8th March 2024

Hypertension

Hypertension

Background

Hypertension is a complex multifactorial disorder, affecting around 1.1 billion people worldwide. It is estimated that globally 1 in 4 men and 1 in 5 women have raised blood pressure. Failure to adequately control hypertension can result in systemic organ dysfunction, including heart failure, nephropathy, retinopathy aneurysms and stroke. 

Given its increasing prevalence, complications, and wide range of therapeutic options, hypertension is frequently tested in MRCP written exams. Candidates are expected to choose appropriate management options for specific population groups and recognise the side effects of commonly used antihypertensive medications.

Principles of blood pressure haemodynamics 

Blood pressure is a product of cardiac output (CO), defined as the volume of blood the heart pumps per minute, and systemic vascular resistance (SVR), which refers to the resistance required to create flow through the circulatory systems. SVR is largely determined by the vascular tone, which itself is under neuronal and hormonal control. The average blood pressure throughout one cardiac cycle can be denoted by the mean arterial pressure (MAP). 

MAP is calculated as follows: 

Taken together, we thus generate the following equation for blood pressure: 

MAP = CO x SVR 

Cardiac output itself is a product of heart rate (HR) and stroke volume (SV), which is defined by the volume of blood pumped by the heart in one cycle.

Therefore, our equation for mean blood pressure can be expanded further to: 

MAP = HR x SV x SVR  

It follows that any condition which raises the cardiac output by causing an elevation in heart rate, stroke volume or an increase in systemic vascular resistance will subsequently lead to a rise in blood pressure. 

Whilst there is far more to haemodynamic principles of blood pressure than outlined above, for the purpose of MRCP written examination, which often tests clinical application, this level of understanding should suffice for candidates to build on the clinical features and management of hypertension. 

Pathophysiology 

Risk factors for hypertension can be categorised as modifiable and non-modifiable. Modifiable risk factors include salt content in diet, obesity, smoking and alcohol, whereas non-modifiable risk factors include age, ethnicity and genetic predisposition. Surprisingly, only 5% of cases of hypertension can be attributed to an underlying cause. For the remaining 95%, the cause is unclear, and these patients are deemed to have Essential hypertension.  Table 1 outlines the common secondary causes of hypertension and their proposed mechanisms.

Table 1. Common causes of secondary hypertension with their proposed mechanism. This table is not exhaustive but rather covers the common conditions encountered in general medicine which results in raised blood pressure.

Clinical manifestations 

Patients may present with varying symptoms depending on how poorly controlled their hypertension is. Many patients may be asymptomatic. However, some may present with shortness of breath, palpitations, chest pain, transient ischemic attack or stroke, encephalopathy, haematuria, visual disturbances, and pulsatile tinnitus.  

Diagnosing hypertension 

NICE recommends that blood pressure is measured in a relaxed, temperature setting with the person quiet, seated with their arms outstretched and supported. Whilst most of these factors can be addressed in the clinic, it is natural for some patients to have raised blood pressure in the presence of a medical professional and this is known as white coat hypertension. When blood pressure is raised between 140/90 mmHg and 180/120 mmHg, further ambulatory blood pressure monitoring (ABPM) should be offered to confirm the diagnosis. ABPM involves wearing a device which measures blood pressure over a 24-hour period. If this is unsuitable or the person is unable to tolerate it, home blood pressure monitoring (HBPM) is advised where the patient takes their own reading, ideally twice a day for a week. 

If the clinic blood pressure reading is >180/120 mmHg, then referral for same-day specialist assessment is warranted in the presence of retinal haemorrhage/papilledema, acute kidney injury, new confusion, chest pain or signs of heart failure.  If there are no signs to indicate same day referral, then further investigation to identify possible target organ damage should be conducted. 

These cut-offs and guidelines can get confusing, therefore the image below outlines the algorithm for diagnosing hypertension, which some readers may find easier to digest.  

Image Source: NICE (2019). visual-summary-pdf-6899919517, Licensed under a creative commons licence. Available for free at: https://www.nice.org.uk/guidance/ng136/resources/visual-summary-pdf-6899919517 (Accessed: 26 January 2024).

Antihypertensives 

Multiple pharmacological agents are licensed for managing hypertension and are included in Table 2. These agents exert their blood pressure lowering properties either through acting on the RAAS pathway or directly impacting cardiac contractility and the vasculature. Several more agents are currently under investigation for management of hypertension; for further details, it is worth reading the comprehensive review of current and emerging antihypertensives by Ojha et al in the American Journal of Cardiovascular Drugs.

Table 2. Commonly used antihypertensive agents categorised by class. Common side effects, contraindications and notable comments are also outlined.

Management 

Management of hypertension is outlined in the image below

Whether a patient should progress to the next line of antihypertensive medications can be determined by observing if they are meeting the blood pressure targets outlined in Table 3. 

Table 3. Blood pressure targets

Hypertension in pregnancy                                                                      

Blood pressure tends to fall in the first trimester and continues to decrease until 20-24 weeks. Following this period, blood pressure starts to rise again and goes back to pre-pregnancy levels by term.  A patient may have pre-existing hypertension, defined as blood pressure > 140/90 mmHg before 20 weeks gestation. Some patients may develop pregnancy induced hypertension or gestational hypertension. This is defined as blood pressure of >140/90 mmHg after 20 weeks. Finally, Pre-eclampsia is defined when gestational hypertension occurs in combination with proteinuria of > 0.3g/24 hours. Patients at high risk of pre-eclampsia (those with hypertension in previous pregnancy, renal impairment, autoimmune disorders or diabetes mellitus) are given Aspirin 75mg once a day from 12 weeks until birth. It is recommended to commence pharmacological agents when blood pressure is > 160/110 mmHg. Commonly, Labetalol is used, however Nifedipine and hydralazine may also be administered.

References

• Hypertension. WHO 2022. Available from: https://www.who.int/health-topics/hypertension#tab=tab_1
• How should I diagnose hypertension?  Available from: https://cks.nice.org.uk/topics/hypertension/diagnosis/diagnosis/ 
• Ojha U, Ruddaraju S, Sabapathy N, Ravindran V, Worapongsatitaya P, Haq J, Mohammed R, Patel V. Current and Emerging Classes of Pharmacological Agents for the Management of Hypertension. Am J Cardiovasc Drugs. 2021 Dec 8:1–15