WO2001003580A1

WO2001003580A1 - Method for continuous monitoring of cardiac output

Info

Publication number: WO2001003580A1
Application number: PCT/FI2000/000608
Authority: WO
Inventors: Börje Rantala
Original assignee: Instrumentarium Oy
Priority date: 1999-07-08
Filing date: 2000-07-03
Publication date: 2001-01-18
Also published as: FI991570A; FI110405B; AU5988600A

Abstract

The invention concerns a method for continuous monitoring of cardiac output, in which method cardiac output is measured by a continuous impedance measurement method. In the method, the cardiac output value (SV) obtained via the impedance measurement is calibrated by a quantitative second measuring method.

Description

METHOD FOR CONTINUOUS MONITORING OF CARDIAC OUTPUT

The present invention relates to a method as defined in the preamble of claim 1.

In prior art, it is known that cardiac output can be monitored by several methods, both non-invasive and invasive, non-continuous and continuous.

A previously known continuous, non-invasive method for measuring cardiac output is based on the measurement of body impedance. In impedance- cardiographic measurement, electrodes are placed on the upper part of the patient's body, and the impedance between the electrodes is measured. The electrical impedance thus measured shows cyclic changes due to cardiac activity, allowing cardiac output to be calculated on the basis of theoretic models and empiric formulas. The principles of the impedance- cardiographic method are described e.g. in the book "Principles and Practice of Intensive Care Monitoring", Martin J. Tobin, McGraw-Hill 1998, ISBN: 007- 0650942, pp. 915-921, to which we are referring here.

Impedance measurement has the advantage of simplicity, and that it allows continuous, fast and non-invasive measurement of cardiac output. However, a significant drawback with the method is inaccuracy, because the correction factors used in the empiric formulas are based on assumptions and are inaccurate. Things like the patient's weight and posture, placement of electrodes etc. and even small changes in these have an effect on the measurement result. Imped- ance cardiography is very sensitive to body structure, the patient's body fluid content, fatness and posture.

A previously known non-continuous , invasive method is so-called bolus thermodilution, i.e. a method of thermal dilution whereby the blending of a cold salt solution bolus is monitored by means of a thermistor placed at the end of a catheter on the cardiac artery. This method is in routine use. Its main disadvantages are its invasive nature and a lack of continuity.

There is also a continuous thermodilution method in which blood is heated using an electric re- sistor, but it has the disadvantages of a still greater degree of invasiveness, an expensive catheter (about three times the price of an ordinary thermodilution catheter) and a slow response time constant (3 - 6 min) . Thermodilution methods are described e.g. in the book "Principles and Practice of Intensive Care Monitoring", Martin J. Tobin, McGraw-Hill 1998, ISBN:0070650942, pp. 801-805, to which we are referring here .

The object of the invention is to eliminate the drawbacks referred to above.

A specific object of the invention is to disclose a method whereby the cardiac output data obtained from a relatively inaccurate impedance measurement sensitive to various influences can be rendered more accurate as necessary.

A further object of the invention is to eliminate the need to use the inaccurate empiric formulas and associated correction factors required in the impedance measurement method. As for the features characteristic of the method of the invention, reference is made to the claims .

In the method of the invention, cardiac output is monitored continuously by measuring it using a continuous impedance measurement method. According to the invention, the cardiac output value obtained via the impedance measurement is calibrated by a quantitative second measuring method.

The invention provides the advantage of al- lowing the cardiac output data obtained via continuous impedance measurement, which is not very accurate, to be rendered more accurate via a calibration measure- ment performed by an accurate second measuring method. The second method used to get more accurate results may be a continuous but slower method, i.e. working with a delay, or it may be a non-continuous method. Further, the calculation of cardiac output can be accomplished without the use of empiric calculation formulae and numerous correction factors. The invention provides a cheap and simple continuous measurement whose results can be rendered more accurate if neces- sary in the circumstances in each case, and preferably without having to attach any extra sensors to the patient .

In an embodiment of the method, the calibration measurement is performed by a second measuring method at predetermined time intervals. The calibration measurement may be repeated periodically at regular time intervals or sporadically at irregular time intervals .

In an embodiment of the method, changes in the cardiac output value obtained by the impedance measurement method are observed, and a calibration measurement is performed by a second measuring method as necessary when a change occurs in the cardiac output value obtained by the impedance measurement method. In a preferred case, an alarm is activated to warn e.g. attendants responsible for patient monitoring when the cardiac output value obtained by the impedance measurement method changes . Based on the alarm, the attendants perform a calibration measure- ment using the said second measuring method.

In an embodiment of the method, the second measuring method is an invasive measuring method more accurate than the impedance measurement method, such as a thermodilution method. If a thermodilution method is used, then it may be either a continuous and slow thermodilution method using electric heating or a non- continuous bolus thermodilution method. Thermodilution with electric heating is a continuous, accurate and slow method requiring a great deal of mean value calculation, measuring cardiac output at with delay of 3 - 6 minutes. Due to the slowness, a sudden change in the cardiac output is only detected after e.g. three minutes . When used in combination with the impedance method, it has the advantage that, based on the inaccurate but fast impedance measurement, it provides a rapid and early warning indicating that the cardiac output value seems to be changing, which can thus be attended to, and that a more accurate measurement result is then automatically obtained after a short delay, e.g. 3 minutes, by a continuous thermodilution method with electric heating, providing an accurate confirmation of the changes.

The second measuring method may also be some other known measuring method, such as a color indicator dilution method, electromagnetic flowmetry, so- called Fick-method or any other method. In an embodiment of the method, an impedance signal is measured from the patient, and from the impedance signal is filtered an impedance-cardiographic signal, from which a cardiac output value is determined. In an embodiment of the method, the impedance signal is filtered to obtain an impedance-respiration signal, from which a respiration frequency value is determined.

In an embodiment of the method, the impedance signal is measured in accordance with the electrocar- diography (ECG) standard by using ECG electrodes conventionally placed on the patient . Regarding placement of the electrodes, it is not necessary to consider any additional requirements of impedance measurement, and no additional sensors need to be attached to the patient for that purpose; instead, the impedance meas- urement can be performed using conventional ECG electrodes .

In an embodiment of the method, an electro- cardiographic signal is measured. From the impedance signal, an impedance-cardiographic signal is filtered in such manner that only signal frequencies close to the heartbeat frequency derived from the ECG signal are passed through.

In an embodiment of the method, the imped- ance-cardiographic signal is displayed as an impedance cardiogram on the same display with the electrocardiogram.

In an embodiment of the method, a respiration curve corresponding to a respiration signal is dis- played on the same display with the impedance cardiogram and/or electrocardiogram.

In an embodiment of the method, the cardiac output value is calculated from the impedance- cardiographic signal on the basis of the respiration signal during a certain respiration phase, preferably during the final phase of exhalation.

In an embodiment of the method, the impedance is measured from several electrode combinations, and of the impedance-cardiographic signals derived from them, the best one, e.g. the one with the largest amplitude, the smoothest one and/or the one bearing the greatest resemblance to a blood pressure signal, is selected for further processing.

In the following, the invention will be de- scribed in detail by the aid of some of its embodiments with reference to the attached drawing, wherein

Fig. 1 presents a diagram representing a measuring arrangement according to the invention,

Fig. 2 and 3 present examples of conventional placement of electrodes used in impedance cardiogra- phy, Fig. 4, 5 and 6 illustrate a standard placement of ECG electrodes in a 3 -lead measurement arrangement ,

Fig. 7 illustrates the placement of ECG elec- trodes in a 5 -lead measurement arrangement,

Fig. 8 illustrates the placement of ECG electrodes in a 12 -lead measurement arrangement, and

Fig. 9 presents an electrocardiogram ECG, an impedance Z, which includes a respiration component resp and an impedance-cardiographic ICG component, and respiration and ICG signal components obtained by filtering from the impedance signal, displayed on the same display device.

Fig. 1 presents a diagram representing a set of equipment for continuous measurement of cardiac output. The equipment comprises means for continuous impedance measurement, including electrodes 1 attached to the patient ' s thorax, a signal processing apparatus 2 and a display device 3. For non-continuous calibra- tion measurement, the equipment also comprises means for thermodilution, of which the figure shows a cardiac artery catheter 4 and an injection means 5, which is used to introduce a bolus through the catheter 4 into the cardiac artery. The measuring instruments and principles used in impedance measurement and bolus thermodilution are known in themselves and are described in detail e.g. in the books "Encyclopedia of Medical Devices and Instrumentation" John G. Webster; John Wiley & Sons 1988, ISBN: 0471829366 , and "Princi- pies and Practice of Intensive Care Monitoring", Martin J. Tobin, McGraw-Hill 1998, ISBN: 0070650942 , so they will not be described in detail in this context.

The measuring method is e.g. a combination of impedance cardiography and bolus thermodilution and an impedance respiration measurement performed as a routine in conjunction with basic ECG monitoring. The impedance respiration method supplies a high-frequency current into the ECG electrodes, thereby qualitatively measuring the patient's respiration activity, in most cases producing an output that shows the patient's respiration frequency and functioning as an apnea alarm when respiration ceases. In respiration monitoring, one component of the impedance signal is originated by the heart. Conventional respiration monitors are designed to filter out this compo- nent .

Conventional impedance cardiography is implemented using so-called four-point measurement (see Fig. 2 and 3) with a high-frequency current of several milliamperes flowing through the entire thorax in or- der to minimize the variability associated with electrode placement. Fig. 2 illustrates the placement of spot electrodes as known in impedance cardiography, and Fig. 3 presents a corresponding placement of band electrodes . However, in the embodiment of the invention to be described next as an example, ordinary standard placement of ECG electrodes is used, so the electrodes can be placed without having to meet any additional requirements imposed by impedance cardiography. Thus, the electrodes may be placed according to any of the arrangements illustrated in Fig. 4 - 8. Fig. 4 - 6 show a standard placement of ECG electrodes in a 3- lead measurement arrangement. Fig. 7 shows a standard ECG electrode placement in a 5 -lead measurement ar- rangement . Fig. 8 shows a standard ECG electrode placement in a 12-lead measurement arrangement.

The use of ECG electrodes for impedance respiration measurement is known in itself from the specification EP 0 747 005 Al by the same applicant. As shown in Fig. 9, the measured impedance signal Z is filtered using conventional signal filtering means to obtain a qualitative impedance- cardiographic signal ICG, which describes the conductivity change caused by heartbeat between two (or more) ECG electrodes. This impedance-cardiographic signal ICG represents the mechanical work done by the heart. To obtain this information, blood pressure measurement or optical plethys ography have generally been used. From the impedance-cardiographic signal ICG, the same information can be obtained directly via ordinary ECG electrodes without any additional sen- sors .

To obtain cardiac output data from the impedance-cardiographic signal ICG, according to the invention, the signal is calibrated at intervals via bolus thermodilution. It is also possible to use some other quantitative method to perform the calibration measurement. This calibration eliminates factors depending on body structure, placement of the electrodes or the patient's posture, and the result obtained is an index that follows the change in cardiac output starting from the instant of calibration. This index can be either scaled via thermodilution and displayed as a continuous cardiac output result as a function of time. Changes in it may trigger an automatic alarm, based on which the user carries out a new thermodilution meas- urement. The cardiac output value SV is calculated from the impedance-cardiographic signal ICG on the basis of the respiration signal (resp) during a given respiratory phase, preferably during the final phase A of exhalation. The cardiac output value SV is propor- tional to the amplitude of the impedance-cardiographic signal ICG.

As in Fig. 9, the electrocardiogram ECG, the impedance signal Z, the respiration signal resp and the impedance-cardiographic signal ICG are displayed as curves in relation to time on the same display 3 to allow them to be observed simultaneously. The invention is not restricted to the examples of its embodiments described above; instead, many variations are possible within the scope of the inventive idea defined in the claims.

Claims

1. Method for continuous monitoring of cardiac output, in which method cardiac output is measured by a continuous impedance measurement method, chara c t e r i z e d in that the cardiac output value (SV) obtained via the impedance measurement is calibrated by a quantitative second measuring method.

2. Method as defined in claim 1, char ac t e ri zed in that the calibration measurement is performed by a continuous second measuring method.

3. Method as defined in claim 1, char a c t e r i z ed in that the calibration measurement is performed by a non-continuous second measuring method.

4. Method as defined in claim 1, char a c t er i z ed in that the calibration measurement is performed by a second measuring method at predetermined regular or irregular time intervals.

5. Method as defined in claim 1, char - ac t e r i z ed in that changes in the cardiac output value (SV) obtained by the impedance measurement method are observed, and a calibration measurement is carried out by a second measuring method as necessary when the cardiac output value obtained via impedance measurement changes.

6. Method as defined in claim 1, char a c t eri z ed in that an alarm is activated when the cardiac output value obtained by the impedance measurement method changes, and, based on the alarm, a cali- bration measurement is performed by said second measuring method.

7. Method as defined in any one of claims 1 - 6, charac t eri zed in that the second measuring method is an invasive measuring method that is more accurate than the impedance measurement method.

8. Method as defined in any one of claims 1 - 7, charact eri z ed in that the second measuring method is a thermodilution method.

9. Method as defined in claim 8, char - ac t e ri z ed in that the second measuring method is a continuous thermodilution method.

10. Method as defined in claim 8, char ac t er i z ed in that the second measuring method is a non-continuous bolus thermodilution method.

11. Method as defined in any one of claims 1

- 10, charac t e ri zed in that an impedance signal (Z) is measured from the patient, and the impedance signal (Z) is filtered so as to obtain an impedance cardiographic signal (ICG) , from which a car- diac output value is determined.

12. Method as defined in any one of claims 1

11, charac t e ri z ed in that the impedance signal is filtered so as to obtain an impedance- respiration signal (resp) , from which the respiration frequency is determined.

13. Method as defined in any one of claims 1

12, charac t e ri z ed in that the impedance signal is measured in accordance with the electrocar- diography (ECG) standard by using ECG electrodes con- ventionally placed on the patient.

14. Method as defined in claim 12 or 13, characteri zed in that an electrocardio- graphic signal (ECG) is measured; and that from the impedance signal (Z) , an impedance-cardiographic sig- nal (ICG) is filtered in such manner that only signal frequencies close to the heartbeat frequency derived from the ECG signal are passed through.

15. Method as defined in any one of claims 1

- 14, c h a r a c t e r i z e d in that the impedance - cardiographic signal (ICG) is displayed as an impedance cardiogram on the same display with the electrocardiogram (ECG) .

16. Method as defined in any one of claims 1 15, charac t er i z ed in that a respiration curve (resp) corresponding to the respiration signal is displayed on the same display with the impedance cardiogram (ICG) and/or electrocardiogram (ECG) .

17. Method as defined in any one of claims 1

- 16, characteri zed in that a cardiac output value (SV) is calculated from the impedance- cardiographic signal on the basis of the respiration signal (resp) during a certain respiration phase, preferably during the final phase of exhalation.

18. Method as defined in any one of claims 13

- 17, charac t e r i z ed in that the impedance is measured from several electrode combinations; of the impedance-cardiographic signals derived from them, the best one, e.g. the one with the largest amplitude, the smoothest one and/or the one bearing the greatest resemblance to a blood pressure signal, is selected for further processing.