Hypertension

What is Hypertension?

Hypertension is increased blood pressure. Two measurements indicate what the blood pressure is. The systolic pressure results from contraction of the left ventricle of the heart, forcing blood into the aorta and out into its branches that form the systemic arterial circulation. The pressure wave of this contraction is measured peripherally. The diastolic pressure results from relaxation of the left ventricle of the heart, and the pressure diminishes to a level sustained by the residual pressure retained by the elasticity of the arteial system.

There can be considerable variation in blood pressure between persons. The average adult blood pressure is around 120/80 mmHg, as measured by a sphygmomanometer with blood pressure cuff around the upper arm while sitting. In general, a sustained diastolic pressure >90 mmHg and a sustained systolic pressure >140 mmHg define hypertension.

Hypertension is a silent disease. It is insidious and relentless. The only reliable way to detect hypertension is to regularly check blood pressure. This should be done as part of a physical exam on every adult.

If hypertension is not treated, there will be organ damage to kidneys, heart, and brain which is generally not reversible. Death in persons with hypertension most often occurs from heart failure, chronic renal failure, and stroke.

Causes for Hypertension

Over 90% of the time, an identifiable cause for hypertension cannot be found. This is known as "primary" or "essential" hypertension. The onset is typically in middle age. Some factors that may contribute to primary hypertension include:

• Genetics: persons whose parents had hypertension are more likely to be hypertensive themselves.

• Diet: more salt (sodium chloride) in the diet promotes increasing blood pressure.

• Stress: native peoples of the world are far less likely to develop hypertension than persons living in cities of developed nations.

• Vascular alteration: over time, hypertension results in thickening of small muscular arteries and arterioles, which makes them less responsive to vasodilators.

Less than 10% of the time, hypertension has an identfiable underlying cause, though this does not necessarily mean that recognition will provide a cure for hypertension. Causes for hypertension may include:

1. Renal Diseases: just about any renal disease leading potentially to renal failure can result in hypertension. Such diseases can include:

   • Diabetic nephropathy

   • Glomerulonephritis

   • Renal vascular diseases (renal artery stenosis, fibromuscular dysplasia, vasculitis)

   • Dominant polycystic kidney disease

   • Renal cell carcinoma

2. Endocrine Diseases:

   • Cushing's syndrome with increased cortisol

   • Pheochromocytoma, with increased catecholamines (tends to be episodic)

   • Aldosterone secreting neoplasm (adrenal cortical adenoma)

3. Neurogenic Causes: such as increased intracranial pressure (tends to be of sudden onset)

4. Vascular Diseases:

   • Aortic coarctation

   • Vasculitis (such as polyarteritis nodosa)

   • Fibromuscular dysplasia of renal arteries

Regulation of Blood Pressure

The kidney contains many mechanisms to control blood pressure. When the glomerular filtration rate (GFR) drops, the stretch receptors in the macula densa signal cells of the juxtaglomerular apparatus to secrete renin.

Renin is converted to angiotensin, which effects vasoconstriction, mainly in peripheral arterioles, which increases peripheral vascular resistance, thereby elevating blood pressure.

In addition, renin stimulates release of aldosterone by adrenal cortical cells in the glomerulosa. Aldosterone exerts an effect on the distal renal tubules, causing them to increase sodium reabsorption while secreting potassium.

Another factor in blood pressure control is atrial natriuretic factor released from the atria of the heart, which senses filling of blood. Increased volume, and subsequent increased filling, results in release of this factor, which inhibits sodium reabsorption at the distal renal tubule.

Consequences of Hypertension

1. Renal Disease: the renal vasculature shows changes with hypertension.

   • "Benign" nephrosclerosis: modest elevations in blood pressure over the years result in thickening of small renal arteries and arterioles, known as hyaline arteriolosclerosis. This vascular disease leads to formation of small cortical scars, with reduction in renal size.

   • "Malignant" nephrosclerosis: in a small number of persons with previously mild hypertension, or as the initial event, there is a marked rise in blood pressure. Diastolic pressure may exceed 120 to 150 mmHg. The changes seen in arterioles may include:

     • Fibrinoid necrosis

     • Necrotizing arteriolitis

     • Hyperplastic arteriolosclerosis

2. Heart Disease: the pressure load placed on the left ventricle results in left ventricular hypertrophy. The heart enlarges and dilates, with hypertrophy more marked than dilation, until the left heart begins to fail, particularly when the heart reaches 500 gm in size. Congestive heart failure and cardiac arrhythmias may result from the failing heart.

3. CNS Disease: the effect of hypertension on small arteries and arterioles in the brain is to cause thickening and loss of resilience. This hypertensive hyalinization may produce occlusion with resultant small lacunar infarcts, or "lacunes" that appear most commonly in the region of the basal ganglia, internal capsule, thalamus, basis pontis, and hemispheric white matter. This arteriolar sclerosis also results in in vessels that are more prone to rupture. The most common site for rupture is the region of the basal ganglia. The hypertensive hemorrhage that results from rupture is one of the causes for a "stroke".