Vascular Hemodynamics and Blood Pressure




(1)
Department of Cardiovascular, Neural and Metabolic Sciences, Istituto Auxologico Italiano, Milano, Italy

 




1.1 Mean Arterial Pressure


In the study of vascular hemodynamics, the cardiovascular system is usually considered to be a simple hydraulic circuit, composed of a pump (heart) with a rhythmic activity (systole → diastole → systole → diastole → systole…) that pushes a liquid (blood) into a tube (the aorta), which divides over and over again (peripheral arteries → arterioles → capillaries) to be able to reach the farthest extremes (tissues).

This hydraulic circuit is very similar to a simple electric circuit and we have to stress that electrical models are often used to verify cardiovascular hemodynamic phenomena (Fig. 1.1).

A299565_2_En_1_Fig1_HTML.gif


Fig. 1.1
Similarities between a simple electric circuit and the circulatory system. (a) Simple electric circuit: V 1 − V 2 is the potential difference measured across the conductor; I is the current through the conductor; R is the resistance of the conductor. (b) Systemic circulation system model: P 1 − P 2 is the difference in blood pressure values between the extreme points of the systemic circulation; CO stands for cardiac output; SVR stands for systemic vascular resistance

According to Ohm’s law, the potential difference between the extreme points of an electric circuit (ΔV = V 1 − V 2) is defined by multiplying the current (I) by the resistance of the circuit (R).


$$ \Delta V=I\times R. $$

The cardiovascular system can be considered in a similar way, and the law which defines blood pressure comes directly from Ohm’s law:



  • The difference in blood pressure values between the extreme points of the systemic circulation (ΔP = P 1 − P 2) represents the potential difference between the extreme points of an electric circuit (ΔV = V 1 − V 2)


  • CO (cardiac output) represents the current (I)


  • SVR (systemic vascular resistance) represents the resistance of the circuit (R)



$$ \Delta P=\mathrm{C}\mathrm{O}\times \mathrm{S}\mathrm{V}\mathrm{R}. $$

As blood pressure back to the heart is very low, let us consider the pressure value as the value of blood pressure in the ascending aorta (P); therefore, the formula can be simplified by writing:


$$ P=\mathrm{C}\mathrm{O}\times \mathrm{S}\mathrm{V}\mathrm{R}. $$

As cardiac output (CO) is given by multiplying stroke volume (SV) by heart rate (HR), the formula can be rewritten as:


$$ P=\mathrm{S}\mathrm{V}\times \mathrm{H}\mathrm{R}\times \mathrm{S}\mathrm{V}\mathrm{R} $$



$$ \left(\mathrm{blood}\kern0.5em \mathrm{pressure}=\mathrm{stroke}\kern0.5em \mathrm{volume}\times \mathrm{heart}\kern0.5em \mathrm{rate}\times \mathrm{systemic}\kern0.5em \mathrm{vascular}\kern0.5em \mathrm{resistance}\right). $$

However, we must point out that blood pressure values change during the cardiac cycle, so the term “P”, defined by the formula above, refers to mean arterial pressure (MAP). Therefore,1


$$ \mathrm{MAP}=\mathrm{S}\mathrm{V}\times \mathrm{H}\mathrm{R}\times \mathrm{S}\mathrm{V}\mathrm{R}. $$

According to this formula, mean blood pressure values depend on just three parameters: stroke volume, heart rate and systemic vascular resistance (Fig. 1.2). It is important to note that, for decades, both research and clinical application focused their attention on these three factors affecting mean arterial pressure.

A299565_2_En_1_Fig2_HTML.gif


Fig. 1.2
Factors defining mean arterial pressure

Mean arterial pressure is a key parameter and its most important aspect is associated with its relative “stability” in the arterial tree. Mean arterial pressure tends to remain unchanged in the arterial system, from the ascending aorta to peripheral arteries (Fig. 1.3).

A299565_2_En_1_Fig3_HTML.gif


Fig. 1.3
Change in blood pressure values from the center to the periphery of the arterial system

But, are these three parameters (SV, HR, and SVR) sufficient to explain the changes in pressure found in both physiological and pathological conditions?

Let us analyze Fig. 1.4. In this figure, we can see the condition of two patients with very different blood pressure levels. The patient on the left (a) has 80 mmHg diastolic blood pressure and 130 mmHg systolic blood pressure. On the contrary, the patient on the right (b) has 60 mmHg diastolic blood pressure and 160 mmHg systolic blood pressure.

A299565_2_En_1_Fig4_HTML.gif


Fig. 1.4
Example of two subjects with the same mean arterial pressure values: (a) normotensive and (b) patient with isolated systolic hypertension. Waveforms were recorded in the brachial artery

Therefore, patient (a) has blood pressure values within the normal range, while patient (b) is characterized by a condition of true isolated systolic hypertension. However, both of these patients present the same mean arterial pressure value (100 mmHg). They both could have the same values of heart rate, stroke volume, and systemic vascular resistance.

We can conclude that different pressure values can correspond to the same mean arterial pressure value. This example is sufficient to answer the question above. The three parameters: heart rate, stroke volume and systemic vascular resistance, define mean arterial pressure, but they are not sufficient, in themselves, to justify blood pressure values.


1.2 Pulse Pressure


Accurate analysis of cardiovascular hemodynamics cannot ignore that blood pressure has two distinct but interdependent components (Fig. 1.5):

A299565_2_En_1_Fig5_HTML.gif


Fig. 1.5
Mean arterial pressure and pulse pressure. Variation of the section of a large artery during a cardiac cycle is schematized in the upper panel of the figure




  • A steady component, namely, mean arterial pressure.


  • A pulsatile component, defined as pulse pressure (PP), which represents the fluctuation in pressure values around the mean value of arterial pressure.

As we have already seen, the steady component, mean arterial pressure, depends on three factors:


  1. 1.


    Heart rate

     

  2. 2.


    Stroke volume

     

  3. 3.


    Systemic vascular resistance

     

The pulsatile component, pulse pressure, depends on two factors (Fig. 1.6):

A299565_2_En_1_Fig6_HTML.gif


Fig. 1.6
Factors defining blood pressure



  1. 1.


    The blood pressure wave originating from the interaction between the left ventricular ejection activity and the mechanical properties of large arteries (forward wave).

     

  2. 2.


    Reflected waves.

     

However, both the steady and pulsatile components of blood pressure should not be thought of as self-contained areas. In fact, there are a number of interactions between the elements defining mean arterial pressure and the ones defining pulse pressure. These interactions will be analyzed later.


1.3 Systemic Circulation: Only One Engine, Two Pumps


When we think of systemic circulation, we bear in mind a clear-cut scheme. We tend to consider systemic circulation as a pump, the left ventricle, which pumps blood into the aorta intermittently (stroke volume) and, from here, to the whole body arterial system, which conveys blood to peripheral tissues. Then blood is sent back to the heart through the veins.

Now, we are going to see that this is a gross oversimplification of the facts and learn that systemic circulation has not only a pump (the left ventricle), which is active only in the systolic phase, but that systemic circulation is composed of two pumps:


  1. 1.


    The left ventricle, which represents the systolic pump. The left ventricle guarantees the pumping action of the circulating system during the systolic phase of the cardiac cycle

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    Related posts:

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    3. Pulse Wave Velocity and Arterial Stiffness Assessment
    4. Central Blood Pressure: Part 2, Pulse Wave Analysis

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Sep 2, 2017 | Posted by in CARDIOLOGY | Comments Off on Vascular Hemodynamics and Blood Pressure

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