Low Blood Pressure vs High Blood Pressure
What is High Blood Pressure?
High blood pressure is defined as Systolic blood pressure above 140 mmHg and diastolic blood pressure above 90 mmHg on average of 2 or more readings taken at 2 separate clinic visits. According to Joint National Committee for Prevention, Detection, Evaluation and Treatment of High Blood Pressure (JNC VII), hypertension is classified into four categories.
1. Normal Systolic less than 120 mmHg, Diastolic less than 80 mmHg
2. Pre-hypertension Systolic 120 – 139 mmHg, Diastolic 80-89 mmHg
3. Stage I Systolic 140 – 159 mmHg, Diastolic 90 – 99 mmHg
4. Stage II Systolic above 160 mmHg, Diastolic above 100 mmHg
Hypertension can be subdivided into primary or essential hypertension and secondary hypertension. Essential hypertension has no detectable cause while secondary hypertension has one. Severe hypertension above 180/110 mmHg is of extreme clinical importance. Hypertensive emergency is blood pressure above 180/110 mmHg with new or ongoing end organ damage. Hypertensive urgency is blood pressure above 180/110 mmHg without end organ features. Hypertensive end organ damage may include encephalopathy, hemorrhagic stroke intracranial hemorrhages, myocardial infarction, left ventricular failure, acute pulmonary edema.
Pathogenesis of essential hypertension is extremely complex. Cardiac output, blood volume, blood viscosity, vessel elasticity, innervations, humoral and tissue factors among many affect blood pressure. Most individuals tend to have an elevated blood pressure as they age.
A variety of disorders can result in secondary hypertension. Endocrinological conditions such as acromegaly, hyperthyroidism, hyperaldosteronemia, corticosteroid over-secretion (Cushing’s), pheochromocytoma, renal disorders such as chronic kidney disease, polycystic kidney disease, systemic conditions such as collagen vascular disease, vasculitis can cause secondary hypertension.
Hypertension in pregnancy is another important area. Hypertension, protienurea, and convulsions characterize eclampsia. Eclampsia can result in abruptio placentae, polyhydramnios, fetal compromise and fetal death.
What is Low Blood Pressure?
Low blood pressure can be caused by various mechanisms. Reduction of the volume of blood, dilatation of peripheral blood vessels, and reduction of cardiac output due to heart failure is the main pathophysiological triad. Reduction of blood volume can be caused by sever hemorrhage, excessive renal loss of water due to polyurea, diuresis, water loss due to sever skin diseases and burns. Dilation of peripheral vessels can be due to drugs such as nitrates, beta blockers, calcium channel blockers, reduced sympathetic tone and vagal stimulation.
During pregnancy, there’s a generalized vasodilatation, reduction of blood viscosity and increase of blood volume culminating in a net reduction of blood pressure especially during the first two trimesters. Endocrinological conditions such as hypoaldosteronism, corticosteroid insufficiency can reduce blood pressure.
Diabetes is known to cause low blood pressure especially due to diabetic autonomic neuropathy. Sever hypotension is known as shock. There are different types of shock. Hypovolemic shock is due to reduction of blood volume. Cardiogenic shock is due to reduced ability of the heart to pump blood. Neurogenic shock is due to reduced sympathetic tone or excessive parasympathetic input. Anaphylactic shock is an exaggerated allergic reaction. Severe reduction of blood pressure may reduce organ perfusion resulting in ischemic stroke, myocardial infarction, acute renal failure, bowel ischemia.