Combining measurements from different sensors

[0001] This application is the U.S. national phase of international application PCT/GB01/02544 filed Jun. 8, 2001, which designated the U.S.

[0002] The present invention relates to a method and apparatus for combining measurements from different sensors in order to provide an improved measurement of a parameter. It is particularly applicable to the measurement of physiological parameters.

[0003] Certain parameters can be measured in more than one way. This is useful in giving independent measures of the same quantity. For instance, in the medical field the heart rate can be measured both from an electrocardiogram (ECG) and from a pulse oximetry waveform (used to calculate oxygen saturation). drawings FIG. 4 illustrates schematically these-two-waveforms, FIG. 4a being the electrocardiogram with the heart rate illustrated as HR₁, and FIG. 4b the pulse oximetry waveform with the heart rate illustrated as HR₂. The heart rate is a parameter which can undergo sudden changes. Some of these changes are valid physiological changes, for example ectopic beats which occur prematurely, and therefore give rise to a temporary increase in the heart rate. FIG. 5 illustrates the occurrence of an ectopic beat 50 found in both the electrocardiogram trace and the pulse oximetry waveform. The shorter interval between the preceeding beat and the ectopic beat 50 manifests itself in a measurement of the heart rate as a sudden increase in the heart rate. FIGS. 1 and 2 of the accompanying drawings show time plots of the heart rate measured by pulse oximetry (FIG. 1) and ECG (FIG. 2). It can be seen that in FIGS. 1 and 2 the heart rate in. the early part of the plot is generally of the order of 80 bpm, but that there are occasional sudden increases in heart rate, such as indicated at 10 and 20 which are caused by ectopic beats and thus appear both in the measurement by pulse oximetry and the measurement by ECG.

[0004] However, in addition to changes in the measured heart rate deriving from valid physiological changes, other changes occur which are not physiologically valid, for instance being caused by sudden movement of the sensors on the body surface (e.g. chest movement with ECG electrodes). FIG. 6 illustrates the presence of artefacts 60 on the pulse oximetry waveform which shorten the interval between apparent beats and thus result in apparent increases in the heart rate. These changes are reflected in one measurement, but not the other, as indicated at 12 and 22 in FIGS. 1 and 2 respectively. The fact that the changes appear in one measurement but not the other means that the two measurements could be combined to help decide which heart rate changes are valid physiological ones, and which are artefacts. However, the normal approach of validating one measurement channel against the other involving cross-correlation of the two measurements invariably fails because it is not possible to know in advance (for each recording, for each patient) what value to give to the threshold for accepting, rather than rejecting a change in the heart rate as being valid. Thus although it would appear from FIGS. 1 and 2 that a threshold could be set which would eliminate changes such as indicated as 22, such a threshold is not appropriate for all patients for all recordings, and does not help with the pulse oximetry waveform. The problems are increased in the event of atrial fibrillation when the heart rate changes rapidly as indicated in the region AF in FIGS. 1, 2 and 3.

[0005] Similar problems arise in other fields where a parameter is measured via two or more measurement channels.

[0006] The present invention provides a method and apparatus for improving measurement of a parameter by combining two measurements of it in a way which allows valid changes to be distinguished from artefacts. Accordingly it provides a method of measuring a parameter comprising the steps of: predicting the value of each of two measurements of the parameter, making two measurements of the parameter to produce two measured values of the parameter, calculating the respective differences between the predicted values and the measured values, and combining the two measured values with weights determined by said differences.

[0007] Thus with the present invention a prediction is made for each measurement and the actual measurement is compared with its prediction. The difference is computed, which is termed the “innovation”, and this innovation is used to calculate a weight which will be given to that measurement when it is combined with the other measurement, also weighted according to its innovation. The weights are calculated so that if the innovation on one measurement channel is high, whereas the innovation on the other measurement channel is low, the measurement from the low innovation channel is more heavily weighted. This is because a high level of innovation from one channel coinciding with a low innovation on the other channel is regarded as indicative of an artefact on the higher innovation channel. Thus, the weight given to each value when the values are combined is inversely related to the square or modulus of the difference between the measured value and its predicted value.

[0008] In one embodiment the measured values can be combined according to the formula:

[0009] $\begin{array}{cc}M=M_1\frac{{\sigma }_2^2}{{\sigma }_1^2+{\sigma }_2^2}+M_2\frac{{\sigma }_1^2}{{\sigma }_1^2+{\sigma }_2^2}& \left(1\right)\end{array}$
where M₁ and M₂ are the two measured values, and σ₁ and σ₂ are the differences between the two measured values and their respective predicted values.

[0010] The steps of prediction, measurement, calculation and combination are preferably repeated continuously, with the predicted value for each of the measurements being based on a linear predictive model, e.g the predicted value is based on its preceding predicted value and the preceding innovation (i.e. the difference between the preceding predicted value and the preceding measurement). The predicted value can be obtained by adding to the preceding predicted value a constant times the innovation. The constant is preferably a positive value less than or equal to unity. Alternatively the predicted value for each of the two independent measurements can be calculated by using. a non-linear, predictive model such as a neural network.

[0011] In one embodiment the predicted values can be based on a mathematical model of the system, which may include estimates for process noise and sensor (measurement) noise. Two independent models may be used, one for each of the measurement channels, and the models can include estimates for the process noise and sensor noise, which can be the same for the two channels. In one embodiment the models are Kalman filters.

[0012] The method is particularly applicable to the measurement of heart rate, in which case the two measurement channels can be from an electrocardiogram and a pulse oximetry waveform, though it is applicable to any other measurement of a parameter which can be derived from two or more sources. Thus the method is applicable for more than two measurement channels, and both where the measurements are independent and where they are not truly independent such as from multiple leads of an ECG.

[0013] The invention can also provide for detection of movement artefacts. In this instance high values of innovation are obtained on both channels for the period of movement, and this can be used as a trigger to discard the sections of data which are corrupted by that movement.

[0014] It will be appreciated that the invention can be embodied using computer software and thus the invention extends to a computer program for controlling and executing the method or parts of it, and to a computer readable storage medium carrying the program. The invention also extends to corresponding apparatus for carrying out the method.

[0026] An embodiment of the invention will now be described in which the invention is applied in the medical field to the measurement of heart rate using ECG and pulse oximetry. As illustrated in FIG. 4a the heart rate measured by ECG is derived from the interval between two successive R-wave peaks. The heart rate measurement derived from the pulse oximetry waveform is obtained from the interval between two successive peaks (or troughs) as illustrated in FIG. 4b. FIGS. 1 and 2 illustrate heart rate plots derived from these two measurements.

[0027] With this embodiment of the present invention a model of the process generating the heart rate is constructed. The same model is run independently for each measurement source (i.e. one for the ECG measurement channel and one for the pulse oximetry measurement channel). In this embodiment the model is a Kalman filter. In general a Kalman filter uses a process model and an observation model. The process model models the state of the system at time t+1 in terms of its state at time t. The measurement or observation model indicates how the measurement at time t is related to the state of the system at time t. Thus in general terms:

• • [0028] X_t+1=Ax₁+w (Process model)
  • [0029] Y_t=Cx_t+v (Observation model)
    where:
  • [0030] w˜N (O, Q)— Gaussian process noise with zero mean and variance Q
  • [0031] v˜N (O, R)— Gaussian measurement noise with zero mean and variance R
    and:
  • [0032] k—vector of state variables x
  • [0033] n—vector of observable or measurements y
  • [0034] State x evolves according to simple first-order Markov dynamics;
  • [0035] A is the k×k state transition matrix
  • [0036] Each measurement vector y is related to the current state by a linear observation process; C is the n×k observation or measurement matrix

[0037] In this embodiment the general Kalman filter is simplified to use scalar quantities and the same process and measurement noise models (w, v) are used on both measurement channels. Thus the simplified Kalman filter is as follows:

• • [0038] x_t+1=Ax_t+w (Process model)
  • [0039] y_t=Cx_t+v (Observation model)

[0040] The model is further simplified by setting C=1, implying that the heart rate is both the state describing the process and the measurement. Further, it is assumed that A=1, implying that the next heart rate is the same as the previous one with the variability allowed for by the process noise model.

[0041] Using this model, on each channel, the process of combining the two measurements then involves the following steps:

• • [0042] 1. From knowledge of previous history up to time t (a) predict the next state xpred; (b) from xpred predict the next measurement ypred
  • [0043] 2. Make the measurement y_t+1
  • [0044] 3. Compute the innovation:
    • [0045] y_t+1=ypred+ε_t+1
    • [0046] where ε_t+1 is the difference between the actual value and the predicted value: the innovation.
  • [0047] 4. Compute the variance σ_t+1²:
    σ_t+1²=ε_t+1²
    • [0048] σ² the variance, is the inverse of the “confidence” which is associated with the prediction
• [0049] 5. Mix the heart rate measurements in inverse proportion to the variance associated with each one:

[0050] $\mathrm{HR}={\mathrm{HR}}_1\frac{{\sigma }_2^2}{{\sigma }_1^2+{\sigma }_2^2}+{\mathrm{HR}}_2\frac{{\sigma }_1^2}{{\sigma }_1^2+{\sigma }_2^2}$
An example of an implementation of this model in MATLAB is given in Appendix 1. That example is general, and will work for vector quantities, though in this embodiment the quantities are scalar. It can be seen from appendix 1 that the predicted heart rate for each new measurement cycle (xnew) is equal to the previously predicted value (xpred) plus the Kalman gain K times the innovation e. The Kalman gain K is derived from the predicted variance Vpred and the measurement variance R. The predicted variance is derived from the previous predicted variance and the process noise Q. To start the process off it is initialized using an initial value of the heart rate as 80 and an initial value of the state variance of 100. The process noise in the Q in this embodiment is set to 5 and the measurement noise variance R is set to 10.

[0051] It will be clear from the implementation that, as normal with a Kalman filter, the variance and Kalman gain are not dependent on the measurement values. The measurement values are only used in the new prediction of heart rate via the innovation e. Thus it will be noted that for the constant values of Q and R used in this example the Kalman gain K tends to 0.5 and the state co-variance V tends to 5. However, K can be made adaptive by modifying the values for the variance constants Q and R, preferably the process variance Q, according to the type of process being encountered, for example atrial fibrillation (where there is a high level of process noise) as opposed to a healthy heart rate (during which there is a low level of process noise).

[0052] FIGS. 8, 9 and 10 illustrate values for the estimated heart rate xpred, the innovation e and the variance σ² for the heart rate plot shown in FIG. 7 (in this case the ECG measurement channel).

[0053] FIG. 3 illustrates the result of combining the two measurement plots from FIGS. 1 and 2 using this embodiment. It can be seen that the movement artefacts 22 on the ECG channel in FIG. 2 have been removed from the combined measurement, even though they occur during a period of atrial fibrillation.

[0054] Thus with this embodiment the difference between the measurement and its predicted value is used to indicated the degree of confidence in that measurement. The higher the difference the lower the confidence. Formula (1) above-is used to combine the two measurements. This can be summarised as follows:

• • [0055] 1) Valid heart rates on both channels: low innovation on both channels; weight both measurements equally
  • [0056] 2) Valid sudden change (e.g. ectopic beat) seen on both channels: high innovation but on both channels; therefore, measurements are again weighted equally
  • [0057] 3) Artefact on one channel: high innovation on one channel only; therefore the information from that channel is ignored by being given a very low weighting (low confidence).

[0058] The method can also be used to provide a movement artefact detector, i.e. to detect when movement artefact is present on both channels and therefore no useful information is available. This is characterised by high values of innovation on both channels for a sustained period of time. This can be used to discard the sections of raw data which are corrupted by this movement and to indicate that no valid heart rate estimate can be derived during those periods.

[0059]

Appendix 1
load -ascii ecghr_13
data_file = ecghr_13;
time = data_file(;,1);
hr = data_file(:,2);
start = 1
stop = size(data_file),1);
fprintf(‘number of R—R intervals detected = %d \n’, stop);
hr_limit = 200;
	% X(t + 1) = A X(t) + noise(Q) - process model with
	Q as variance of noise w
	% Y(t) = C X(t) + noise(R) - measurement model with
	R as variance of noise v
ss = 1;	% state size - sets to one dimensional, ie scalar though
	routine works for vectors
os = 1;	% observation size - sets to one dimensional, ie scalar
A = [1];	% assume x(t + 1) = x(t)
C = [1];	% assume y = x
Q = 5.0*eye(ss);	% process noise - eye is the identity matrix in
	MATLAB-here just unity
R = 10.0*eye(os);	% measurement noise variance
initx = [80];	% initial state value (HR of 80 bpm)
initV = 100*eye(ss);	% initial state variance
xnew = initx;	% - initialisation
Vnew = initV;
for i = start:stop %-start of cycle
	x = xnew;	% update from previous cycle
	V = Vnew;	% update from previous cycle
	xpred = A*x;	% prediction of state
	Vpred = A*V*A′ + Q;	% prediction of state covariance, A′ is
		transpose of A
	ypred(i) = C*xpred;	% prediction of measurement
	y(i) = hr(i);	% “make measurement”
	e = y(i) − ypred(i);	% calculate innovation
	innov(i) = e;	% for plotting
	sigma2(i) = e*e;	% variance for saving
	S = C*Vpred*C′ + R;	% innovation covariance
	Sinv = inv(S);	% invert S
	K = Vpred*C′ *Sinv;	% compute Kalman gain
	xnew = xpred + K*e;	% update state by the innovation controlled
		by the Kalman gain
	Vnew = (eye (ss) − K*C) *Vpred;	% update state covariance
	end
end