Casual and inaccurate measurements of a continuous fluctuating variable: the original sin of the concept of arterial hypertension?

Blood pressure (BP) and its progressive increase with age are among the main biological phenomena that affect the longevity of human life. As a result of high BP, arterial hypertension constitutes the most frequent cause of preventable premature death in humans ^[1].

Elevation of BP is a strong predictor of cardiovascular events, and the higher the BP, the greater the risk of complications ^[2].

BP management helps reduce complications. Its relative benefits are proportional to the decrease in BP figures and are independent of the level of BP or risk at the initiation of treatment. Evidence supporting this notion is summarized in the 2021 Blood Pressure Lowering Treatment Trialists Collaboration meta-analysis ^[3].

BP is a clinical parameter with constant fluctuations and may vary based on circadian or seasonal rhythms.

Increased BP variability is another significant risk factor for hypertension, mortality, cerebrovascular disease, and cardiovascular and renal diseases ^[4]. It is also significant in explaining the progressive damage in cognitive function ^[5] and probably left ventricular hypertrophy and arterial stiffness ^[6].

Seasonal BP variability is not well understood. Some studies have shown that there may be a slightly high BP during the fall and winter months ^[7], whereas other studies have not found any significant seasonal variations ^[8]; there are also reports of an inverse seasonal variation: higher BP levels in summer than in winter ^[9].

This issue of the journal reports the study titled: ‘Seasonal variation in blood pressure control across US health systems,’ conducted among 1 818 041 people with hypertension, from a national electronic health record-based BP surveillance system ^[10]. The authors analyzed seasonal variation in BP control and its association with outdoor temperature using state-of-the-art statistical techniques and concluded that BP control improved during spring/summer months compared to fall/winter months. This study provides information not only on the seasonal variability of BP but also on an extensive sample showing that BP control also varies with seasonal rhythm. This is the only or at least the most extensive study on the seasonal variability of arterial hypertension treatment in the United States.

One of the main issues with clinical research on hyper-tension, in Nilles et al. ^[10] that is commented upon herein, as with most studies published in the twentieth century, that BP was measured in the 20th century with a nonhomogenous technique in the physician's office. These were based on one or a few time point estimates of a continuous variable, obtained randomly, with an inaccurate measurement method, and minimal to no quality control.

Inaccuracy in the daily measurement of pressure is not a futility, imprecisions of just 5 mmHg correspond to an incorrect classification of hypertension in 84 million people worldwide ^[11].

In the 2005 AHA guidelines for BP management, emphasis was placed on the quality control of the instruments used and trimmed on the pressure measurement technique or the crucial need to repeat the measurement several times ^[12]. The validation of BP-measuring devices began in the 1980s with a series of ad-hoc validation protocols ^[13]. The oscillometric technique has been used in trials in the past 20–30 years after algorithms to estimate BP were developed and refined ^[14].

If BP is a continuous fluctuating variable, how can it be defined with only one reference point? BP varies, but it does not do so chaotically, primarily within tracks with maximum and minimum limits. These lanes move to higher levels with age. When these upper displacements happen in a sustained and faster way, the process we call arterial hypertension is happening.

We might think that when several BP points measured with reasonable accuracy are within the limits of a track, we can characterize the entire lane with just a few points. This could explain the reasonable correlation between the punctual measurement of hypertension and the reality of a continuous and variable process.

By taking casual and inaccurate measurements, we can still obtain a general idea of how a variable changes over time; thus, such measurements can help identify potential trends or changes.

Moreover, another concept that could explain why isolated measurements of BP have helped predict the risk imposed by this disease would be the fact that the nature of hypertensive damage does not depend on transient elevations in BP but on the accumulated load of inadequate pressure, defined as the percentage of abnormally elevated BP readings over time (the higher the pressure and the longer the time, the greater the load and therefore the greater the damage). This load integrates a growing mass of damage, which triggers the clinical complications of arterial hypertension (acute or chronic coronary syndromes, stroke, kidney damage, heart failure, cognitive impairment) when it reaches a critical level. The relative importance of this accumulated mass of pressure load is given by its interaction with the environment of other risk factors.

Thus, the parameter that would be used to qualify the importance of hypertension would be the time out of the target range ^[15]. However, this depends on the same BP measurement technique and requires information on the time of the start of the process, which is very difficult to obtain.

The casual measurement of BP, due to its accessibility, low cost, and tradition of use, cannot be ruled out as the primary method to characterize the increase in pressure figures in daily life.

Soon, the scenery of uncertainty we have described regarding the quality of BP measurement and its changes over time will only be part of history. The casual and inaccurate measurement of pressure, in the form of solo shots, will be replaced by wearable monitors (watches, belts, etc.), which will yield continuous measurements, which instead of being tens or hundreds as up to now, there will be trillions of figures, in periods that can include up to the whole of life and that will be analyzed with the ‘big data’ technique and can be clearly understood with the help of ‘Augmented Intelligence’ and ‘Learning Machines,’ we will then have a piece of factual information on the characteristics the progressive increase in BP, which constitutes the essence of the complex biology that we call arterial hypertension, and we will be able to assess for example, the real importance of BP variability and its control.

In the meantime, occasional measurements with time-honored cuff BP alongside protocolized home measurements and ambulatory BP measurements are preferred. The current guidelines recommend these measurement methods over certified quality equipment and standardized measurement techniques when making decisions about diagnosing and treating arterial hypertension ^[16].

ACKNOWLEDGEMENTS

Conflicts of interest

There are no conflicts of interest.