The present invention relates generally to the field of impedance determination in biological tissues and more specifically to methods and devices for performing impedance measurements using an impedance sensor having a pair of electrodes which are positioned within a blood vessel. 
The use of impedance measurements of body tissues or organs for obtaining various mechanical properties and physiological parameters of different organs or body parts are well known in the art.  K N Hoekstein and G F Inbar, in a paper titled xe2x80x9cCardiac Stroke Volume Estimation from Two Electrodes Electrical Impedance Measurementsxe2x80x9d published as Technion Department of Electrical Engineering Publication EE PUB No.  911, February 1994, disclose, inter alia, the use of a two electrode based impedance measurement method and device for estimating cardiac stroke volume of. 
Various other applications of Intracardiac impedance methods for measurement of various hemodynamic parameters are well know in the art.  Measurement of intracardiac and transcardiac impedance has been described for use in the control of pacemakers and defibrillators.  It is commonly accepted that the impedance signal derived from electrodes attached to the heart holds information regarding the cardiac hemodynamics of the patient. 
Salo et al.,  in U.S. Pat.  No.  4,686,987 assume that the amplitude of the impedance signal detected through a tripolar lead implanted in the right ventricle (RV) correlates with the heart""s stroke volume. 
Chirife, in U.S. Pat.  No.  5,154,171 has proposed that intracardiac impedance is representative of the volume of the heart and therefore, ejection fraction may be estimated by assuming that the impedance at end-diastole is representative of end-diastolic volume, and the impedance at end-systole is representative of end-systolic volume. 
Impedance measurements have also been used to estimate respiratory minute ventilation.  For example, U.S. Pat.  4,702,253 to Nappholz et al.  discloses a metabolic demand pacemaker utilizing tripolar leads implanted in the right ventricle (RV), or the left atrium for determining the respiratory minute volume by measuring the impedance between a lead electrode and the pacemaker case. 
U.S. Pat.  No.  4,901,725 to Nappholz et.al disclose a minute volume rate responsive pacemaker utilizing a bipolar lead implanted in the right ventricle for determining the respiratory minute volume by measuring the impedance between a lead electrode and the pacemaker case.  The impedance methods disclosed hereinabove by Nappholz et al.  have the disadvantage of being sensitive to patient postural changes and to patient activity because of variations in the distance and impedance between the lead electrode and the case due to the posture changes or the patient""s activity, respectively. 
U.S. Pat.  No.  4,773,401 to Citak et al.,  discloses a quadrupolar electrode implanted in the right ventricle for determining pre-ejection interval to control the rate of a pacemaker. 
U.S. Pat.  No.  5,235,976 to Spinelli discloses a method and apparatus for managing and monitoring cardiac rhythm using intra-ventricular impedance measurements. 
U.S. Pat.  No.  5,197,467 to Steinhaus et al.,  discloses a multiple parameter rate responsive cardiac stimulation apparatus using impedance measurement methods. 
U.S. Pat.  No.  5,531,772 to Prutchi, U.S. Pat.  No.  5,735,883 to Paul et al.,  and U.S. Pat.  No.  5,507,785 to Deno, disclose pacemakers incorporating improved circuitry for cardiac impedance determination for eliminating various types of background interference using various combinations of standard ventricular and/or atrial leads. 
U.S. Pat.  No.  5,578,064 to Prutchi, discloses a rate responsive cardiac pacemaker with impedance sensing, having impedance measuring circuits using a Wein bridge for eliminating baseline impedance. 
A known problem encountered in impedance measurements is that measuring the impedance over a relatively long path results with an impedance signal which is only partially correlated to the mechanical property or to the physiological parameter which one seeks to determine.  Additionally, the resulting impedance signal may include signal components which are unrelated to the mechanical property or the physiological parameter which one desires to determine.  Such signal components may be due to, inter alia, patient""s postural changes, patient""s physical activity, or other different physiological parameters or mechanical properties unrelated to the property or parameter that needs to be determined. 
Prior art impedance measurements rely on relatively large changes in impedance by using electrodes that are separated widely apart from each other.  The advantages of placing the electrodes far apart is that relatively large changes in impedance are measured between the electrodes.  This allows a relatively simple circuit to be employed (such as described by Hoekstein and Inbar) which gives relatively large sensitivity of the measured parameter.  For example, Hoekstein and Inbar disclose an electrode separation of 2.5 centimeters for measuring right ventricular volume. 
Thus, prior art methods suffer from artifacts that are related to posture and movements.  This is due to the fact that when widely separated electrodes are used, changes in the distance between the electrodes which are related to posture changes and movements are strongly reflected in the impedance measurement.  This effect introduces noise over the desired measured parameter which greatly limits the application of these measurements.  Even in the case of two relatively close electrodes of a lead disposed in the right ventricle, there still is the problem of posture or movement since the lead which includes the electrodes bends during movements and causes posture or movement related changes of the measured impedance
The present invention provides an improved impedance measurement method and device for providing an impedance signal correlated to a mechanical or physiological property of an organ or a part of a body. 
The impedance measurement device includes an impedance sensor and an impedance determining unit suitably connected thereto. 
A feature of the impedance sensor is that it includes two or more electrodes for impedance determination and that all of the electrodes are disposed within a blood vessel which is mechanically or physically coupled to the organ or the part of the body.  The sensor is positioned in the blood vessel such all of the electrodes used for determining the impedance are disposed within the blood vessel.  The intra-vessel impedance measured by the device is correlated to the mechanical or physiological property of the organ or the part of a body to which the blood vessel is mechanically or physically coupled. 
The impedance measuring unit and the impedance sensor may be adapted for using various impedance measuring methods known in the art, including, but not limited to impedance determining methods using high frequency modulated currents or current pulses, and methods using various test current pulses. 
The number of the electrodes included in the impedance sensor may vary depending on the specific impedance measuring method used.  An electrode pair configuration is suitable for use in methods which apply, a modulated current or a current pulse or any other current waveform between two electrodes and senses the voltage developed across the same two electrodes.  However, more than one electrode pairs can be used. 
Alternatively, two electrode pairs may be used in the sensor, a first pair of electrodes for applying a modulated current or a current pulse or any other current waveform therebetween, and a second pair of electrodes for sensing the voltage difference due to the current applied through the first pair of electrodes.  However, all the electrodes of the two pairs of electrodes are disposed within the blood vessel.  The application of the current to the first pair of electrodes and the measurment of the voltage difference across the second pair of electrodes are performed by the impedance determining unit.