Method and apparatus for excluding artifacts from automatic blood pressure measurements

A method and system for detecting blood pressure pulses and screening out artifact-induced pulses when an automatic blood pressure monitor is provided. In preferred embodiments, the system detects the occurrence of an oscillometric pulse, determines the amplitude and rise time of the pulse, determines whether the amplitude and rise-times are within patient-specific screening limits and disregards pulses not within the limits, determines whether a pulse matches other pulses and saves the matching pulse data, determines the diastolic and systolic pressure based on the saved matched pulse data, and updates the patient-specific screening limits. In a preferred embodiment, pulses match based on similar amplitude and rise time characteristics. The amplitude and rise time screening limits are updated based on average values of prior blood pressure measurements for the patient.

TECHNICAL FIELD 
This invention relates to the automatic measurement of blood pressure, and 
more particularly to a method and apparatus for detecting blood pressure 
pulses and screening out artifact-induced pulses. 
BACKGROUND OF THE INVENTION 
Automatic blood pressure monitors are commonly used to periodically measure 
the blood pressure of a patient. In most automatic blood pressure 
monitors, a pressure cuff is attached to a patient's arm over the brachial 
artery. The cuff is first pressurized with an applied pressure that is 
high enough to substantially occlude the brachial artery. The cuff 
pressure is then gradually reduced, either continuously or in increments. 
As the pressure is reduced to systolic pressure, the flow of blood through 
the brachial artery beneath the cuff increases substantially. 
When the blood flows through the brachial artery following each contraction 
of the heart, it imparts a pulsatile movement to the wall of the artery. 
This pulsatile movement is coupled to a blood pressure cuff extending over 
the artery as minute changes in the cuff pressure, which are known as 
oscillometric pulses. Automatic blood pressure monitors measure and record 
the amplitude of the oscillometric pulses at a number of cuff pressures. 
After the blood pressure measurement had been completed, a table contains 
the oscillometric pulse amplitudes recorded at each cuff pressure. 
In theory, the systolic, diastolic, and mean arterial blood pressures can 
then be determined from the values in the table using empirical 
definitions of these parameters as a function of the amplitudes of these 
oscillometric pulses. However, blood pressure measurements are often 
adversely affected by artifact, generally produced by patient movement. 
Motion-induced artifacts can substantially alter the measured amplitude of 
oscillometric pulses thus introducing inaccuracies in the measurement of 
the patient's blood pressure. 
Prior systems use various techniques to minimize the effects of artifacts. 
Some prior systems screen oscillometric pulses based on their amplitude. 
Pulses with amplitudes outside the screen are considered artifact-induced. 
The screen is generally a population-based screen and not specific to any 
patient. Some previous systems also compare sequential pulses to ensure 
they are blood pressure induced. If two sequential pulses have similar 
amplitudes, these systems assume the pulse is blood pressure induced. 
However, these screening and comparison techniques do not always produce 
acceptable results. It would be desirable to have a system in which the 
screening and comparing of pulses more accurately identifies blood 
pressure induced pulses. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved method and 
system for detecting blood pressure induced pulses in an automatic blood 
pressure measurement device. 
It is another object of the present invention to provide a method and 
system in which oscillometric pulses are matched based on the amplitude 
and rise time characteristics of the pulses. 
It is another object of the present invention to provide a method and 
system in which artifact-induced pulses can be detected by the use of 
patient-specific amplitude and rise time screening limits. 
It is another object of the present invention to provide a method and 
system in which the patient-specific screening limits are automatically 
updated based on data gathered during the blood pressure measurement 
process. 
These and other objects, which will become apparent as the invention is 
more fully described below, are obtained by an improved blood pressure 
measurement device. In preferred embodiments, the device detects the 
occurrence of an oscillometric pulse, determines the amplitude and rise 
time of the pulse, determines whether the amplitude and rise time are 
within patient-specific screening limits and disregards pulses not within 
the limits, determines whether a pulse matches other pulses and saves the 
matching pulse data, determines the diastolic and systolic pressure based 
on the saved matched pulse data, and updates patient-specific screening 
limits. In a preferred embodiment, pulses match based on similar amplitude 
and rise time characteristics. The amplitude and rise time screening 
limits are updated based on average values of prior blood pressure 
measurements for the patient.

DETAILED DESCRIPTION OF THE INVENTION 
One embodiment of a system for detecting blood pressure pulses and 
screening out artifact-induced pulses in an automatic blood pressure 
measuring system is illustrated in FIG. 1. The system 10 comprises a 
number of hardware components, all of which are conventional. The system 
includes a conventional blood pressure cuff 12 in fluid communication with 
conduits 14 and 16, a conventional pump 18, a conventional valve 20, and a 
conventional pressure transducer 22. The pump 18 and valve 20 are operated 
by a conventional microprocessor 30. 
As explained in greater detail below, during the operation of the automatic 
blood pressure measuring system, the blood pressure cuff 12 is inflated to 
a pressure that is greater than systole as indicated by the pressure 
transducer 22. The valve 20 is then opened, usually for a predetermined 
period, although it may be continuously open to allow a slight leakage of 
air from the blood pressure cuff 12. However, the valve 20 normally allows 
air to escape from the cuff 12 fairly rapidly in relatively small 
increments. As the pressure in the cuff 12 is reduced, either gradually or 
incrementally, the pressure in the cuff 12 is measured by the pressure 
transducer 22. 
The pressure in the blood pressure cuff 12 consists of two components, 
namely, a relatively constant, or "DC," component and a relatively 
variable, or "AC," component. The relatively constant component is a 
function of the pressure in the blood pressure cuff 12. The relatively 
variable component is produced by the minute change in the pressure of the 
cuff 12 following each contraction of the heart. Thus, the relatively 
constant DC component of the pressure in the cuff can be used as an 
indication of cuff pressure, while the relatively variable AC component of 
the pressure in the cuff 12 can be used as an indication of an 
oscillometric pulse. 
Two signals are obtained from the pressure transducer 22. One set of 
circuitry 34 supplies a DC component to an analog-to-digital (A/D) 
converter 32. Another set of circuitry 36 supplies an AC component to the 
A/D converter 32. The signal supplied through the DC circuitry 34 is thus 
an indication of the cuff pressure, while the signal supplied through the 
AC circuitry 36 is an indication of the oscillometric pulse. The A/D 
converter 32 digitizes the DC and the AC signals and outputs digital bytes 
indicative of their values through a bus 38 to the microprocessor 30. 
As mentioned above, the microprocessor 30 is of conventional variety and, 
as is typical with such devices, is connected to a random access memory 40 
used for the storage of data, and to either random access memory or 
read-only memory 42 that contains the software for operating the 
microprocessor 30. Operator controls 44, such as a keyboard or buttons, 
are also connected to the microprocessor 30. 
Although the measuring system 10 illustrated in FIG. 1 utilizes a pressure 
transducer 22 and separate circuitry for the AC and the DC pressure 
signals, it will be understood that other implementations are possible. 
For example, a single circuit providing a signal corresponding to both the 
steady-state and the variable pressures in the cuff 12 can be supplied to 
the analog-to-digital converter 32. After the signal is digitized by 
analog-to-digital converter 32 and applied to the microprocessor 30, 
algorithms executed by the microprocessor 30 can detect the steady-state 
component of the cuff pressure and the variable component variations in 
the cuff pressure. 
In a preferred embodiment, the present invention is used to periodically 
determine the systolic and diastolic blood pressure of a patient. To 
determine the systolic and diastolic blood pressure, the preferred system 
of the present invention collects blood pressure readings at several cuff 
pressures, referred to as target pressure levels. When the cuff is 
inflated or deflated to the target pressure, the system samples the output 
of the A/D converter 32 to determine when an oscillometric pulse occurs. 
The occurrence of an oscillometric pulse is detected when the signal 
applied to the A/D converter 32 has predetermined characteristics, as 
described in greater detail below. The system then determines whether 
these pulses are within pulse amplitude and rise time screening limits. If 
an oscillometric pulse is not within the screening limits, it is 
disregarded. The screening limits for the patient are updated after the 
systolic and diastolic blood pressure are determined. This effects the 
automatic updating of patient-specific screening limits. The system 
collects data on pulses at a particular target pressure level until two 
matching pulses are detected. Pulses match when they have similar 
amplitude and rise time characteristics. When a match is found, the system 
saves the matching pulse data. The system collects matching pulse data for 
various target pressure levels. The system then evaluates the matching 
pulse data to determine the systolic and diastolic pressure. Algorithms to 
determine the systolic and diastolic blood pressures are well-known as are 
various algorithms to reduce the effects of artifacts. An algorithm to 
determine systole and diastole is described in U.S. Pat. No. 4,785,820 
entitled Method And Apparatus For Systolic Blood Pressure Measurements, 
which is incorporated herein by reference. Algorithms to reduce the 
effects of artifacts are described in U.S. Pat. No. 4,777,959 entitled 
Artifact Detection Based On Heart Rate In A Method And Apparatus For 
Indirect Blood Pressure Measurement and U.S. Pat. No. 5,014,714 entitled 
Method And Apparatus For Distinguishing Between Accurate And Inaccurate 
Blood Pressure Measurements In The Presence Of Artifact, which are hereby 
incorporated by reference. If the system successfully determines systolic 
and diastolic blood pressure, the system then updates the screening 
limits. 
FIG. 2 is a flow diagram of an overview of the processing of the present 
invention. In a preferred embodiment, the processing is implemented on a 
computer program, which executes on microprocessor 30. In block 201, the 
system sets the cuff pressure to the target pressure. In a preferred 
embodiment, the target pressure is initially set in excess of the 
anticipated systolic pressure. The target pressure is then decremented 
sequentially. In block 202, the system waits until the interrupt routine 
detects a peak in the oscillometric data. A peak usually indicates that a 
pulse has occurred, although a peak could be produced by an artifact. In a 
preferred embodiment, a peak is detected when the AC pressure is 
increasing and passes through a "high trigger" level and then passes 
through a "low trigger" level while decreasing. In block 203, the system 
determines whether the detected pulse is within the screening limits. In a 
preferred embodiment, the screening limits are patient-specific limits 
based on the amplitude and the rise time of the pulse. In alternate 
embodiments, the pulse fall time, pulse width, integral of the pulse, and 
derivatives of the pulse can be used as screening limits (and for matching 
pulses as discussed below). If the pulse data is within the screening 
limits, then the system continues at block 204, else the system disregards 
the pulse data and loops back to block 202 to await the next peak. The 
system maintains a Temporary Table of pulse data that is collected at a 
particular target pressure and that is within the screening limits. In 
block 204, the system determines whether the current pulse data matches 
any of the data stored in the Temporary Table. In a preferred embodiment, 
a match is detected based on the amplitude and the rise time of the 
pulses. If a match is detected with any of the pulse data in the Temporary 
Table, the system continues at block 206, else the system adds the current 
pulse data to the Temporary Table in block 205 and loops to block 202 to 
detect the next peak. The system maintains an Oscillometric Table that 
contains the DC pressure, AC pressure, rise time, and heart rate 
associated with each matched pulse. (The rise time is used to update the 
screening limits.) In block 206, the system adds the average pulse data of 
the matched pulses to the Oscillometric Table. In block 207, the system 
clears the Temporary Table in preparation for the collecting of data at 
the next target pressure. In block 208, the system determines whether 
there is enough data in the Oscillometric Table to perform a blood 
pressure evaluation. If there is enough data in the Oscillometric Table, 
the system continues at block 209, else the system loops to block 201 to 
collect more data. In block 209, the system evaluates the Oscillometric 
Table. In block 210, the system determines if the evaluation was 
successful. If the evaluation was successful, the system continues at 
block 211, else the system loops to block 201 to set the cuff pressure at 
the next target pressure. In block 211, the system updates the screening 
limits for the patient and the processing for the particular blood 
pressure reading is complete. 
FIGS. 3A, 3B, and 3C are a detailed flow diagram of the processing routine 
of the present invention. The procedure that invokes this routine performs 
a number of conventional tasks. These tasks include, upon completion of a 
measurement, opening the valve 20 to fully deflate the cuff 12, displaying 
or storing the results of the measurement, and scheduling the next 
automatic measurement. Also, the invoking procedure initializes the 
screening limits before invoking the processing routine the first time for 
a particular patient. In a preferred embodiment, the amplitude and rise 
time screening limits are set to a level that will, in effect, allow any 
pulse to pass through the screening test initially. The screening limits 
will be automatically adjusted to the patient's characteristics as 
described below. Referring now to FIG. 3A, in block 301, the system 
performs initialization for the subsequent processing. The system 
initializes the Temporary Table, Oscillometric Table, and various flags. 
In block 302, the system determines if there is enough data in the 
Oscillometric Table to calculate a blood pressure reading. In a preferred 
embodiment, there is enough data in the Oscillometric Table when there are 
three entries. One skilled in the art would appreciate that other criteria 
can be used to determine whether there is enough data in the Oscillometric 
Table. If there is enough data in the Oscillometric Table, the system 
continues at block 330 in FIG. 3C, else the system continues at block 303. 
In blocks 303 through 308, the system sets the cuff pressure to the target 
pressure and loops waiting for a peak to be detected. If a peak is 
detected the system continues at block 310 in FIG. 3B. In block 303, the 
system calculates a target pressure. In a preferred embodiment, the target 
pressure is initially set higher than the anticipated systolic pressure. 
In a preferred embodiment, each pass through block 303, the system 
decrements the target pressure by 8 mm. However, one skilled in the art 
would appreciate that other methods of calculating the target pressure 
would be acceptable. In block 304, the system sets the cuff pressure to 
the target pressure. Initially, the cuff pressure starts off well below 
the target pressure. If the cuff pressure is below the target pressure, 
the system energizes the pump 18 to increase the pressure. Conversely, if 
the target pressure is below the cuff pressure, then the system will 
release pressure from the cuff 12 through the valve 20. In block 305, the 
system determines whether a peak has been detected. The interrupt routine, 
as described in FIG. 4, determines whether a peak has been detected and 
sets an appropriate flag. If a peak is detected, then the system continues 
at block 310 of FIG. 3B, else the system continues at block 307. In blocks 
307 and 308, the system checks various flags that are set by the interrupt 
routine. These flags are used to determine whether the time at the target 
pressure has been too long and whether the cuff pressure is near enough to 
the target pressure. In block 307, if the time at the particular target 
pressure has been too long, then the system loops to block 303 to 
calculate a new target pressure and continue processing, else the system 
continues at block 308. In block 308, the system determines whether the 
cuff pressure is near the target pressure. If the cuff pressure is near 
the target pressure, then the system continues to wait for a peak by 
looping to block 305, else the system loops to block 304 to reset the cuff 
pressure to the target pressure. 
Referring now to FIG. 3B, in blocks 310 and 311, the system determines 
whether the detected pulse data is within the amplitude and rise time 
screening limits. The screening limits are updated in block 336 of FIG. 
3C. In block 310, if the amplitude of the detected pulse is within the 
amplitude screening limits, the system continues at block 311, else the 
system disregards the pulse data and loops to block 305 of FIG. 3A to wait 
for the next peak. In block 311, if the rise time of the detected pulse is 
within the rise time screening limits, then the system continues at block 
312, else the system loops to block 305 in FIG. 3A to wait for the next 
peak. 
In blocks 312 through 319, the system determines whether the detected pulse 
matches any other pulse data stored in the Temporary Table at the 
particular target pressure. If the detected pulse matches a pulse in the 
Temporary Table, then the system updates the Oscillometric Table and 
continues at block 302 of FIG. 3A to determine if there is enough data in 
the Oscillometric Table to be evaluated. If no match is found, then the 
system adds the detected pulse data to the Temporary Table and continues 
at block 305 in FIG. 3A to wait for the next peak. In block 312, if the 
detected pulse is the first pulse at the target pressure to pass the 
screening limits, then there is no data in the Temporary Table to match 
and the system continues at block 317, else the system continues at block 
313. In blocks 313 through 316, the system loops comparing the detected 
pulse with each of the entries in the Temporary Table. If a match is 
found, the system continues at block 318. In block 313, the system selects 
the next pulse in the Temporary Table starting with the last pulse stored 
in the table. In block 314, the system determines whether the amplitude of 
the selected pulse matches the amplitude of the detected pulse. In a 
preferred embodiment, the amplitude of two pulses match when the absolute 
value of their differences is less than or equal to a constant value plus 
9% of the amplitude of the detected pulse. In a preferred embodiment, the 
constant is set to account for the inherent noise in the detection 
equipment. One skilled in the art would appreciate that although 
amplitudes preferably match when they are within 9%, other matching 
criterions are acceptable. If an amplitude match is found, the system 
continues at block 315, else the system continues at block 316. In block 
315, the system determines whether the rise time of the selected pulse 
matches the rise time of the detected pulse. In a preferred embodiment, 
the rise times of pulses match when they are within approximately 8 
milliseconds. One skilled in the art would appreciate that other rise time 
matching criteria would produce acceptable results. If the rise times 
match, then the system continues at block 318, else the system continues 
at block 316. In block 316, if all the data in the Temporary Table has 
been checked for a match, then the system continues at block 317, else the 
system loops to block 313 to select the next pulse in the Temporary Table. 
In block 317, no match has been found, and the system stores the detected 
pulse data in the Temporary Table and continues at block 305 of FIG. 3A to 
wait for the next peak. In block 318, the system places the matched pulse 
data in the Oscillometric Table. In a preferred embodiment, the average 
value of the amplitude and rise times and associated cuff DC pressure for 
the selected and detected pulses are stored in the Oscillometric Table. 
The time interval between the two matching pulses is also stored to be 
used to determine the heart rate. In block 319, the system clears the 
Temporary Table for processing at the next target pressure level, and the 
system continues at block 302 of FIG. 3A to determine if there is enough 
data in the Oscillometric Table for evaluation. 
Referring to FIG. 3C, in block 330, the system evaluates the Oscillometric 
Table to determine the systolic and diastolic blood pressures. Three 
outcomes of this evaluation are possible. First, the evaluation was 
incomplete because of insufficient data. Second, the evaluation found data 
inconsistencies although there was sufficient data. During evaluation, 
each amplitude and heart rate interval in the Oscillometric Table is 
checked for internal consistency with the other entries. These checks are 
fully described in the earlier cited patents. Third, the evaluation was 
successful. In block 331, if the data is insufficient, then the system 
loops to block 303 in FIG. 3A to collect additional data at the next lower 
target pressure, else the system continues at block 332. In block 332, if 
inconsistencies with the data are found, then the system returns with an 
error code, else the system continues at block 333. In block 333, the 
system determines whether the screening limits should be updated. In a 
preferred embodiment, the screening limits should not be updated when too 
many pulses fail to pass the screening limits or when too many nonmatching 
pulses were detected. This test ensures that the screening limits are set 
as a function of the patient oscillometric data rather than as a function 
of artifacts. In alternate embodiments, the screening limits are updated 
when a majority of the pulses fall outside the screening limits. If the 
screening limits are to be updated, the system continues at block 334, 
else the system returns. In block 334, the system calculates the average 
highest amplitude for several blood pressure readings. In block 335, the 
system calculates the average longest rise time for several blood pressure 
readings. In block 336, the system updates the screening limits based on 
the average highest amplitude and the average longest rise time. In a 
preferred embodiment, the maximum amplitude screening limit is set to 150% 
of the average highest amplitude, and the minimum amplitude screening 
limit is set to 19% of the average highest amplitude. In a preferred 
embodiment, the maximum rise time screening limit is set to 20 
milliseconds longer than the average longest rise time. There is no 
minimum rise time screening limit. In a preferred embodiment, the averages 
are calculated using the last five blood pressure measurements that 
resulted in an update of the screening limits. One skilled in the art 
would appreciate that the updating of these screening limits can be varied 
and still produce acceptable results. The system then returns. 
FIG. 4 is a flow diagram of the interrupt routine. The interrupt is 
timer-driven. The interrupt routine inputs digitized data, detects when a 
peak occurs, and checks for timing and pressure measurements. The 
interrupt routine sets flags that are used by the main processing routine 
for detection of certain conditions. Because of the potential danger to a 
patient, the interrupt routine first determines whether the allowed time 
to have the cuff pressurized is exceeded. In block 401, if the time is 
exceeded, the routine continues at block 402, else the routine continues 
at block 403. In block 402, the routine deflates the cuff and returns to 
the calling routine, rather than the interrupted routine. In block 403, 
the routine inputs the digitized AC oscillometric pressure and the DC cuff 
pressure and initiates the next sampling by the A/D converter. In block 
404, the routine determines if a peak occurred. A peak occurs when the 
data has passed from below a high trigger level to above the high trigger 
level, and then passes from above a low trigger level to below the low 
trigger level. One skilled in the art would appreciate that other peak 
detection algorithms would produce acceptable results. In block 405, if a 
peak was detected, then the routine sets the peak detected flag in block 
406. In block 407, if the time at the target pressure has exceeded a 
predetermined time, then the routine sets the target pressure time 
exceeded flag. The routine stores the raw digitized data, and the main 
processing routine calculates the amplitude and rise time. In a preferred 
embodiment, the rise time is the amount of time it takes the pressure to 
rise between 25% and 88% of the pulse amplitude. The routine then returns 
to the interrupted routine. 
Although the present invention has been described in terms of preferred 
embodiments, it is not intended that the invention be limited to these 
embodiments. Modification within the spirit of the invention will be 
apparent to those skilled in the art. The scope of the present invention 
is defined by the claims that follow.