Method of measuring blood pressure with a plethysmograph

• US 5,269,310 A
• Filed: 08/17/1992
• Issued: 12/14/1993
• Est. Priority Date: 09/06/1990
• Status: Expired due to Fees

• Current Assignee: Spacelabs Medical Incorporated (OSI Systems Incorporated)
• Original Assignee: Spacelabs Medical Incorporated (OSI Systems Incorporated)
• Inventors: Wang, Wei-Min, Jones, Paul H.
• Primary Examiner(s): Cohen, Lee S.
• Assistant Examiner(s): Nasser, Robert L.
• Application Number:
  US07/931,578
• Time in Patent Office: 484 Days
• Field of Search: 128/664.7, 128/672, 128/674, 128/713, 128/687-690
• US Class Current: 600/480
• CPC Class Codes: A61B 5/021 Measuring pressure in heart or blood vessels A61B5/0205 takes precedence
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence

Abstract

A method and apparatus for determining arterial blood pressures using a photoplethysmograph is provided. In the inventive method the photoplethysmograph output is calibrated to the patient using an auxiliary blood pressure determining instrument, a constant k particular to the patient'"'"'s arterial blood pressure-volume relationship is determined and is stored for later use; at the time of measurement information is obtained from the photoplethysmograph output; and a computer determines the patient'"'"'s systolic and diastolic blood pressures.

Claims

1. A method of determining arterial blood pressures, comprising:

attaching a plethysmograph to a patient so that said plethysmograph interacts with an artery of said patient, said plethysmograph generating an output signal having a predetermined relationship to a characteristic of blood in said artery;

calibrating said plethysmograph during a calibration period by determining the patient'"'"'s actual arterial blood pressure by means other than said plethysmograph, and then determining the value of a first arterial characteristic in a predetermined relationship between said first arterial characteristic, arterial blood volume as indicated by said plethysmograph output signal, a conversion value corresponding to arterial blood volume at infinite pressure, and said actual arterial blood pressure during said calibration period; and

analyzing said plethysmograph output signal during a measurement period to determine an arterial blood pressure corresponding to said output signal in accordance with said predetermined relationship.

2. The method of claim 1 wherein said conversion value corresponding to arterial blood volume at infinite pressure is determined by examining the relationship between arterial blood volume and arterial blood pressure, and then determining said arterial blood volume at infinite pressure as the asymptotic value of arterial blood volume in said relationship.

3. The method of claim 1 wherein said step of determining the value of said first arterial characteristic includes the step of expressing said first arterial characteristic as a logarithmic function of said plethysmograph output signal.

4. The method of claim 1 wherein said step of determining the value of said first arterial characteristic includes the step of expressing said first arterial characteristic as an exponential function of said actual arterial pressure during said calibration period.

5. The method of claim 4 wherein said predetermined relationship is defined by the formula:

space="preserve" listing-type="equation">ψ

=ψ

.sub.inf (1--Kexp(kp))

where ψ

corresponds to the arterial blood volume as indicated by said plethysmograph output signal, ψ

_inf is the conversion value corresponding to said arterial blood volume at infinite pressure, k corresponds to said first arterial characteristic, K corresponds to a second arterial characteristic, and P corresponds to said actual arterial pressure during said calibration period.

6. The method of claim 1, further including the steps of determining from said plethysmograph output signal a value t_d corresponding to a duration of the cardiac cycle during said measurement period, a value S corresponding to systolic pressure during said measurement period, a value D corresponding to diastolic pressure during said measurement period, and a value ARC corresponding to an integral with respect to time of the difference between the plethysmograph output signal during said measurement period and a value of said plethysmograph output signal corresponding to diastolic pressure.

7. The method of claim 1, wherein said step of calibrating said plethysmograph during a calibration period includes the steps of:

determining an actual arterial systolic, diastolic, and mean blood pressures, P_s P_d, and P_m, respectively, during said calibration period by means other than said plethysmograph;

determining values V_d and V_s of said plethysmograph output signal corresponding to respective diastolic and systolic arterial pressures during said calibration period;

determining a value V_m corresponding to a mean of said plethysmograph output signal during said calibration period; and

calculating said first arterial characteristic k from the relationship;

##EQU7## where V_d and V_s are respective values of said plethysmograph output signal corresponding to diastolic and systolic pressures during said calibration period, P_d, P_s, and P_m are respective values corresponding to said actual arterial diastolic, systolic and mean arterial pressures during said calibration period, and k is the value of said first arterial characteristic determined during said calibration period.

8. The method of claim 7, wherein said step of determining a value V_m corresponding to the mean of said plethysmograph output signal during said calibration period is accomplished by calculating V_m from the relationship:

space="preserve" listing-type="equation">V.sub.m =V.sub.d -(ARC/t.sub.d)

where V_d is a value of said plethysmograph output signal corresponding to diastolic pressure during said calibration period, ARC is an area between the output of said plethysmograph and V_d, and t_d is a period of the output of said plethysmograph.

9. The method of claim 1 further including the step of determining from said plethysmograph output signal a value X corresponding to arterial pulse pressure during said measurement period by the steps of:

determining values V_d and V_s of said plethysmograph output signal corresponding to respective diastolic and systolic arterial pressures during said calibration period;

determining a value V_m corresponding to a mean of said plethysmograph output signal during said calibration period; and

calculating the value X corresponding to arterial pulse pressure during said measurement period from the relationship;

##EQU8## where V_d, V_s and V_m are respective values of said plethysmograph output signal corresponding to diastolic, systolic, and mean arterial pressures during said calibration period, and k is the value of said first arterial characteristic determined during said calibration period.

10. The method of claim 9 wherein said step of calibrating said plethysmograph during said calibration period further includes the steps of:

determining an actual arterial systolic and diastolic blood pressures, P_s and P_d, respectively, during said calibration period by means other than said plethysmograph;

calculating the ratio V_o /V_inf during said calibration period from the relationship;

##EQU9## where V_d and V_s are respective values of said plethysmograph output signal corresponding to diastolic and systolic arterial pressures during said calibration period, P_d and P_s are respective values corresponding to said actual arterial diastolic and systolic arterial pressures during said calibration period, V_o is a value corresponding to arterial blood volume at zero pressure, V_inf is a value corresponding to arterial blood volume at infinite pressure, and k is the value of said first arterial characteristic determined during said calibration period.

11. The method of claim 10 further including the step of determining from said plethysmograph output signal a value D corresponding to diastolic pressure during said measurement period by calculating D from the relationship:

##EQU10## where V_d and V_s are the respective values of said plethysmograph output signal during said calibration period corresponding to diastolic and systolic pressure, X is a value corresponding to said arterial pulse pressure during said measurement period determined in accordance with the method of claim 9, the ratio V_o /V_inf was determined in accordance with the method of claim 10, and k is the value of said first arterial characteristic determined during said calibration period.

12. The method of claim 9, wherein said step of calibrating said plethysmograph during said calibration period further includes the step of:

determining V_inf at said calibration period from the relationship;

space="preserve" listing-type="equation">V.sub.inf =exp{[1n(V.sub.s)-(1nV.sub.d) exp(-kX)]/[1-exp(-kX)]};

determining V_O from the relationship;

##EQU11## where V_d and V_s are the respective values of said plethysmograph output signal corresponding to diastolic and systolic pressure during said calibration period, P_d and P_s are respective values corresponding to said actual arterial diastolic and systolic pressures during said calibration period, X is the value corresponding to said arterial pulse pressure during said measurement period determined in accordance with the method of claim 11 V_o is a value corresponding to arterial blood volume at zero pressure, V_inf is the value corresponding to arterial blood volume at infinite pressure, and k is the value of said first arterial characteristic determined during said calibration period.

13. The method of claim 12 further including the step of determining from said plethysmograph output signal a value D corresponding to diastolic pressure during said measurement period by calculating D from the relationship:

##EQU12## where V_d and V_s are the respective values of said plethysmograph output signal during said calibration period corresponding to diastolic and systolic pressure, X is the value corresponding to said arterial pulse pressure during said measurement period determined in accordance with the method of claim 9, V_o and V_inf were determined in accordance with the method of claim 12, and k is the value of said first arterial characteristic determined during said calibration period.