Hemodynamic monitor and method of hemodynamic monitoring

1. An apparatus for determining stroke volume comprising:

an input for receiving an arterial pressure waveform;

means arranged to define a start point within the arterial pressure waveform at a beginning of a systolic arterial pressure increase due to a ventricular contraction phase of a heart beat;

means arranged to determine an end point of the heart beat for the purpose of calculating stroke volume by

determining whether there is a part or point of the arterial pressure waveform that has substantially the same arterial pressure value as said starting point,

responsive to said arterial pressure value at said starting point being regained, setting the end point to be the part or point of the arterial pressure waveform that has substantially the same arterial pressure value as said starting point, and

responsive to said arterial pressure value at said starting point not being regained, extrapolating the end point of the heart beat to a point where the arterial pressure waveform has substantially the same arterial pressure value as said starting point to define the end point;

means arranged to calculate a stroke volume from a part of the arterial pressure waveform or of the extrapolated arterial pressure waveform between the start point and the end point; and

an output means and configured to output, via the output means, an indication of at least one of a current stroke volume, a current heart rate, a number of previous stroke volumes and a number of previous heart rates.

2. The apparatus according to claim 1 further comprising:

means arranged to identify a baseline change in the arterial pressure waveform; and

means arranged to correct the part of the arterial pressure waveform or of the extrapolated arterial pressure waveform between the start point and the end point to account for said baseline change;

wherein the stroke volume is calculated from the corrected part of the arterial pressure waveform or of the extrapolated arterial pressure waveform.

3. The apparatus of claim 1 , the apparatus configured to correct part of the arterial pressure waveform or of the extrapolated arterial pressure waveform between the start point and the end point for an influence of an ectopic heart beat or of atrial fibrillation or of respiratory changes on the pressure waveform, wherein the stroke volume is calculated from the corrected part of the arterial pressure waveform or of the extrapolated arterial pressure waveform.

4. An apparatus according to claim 1 , further configured to identify a trend in the last half, third, quarter or fifth of a part of the arterial pressure waveform relating to one heart beat and extrapolating the arterial pressure waveform by continuing the identified trend until a value substantially corresponding to the value of the start point is reached.

5. An apparatus according to claim 4 , wherein identifying said trend comprises determining a gradient in the said part of the arterial pressure waveform and wherein continuing the trend comprises extending the arterial pressure waveform using the determined gradient.

6. An apparatus according to claim 1 , further configured to apply a non-linear correction to the arterial pressure waveform.

7. An apparatus according to claim 1 , further configured to calculate the stroke volume by determining the pulsatility of the part of the arterial pressure waveform relating to a heart beat between the start point and the end point.

8. An apparatus according to claim 7 , further configured to determine an absolute stroke volume by multiplying the pulsatility with a calibration factor specific to a patient.

9. An apparatus according to claim 1 , configured to determine cardiac output by multiplying stroke volume with a heart rate.

10. An apparatus according to claim 1 , configured to store the stroke volumes or cardiac outputs for a number of heart beats and to provide an indication should a fluctuation in the stroke volume or the cardiac output over time exceed a predetermined value.

11. An apparatus according to claim 1 , further configured to determine a heart rate based on the received arterial pressure waveform by performing autocorrelation on the arterial pressure waveform over a number of consecutive beats.

12. An apparatus for monitoring patient respiration comprising an apparatus according to claim 1 , the apparatus configured to determine a respiratory rate based on cyclic changes in the stroke volume or cardiac output.