Method and apparatus for monitoring cardiac output of a patient in a home environment
There is provided a method of determining a measure of the cardiac output, CO, of a patient, the method comprising obtaining measurements of one or more physiological characteristics of the patient, the physiological characteristics including at least the heart rate, HR, of the patient, the systolic blood pressure, S, of the patient and the diastolic blood pressure, D, of the patient; and processing the measurements to determine a measure of the cardiac output of the patient; wherein the measure of the cardiac output of the patient is derived using the relationship CO = K 1 ( HR S - D S + D ) , CO = K 2 HR ( S - D ) ( PTT ) 2 , and / or CO = K 2 HR ( S - D ) ( PAT - PEP ) 2 where K 1 and K 2 are patient-specific calibration factors, PAT is the pulse arrival time, PEP is the pre-ejection period and PTT is the pulse transit time, the time taken for a pulse wave to travel between two points in the body of the patient.
1. A method of determining a measure of the cardiac output, CO, of a patient with a monitoring apparatus, the monitoring apparatus comprising a hardware processing unit and one or more physiological sensors, the method comprising:
obtaining, with the processing unit, measurements of one or more physiological characteristics of the patient from the one or more physiological sensors, the physiological characteristics including at least the heart rate, HR, of the patient, the systolic blood pressure, S, of the patient and the diastolic blood pressure, D, of the patient; and
processing, with the processing unit, the measurements to determine a measure of the cardiac output of the patient;
wherein the measure of the cardiac output of the patient is derived using the relationship:
CO
=
K
1
(
HR
S
-
D
S
+
D
)
,
CO
=
K
2
HR
(
S
-
D
)
(
PTT
)
2
,
and
/
or
CO
=
K
2
HR
(
S
-
D
)
(
PAT
-
PEP
)
2
,
where K 1 and K 2 are patient-specific calibration factors, PAT is the pulse arrival time, PEP is the pre-ejection period and PTT is the pulse transit time, the time taken for a pulse wave to travel between two points in the body of the patient.
2. A method as claimed in claim 1 , wherein K 1 and/or K 2 are determined by the processing unit from a reference measurement of the cardiac output of the patient.
3. A method as claimed in claim 1 , wherein the measure of the cardiac output of the patient is a measure of the change in the cardiac output from an earlier cardiac output measurement or a reference measurement for cardiac output, and the measure of the change in the cardiac output is given by
CO
CO
ref
=
(
HR
S
-
D
S
+
D
)
/
(
HR
S
-
D
S
+
D
)
ref
,
CO
CO
ref
=
(
HR
(
S
-
D
)
PTT
2
)
/
(
HR
(
S
-
D
)
PTT
2
)
ref
,
and
/
or
CO
CO
ref
=
(
HR
(
S
-
D
)
(
PAT
-
PEP
)
2
)
/
(
HR
(
S
-
D
)
(
PAT
-
PEP
)
2
)
ref
where ( . . . ) ref denotes the earlier cardiac output measurement or the reference measurement for cardiac output.
4. A method as claimed in claim 1 , wherein the step of obtaining measurements comprises obtaining, from the one or more physiological sensors, an electrocardiogram signal, a photoplethysmogram signal, and at least one of heart sounds and an impedance cardiogram signal; and wherein the step of processing comprises processing the electrocardiogram signal and photoplethysmogram signal to determine the pulse arrival time, and processing the at least one of the heart sounds or the impedance cardiogram signal and the electrocardiogram signal to determine the pre-ejection period.
5. A method as claimed in claim 1 , further comprising the steps of:
comparing, with the processing unit, the measure of the cardiac output of the patient or measure of the change in the cardiac output of the patient to one or more threshold values; and
providing, with the processing unit, an output indicating the result of the step of comparing.
6. A method as claimed in claim 1 , the method further comprising the steps of:
obtaining, with the processing unit, measurements of acceleration acting on the patient during the measurement of the physiological characteristics of the patient;
processing, with the processing unit, the measurements of acceleration to determine the posture of the patient;
comparing, with the processing unit, the determined posture of the patient to a predetermined posture for the patient; and
providing, with the processing unit, feedback to the patient regarding their posture if the determined posture is not the same or sufficiently close to the predetermined posture.
7. A method as claimed in claim 6 , wherein the predetermined posture corresponds to the posture of the patient during a previous set of measurements of the physiological characteristics of the patient.
8. A method as claimed in claim 1 , further comprising the step of:
processing, with the processing unit, the measurements to determine a measure of the systemic vascular resistance, SVR, of the patient from the measure of the cardiac output of the patient using the relationship:
SVR=MAP/CO
where MAP is the mean arterial blood pressure and is given by:
MAP= S/ 3+2 D/ 3.
9. An apparatus for use in monitoring the cardiac output, CO, of a patient, the apparatus comprising:
a hardware processing unit configured to:
receive measurements of one or more physiological characteristics of the patient from one or more physiological sensors, the physiological characteristics including at least the heart rate, HR, of the patient, the systolic blood pressure, S, of the patient and the diastolic blood pressure, D, of the patient; and
process the measurements to determine a measure of the cardiac output of the patient;
wherein the measure of the cardiac output of the patient is derived using the relationship:
CO
=
K
1
(
HR
S
-
D
S
+
D
)
,
CO
=
K
2
HR
(
S
-
D
)
(
PTT
)
2
,
and
/
or
CO
=
K
2
HR
(
S
-
D
)
(
PAT
-
PEP
)
2
where K 1 and K 2 are patient-specific calibration factors, PAT is the pulse arrival time, PEP is the pre-ejection period and PTT is the pulse transit time, the time taken for a pulse wave to travel between two points in the body of the patient.
10. An apparatus as claimed in claim 9 , wherein the measure of the cardiac output of the patient determined by the processing unit is a measure of the change in the cardiac output from an earlier cardiac output measurement or a reference measurement for cardiac output, and the processing unit is configured to process the measurements to determine the measure of the change in the cardiac output using:
CO
CO
ref
=
(
HR
S
-
D
S
+
D
)
/
(
HR
S
-
D
S
+
D
)
ref
,
CO
CO
ref
=
(
HR
(
S
-
D
)
PTT
2
)
/
(
HR
(
S
-
D
)
PTT
2
)
ref
,
and
/
or
CO
CO
ref
=
(
HR
(
S
-
D
)
(
PAT
-
PEP
)
2
)
/
(
HR
(
S
-
D
)
(
PAT
-
PEP
)
2
)
ref
where ( . . . ) ref denotes the earlier cardiac output measurement or the reference measurement for cardiac output.
11. An apparatus as claimed in claim 9 , wherein the processing unit is further configured to:
compare the measure of the cardiac output of the patient or measure of the change in the cardiac output of the patient to one or more threshold values; and
provide an output indicating the result of the comparison.
12. An apparatus as claimed in claim 9 , further comprising the one or more physiological sensors, the one or more physiological sensors including: a blood pressure monitor, a heart rate sensor, an electrocardiogram sensor, a photoplethysmogram sensor, an impedance cardiogram sensor and one or more microphones.
13. An apparatus as claimed in claim 12 , wherein the one or more physiological sensors are integrated into an item of clothing to be worn by the patient.
14. An apparatus as claimed in claim 9 , wherein the processing unit is further configured to receive measurements of the acceleration acting on the patient and to process the measurements of the acceleration to determine the posture of the patient.