(1)
Department of Cardiovascular, Neural and Metabolic Sciences, Istituto Auxologico Italiano, Milan, Italy
3.1 Blood Pressure Variability
“Blood pressure variability” refers to the spontaneous instability of arterial blood pressure values over short- and long-term periods of time. The easiest way to evaluate blood pressure changes is to assess the size of the standard deviation relative to a set of data. Consequently, the standard deviations of systolic, diastolic, mean arterial blood pressure, and pulse pressure values can be analyzed separately.
Blood pressure variability consists of different components, which can be assessed over time intervals of different widths and with different methods:
- 1.
Long-term visit-to-visit blood pressure variability, assessed with repeated office blood pressure measurements or home blood pressure monitoring
- 2.
Mid-term blood pressure variability, assessed day-by-day using home blood pressure monitoring
- 3.
Short-term blood pressure variability (assessed over a 24-hour period), using a 24-hour ambulatory blood pressure monitoring (ABPM)
The standard deviation of the mean of the values obtained by multiple measurements over 24 hours reflects short-lasting blood pressure changes. It is also strongly influenced by the amount of day–night blood pressure changes. This is the reason why additional more specific indices of short-term blood pressure variability, able to avoid the contribution to overall variance of nocturnal blood pressure fall, may be computed. They may refer to:
- (a)
The standard deviation of daytime blood pressure
- (b)
The standard deviation of nighttime blood pressure
- (c)
“Weighted” 24-hour blood pressure standard deviation, defined as the mean of daytime and nighttime blood pressure standard deviation weighted by the duration (in number of hours) of each time period
- (d)
Average real variability (ARV) of 24-hour blood pressure, defined as the average of the absolute differences of consecutive measurements. ARV better predicts cardiovascular risk in hypertensive patients in comparison with the traditional standard deviation of short-term blood pressure variability
- (a)
- 4.
Very short-term blood pressure variability, assessed by means of continuous beat-to-beat blood pressure recordings.
Each of these blood pressure variability components has been related to the development or the progression of hypertensive target organ damage and to an increased risk for cardiovascular events and mortality.
3.2 Blood Pressure Variability and Cardiovascular Risk
Hypertension is considered to be the major risk factor for cardiovascular events and mortality. We have already seen that blood pressure has two distinct but interdependent components:
- 1.
Mean arterial pressure, depending on cardiac output and systemic vascular resistance, and
- 2.
Pulse pressure, depending mainly on the viscoelastic properties of large elastic arteries, which represents the fluctuation in pressure values around the mean value of blood pressure.
We can consider blood pressure variability as a third component characterizing blood pressure properties (Fig. 3.1).
Fig. 3.1
The three components characterizing blood pressure properties. (a) Mean arterial pressure, the steady component. (b) Pulse pressure represents the fluctuation in pressure values around the mean value of blood pressure. (c) Blood pressure variability represents spontaneous oscillations of mean arterial pressure and pulse pressures over short-term and long-term periods
These three components of blood pressure must be considered in the clinical assessment of patients. None of them must be overlooked. Let us try to better understand the importance of them. Suppose you live in Venice (Table 3.1). Venice is a wonderful Italian city, which is built on water and is situated on the Adriatic Sea. The streets of Venice are made of canals, and cars are, therefore, replaced by boats and motor-boats. Residents in Venice have always had a very close relationship with the sea and when sea level rises, life is difficult there.
Table 3.1
Similarities between the factors affecting blood pressure values and those affecting sea level in Venice
Sea | Blood pressure | |
|---|---|---|
Steady condition | Tide | Mean arterial pressure |
Dynamic condition | Wave motion | Pulse pressure |
Variable dynamic condition | Change in the width of waves coming from boats | Blood pressure variability |
If you live in Venice, you must, therefore, be aware of tides, i.e., the average sea-level, and you cannot ignore either wave motion, i.e., the vivacity and width of the waves traveling to town, or the waves produced by motorboats, steamboats, or cruising ships. Similarly, in the clinical assessment of our patients, we cannot only take into account mean arterial pressure value (tide), but we must also consider the dynamic variation in blood pressure values with respect to mean arterial pressure value (wave motion) and daily changes in mean and pulse pressures (wave motion created by motorboats).
Actually, cardiovascular risk is not only related to an increase in arterial pressure values but also to a degree of short-term blood pressure variability, unrelated to average values of systolic or diastolic blood pressure. The extent of fluctuations of blood pressure over time may provide additional, independent prognostic information compared with both isolated office readings and average ambulatory blood pressure levels, respectively. Prospective studies have clearly demonstrated that short-term blood pressure variability is an independent predictor of the progression of subclinical organ damage, cardiovascular events, and cardiovascular mortality [1–3]. These clinical studies support the concept that the adverse cardiovascular consequences of high blood pressure depend not only on mean blood pressure values but also on fluctuation of blood pressure values.
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