Low blood pressure

Introduction

It is appropriate at this point in our series on hypertension to consider the issue of low blood pressure. While this condition has not attracted enough attention from the medical world [1], recent information suggests that the total cardiovascular risk in these patients is increased [2]. The adage, “the lower, the better”, most likely is not fully applicable to blood pressure in man.

Blood pressure regulation

Blood pressure is regulated by several mechanisms, among which the baroreceptors play an important role. The goal of the baroreceptors is to adapt blood pressure to the needs of the body by acting on haemodynamic parameters such as cardiac output, local and systemic peripheral resistance and circulating blood volume. Changes in any of these can quickly influence blood pressure followed by a variety of consequences, with the final goal being to ensure satisfactory perfusion of vital organs such as the brain, coronary and renal systems.

In relation to low blood pressure, an important role is played by body position. When we move from the supine to the sitting or standing position, a substantial amount of blood is displaced to the lower parts of the body (abdomen, limbs) that can lead to a blood pressure fall. However, due to the rapid action of the baroreceptors in adapting the vasoconstrictor tone and heart rate, blood pressure is kept largely constant; in patients having a less active baroreceptor function, blood pressure falls can lead to syncope and are accompanied by several other unpleasant complications. The wider implications of syncope are addressed in a recent European Society of Cardiology guideline [3]. In the present article, however, we will focus on low blood pressure itself, reviewing the definition of low blood pressure as well as looking at its causes, prevention and treatment.

Definitions

There are different presentations of low blood pressure: some patients show a blood pressure drop specifically while shifting to standing; others have permanently low figures. The former is generally called “orthostatic hypotension” (OH), while the latter is called “constitutional hypotension”. Here we will focus primarily on OH, defined by a blood pressure decrease of 20 mmHg systolic and/or a diastolic pressure of 10 mmHg within three minutes of standing [4].

This definition has shortcomings. In many patients the blood pressure drop may be rapid with a quick recovery to normal levels, making it difficult to record the blood pressure correctly with regular blood pressure monitors. Also, it is the general experience that frail patients have greater difficulties in standing for more than one minute. Therefore, in a more recent paper, it was proposed that a drop of 15/7 mmHg when shifting from sitting to standing might be sufficient to make the diagnosis [5]. Besides, 24-hour recordings often do not provide additional information because pressure drops do not occur every day and, even when they do occur, they are too short to allow for proper recording.  

It is probably true that measurement on a tilting table is a better technique; even then, to get the blood pressure drop defined as precisely as possible, invasive blood pressure recordings are advised. Obviously, this is not applicable in routine cardiology practice.

Prevalence, risk and causes

There are no definitive or generally accepted figures on prevalence. Constitutional hypotension is seen more often in younger patients while OH is observed more frequently in elderly patients.

Risk is increased, especially in OH. The risk for syncope along with the danger of falling – with all its consequences – is the “immediate” risk in these patients, as fractures often occur in these conditions. Also, the impact on quality of life should not be underestimated.

On top of these, there is, according to several papers, a clear increase in cardiovascular risk in OH patients. Remarkably, this increased risk is also present in patients who are totally asymptomatic. There seems to be a link to heart failure and atrial fibrillation [6], maybe to dementia as well. In the recent ARIC study [2], an increased prevalence of stroke and coronary artery disease was documented in all age classes. On the other hand, it should be mentioned here that this increase in risk has not been confirmed in all studies and that the age of many patients obviously by itself is a major and thoroughly realistic factor in explaining the increased risk.

In relation to causes (Table 1), there is, besides age, a close link with diabetes in patients with OH. Even antihypertensive drugs interfering with the alpha-adrenergic receptors can cause OH. Some neurological syndromes are accompanied by OH such as pure autonomic failure or multiple system atrophy (Shy-Drager syndrome). 

Table 1. Causes of orthostatic hypotension (classified by prevalence).

Triggers leading to syncope

OH patients are highly sensitive to hot temperatures or volume changes (for instance, those caused by diuretics). Patients tend to forget about their problems and do not take all the necessary precautions when standing up too quickly. A typical example is when standing up at night to visit the toilet. In today’s social context, “standing” receptions are a usual scenario for syncope or fainting, with the association of standing, advanced age, some alcohol, high temperatures and some emotional aspects bringing together all the necessary triggers to develop fainting. Organisers of standing receptions are thus advised to have a number of chairs available to permit sensitive and elderly patients to sit, preventing, in this simple way, quite unpleasant medical events.

Treatment

It should be underlined that, before starting a specific treatment for low blood pressure, all other reasons that could possibly be implicated in blood pressure decrease should be detected and treated such as haemorrhaging or low cardiac output, etc.

Teaching your patients what to do when feeling faint or weak

Patients should be instructed that they can effectively help themselves in the prevention of fainting and syncope. A number of easy manoeuvres are useful in case the patient feels that fainting may occur. All of these act on either mobilising volume from the lower parts of the body or stimulating pressure receptors leading to vasoconstriction. Repeated rising on your heels mobilises blood volume in the lower part of the body due to compression of the veins in the limbs by the contracting muscles; this volume is sent to the heart and can increase cardiac output very quickly. Isometric handgrips (for instance, by compressing a small rubber ball) can jump start a quickly acting reflex resulting in sympathetic vasoconstriction and, as a result, a blood pressure rise. Obviously, especially when a pre/syncope is beginning, sitting or lying down in a cooler room can provide substantial additional help.

Non-pharmacological treatment of low blood pressure

For many patients, these methods are helpful both in preventing fainting as well as in correcting the whole 24-hour blood pressure profile. At night, tilting the bed so that the patient is in a “head-up” position is quite effective in this respect though, remarkably, the mechanism of this positive effect is not fully understood. Because many OH patients are, in fact, also hypertensive during the day, tilting the bed may prevent a further blood pressure rise during the night. A similar beneficial effect could be exerted by drinking a glass of ice-cold water prior to going to bed. Most likely, the cold drink causes venoconstriction improving venous return to the heart. Tilt training has been suggested as well, allowing neuro-cardiogenic mechanisms to achieve a better balance [7].

OH patients as well as patients with constitutional hypotension can benefit immensely from wearing strong compression stockings. Such stockings seem to counteract exaggerated venous pooling in the limbs, something which is especially useful in patients with venous disease. In very advanced conditions, compression of the limbs, including the lower part of the abdomen, might be necessary [8].

In patients with constitutional hypotension, an increase of salt intake can be suggested, though this is often not well accepted as many of these patients dislike the taste of salt. Obviously, great care should be taken with an increase of salt intake in patients having hypertension combined with OH episodes.

Pharmacological treatment

Many drugs have been proposed for the prevention and control of low blood pressure episodes in conditions such as OH or for increasing blood pressure in patients with constitutional hypotension. Hard proof of the efficacy of these pharmacological treatments is often lacking. For many, the whole 24-hour pressure may be increased, which could be useful in patients with constitutional hypotension, but much less so in OH patients, especially when, besides the OH episodes, blood pressure is high rather than low. Also, one needs to be careful, as mentioned above, because OH patients are often quite sensitive to a variety of stimuli such as volume and temperature changes, vasodilators and diuretics.

In these patients, fludrocortisone acetate is seen to have a favourable effect on the sodium/potassium balance and, in this fashion, lead to fluid retention and increases in central volume. As a consequence of this action, blood pressure can increase. While this increase is often rather small, it is sufficient to offer a better quality of life in constitutional hypotension; however, in OH patients, it is often not enough to help them substantially. The eventual exaggerated loss of potassium should also be carefully controlled.

A long series of substances/drugs has been used in treating OH with varying success such as ephedrine, caffeine, etilefrine, dihydroergotamine, or midodrine. In the majority of cases they act by arterial or even venous constriction. For midodrine, favourable clinical responses were described and accepted as such by the 2018 ESC guidelines [3]. However, for several of the other drugs listed above, a beneficial clinical effect is hard to prove.

Recently, interest has been focused on droxidopa, a precursor of noradrenaline. Encouraging results have been published [9], but these should be explored further and especially confirmed at long term.

In a number of patients, the orthostatic changes are linked to drugs given for accompanying conditions including hypertension, systemic and neurologic disorders. Often, in such cases, the problems can be resolved by stopping or reducing the dosage of the responsible drugs.

From all of the above, it should be clear that there is still room for more studies on the processes involved in low blood pressure as well as its management.

Conclusions and summary

Several papers have shown that OH is accompanied by a higher cardiovascular risk. OH is generally defined by a decrease of 20 mmHg systolic and/or 10 mmHg diastolic within three minutes of standing. This definition is, however, difficult to employ. The pressure drop is often short; it requires measurements in the supine and standing positions and is often missed in 24-hour pressure recordings. 

For preventing fainting, simple manoeuvres (heel raising, handgrips) can help. To start treatment of OH, non-pharmacological drugs should be preferred. Patients should also be aware that the pressure drop often occurs when getting up at night.

Mineralocorticoids can be helpful; droxidopa shows promising results. For many other drugs, results are variable.