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AN EXPERIMENTAL SYSTEM FOR ESTIMATION OF BLOOD PRESSURES AND HEMODYNAMICS FROM OSCILLOMETRIC WAVEFORMS. Jiri Jilek, Milan Stork Carditech, Culver City, California, USA. Abstract The article describes an experimental system which noninvasively determines blood pressure, cardiac output, total peripheral resistance, and systemic arterial compliance. Systolic, diastolic and mean arterial pressures are determined by oscillometric method from a wrist cuff. Radial artery waveforms are analyzed to obtain stroke volume. Stroke volume is adjusted for body area and the adjusted value is used to compute cardiac output.Total peripheral resistance and systemic arterial compliance are computed from arterial pressures, cardiac output and stroke volume. The computed blood pressure determinants are displayed on the computer screen in a “quadrant” graphic format. Introduction. Cardiovascular disease (CVD) is the leading cause of death in many countries [1]. An important contributor to CVD is hypertension. The incidence and prevalence of hypertension is very high despite efforts focused on its detection, evaluation and treatment [2]. Clinical hypertension can be characterized by an increase of the ratio of total peripheral resistance (TPR) to flow (cardiac output) [3]. Mean arterial pressure (MAP) is the product of cardiac output and total peripheral resistance given by: MAP = CO * TPR

[ mmHg, L/min, dyn]

(1)

This relationship refers only to the steady phenomena and it does not take into account the fact that blood pressure fluctuates about the mean pressure during the cardiac cycle [4]. The more accurate approach is that arterial pulse pressure (PP) is influenced by systemic arterial compliance (SAC). Pulse pressure (PP) is the difference between systolic blood pressure (SBP) and diastolic blood pressure (DBP) as expressed by (2): PP = SBP – DBP

[mmHg]

(2)

In the arterial system with decreased SAC, pulse pressure is higher for the same stroke volume (SV). The importance of arterial compliance has recently been recognized and its surrogate pulse pressure has been found to be a significant risk factor of mortality in older people [5]. Increased systolic and pulse pressures are characteristic for isolated systolic hypertension. The most common essential hypertension is characterized by increased total peripheral resistance (TPR).

Figure 1. The system with notebook, module and wrist cuff 1

Description of the system The system consists of a compact, battery powered module, a wrist cuff, and a notebook computer as shown in Figure 1. Fully automatic operation of the system is controlled by the computer and a test takes less than one minute. Block diagram of the module and the cuff is in Fig. 2. The module’s microcontroller (Intel

Figure 2. Block diagram of the module with wrist cuff. 87C51) communicates with the notebook via serial interface (Comm). The notebook controls inflation and deflation of the cuff and acquisition of data. Operation of the system starts with cuff inflation to about 30 mmHg above expected SBP. Cuff pressure is converted to analog voltage by pressure sensor (piezoresistive bridge type, range 0-250). The analog voltage is amplified by an instrumentation amplifier (Burr-Brown INA118) and filtered by a low-pass filter with cutoff frequency of 35 Hz. The pressure voltage is digitized by a 12-bit A/D converter with serial output (MAX1247). The A/D converter operation is controlled by the microcontroller. Sampling rate is 11.6 mS. The deflation of the cuff is controlled by a current controlled air-flow valve (Omron 608) . Valve is open when current=0, closed when current= 22 mA.Valve control is accomplished by 8 resistors (2.2 k each) are connected to port 0 of 87C51 (8 bits). The other “ends”of the resistors are tied together and connected to “minus” side of valve. This configuration forms a simple digital-to-current converter. “Plus” side of valve is connected to plus 5 volt power supply. Deflation rate is controlled by notebook software. Cuff pressure drop (CPD) is computed at 300 mS intervals. Heart rate factor (HRF) is computed from time intervals between pulse peaks. HRF=constant/interval. HRF causes the deflation rate to speed-up when heart rate is high and slow down when heart rate is low. The valve control is also dependent on the pressure drop CPD. Control byte (CB) is computed according to CB=HRF/CPD*CONSTANT. Control byte is sent to the module at 300 mS intervals. The microcontroller only passes the CB value to the port 0. Higher value of CB (less current) causes air flow to increase and lower value decreases the flow. This air-flow control linearizes the cuff deflation rate to about 4-6 mmHg/sec. When cuff pressure drops below diastolic pressure, the valve opens and the cuff is rapidly deflated. Computation of blood pressures and hemodynamics takes place next. Oscillometric mean arterial pressure (MAP) is computed first. Mean pressure in the cuff corresponds to the largest amplitude of pressure pulses [7]. Heart rate is computed from the peak –to-peak intervals between individual pressure pulses. Systolic and diastolic pressures are computed next. Algorithmic procedure based on the rate of change of pulse waveform amplitudes is used [8]. The arterial pulse waveforms are then analyzed to obtain stroke volume. Since the stroke volume is not obtained by estimating actual left ventricular volumes, the stroke volume computed from radial artery must be adjusted for body surface area (BSA).Height, weight and 2

age of the tested subject is manually entered and the stroke volume is adjusted according to the formula (3). BSA = (weight + height – 60) / 100 [m2, kg, cm] (3) Cardiac output is then computed by multiplying stroke volume by heart rate: CO = SV * HR

[L/min, mL, bpm]

(4)

Total peripheral resistance (TPR) is obtained by dividing mean arterial pressure by cardiac output: TPR=80 * MAP/CO

[dyn , mmHg, L/mi]

(5)

Systemic arterial compliance (SAC) is computed according to the formula (5): SAC = SV/PP

[mL, mL, mmHg]

(6)

This measure of compliance was used because both of the variables used (SV, PP) are already available. Moreover, pulse pressure is now widely recognized as a surrogate measure of arterial compliance [9]. The computed blood pressure and hemodynamic variables are displayed on the computer screen as numeric values and as a “quadrant” graphic format (Fig.3). The quadrant shows the relationships of cardiac output (CO), total peripheral resistance (TPR), and systemic arterial compliance (SAC). TPR and SAC are graphically represented by small rectangles and they move together on the vertical (CO) axis according to the value of CO. TPR and SAC rectangles are positioned on the horizontal axis according to their values. Higher SAC and lower TPR values move the rectangles to the right. Normal values of TPR and SAC are displayed graphically in the right half of the quadrant. Abnormal values (usually accompanied by hypertension) are located in the left half.

Figure 3. Graphic and numeric results of a “normal” test. The values displayed in Figure 3 are typical values of a normotensive, middle-age male. TPR and SAC values are graphically represented in the right “good” half of the quadrant. Figure 4. shows hemodynamic values corresponding to chronic hypertension in an elderly woman. Blood pressures are elevated, cardiac output is within normal range and total peripheral resistance (TPR) is high. Systemic arterial compliance (SAC) is substantially reduced.

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Figure 4. Test results of a hypertensive woman. Both TPR and SAC are graphically represented in the left “bad” half of the quadrant. Summary. A simple, noninvasive system for determination of blood pressures, cardiac output, total peripheral resistance, and systemic arterial compliance was described. The ease of use and low cost could make the system useful in many applications where other methods of measuring hemodynamics may not be justified in terms of cost and complexity of operation. References: [1] Lenfant, C. High Blood Pressure: Some Answers, New questions, Continuing Challenges. JAMA 1996;275:1604. [2] Perry, H.M. et al. Difficulties in diagnosing hypertension: Implications and alternatives. J Hypertension 1992;10:887-96 [3] Laragh, J.H., Brenner, B.M. Hypertension: Pathophysiology, Diagnosis and Management, Raven Press, 1995. [4] Safar, M.E. et al. Artertial alterations in hypertension with disproportionate increase in systolic over diastolic blood pressure. J Hypertension 1996 14: S78-79. [5] Glynn, R.J. et al. Pulse Pressure and Mortality in Older People. Arch Intern Med 2000;160: 2765-2772. [6] Petrin, J. et al. Increased beta-adrenergic tone enhances arterial compliance in hyperkinetic borderline hypertension. Journal of Hypertension 1989;7:S78-9. [7] Geddes, L.A. Characterization of the oscillometric method for measuring indirect blood pressure. Annals of biomedical engineering 1982;10:271-280. [8] Ng K.G. Blood pressure measurement. Medical Electronics Feb. 1999:61-65. [9] Arnett, D.K. Arterial Stiffness: A New Cardiovascular Risk Factor? Am J Epid Oct 1994:669

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