Biophysics and Biomechanics - V. S. Antoniuk - 2012

Chapter 4. BIOPHYSICS OF COMPLEX BIOLOGICAL SYSTEMS

4.6. Biophysics of the Circulatory System

4.6.3. Blood Flow Through Vessels. Arterial Blood Pressure and Methods for Its Determination

ARTERIAL Blood PRESSURE (BP) refers to the pressure exerted by blood on the arterial walls. The level of arterial pressure depends on the volume of blood entering The Vascular System per unit of time, the capacity of the vascular system, the tension of the arterial walls, and blood viscosity.

Throughout the cardiac cycle, blood pressure in the Arteries varies cyclically, reaching its maximum during Heart contraction and blood ejection into the aorta (systole). Minimum pressure corresponds to the period of heart relaxation (diastole). As blood moves through the vascular bed, the amplitude of pressure fluctuations decreases, and venous and capillary pressures become largely independent of the cardiac cycle phase. Therefore, several types of blood pressure are distinguished.

Minimum or diastolic pressure is the lowest blood pressure in an artery prior to the end of the diastolic period. Its value primarily depends on the patency of the precapillary bed and The rate of blood outflow through it. The greater the resistance of the precapillary system (i.e., the higher the arteriolar tone), the higher the minimum pressure must be. To a lesser extent, the level of minimum pressure depends on heart rate and the Elastic properties of large arterial vessels. The lower The Heart rate, the longer the diastole duration, and the more blood flows from the Arterial System into the Venous system, resulting in a drop in minimum pressure. The lower the visco-elastic state of the walls

of large arteries, the greater the capacity of the arterial system and the higher the minimum pressure.

Mean dynamic pressure is the resultant of all fluctuating pressure values throughout a single cardiac cycle. This type of pressure is not simply the arithmetic mean of the maximum and minimum pressures, but is closer to the minimum pressure. Mathematically, it is the integral or the average of infinitely small pressure changes over one cardiac cycle (N. N. Savitsky). Since Other types of pressure represent transient pressure levels within the artery, mean dynamic arterial pressure exhibits a certain stability. Blood Flow through arterioles and capillaries is driven by the mean arterial pressure, meaning that mean pressure determines the energy of continuous blood flow from the arterial system into the venous system.

Lateral (purely systolic) pressure is the pressure exerted on the lateral wall of an artery during ventricular systole.

Maximum or systolic pressure is the value that determines the entire energy reserve of the moving blood Column during systole. Maximum pressure consists of lateral and impact pressure—that is, the pressure generated when an obstacle appears in the path of the blood flow within the artery (e.g., when compressing the artery with a cuff).

Impact pressure, or hemodynamic Shock, determines the kinetic energy of the moving blood stream.

The difference between maximum and minimum pressure values is called pulse pressure. However, true pulse pressure should be considered the difference between lateral and minimum pressure.

Invasive (direct) measurement method. The invasive (direct) method of BP measurement is used exclusively in hospital settings during surgical interventions, when inserting a probe with a pressure transducer into the patient's artery is required for continuous monitoring of pressure levels. The advantage of this method is that pressure is measured continuously and displayed as a pressure-to-time curve. However, patients undergoing invasive BP monitoring must be closely observed to prevent severe Hemorrhage in the event of

probe disconnection, hematoma formation, thrombosis at the puncture site, or infectious complications. Non-Invasive Methods of BP determination have become much more widely adopted in clinical practice.

Non-invasive measurement methods. In routine medical practice, non-invasive methods of BP measurement are the most common and are classified According to the underlying principle used to determine BP. These include Methods based on Palpation, arterial Auscultation, and oscillometric recording.

Palpatory method. This method involves the gradual compression or decompression of a limb or tissue over an arterial segment and the palpation of the artery distal to the occlusion site. One of the earliest devices (S. von Basch, 1876) consisted of a small Water-filled rubber bulb connected by a tube to a manometer. The bulb was placed over the limb's artery and compressed in a controlled manner using the palm. Simultaneously, the artery beyond the applied bulb was palpated. The pressure at which arterial pulsation disappeared was determined as the systolic pressure. A convenient compression cuff combined with a vertical mercury manometer for the palpatory method was proposed by the Italian physician Riva-Rocci (1896).

The Riva-Rocci cuff was only 4...5 cm wide, which allowed BP to be elevated up to 30 mmHg. Five years later, this inadequate cuff was replaced by F. von Recklinghausen's cuff, which was 12 cm wide—a design that remains in use today. The pressure in the cuff was raised until the pulse completely disappeared, and then gradually lowered. Systolic blood pressure (SBP) was determined by the cuff pressure at the onset of the pulse, while diastolic blood pressure (DBP) was evaluated at the moments when pulse amplitude began to drop noticeably or an apparent pulse acceleration was perceived.

Auscultatory method. On November 8, 1905, at a meeting of the Scientific Assembly of the Military Clinical Hospital, surgeon Nikolai S. Korotkov presented his report "On the Selection/31.html">Methods of Studying blood pressure." This presentation marked the begin-

ning of the auscultatory method for non-invasive BP measurement, which remains the most widespread and reliable method in clinical practice.

The method has not undergone substantial changes in over 100 years of Structure/182.html">Practical Application. The Classification of the sounds, known as Korotkoff sounds, has evolved: The first phase consists of their appearance; the second, their muffling (sometimes disappearing entirely, known as an auscultatory gap) and replacement by compression murmurs; the third, the intensification of the sounds; the fourth, their abrupt muffling; and the fifth, their complete cessation (sometimes due to The phenomenon of an "infinite tone").

A typical device for measuring BP using N. S. Korotkoff's method (sphygmomanometer, or tonometer) consists of an inflatable occlusion cuff, a rubber bulb for air inflation with an adjustable release valve, and a device that measures pressure within the cuff. The measuring device may be a mercury manometer—The production of which is being phased out globally to prevent mercury poisoning and for environmental safety (and has been discontinued in Russia for several years)—an aneroid (dial) manometer, or an electronic manometer. To ensure accurate readings, the scale of the dial or mercury manometer should be positioned at eye level to minimize reading errors.

Pressure values are rounded to the nearest even number. Rounding to 5 and 0 (i.e., fixed recordings such as 145/95 or 160/100) is strictly prohibited. Like all measuring instruments, these devices require careful handling and regular inspection (accuracy checks and calibration) performed by specialized metrological organizations. The intervals between inspections are specified in the device documentation, but they must not exceed one year. If there is any doubt regarding the accuracy of an aneroid manometer's readings between regular inspections, these readings should be compared against those of other regularly verified devices.

The cuff is applied to the upper arm. It is not recommended to place it over clothing, and rolling up sleeves tightly to form constrictive bands of fabric is strictly forbidden. Both overly loose and excessively tight cuff placement must be avoided. The distance from the lower edge of the cuff to the elbow crease should be 3...4 cm (for children, 2...3 cm), while the clearance between the upper edge and the shoulder should be 3...4 cm.

The dimensions of the cuff (most notably the width and length of the internal inflatable bladder) must correspond to the circumference of the upper arm: the length should be at least 80%, and the width approximately 40% of the arm circumference. The bladder dimensions of a standard adult medium arm cuff are 13×24 cm, which is suitable only for arm circumferences between 22 and 33 cm. For a large proportion of the adult population, upper arm circumferences significantly exceed 32 cm, and using standard cuffs leads to a substantial overestimation of BP values. Conversely, using such cuffs on arms with a circumference of less than 22 cm results in underestimated BP readings. Specialized cuffs are required for children and for measuring BP on the legs. A complete set of occlusion cuffs consists of 5–7 types. The pressure in the cuff is rapidly inflated to a level exceeding the SBP by 30 mmHg. To estimate SBP during compression, the radial artery is palpated. It should be noted that excessively high cuff pressure can cause additional pain and an elevation in BP.

The rate of decompression is 2–3 mmHg per second or per the time interval between successive heartbeats (if the pressure exceeds 200 mmHg, an increase in the decompression rate up to 4.5 mmHg is permissible).

Auscultation is performed using either a stethoscope (preferably) or a membrane phonendoscope (including those adapted for BP measurement and supplied with tonometers). The sensitive HEAD is placed on the lower edge of the cuff over the PROJECTION OF THE brachial artery (the position is determined beforehand by palpation and adjusted to ensure maximum sound intensity). The head should be secured with devices that do not exert excessive pressure on the Skin. Securing it with significant pressure, as well as placing the head under the cuff, primarily distorts DBP.

During decompression, SBP is determined at the moment Korotkoff sounds appear (the first phase of the sounds). If determining SBP is difficult—for instance, in the case of an auscultatory gap (a sharp attenuation and disappearance of sounds after Hearing the first two or three distinct sounds) or low sound intensity—the moment blood begins to flow under the cuff is also recorded. In complex cases, ultrasonic Doppler devices can be used. A pronounced auscultatory gap may lead to an underestimation of SBP if the researcher relies solely on regular sounds.

Diastolic blood pressure is, in the vast majority of cases, determined by the disappearance of Korotkoff sounds (the fifth phase). To confirm the complete disappearance of sounds, auscultation must be continued while lowering the cuff pressure by 10–20 mmHg down to the last sound heard.

Determining DBP by the fourth phase of the sounds (the moment of their abrupt muffling) is recommended when measuring BP in children under 12 years of age, pregnant women, and patients with high Cardiac Output caused by physical exertion, disease, or physiological characteristics. One should switch to the fourth phase when a clearly defined fifth phase—the phenomenon of the "infinite tone"—is absent.

Measuring BP in patients with Cardiac Arrhythmias is extremely difficult. It is necessary to palpate the radial artery to assess the irregularity of heart contractions during the measurement. In cases of rare extrasystoles, it is advisable to repeat the measurement and rely on the BP values obtained during a regular rhythm. In cases of frequent extrasystoles and atrial fibrillation, one should rely on the average BP values from 4–6 consecutive measurements.

Blood pressure can be measured in sitting, lying, and standing positions; however, in all cases, the arm must be positioned so that the middle

of the cuff is at heart level. This avoids The Effect of the hydrostatic column on the measured BP value. Every 5 cm shift of the middle of the cuff relative to heart level leads to an overestimation (if the arm is lowered) or an underestimation (if the arm is raised) of SBP and DBP by 4 mmHg. The sitting position is the most appropriate for measuring BP in outpatient settings and clinical offices.

Recent studies have shown that adhering to measurement rules increases the reliability of BP values and, consequently, their correlation with target organ damage and disease prognosis. According to the 1999 World Health Organization recommendations, BP measurement using M. S. Korotkoff's method, performed by a trained specialist, is the "gold standard" and can only be supplemented by measurements using automated devices.

Automated devices based on the auscultatory method replicate M. S. Korotkoff's measurement algorithm and, in some cases, employ additional measures to enhance its reliability. Currently, they are used for stress tests and 24-hour ambulatory blood pressure monitoring.

The oscillometric method. The original oscillometric technique by E.-J. Marey (1876) involved placing a human limb into a water plethysmograph, which allowed for creating a controlled compressive pressure around it while simultaneously recording small limb volume pulsations associated with the pulse blood Filling of the arteries. After analyzing how the amplitude of these pulsations depends on the compression pressure, the author proposed criteria for evaluating BP. The compressive pressure (during decompression) at which pulsations begin to rise sharply corresponds to SBP; when they reach maximum values, it corresponds to mean BP; and when they drop sharply, it corresponds to DBP.

The method had two drawbacks: it required special equipment in the form of a plethysmograph and caused difficulties in data interpretation because the characteristic points for SBP and DBP were not clearly distinguishable on all recordings. The first drawback was eliminated by simplifying the plethysmographic

method and, ultimately, replacing the plethysmograph with a standard occlusion cuff. It successfully combined both the external compression device and an arterial pulse sensor that, while not highly precise, was acceptable for BP measurement. Indeed, pulse-induced changes in arterial volume under the cuff are transformed into small pressure oscillations within the occlusion cuff, which are clearly visible even with slight Movements of the aneroid manometer needle or the mercury level in tonometers. A simplified algorithm for determining BP based on cuff pressure data and the amplitude of micro-pressure oscillations in that cuff is shown in Fig. 4.17.

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Fig. 4.17. Simplified algorithm for determining BP using the oscillometric method

The technical challenge of measuring the amplitude of these small oscillations was solved in the 1930s. However, a formalized, objective, and accurate interpretation of the results of measuring small cuff pressure pulse characteristics was not achieved until the 1970s. This problem was resolved thanks to technological progress (primarily in digital microelectronics), which made it possible to apply quite complex digital signal Processing methods for these purposes.

In 1976, Criticon introduced and released the first bedside automated BP monitor (Dinamap 825), which successfully implemented a modified version of E. J. Marey's oscillometric method. When measuring BP using this method, the pressure in the occlusion cuff decreases stepwise—by 6–8 mmHg per step—and at each pressure step, the amplitude of the cuff micro-pulsations arising from the transmission of arterial pulses to the cuff is analyzed.

Since the 1980s, this method has been used in bedside and 24-hour BP monitors, as well as in devices for self-monitoring of BP. Manufacturers continuously improve operating algorithms to enhance the accuracy and reliability of the obtained results. The latest models of home BP monitors feature an increased "intelligence level" (fuzzy logic models). This raises hopes for The Development of devices that are resistant to cardiac arrhythmias and arm movements during measurement. Currently, devices based on the oscillometric method account for about 80% of all automated and semi-automated blood pressure monitors. Among portable 24-hour monitors, this percentage drops to 30% (while auscultatory-based monitors account for 38%, and combined methods for 24%).

Methods for rapid (beat-to-beat) BP measurement. Cyclic BP measurement methods are the most accurate, but they determine BP at intervals of no less than 2–3 minutes (momentarily and at low BP values, this interval can be shortened to 1 min). Certain methods make it possible to increase the promptness of BP monitoring and determine all or some BP parameters during every heartbeat.

In 1969, J. Penaz obtained a patent for a method referred to in English-language literature as the volume-clamp method. It is based on the continuous assessment of finger arterial volume via photoplethysmography and The Use of an electropneumatic system to generate a counter-pressure in a finger cuff that prevents the stretching of arterial walls. To keep the diameter of the digital arteries constant despite fluctuations in their blood pressure, a constant pressure close to zero is maintained in the arteries, and the cuff pressure begins to "track" the blood pressure in the finger arteries. As a result, the device provides a unique

opportunity for continuous non-invasive recording of the entire blood pressure waveform, which was previously achievable only via the invasive Oxford method. The stationary device implementing this method is known as the vascular unloading technique, and a more recently developed variant is the continuous blood pressure monitor. The device features a hydrostatic correction system to compensate for errors arising from different finger positions relative to heart level. Unfortunately, the method is not without significant drawbacks. The measured DBP value is lower than that in the brachial artery, with the correction factor depending on the vasospastic state of the finger arteries. Systolic blood pressure in young subjects is usually higher than in the brachial artery, whereas in older individuals, it is lower. The correction also depends on arterial tone.

Advantages and disadvantages of the auscultatory and oscillometric methods. Advantages of the auscultatory method:

- currently recognized as the official standard for non-invasive BP measurement for diagnostic purposes and validation measurements;

- exhibits enhanced resistance (compared to the oscillometric method) to arm movements, especially when coordinating the analysis of sound phenomena relative to the ECG R-wave, employing two or more microphones, and utilizing complex spectral algorithms for useful signal recognition (for example, the Accutracker-2 device successfully performs about 93% of BP measurements during cycle ergometer stress testing);

- potentially more resistant to cardiac arrhythmias.

Disadvantages of the auscultatory method: sensitive to ambient room noise, the precision of microphone placement relative to the artery, shifting of the microphone cuff on the arm during long-term monitoring, and the requirement for direct contact between the cuff or microphone and the patient's skin. Operational experience shows that the microphone is frequently the most vulnerable component of the device, prone to damage and requiring repairs.

Advantages of the oscillometric method:

- relatively resistant to noise Interference, making it suitable for high-noise environments (such as a helicopter cockpit);

- enables blood pressure determination in cases of pronounced auscultatory gap, weak Korotkoff sounds, or an "endless tone";

- pressure values are independent of cuff rotation on the arm and largely unaffected by its displacement along the limb (provided the cuff does not reach the antecubital fossa);

- allows blood pressure measurement without loss of accuracy through thin clothing fabric;

- ensures a lower percentage of failed measurements during 24-hour ambulatory monitoring compared to the auscultatory method, as proven by operational practice.

Disadvantages of the oscillometric method:

- relatively low resistance to arm movements: for instance, the SL90202 device fails to provide accurate blood pressure measurements during a cycle ergometer test in 82% of readings;

- in a small percentage of patients (about 5%), a persistent discrepancy exists compared to Korotkoff method values, which complicates the Interpretation of Results.

Both methods have proven ineffective in the presence of severe cardiac arrhythmias.

Thus, vascular pressure measurement methods are categorized into invasive and non-invasive. The invasive (direct) blood pressure measurement method is used exclusively in hospital settings during surgical Procedures. Non-invasive methods have become widespread in clinical practice and are classified according to their underlying physical principle. These include methods based on palpation, arterial auscultation, and oscillometric recording.