. The American College of Veterinary Internal Medicine classifies risk of target-organ damage from hypertension into four categories based on systolic blood pressure:
I: <150 mmhg="minimal">
II: 150–159 mmHg = mild risk
III: 160–179 mmHg = moderate risk
IV: >180 mmHg = severe risk
Hypertension can lead to retinal detachment or to neurologic derangements from intracranial hemorrhage and can exacerbate proteinuria in animals with chronic kidney disease. Moderate to severe hypertension can be treated with oral antihypertensive agents such as angiotensin-converting enzyme inhibitors (eg, benazepril) in nonazotemic animals, calcium channel blockers (eg, amlodipine), direct arterial dilators (eg, hydralazine), or systemic injectable antihypertensive agents such as nitroprusside (0.5–10 mcg/kg/minute), titrated to effect. Blood pressure must be monitored continuously to assess response to therapy. Chronic hypertension that is rapidly decreased may result in decreased renal perfusion; the goal should be to decrease blood pressure by no more than 25% in the acute setting.