Device for monitoring fluid status
A technique for determining a patient's hematocrit by an electrical impedance measurement is described. An implantable device may be configured to utilize the technique in order to assess a cardiac patient's fluid status. In order to determine the hematocrit, the electrical impedance of the blood is measured and mapped by a mapping function to a corresponding hematocrit value.
1. A device, comprising:
an exciter for outputting an excitation current waveform and impressing a current field between a pair of excitation current electrodes;
voltage sensing circuitry for sensing a voltage difference signal between a pair of voltage sense electrodes disposed in a patient's blood and in the current field; and,
a controller interfaced to the voltage sensing circuitry which is programmed to estimate the patient's hematocrit from the voltage difference signal with a mapping function that maps a capacitance value derived from the voltage difference signal to a hematocrit value.
2. The device of claim 1 further comprising a lead which incorporates the excitation current electrodes and the voltage sense electrodes, wherein the voltage sense electrodes are located between the excitation current electrodes.
3. The device of claim 1 wherein the controller is programmed to estimate the patient's hematocrit with a mapping function which maps the amplitude of the voltage difference signal to a hematocrit value.
4. The device of claim 1 wherein the controller is programmed to derive a capacitance value from the voltage difference signal by deriving a resistance value from the voltage difference signal at a first excitation frequency and measuring a phase angle between the excitation current waveform and the voltage difference signal at a second excitation frequency.
5. The device of claim 1 wherein the controller is programmed to derive a capacitance value from the voltage difference signal by deriving a resistance value from the voltage difference signal at a first excitation frequency and measuring the amplitude of the voltage difference signal at a second excitation frequency.
6. The device of claim 1 further comprising means for measuring a resistivity of the patient's plasma and wherein the controller is programmed to estimate the patient's hematocrit with a mapping function which maps the amplitude of the voltage difference signal and the resistivity of the patient's plasma to a hematocrit value.
7. The device of claim 1 wherein the controller is programmed to periodically estimate the patient's hematocrit which is stored and trended over time.
8. The device of claim 1 wherein the controller is programmed to compare the estimated hematocrit value with a specified limit range and set an alarm flag if the estimated hematocrit value is out of the specified limit range.
9. The device of claim 8 wherein the controller is programmed to transmit an alarm message to a remote monitoring device if the alarm flag is set.
10. A method for monitoring the fluid status of a patient, comprising:
outputting an excitation current waveform and impressing a current field between a pair of excitation current electrodes;
sensing a voltage difference signal between a pair of voltage sense electrodes disposed in the patient's blood and in the current field; and,
estimating the patient's hematocrit from the voltage difference signal with a mapping function that maps a capacitance value derived from the voltage difference signal to a hematocrit value.
11. The method of claim 10 wherein the voltage sense electrodes are located between the excitation current electrodes in a single lead.
12. The method of claim 10 further comprising estimating the patient's hematocrit with a mapping function which maps the amplitude of the voltage difference signal to a hematocrit value.
13. The method of claim 11 further comprising deriving a capacitance value from the voltage difference signal by deriving a resistance value from the voltage difference signal at a first excitation frequency and measuring a phase angle between the excitation current waveform and the voltage difference signal at a second excitation frequency.
14. The method of claim 11 further comprising deriving a capacitance value from the voltage difference signal by deriving a resistance value from the voltage difference signal at a first excitation frequency and measuring the amplitude of the voltage difference signal at a second excitation frequency.
15. The method of claim 10 further comprising measuring a resistivity of the patient's plasma and estimating the patient's hematocrit with a mapping function which maps the amplitude of the voltage difference signal and the resistivity of the patient's plasma to a hematocrit value.
16. The method of claim 10 further comprising periodically estimating the patient's hematocrit and trending the estimated hematocrit values over time.
17. The method of claim 10 further comprising comparing the estimated hematocrit value with a specified limit range and setting an alarm flag if the estimated hematocrit value is out of the specified limit range.
18. The method of claim 17 further comprising transmitting an alarm message to a remote monitoring device if the alarm flag is set.