Figure 13.1 – Blood pressure measurement.

13.1.1 Definition

Blood pressure (BP) and cardiovascular disease risk have a continuous relationship, so it is impossible to precisely define hypertension. However, for practical purposes, hypertension is when arterial blood pressure reaches levels which significantly increase cardiovascular risk; and when treatment can provide a clear-cut benefit. Hypertension is usually diagnosed by measuring the average BP from multiple readings. This will be explained further below. A consistently high BP over 140/90 is considered as hypertension.

13.1.2 Epidemiology

• 31% of males and 28% of females over the age of 35 have, or are being treated for hypertension.
  
• This prevalence increases with age:
  
  
  
  • in those aged 45–54 years: 33% of males, 25% of females
    
  • in those aged >75 years: 66% males, 78% females

GUIDELINES: Diagnosis of hypertension (NICE, 2011)

NICE employs the following definitions in its guidelines:

Stage 1 hypertension:

• Clinic blood pressure (CBP) = 140/90 mmHg or higher, and

• Ambulatory blood pressure monitoring (ABPM) average or home blood pressure monitoring (HBPM) average = 135/85 mmHg or higher

Stage 2 hypertension:

• CBP = 160/100 mmHg or higher, and

• ABPM/HBPM average = 150/95 or higher

Severe hypertension:

• Clinic systolic blood pressure = 180 mmHg or higher, or

• Clinic diastolic blood pressure = 110 mmHg or higher

13.1.3 Aetiology

95% of cases are classified as ‘essential’ – i.e. the underlying cause has not been identified. The other 5% of causes are classified as ‘secondary’, which can be due to the following:

• Renal disease
  
  
  
  • diabetic nephropathy, renovascular disease, glomerulonephritis, vasculitides, chronic pyelonephritis, polycystic kidneys
  
  
• Endocrine disease
  
  
  
  • Conn’s and Cushing’s syndromes, glucocorticoid remediable hypertension, phaeochromocytoma, acromegaly, hyperparathyroidism
  
  
• Other
  
  
  
  • aortic coarctation, pregnancy-induced hypertension and pre-eclampsia, obesity, excessive dietary salt, drugs (e.g. NSAIDs, sympathomimetics, illicit stimulants such as amphetamine, MDMA, and cocaine).

13.1.4 Pathophysiology

Normal blood pressure regulation

There are four main physiological mechanisms in the human body which regulate blood pressure:

• Cardiac contractility and pumping pressure
  
• Blood vessel tone and systemic resistance
  
• Intravascular volume regulation, controlled by the kidneys
  
• Hormones – which control the other three mechanisms (refer to Chapter 1)

These mechanisms form the process of ‘pressure natriuresis’ in normal functioning kidneys.

Blood pressure regulation in hypertensive patients

An increase in BP leads to an increase in urine production and sodium excretion, in an attempt to reduce intravascular volume and return arterial pressure to normal. Pressure natriuresis is less efficacious in hypertensive patients because of:

• Microvascular and tubulointerstitial kidney damage due to hypertension
  
• Defects in hormonal regulation of blood pressure such as the renin–angiotensin system (refer to Chapter 1).
  
  
  
  • hormonal regulation anomalies are the target of a host of antihypertensive medications, including ACE inhibitors and diuretics (refer to Chapter 5).

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