Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-10-04
2002-07-23
Shaver, Kevin (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S493000, C600S495000
Reexamination Certificate
active
06423010
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a blood pressure monitoring device which employs the oscillometric method of determining blood pressure, and a method for improving the performance of such a device in the presence of arrhythmias.
The basis for the oscillometric method of measuring blood pressure is disclosed in U.S. Pat. Nos. 4,349,034 and 4,360,029 both to Ramsey, III the disclosures of which are incorporated herein by reference. Using the technique disclosed by Ramsey, III the oscillometric method of measuring blood pressure involves applying an inflatable cuff around an extremity of a patient's body, such as the a patient's upper arm. The cuff is then inflated to a pressure above the patient's systolic pressure and then incrementally reduced in a series of small steps. A pressure sensor measures the cuff pressure at each step. The sensitivity of the sensor is such that pressure fluctuations within the artery resulting from the beats of the patient's pulse may be detected. These pulses are transferred to the inflated cuff causing slight pressure variations within the cuff which are detected by the pressure sensor. The pressure sensor produces an electrical signal which typically comprises a DC component representing the incremental cuff pressure and a series of small periodic variations associated with the beats of the patient's pulse. These small variations are often referred to as “oscillation complexes” or simply “oscillations”.
A patient's blood pressure may be estimated based on an analysis of these oscillation complexes. After filtering out the DC component and amplifying the signal generated by the cuff pressure sensor, peak pulse amplitudes (PPA) may be determined for each oscillometric complex. The PPA will tend to increase as the cuff pressure is reduced until a peak amplitude is reached. Once this peak has been reached, the PPA will begin to decrease with further reductions in cuff pressure. The peak pulse amplitudes thus form an oscillometric blood pressure envelope for the patient. The cuff pressure at which the oscillations have a maximum value has been found to be representative of the patient's mean arterial pressure (MAP). The systolic and diastolic pressures can be derived either as predetermined fractions of MAP, or by more sophisticated estimating techniques using direct processing of the oscillation complexes.
FIG. 1
shows the basic elements of measuring a patient's blood pressure using the oscillometric method. Three waveforms are shown. Curve A represents the overall cuff pressure of the inflatable cuff, curve B represents an invasive arterial waveform showing the periodic pressure variations corresponding to the patient's pulse, and curve C represents the measured peak pulse amplitudes for the oscillometric complexes associated with each pulse of waveform B. As can be seen, the cuff is first inflated to a maximum pressure
10
, and then reduced in a series of small incremental steps such as steps
12
,
14
, and
16
. Oscillations
18
corresponding to each pulse of the arterial waveform B are measured at each incremental cuff pressure. The PPA of the oscillations increases with each decrement of cuff pressure until the PPA reach a maximum at cuff pressure
14
. The PPA are diminished with every subsequent reduction in cuff pressure. Thus, the cuff pressure at step
14
represents the patient's mean arterial pressure, and the patient's systolic and diastolic pressures can be determined therefrom.
A problem with this method of measuring blood pressure is that blood pressure measurements can be skewed due to artifacts caused by patient motion or by the presence of arrhythmias. Events such as these can adversely affect the peak pulse amplitudes detected by the cuff's pressure sensor, resulting in erroneous blood pressure measurements. The Ramsey, III patents listed above disclose a first technique for rejecting artifacts. There, a plurality of oscillometric complexes are measured at each incremental cuff pressure. Selected parameters such as peak height and time rate of change of successive complexes, and series of complexes are evaluated relative to specific artifact discrimination criteria. Complexes which do not fall within the predefined criteria are rejected and are not used in forming the blood pressure measurement. Thus, as shown in
FIG. 1
, two matched oscillation complexes are measured at each cuff pressure before the cuff pressure is reduced to the next step. This technique works well to reject artifacts due to motion, but it can unduly lengthen the procedure for obtaining a blood pressure measurement in the presence of arrhythmias. In fact, in some cases it can prevent a measurement from being completed.
FIG. 2
shows how the presence of an arrhythmia affects blood pressure measurements using the oscillometric method. The three curves A′, B′ and C′ are identical to those of
FIG. 1
except for the presence of non-sinus beats
22
,
24
which are best seen on the invasive arterial waveform B′. Non-sinus beats
22
,
24
occur prematurely. Therefore, the period between the preceding sinus beats
21
,
23
and the non-sinus beats
22
,
24
will be shorter than the normal period between sinus beats, and the period between the non-sinus beats
22
,
24
and the immediately following sinus beats
25
,
26
will be extended. Furthermore, because the non-sinus beats
22
,
24
occur prematurely, a smaller amount of blood is pumped from the heart than would otherwise be the case and the arterial pressure associated with the non-sinus beats is reduced. As can be seen in
FIG. 2
, the PPA measured from the oscillations
28
,
30
associated with the non-sinus beats
22
,
24
are subsequently reduced. Likewise, because the time between the non-sinus beats
22
,
24
and the next following sinus beats
25
,
26
is extended, beats
25
,
26
pump a greater amount of blood than normal sinus beats and the pulse pressure is increased as reflected in the higher than normal PPA of oscillations
32
,
34
associated with beats
25
,
26
following non-sinus beats
22
,
24
. Thus, using the method for rejecting artifacts disclosed by Ramsey, III at cuff pressure increments
36
and
37
no two adjacent oscillometric complexes have the same PPA. Thus, if the matching criteria for determining matching oscillation complexes are drawn toward a narrow range of peak-pulse amplitude variations no match would be found at pressure steps
36
and
37
.
A number of other references disclose using various methods for detecting the presence of arrhythmias and rejecting the oscillometric complexes associated with irregular heart beats. For example, European Patent No. EP 0895748 to Sohma et al. discloses a blood pressure monitoring device which uses an ECG and a pulse sensor to measure pulse transit time (PTT). Changes in PTT from sample to sample as well as changes in PPA are used to detect the presence of arrhythmias. However, Sohma et al. do not utilize this information to alter the function of a non-invasive blood pressure monitor. Similarly, European Patent 960598 to Forstner discloses an NIBP monitor which uses variances in pulse period to detect arrhythmias. Variations in the pulse period are used to reject those oscillometric complexes associated with arrhythmias or to send a signal indicating the presence of an arrhythmia.
U.S. Pat. No. 5,404,878 to Frankenreiter discloses a method and apparatus to measure and sort oscillations by amplitude and period and reject those that fall outside predefined limits.
U.S. Pat. No. 5,865,756 discloses an NIBP which uses an ECG to detect arrhythmias and a pulse oximeter to measure blood volume. This apparatus corrects the amplitude of oscillometric complexes which are “corrupted” by the presence of arrhythmias. The pulse volume measurement is used to correct the size of the oscillations so that the complexes may be used in calculating blood pressure.
The prior art recognizes the problems associated with calculating b
Dedyo Christopher J.
Friedman Bruce
Hersh Lawrence T.
Critikon Company, L.L.C.
Mallari Patricia
Michael Best & Friedrich LLC
Saret Larry L.
Shaver Kevin
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