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See also CHROMATOGRAPHY; FIBER OPTICS IN MEDICINE; PERIPHERAL VASCULAR NONINVASIVE MEASUREMENTS.

BLOOD PRESSURE MEASUREMENT

CAN ISIK

Electrical Engineering and

Computer Science Department,

Syracuse University Syracuse,

New York

INTRODUCTION

Blood pressure is an important signal in determining the functional integrity of the cardiovascular system. Scientists and physicians have been interested in blood pressure measurement for a long time. The first blood pressure measurement is attributed to Reverend Stephen Hales, who in the early eighteenth century connected water-filled glass tubes in the arteries of animals and correlated their blood pressures to the height of the column of fluid in the tubes. It was not until the early twentieth century that the blood pressure measurement was introduced into clinical medicine, albeit with many limitations.

Blood pressure measurement techniques are generally put into two broad classes: direct and indirect. Direct techniques of blood pressure measurement, which are also known as invasive techniques, involve a catheter to be inserted into the vascular system. The indirect techniques are noninvasive, with improved patient comfort and safety, but at the expense of accuracy. The accuracy gap between the invasive and the noninvasive methods, however, has been narrowing with the increasing computational power available in portable units, which can crunch elaborate signal processing algorithms in a fraction of a second.

During a cardiac cycle, blood pressure goes through changes, which correspond to the contraction and relaxation of the cardiac muscle, with terminology that identifies different aspects of the cycle. The maximum and minimum pressures over a cardiac cycle are called the systolic and diastolic pressures, respectively. The time average of the cardiac pressure over a cycle is called the mean pressure, and the difference between the systolic and diastolic pressures is called the pulse pressure.

Normal blood pressure varies with age, state of health, and other individual conditions. An infant’s typical blood

pressure is 80/50 mmHg (10.66/6.66 kPa) (systolic/diastolic). The normal blood pressure increases gradually and reaches 120/80 (15.99/10.66 kPa) for a young adult. Blood pressure is lower during sleep and during pregnancy. Many people experience higher blood pressures in the medical clinic, a phenomenon called the ‘‘white coat effect.’’ Therefore, the ranges given in Table 1 are used as guidelines rather than as diagnostic facts.

DIRECT TECHNIQUES

The operation of direct measurement techniques can be summarized in very simple terms: They all use a pressure transducer that is coupled to the vascular system through a catheter or cannula that is inserted to a blood vessel, followed by a microcontroller unit with electronics and algorithms for signal conditioning, signal processing, and decision making. There are many advantages of this set of techniques, including:

The pressure is measured very rapidly, usually within one cardiac cycle.

The measurement is done to a very high level of accuracy and repeatability.

The measurement is continuous, resulting in a graph of pressure against time.

The measurement is motion tolerant.

Therefore, the direct techniques are used when it is necessary to accurately monitor patients’ vital signs, for example, during critical care and in the operating room. Although direct techniques have a lot in common, there are differences in the details of various approaches.

Extravascular Transducers

The catheter in this type of device is filled with a saline solution, which transmits the pressure to a chamber that houses the transducer assembly. As a minor disadvantage, this structure affects the measured pressure through the dynamic behavior of the catheter. As the catheter has a known behavior, this effect can be minimized to insignificant levels through computational compensation (1).

Intravascular Transducers

The transducer is at the tip of the catheter in this type of device. Then the measured signal is not affected by the hydraulics of the fluid in the catheter. The catheter diameter is larger in this class of transducers.

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Transducer Technology

A wide spectrum of transducer technologies is available to build either kind of transducer. They include metallic or semiconductor strain is gauges, piezoelectric, variable capacitance, variable inductance, and optical fibers. Appropriate driver and interface circuitry accompanies each technology (2).

Other Applications of Direct Pressure Measurement

Another advantage of direct measurement techniques is that they are not limited to measuring the simple arterial pressure. They can be used to obtain central venous, pulmonary arterial, left atrial, right atrial, femoral arterial, umbilical venous, umbilical arterial, and intracranial pressures by inserting the catheter in the desired site

(3).

Sources of Errors

Direct blood pressure measurement systems have the flexibility of working with a variety of transducers/probes. It is important that the probes are matched with the appropriate compensation algorithm. Most modern equipment does this matching automatically, eliminating the possibility of operator error. An additional source of error occurs when air bubbles get trapped in the catheter. This changes the fluid dynamics of the catheter, causing an unintended mismatch between the catheter and its signal processing algorithm. This may cause distortions in the waveforms and errors in the numeric pressure values extracted from them. It is difficult to recognize this artifact from the waveforms, so it is best to avoid air bubbles in the catheter.

NONINVASIVE (INDIRECT) TECHNIQUES

An overwhelming majority of blood pressure measurements do not require continuous monitoring or extreme accuracy. Therefore, noninvasive techniques are used in most cases, maximizing patient comfort and safety. Currently available devices for noninvasive measurement are

Manual devices: These devices use the auscultatory technique.

Semiautomatic devices: These devices use oscillatory techniques.

Automatic devices: Although most of these devices use oscillatory techniques, some use pulse-wave velocity or plethysmographic methods.

The Auscultatory Technique

In the traditional, manual, indirect measurement system, an occluding cuff is inflated and a stethoscope is used to listen to the sounds made by the blood flow in the arteries, called Korotkov sounds. When the cuff pressure is above the systolic pressure, blood cannot flow, and no sound is heard. When the cuff pressure is below the diastolic pressure, again, no sound is heard. A manometer connected to the cuff is used to identify the pressures where the transi-

Figure 1. Blood pressure waveform, and systolic, diastolic, and mean pressures, from an invasive monitor screen (4).

tions from silence to sound to silence are made. This combination of a cuff, an inflating bulb with a release valve, and a manometer is called a sphygmomanometer and the method an auscultatory technique. Usually, the cuff is placed right above the elbow, elevated to the approximate height of the heart, and the stethoscope is placed over the brachial artery. It is possible to palpate the presence of pulse under the cuff, rather than to use a stethoscope to listen to the sounds. The latter approach works especially well in noisy places where it is hard to hear the heart sounds.

This method has various sources of potential error. Most of these sources are due to misplacement of the cuff, problems with hearing soft sounds, and using the wrong cuff size. Using a small cuff on a large size arm would result in overestimating the blood pressure, and vice versa. Nevertheless, an auscultatory measurement performed by an expert healthcare professional using a clinical grade sphygmomanometer is considered to be the gold standard in noninvasive measurements.

Oscillatory Techniques

Most automatic devices base their blood pressure estimations on the variations in the pressure of the occluding cuff, as the cuff is inflated or deflated. These variations are due to the combination of two effects: the controlled inflation or deflation of the cuff and the effect of the arterial pressure changes under the cuff. The Korotkov sounds are not used in the oscillatory techniques.

The cuff pressure variation data may be collected while the cuff is being inflated or deflated. Furthermore, the inflation or deflation during the data collection may be controlled in a continuous fashion or in a step-wise fashion. This variability gives four different strategies in data collection. Their differences may seem insignificant at first, but they have significant effects on the way a variety of algorithms are designed.

Data in Fig. 2 were collected using an experimental system. The cuff is first rapidly inflated to a value higher than the anticipated systolic pressure, an approximate pressure of 170 mmHg (22.66 kPa) in this case. Then it is deflated in small steps until the cuff pressure is below the anticipated diastolic pressure, 50 mmHg (6.66 kPa). Please note that when the cuff pressure is very high or very low, the arterial blood pressure variations contribute very little to the cuff pressure trajectory. As a matter of fact, the height of those pulses above the cuff pressure baseline is at their maximum when the baseline pressure is equal to the mean arterial pressure (MAP). We demonstrate this in Fig. 3, with a plot of pulses relative to their baseline pressure (pulse-wave amplitude), against their respective baseline cuff pressures. Please note that only

Figure 2. Cuff pressure trajectory when data are collected during step-wise deflation of the cuff (5).

a few of the pulses observed in Fig. 2 are transferred to Fig. 3 to maintain clarity.

Figure 4 shows a cycle of data collected during the continuous inflation of the cuff as well as the pulse-wave amplitude. The pulse-wave amplitude is obtained by subtracting the baseline cuff pressure from the raw pressure data. Next, we will return to the example developed in Figs. 2 and 3 and continue with the estimation of blood pressure values.

It seems trivial to pick the pulse with the tallest height above baseline and to select its baseline pressure to be the MAP. So, for the example at hand, MAP would be just under 100 mmHg (13.33 kPa), as shown in Fig. 5. The systolic and diastolic pressures are then estimated from the MAP using a variety of heuristic rules. A common class of these heuristic rules works as follows. First, the peak values (heights) of the pulse-wave amplitudes are connected to form an envelope. Again, the baseline pressure at the peak of this envelope is the MAP value. Then, the height of the MAP pulse is reduced by a predetermined systolic ratio, and the intersection of this ‘‘systolic height’’ with the envelope to the right of the MAP pulse is selected as the systolic location. The baseline pressure at this location is assigned as the estimate of the systolic pressure, as depicted in Fig. 5. The diastolic pressure is estimated in a similar fashion by using a ratio of its own to arrive at the

Figure 3. Pulse-wave amplitude profile (6).

Figure 4. Cuff pressure trajectory and pulse-wave amplitude when data are collected during continuous inflation of the cuff.

diastolic height and then by finding the corresponding intersection with the envelope to the left of the MAP pulse.

In the example shown in Fig. 5, the systolic ratio and diastolic ratio were arbitrarily selected as 0.5 and 0.7, respectively. In a realistic system, those ratios would be found statistically (using methods such as regression, fuzzy rule-based systems, neural networks, or evolutionary algorithms) to minimize deviations between estimated and actual blood pressure values.

Algorithmic Components of Blood Pressure Measurement

In the earlier measurement units, it was a combination of hardware and software that controlled the various aspects of the automated measurement (or estimation) of blood pressure. With the ever increasing computational power of microcontrollers, all decision making and control are now implemented in software and with more elaborate algorithms. Here are some functions that are included in a measurement system. Please refer to Fig. 6 for a typical organization of such algorithms in a blood pressure measurement system.

Inflation/deflation control: Whether data collection is done during inflation or deflation, continuously

Figure 5. Blood pressure estimation from pulse-wave amplitude profile.

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Inflation/

Deflation Cuff

Control

Algorithm

Management/

Cycle Control

Figure 6. A typical organization of algorithmic components of (oscillatory) blood pressure measurement.

or in steps, there are many challenges to appropriately controlling the air pump. They include maintaining a smooth baseline cuff pressure without filtering out the arterial variations; adjusting the pump speed to variations arising from different cuff sizes, arm sizes, and cuff tightness; and selecting the range of cuff pressures for which data will be collected.

Pulse detection: This is a fundamental part of extracting features from raw cuff pressure data. It becomes especially challenging when conditions such as arrhythmia, or tremors, affect the regularity of pulses. Pattern recognition techniques with features found in time, frequency, or wavelet domains are used to deal with difficult situations.

Blood pressure estimation: The indirect method of measurement is a process of estimating pressures with the use of features extracted from cuff-pressures or other transducer data. This algorithm used to be limited to linear interpolation, as described in the example of Fig. 5. Recently, more elaborate deci- sion-making and modeling tools such as nonlinear regression, neural networks, and fuzzy logic also are being used for this purpose.

Sources of Inaccuracy

Many factors contribute to the inaccuracies in the automated measurement of blood pressure. The following are some of the more significant sources of error:

Sparseness of data: An important design criterion of a blood pressure monitor is to go through a cycle as quickly as possible. However, the faster a device functions, the fewer pulses it will have in a cycle. A cycle time of 1 min would yield about 60–70 pulses, whereas a 20-min cycle would have only 20–23 pulses. The oscillatory techniques are based on collecting cuff pressure due to pulses at baseline pressures that change from above systolic to below diastolic. If we divide a cuff pressure range of about 150–180 mmHg (10.99–23.99 kPa) by the number of pulses in a cycle, we can see that the baseline increment between successive pulses varies from 2 to 3 mmHg (0.26 to 0.39 kPa) in a 1 min cycle to 6 to 9 mmHg (0.79 to 1.19 kPa) in a 20 s cycle. This quantization error affects the accuracy in the estimate of the mean arterial pressure as well as

the shape of the pulse envelope, hence, the accuracy of the systolic and diastolic values. Various curve-fitting and interpolation techniques are used to remedy this problem.

Pulse extraction uncertainty: Whether the baseline cuff pressure is varied continuously or in steps, figuring out where one pulse ends and another one starts is not a trivial matter. An inspection of Fig. 2 will show that many artifacts in the data stream may confuse a pulse extraction algorithm and cause errors in the pulse-wave amplitude profile in Fig. 3. In addition, common factors such as an irregularity in the pulses as in arrhythmia, small wrinkles, or folds in the cuff changing its volume suddenly during data collection, or small movements of the patient may amplify those artifacts. A variety of pattern recognition techniques are employed to improve the accuracy of pulse detection (7).

Motion artifacts: The performance of the oscillatory techniques depends on all measurements during a cycle. Therefore, any error caused by a motion of the patient may affect the accuracy of the blood pressure estimations. A comparative study of six noninvasive devices has found that average percent errors due to motion artifacts may be as high as 39%

(8). Remedies to this source of error may be a combination of three strategies: (1) to identify and compensate for minor artifacts, (2) to identify and discard data that include significant artifacts or to repeat the entire cycle if the estimates are deemed unreliable, and (3) to incorporate features from additional sensors or monitors such as electrocardiogram (EKG) to help identify motion artifacts (8,9).

Other Blood Pressure Measurement Techniques

Oscillometry is by far the most common technique in automatic noninvasive blood pressure measurement. However, other methods are found in commercial units or in units that are being developed. In this section, a few of these methods are summarized and references are given for further information. It should be noted that algorithmic components and sources of inaccuracy presented within the context of oscillatory technique may apply to other automated measurement methods.

Arterial Tonometry. This relatively new technique in blood pressure measurement is inspired by the tonometry devices that were made in the mid-1950s to measure intraocular pressure. The arterial tonometry device is based on a pressure sensor and pneumatic actuator combination, which is placed on the wrist, above the radial artery. When the pressure applied on the artery is adjusted to the appropriate level (called the hold-down pressure), the portion of the artery wall that is facing the actuator is partially flattened. This configuration maximizes the energy transfer between the artery and the sensor, yielding pulses with the highest amplitude. The relative amplitudes of the tonometry pulses are calibrated to the systolic and the diastolic pressures. Tonometry is suitable for continuous monitoring applications. Sensor placement sensiti-

vity, calibration difficulties, and motion sensitivity are problems that need improvement (10,11).

Pulse-Wave Velocity. A pulse wave is generated by the heart as it pumps blood, and it travels ahead of the pumped blood. By solving analytical equations of fluid dynamics, it has been shown that changes in blood pressure heavily depend on changes in pulse-wave velocity. Blood pressure can be continuously calculated from pulse wave velocity, which in turn is calculated from EKG parameters and peripheral pulse wave measured by an SpO2 probe on the finger or toe. This method is suitable for continuous monitoring as well as for detecting sudden changes in blood pressure to trigger an oscillometric cycle (12).

Plethysmographic Methods. In this method, changes in the blood volume during a cardiac cycle are sensed using a light emitter and receiver at the finger. Tissue and blood have different infrared light absorbance characteristics. That is, the tissue is practically transparent to the infrared light, whereas blood is opaque to it. A prototype of a ringlike sensor/signal processor/transmitter combination has been reported (13,14)

DIFFERENT FORMS OF BLOOD PRESSURE MEASUREMENT DEVICES

The techniques, algorithms, and transducers discussed in the previous sections have led to a variety of forms of devices, differentiated by where in the body the measurements are taken, or for what purpose the device is used.

Ambulatory Blood Pressure Monitoring

These portable and wearable devices monitor the patient’s blood pressure over a long period, say for 24 h. While the patient is following her daily routine, the device periodically takes measurements and saves the results. These measurements are later downloaded for analysis by a physician. The first ambulatory devices, introduced in the early 1960s, were rudimentary and used tape recorders to capture the Korotkoff sounds with an occluding cuff. Most current ambulatory devices use the oscillatory technique. As the patient is subjected to repeated blood pressure measurements with an ambulatory device, it is essential to improve motion tolerance, patient comfort, measurement time, and of course overall accuracy of measurement algorithms that are employed in ambulatory monitors (15).

Ambulatory devices have been instrumental in clinical research and practice. Through their use, there have been significant improvements in our understanding of blood pressure dynamics in a variety of physiological and psychological conditions, and concepts such as ‘‘white-coat hypertension,’’ ‘‘episodic hypertension,’’ and ‘‘circadian rhythm of blood pressure’’ (e.g., daytime/nighttime variations of blood pressure) have been investigated and added to the medical lexicon (16).

Wrist Blood Pressure Monitoring

These monitors have smaller cuffs than their upperarmattached counterparts. Hence, they are more compact and

more conducive to self-measurement. It is important that the monitors are held at the heart level for correct measurement. They are popular with the home users but typically less accurate than the full-size arm monitors.

Finger Blood Pressure Monitoring

Finger monitors are not nearly as common as the arm or wrist monitors. The approaches used are auscultatory and plethysmographic.

Semiautomatic Blood Pressure Monitoring

The semiautomatic devices have cuffs that are inflated manually by an attached bulb, like a sphygmomanometer. Once the cuff is inflated, the monitor functions in the same manner as an automatic device, taking cuff-pressure measurements while releasing the pressure in a controlled way. These devices are more economical and have longer battery lives than their fully automated counterparts.

ACCURACY OF BLOOD PRESSURE MEASUREMENT DEVICES

Blood pressure measurement devices play an important role in medicine, as they measure one fundamental vital sign. In addition to this traditional use, noninvasive blood pressure devices, especially the automated ones, have become ubiquitous in the home, regularly used by lay people. Two widely used protocols for testing the accuracy of these devices are those set by the Association for the Advancement of Medical Instrumentation (AAMI), a pass/ fail system published in 1987 and revised in 1993, and the protocols of the British Hypertension Society (BHS), an A– D graded system, established in 1990 and revised in 1993. These protocols describe in detail the process manufacturers should follow in validating the accuracy of their devices. Their numeric accuracy thresholds can be summarized as follows. A device would pass the AAMI protocols if its measurement error has a mean of no >5 mmHg (0.66 kPa) and a standard deviation of no >8 mmHg (1.06 kPa). The BHS protocol would grant a grade of A to a device if in its measurements 60% of the errors are within 5 mmHg, 85% of the errors are within 10 mmHg (1.33 kPa), and 95% within 15 mmHg (1.99 kPa). BHS has progressively less stringent criteria for the grades of B and C, and it assigns a grade D if a device performs worse than C.

The European Society of Hypertension introduced in 2002 the International Protocol for validation of blood pressure measuring devices in adults (17). The working group that developed this protocol had the benefit of analyzing many studies performed according to the AAMI and BHS standards. One of their motivations was to make the validation process simpler, without compromising its ability to assess the quality of a device. They achieved it by simplifying the rules for selecting subjects for the study. Another change was to devise a multistage process that recognized devices with poor accuracy early on. This is a pass/fail process, using performance requirements with multiple error bands.

Whether blood pressure measurement devices are used by professionals or lay people, their accuracy is important.