Introduction

Approximately 45% of patients admitted to the hospital become dehydrated at admission.¹ Dehydration can cause several complications for pre-existing conditions that individuals may already be experiencing difficulties with. Pulse pressure has demonstrated superiority as a predictor of cardiovascular disease and death. You’re going to find out how these two are linked and the relevance of this within clinical practice.

Understanding pulse pressure

Pulse pressure can provide valuable information to healthcare professionals about the health of your arteries. Arteries are blood vessels that carry oxygen-rich blood around the body to the tissues and organs.

Pulse pressure is the difference between the systolic and diastolic blood pressure readings. The systolic blood pressure (the upper reading) represents the maximum pressure experienced in the aorta when the heart contracts to pump blood through to the body. The diastolic blood pressure (the lower reading) represents the minimum pressure in the aorta when the heart is relaxing before the next contraction.²

These pressures are measured in millimetres of mercury represented as mmHg. This is because blood pressure measuring equipment used to contain mercury, so the measurements were represented in this way. Modern-day equipment no longer uses mercury; however, the measurements have remained in this form.

Calculation of pulse pressure

To calculate your pulse pressure, you must subtract the diastolic from the systolic reading. For example, in blood pressure of 120/80mmHg, the calculation would be 120-80=40mmHg. The normal range for pulse pressure is approximately 40mmHg.

Pulse pressure and cardiovascular health

Arteries consist of three layers. The inner layer is called the tunica intima, the middle layer is the tunica media which contains the smooth muscle and elastic cells and the outermost layer is known as tunica externa, containing collagen cells. These combined layers and cells make arteries flexible to adequately carry the volumes of blood being pumped out of your heart. This is known as arterial compliance.

As we age, we lose collagen, and this includes those cells within our artery walls. This can lead to arterial stiffness. You may have heard of this being described as the hardening of your arteries. This means the arteries are not as flexible or able to cope with the volume and flow of blood being pumped out of the heart. As a result, when the heart contracts to pump the blood out to the body, there is a greater pressure required to force the blood through the hardened arteries. This increases the systolic blood pressure and decreases the diastolic, which leads to a widened pulse pressure. Studies have shown that an increase of just 10mmHg in pulse pressure is associated with a 20% increased risk of cardiovascular disease.³

Dehydration – causes and effects

Dehydration occurs when you are losing more fluid than you are taking into your body. Your body is composed of approximately 75% of water and maintaining this is vital to our survival.⁴ Dehydration can occur from not drinking adequate amounts of fluid. The NHS recommends you should drink 6-8 glasses of fluid each day, equating to roughly 2 litres.⁵

Loss of fluid can result in dehydration if not adequately replaced. This could occur when you are unwell with vomiting or diarrhoea, excessively sweating due to heat exposure or prolonged exertional exercise and can be the result of taking certain medications called diuretics (water tablets).

Symptoms of dehydration

Some of the symptoms of dehydration include:

• Headache
• Concentrated urine (dark yellow)
• Feeling thirsty
• Feeling light-headed or dizzy
• Dry mouth
• Feeling tired and lethargic

Signs of dehydration:

• Dry skin, less elasticity
• Dry or cracked lips
• Low blood pressure
• Low urine output

Impact of dehydration on cardiovascular function

When there is insufficient fluid circulating in the body it can have a negative impact on your normal cardiovascular functioning. Fluid is required to contribute to overall blood volume circulating through your body, when there is less fluid it means there will be less blood in circulation. This can lead to low blood pressure. As a result, the heart may begin to increase the number of times it beats per minute in an attempt to compensate for the loss of volume and to try to get more blood circulating to the vital tissues and organs that require it. This inevitably puts a strain on the heart the longer this state is maintained.

Dehydration has been demonstrated to increase the thickness of blood including an increase in factors including haematocrit and fibrinogen.⁶ A loss of fluid and an increase in these factors could lead to the development of blood clots causing heart attacks and strokes.

Relationship between pulse pressure and dehydration

As previously mentioned, inadequate levels of fluid in your circulating blood lead to a lower blood volume causing hypotension, or low blood pressure. When your blood pressure remains low, your body has a mechanism to compensate for this to maintain an adequate circulation of blood. Your body stimulates your kidneys to produce arginine vasopressin, also known as antidiuretic hormone. This hormone acts by increasing the amount of water that is reabsorbed in the kidneys in an attempt to increase blood volume, thus increasing arterial blood pressure.⁷

Evidence of correlation

Studies have examined the effect of low blood volume, also known as hypovolaemia on pulse pressure. Hypovolaemia has been linked with a narrowing in pulse pressure and demonstrated the activation of the arginine vasopressin response.⁷ This response appears to be able to continue for a number of hours, however as with most things it is unable to sustain this state permanently.⁸ Narrowed pulse pressures are linked with cardiovascular conditions where there are reduced volumes of blood or the ability to pump the blood such as heart failure, blood loss and aortic stenosis.²
Narrowed pulse pressures have been linked with cardiac dysautonomia conditions. These have an effect on blood pressure regulation, heart rate and maintaining balance when you are doing activity or standing upright. These can cause symptoms of fainting and irregular heart rhythms.⁹

Clinical considerations and management

It is important to consider the potential implications of an obscure pulse pressure when carrying out clinical assessment and management of an individual. Several studies have shown abnormal pulse pressure to be a predictor of cardiovascular events.¹⁰ A number of studies have found pulse pressure to be a more accurate predictor of cardiovascular death than monitoring of systolic blood pressure alone.^11,12

So incorporating pulse pressure measurements into clinical practice may prove beneficial in managing at-risk patients.

Assessment

The easiest way to get an idea of pulse pressure is through measurement of the systolic and diastolic blood pressure. This is the most readily available option in clinical practice; however, this is not the most accurate method as the readings are taken away from the centre of the body where the most accurate readings can be obtained.¹³

 Other methods include:

• Pulse wave velocity – this is the measurement of the pulse wave as it moves through the circulatory system, noting the time it takes to pass between two sections of the vessels.
• Ultrasound indices – this involves taking multiple images of the artery walls to create a calculation of the maximum and minimum areas of the vessel to give an idea of arterial compliance and distensibility.
• MRI–derived indices – this scan is able to give similar information to the ultrasound scan in the form of images to produce calculations on the ability of the vessels to respond to the pressures and volumes of fluid circulating.

Clinical management of dehydration

In identifying dehydration, it is important to commence treatment as soon as possible.

Firstly by identifying the cause of the dehydration. This could be a loss of fluids through bleeding from an injury or internal cause such as vomiting or diarrhoea. It is important to treat the underlying cause and to focus on fluid replacement.

 The treatment options for managing dehydration include:

• Encouraging increased intake of water or other types of fluids, where possible
• Replacement of electrolytes that may have become depleted through dehydration such as potassium and sodium
• Intravenous or subcutaneous replacement of fluids in acutely unwell individuals.
• Consideration of individuals with heart failure or kidney disease when replacing fluids to reduce the risk of overloading these vital organs.
• Close monitoring of individuals whilst re-hydrating

Summary

Understanding dehydration and its potential effects on individuals is an important consideration within clinical practice. Prompt, safe treatment of dehydration is of paramount importance. However, firstly understanding and treating the underlying cause of dehydration should be considered.
The measurement of pulse pressure in clinical practice may give further insight into the artery's ability to adapt to varying blood volumes. However, further research is required to consolidate this.

FAQs

What can I do to manage my pulse pressure?

1. Having reviews with your healthcare professional is the first port of call. A yearly check-up of your blood pressure and discussing relevant risk factors is important to monitor.
2. Ensuring you maintain a healthy lifestyle including regular exercise, refraining from smoking and eating a healthy well-balanced diet can contribute to maintaining cardiovascular health.
3. If your healthcare provider has prescribed medications for you, it is important to follow the instructions they have given you and be compliant with your medication regime. If you have any concerns or are uncertain of any medicines, consult your medical professional for advice.