Automatic blood pressure monitor having reduced data loss sensitivity to cuff pressure changes

• US 5,280,790 A
• Filed: 01/25/1993
• Issued: 01/25/1994
• Est. Priority Date: 01/25/1993
• Status: Expired due to Fees

• Current Assignee: Spacelabs Medical Incorporated (OSI Systems Incorporated)
• Original Assignee: Spacelabs Medical Incorporated (OSI Systems Incorporated)
• Inventors: Brooks, James R.
• Primary Examiner(s): Cohen, Lee S.
• Assistant Examiner(s): NASSER, ROBERT L
• Application Number:
  US08/008,731
• Time in Patent Office: 365 Days
• Field of Search: 128/672, 128/677-686, 364/413.05
• US Class Current: 600/494
• CPC Class Codes: A61B 5/02225 using the oscillometric method
• CPC Class Codes: A61B 5/0225 the pressure being controlled by electric signals, e.g. derived from Korotkoff sounds

Abstract

An automatic blood pressure monitor having a blood pressure cuff, an air pump applying pressurized air to the cuff, an air valve selectively venting the cuff, and a pressure transducer generating an output signal having a D.C. component indicative of the steady-state pressure in the cuff, and an A.C. component indicative of oscillometric pulses. The pressure transducer output is amplified and applied to an analog-to-digital converter which generates a digital word indicative of the steady-state cuff pressure. The amplified pressure transducer output is also applied to one input of a differential amplifier having an output that is applied to an analog-to-digital converter having a limited operating range. The other input of the amplifier receives an offset signal from a voltage source to offset the amplified pressure transducer output so that the output of the differential amplifier will not exceed the operating range of the analog-to-digital converter.

Claims

1. An improved method for use in a blood pressure monitor of the type having a blood pressure cuff, an air pump in fluid communication with said cuff to direct pressurized air into said cuff, an air valve in fluid communication with said cuff to selectively vent said cuff to atmosphere, a pressure transducer in fluid communication with said cuff generating a pressure signal indicative of the steady state fluid pressure in said cuff corresponding to an occlusive pressure exerted by said cuff on an artery and a transient fluid pressure in said cuff indicative of the amplitude of an oscillometric waveform, a processor in electrical communication with said air pump and said air valve for selectively energizing said air pump or valve to pressurize or depressurize said cuff, respectively, said method allowing said oscillometric waveform to be sampled despite relatively large changes in the steady state fluid pressure in said cuff, said method comprising:

setting the steady state fluid pressure in said cuff to a predetermined value;

sampling the pressure signal output by said pressure transducer resulting from the steady state fluid pressure in said cuff, and generating an offset signal corresponding thereto;

subtracting said offset signal from the pressure signal output by said pressure transducer, thereby generating an offset pressure signal;

sampling said offset pressure signal;

generating a plurality of digital numbers corresponding to a respective plurality of samples of said offset pressure signal; and

analyzing said digital numbers to determine blood pressure.

2. The method of claim 1 further including the step of adjusting magnitude of said offset signal by the steps of:

comparing each digital number to a predetermined range of said digital numbers; and

adjusting said offset signal in the event that said digital number is outside a predetermined range of said digital numbers so that said offset pressure signal corresponds to a value that is closer to the center of said predetermined range of said digital numbers.

3. An improved method for use in a blood pressure monitor of the type having a blood pressure cuff, an air pump in fluid communication with said cuff to direct pressurized air into said cuff, an air valve in fluid communication with said cuff to selectively vent said cuff to atmosphere, a pressure transducer in fluid communication with said cuff generating a pressure signal indicative of the steady state fluid pressure in said cuff corresponding to an occlusive pressure exerted by said cuff on an artery and a transient fluid pressure in said cuff indicative of the amplitude of an oscillometric waveform, a processor in electrical communication with said air pump and said air valve for selectively energizing said air pump or valve to pressurize or depressurize said cuff, respectively, said method allowing said oscillometric waveform to be sampled despite relatively large changes in the steady state fluid pressure in said cuff, said method comprising:

setting the steady state fluid pressure in said cuff to a predetermined value;

sampling the pressure signal output by said pressure transducer resulting from the steady state fluid pressure in said cuff, and generating an initial offset signal corresponding thereto;

subtracting said initial offset signal from the pressure signal output by said pressure transducer, thereby generating an initial offset pressure signal;

sampling said initial offset pressure signal, and generating an initial digital number corresponding thereto;

determining an initial digital offset value;

adding said initial digital offset value to said initial digital number, thereby generating an initial oscillometric value;

continuing to subtract said initial offset signal from the pressure signal output by said pressure transducer, thereby generating a continuous offset pressure signal;

periodically sampling said continuous offset pressure signal, and generating a respective digital numbers corresponding thereto;

adjusting the value of said initial offset signal if said offset signal falls outside a predetermined range;

after each adjustment of said initial offset signal, calculating a new digital offset value by summing the prior digital offset value to the difference between two of said digital numbers taken at successive times;

adding said new digital offset value to each of said digital numbers, thereby generating a plurality of oscillometric values;

analyzing said oscillometric values to determine blood pressure.

4. An improved method for use in a blood pressure monitor of the type having a blood pressure cuff, an air pump in fluid communication with said cuff to direct pressurized air into said cuff, an air valve in fluid communication with said cuff to selectively vent said cuff to atmosphere, a pressure transducer in fluid communication with said cuff generating a pressure signal indicative of the steady state fluid pressure in said cuff corresponding to an occlusive pressure exerted by said cuff on an artery and a transient fluid pressure in said cuff indicative of the amplitude of an oscillometric waveform, a processor in electrical communication with said air pump and said air valve for selectively energizing said air pump or valve to pressurize or depressurize said cuff, respectively, said method allowing said oscillometric waveform to be sampled despite relatively large changes in the steady state fluid pressure in said cuff, said method comprising:

setting the steady state fluid pressure in said cuff to a predetermined value;

sampling the pressure signal output by said pressure transducer resulting from variations in the fluid pressure in said cuff caused by oscillometric pressure pulses, and generating a pressure signal corresponding thereto;

offsetting said pressure signal by an offset value, thereby generating an offset pressure signal;

periodically sampling said offset pressure signal and generating respective digital numbers corresponding thereto;

summing each of said digital numbers with an offset value, thereby generating respective oscillometric values;

adjusting the magnitude of said offset pressure signal if said offset value falls outside a predetermined range;

after each adjustment of said offset value, calculating a new offset value by summing the prior offset value with the difference between two of said digital numbers taken at successive times;

analyzing said oscillometric values to determine blood pressure.

5. A blood pressure monitor, comprising:

a blood pressure cuff;

a pneumatic controller in fluid communication with said cuff to selectively vary a fluid pressure in said cuff responsive to a pneumatic control signal;

a pressure transducer in fluid communication with said cuff, said pressure transducer generating a pressure signal indicative of the fluid pressure in said cuff corresponding to an occlusive pressure exerted by said cuff on an artery and a transient fluid pressure in said cuff indicative of the amplitude of an oscillometric waveform;

voltage generating means for generating an offset voltage corresponding to a steady state pressure in said cuff;

offset means coupled to said voltage generating means and to said pressure transducer for generating an offset pressure signal corresponding to the difference between the pressure signal output by said pressure transducer and said offset voltage;

analog-to-digital converter means having a predetermined operating range coupled to said offset means, said analog-to-digital converter means sampling said offset pressure signal, thereby generating a plurality of digital numbers corresponding to a respective plurality of samples of said offset pressure signal; and

processing means in electrical communication with said pneumatic controller for generating said pneumatic control signal to cause said pneumatic controller to vary the fluid pressure in said cuff, said processing means being connected to an output of said analog-to-digital converter means to receive said digital numbers and analyze said samples to determine the blood pressure in an artery beneath said cuff whereby oscillometric pressure pulses imparted to said cuff can be sampled despite relatively large changes in the steady state fluid pressure in said cuff.

6. The blood pressure monitor of claim 5 wherein said offset means amplifies changes in the magnitude of said pressure signal, and wherein said analog-to-digital converter means receives said pressure signal as well as said offset pressure signal, said processing means causing said voltage generating means to initially generate said offset voltage from said pressure signal.

7. The blood pressure monitor of claim 5 wherein said processing means further includes means for adjusting the magnitude of said pressure offset signal by comparing each sample of said offset pressure signal to the operating range of said analog-to-digital converter means, and then adjusting said pressure offset signal in the event that a sample of said offset pressure signal is outside a predetermined portion of the operating range of said analog-to-digital converter means so that said offset pressure signal is closer to the center of the operating range of said analog-to-digital converter means.

8. The blood pressure monitor of claim 5 wherein said processing means includes means for adjusting the magnitude of said offset voltage generated by said voltage generating means if said offset voltage falls outside a predetermined portion of the operating range of said analog-to-digital converter means.

9. The blood pressure monitor of claim 8 wherein said processing means includes means for calculating oscillometric signal samples from said digital numbers, comprising:

means for summing each of said digital numbers with an offset value, thereby generating respective oscillometric signal samples; and

after each adjustment of said offset value, calculating a new offset value by summing the prior offset value with the difference between two of said digital numbers taken at successive times.