Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-04-07
2002-06-11
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S485000, C600S490000, C600S496000
Reexamination Certificate
active
06402696
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to measurement of systolic blood pressure and, in particular, it concerns a method for measuring systolic blood pressure using photoplethysmographic (PPG) sensors.
General Background
The assessment of the systolic and the diastolic arterial blood pressure has both physiological and clinical significance, and tremendous efforts have been applied to the development of a reliable noninvasive method for their measurement. Manual sphygmomanometry, which is based on an external cuff and audible detection of Korotkoff sounds, is considered to be the most accurate method, to which other methods should be compared, though it is prone to several sources of error. These include insufficient hearing acuity of the user, the auscultatory gap, and behavioral factors which influence the level of blood pressure, such as the presence of a physician. These errors are avoided when automatic measurement of the blood pressure is performed, but the available automatic noninvasive blood pressure (NIBP) meters and monitors have other sources of error, significantly reducing their accuracy.
Several methods have been suggested for automatic NIBP measurement. The most widely used of these, together accounting for about 96% of all NIBP monitors currently in use, are oscillometry and the auscultatory method. Oscillometry is based on the measurement of the change in the cardiac induced air pressure oscillations in the pressure cuff during cuff deflation after the elevation of the cuff air pressure above the systolic blood pressure. The cuff pressure at which the oscillometric pulse amplitude is maximal is generally regarded as the mean arterial pressure (MAP). The systolic (SBP) and the diastolic (DBP) blood pressures are determined from the envelope of the oscillometric curve using empirical criteria, such as the cuff pressure of maximal (or minimal) slope or of a determined value of the amplitude relative to maximal amplitude. These empirical criteria are the main source of error in oscillometry since they depend on the character of the cuff and since they are not appropriate for all patients. The automatic auscultatory method is also prone to artifacts due to external noise and vibrations.
The accuracy of the available automatic NIBP meters is very low, as can be understood from the standards imposed by the Association for the Advancement of Medical Instrumentation (AAMI) and the British Hypertension Society (BHS). Both standards are based on comparing the automated NIBP meter to manual sphygmomanometry (which is taken as a “gold standard”) for at least 85 subjects. The standards require that the mean difference between the SBP (or DBP) values measured by the sphygmomanometer and the device under examination do not exceed 5 mmHg, and that the standard deviation of that difference should not exceed 8 mmHg. In other words, a device for which 37% of the examinations differ from the reference manual sphygmomanometer by more than 8 mmHg and 5% of the examinations differ from the reference device by more than 16 mmHg is acceptable. Such low accuracy is permitted because the known methods are not capable of providing measurements of higher accuracy.
PPG-Based Techniques
Both the auscultation method and oscillometry are based upon indirect physiological effects such as Korotkoff sounds, whose origin is not clear, or upon empirical parameters of the oscillometric envelope curve.
The volume-oscillometric method, like the oscillometric method, is based on the measurement of the arterial blood volume oscillation changes as a function of the pressure of an external cuff placed over the artery. The difference between the two methods is that in oscillometry the arterial volume oscillations are measured by means of pressure oscillations in the cuff itself, while in volume-oscillometry these oscillations are measured by photoplethysmography (PPG) or by another plethysmographic device placed under the cuff. As a result, in contrast to oscillometry, no oscillations are detected by the volume-oscillometry sensor, which is located at the distal end of the cuff, when the cuff pressure is above the systolic blood pressure. (In oscillometry the pressure in the cuff continues to oscillate even in high cuff pressure due to the impact of the arterial blood on the tissue under the proximal end of the cuff). Hence, volume oscillometry enables the measurement of systolic blood pressure more directly than oscillometry, with no need for empirical formula.
Systolic Blood Pressure Measurement By PPG
In principle, the measurement of systolic blood pressure using a pressure cuff and a PPG sensor is very straightforward. At cuff pressures below the SBP, some blood passes through the arteries under the cuff, producing pulsatile tissue blood volume variations which generate pulses in the PPG signal. Above the SBP, the artery under the cuff collapses, resulting in interruption of the pulsatile blood volume variations.
FIG. 1
illustrates schematically a system for the measurement of the SBP using a pressure cuff
1
and a PPG sensor
2
. A mercury manometer
3
is typically provided for calibration, while primary measurement and control of pressure in the cuff are performed, respectively, by a piezoelectric transducer
4
and a pressure pump (and its electronic control)
5
. These cuff control components, as well as the electronic control
6
of the PPG device are all connected via an A/D card to a computer
7
which analyzes the results.
FIG. 2
shows typical curves of the PPG signal and of the air pressure as a function of time during the decrease in the cuff air pressure. Note the start of the PPG pulses when the air pressure decreases below SBP value.
Although theoretically simple, the actual measurement of SBP by this technique presents significant problems. Firstly, the amplitude of the first PPG pulses immediately after the air pressure falls below the SBP is small, often making it difficult to identify reliably from background noise in the PPG signal. More importantly, it has been found that, in 15-20% of subjects, no pulses are actually detected in the PPG signal until the pressure has decreased beyond the actual SBP (measured by Korotkoff sounds) by as much as 10-20 mmHg. Without in any way limiting the scope of the present invention, this latter problem is believed to be attributable to the mechanical pressure applied by the PPG sensor itself on the arteries underneath. When the cuff air pressure is above SBP, the arteries distal to the cuff drain into the veins so that the arterial blood pressure becomes relatively low and the small pressure exerted by the sensor can make them collapse (close them). In some cases the small blood volume pulses entering the arteries distal to the cuff when the cuff air pressure is just below SBP value cannot open the arteries under the sensor.
Finally, reference is made to U.S. Pat. No. 5,447,161 to Blazek et al. which proposes a method for measuring venous blood pressure, SBP and DBP during a slow increase (4 mmHg/s) in applied cuff pressure. When the applied external pressure is above the venous blood pressure, the veins collapse. As a result, the slow air pressure increase leads to accumulation of much blood in the hand vascular system. As will be detailed below, the slow increase leads to reduction in the distal PPG signal amplitude so that the PPG signal may initially be undetectable when the cuff air pressure decreases to slightly below the SBP. This renders SBP measurement unreliable.
There is therefore a need for a method for measuring arterial systolic blood pressure using a cuff and a PPG sensor which would provide accurate and reliable results.
SUMMARY OF THE INVENTION
The present invention is a method for measuring arterial systolic blood pressure in a subject.
Conceptually, the present invention provides two approaches to overcoming the problem of delayed PPG sensing of cardiac induced pulsatile variations in tissue blood volume just below the systolic blood pressure.
According to a first, particularly simple approach,
Babchenko Anatoly
Nitzan Meir
Rosenfeld Chaim
Friedman Mark M.
Nasser Robert L.
Ninbar Ltd.
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