NON INVASIVE DETERMINATION BY BIOIMPEDANCE

Enrico Tam SS.MM. Università degli Studi di Verona

SS.MM._UNIVR 2012/2013 Potential Methods To Measure Cardiac Output • Invasive methods:  Thermodilution  Lithium kinetics  Esophageal Doppler  Pulmonary arterial catheterization and Dyedilution • Non Invasive methods:   Pulse waveform methods  Bioimpedance Electrical impedance: measure of the opposition that a circuit presents to the passage of a current when a voltage is applied.

Impedance (Z) extends the concept of resistace (R) to AC circuit.

Ohm’s law: (DC) R=V/I (AC) Z = V/I Bioimpedance Technology

The electrical bioimpedance technology uses constant electric current stimulation for identification of electrical impedance variations, which are associated with physiologic or pathological changes in the body.

• Intra/extracellular fluids, membranes behave like a resistor-capacitor network, whereas blood vessels and lymphatic channels behave like electrical conductors

• When alternating current (AC) is used, the capacitance component of the impedance reacts differently to the various electrical frequencies.

• Basic tool for measuring resistance in the body is the potentiometer, and its two bipolar ( or four tetrapolar) electrodes. Impedance Cardiography With Impedance Cardiography (ICG), disposable sensors on the neck and chest are used to transmit and detect electrical and impedance changes in the thorax, which are used to measure and calculate hemodynamic parameters. thoracic electrical bioimpedance (TEB) whole body electrical bioimpedance (WBEB) Principles When AC current from 20 to 100 kHz The resistivity (Ω/cm) is: is applied, it is primarily distributed via . 150 blood, the extracellular fluid and the blood vessels. . 63 plasma, . 750 , The Electric current passes through . 1275 lungs, conduits of higher conductance. . 2500 fat

For each systolic increase in the aortic blood volume, there is a proportional increase in the aortic electrical conductance, expressed as: ΔV equals the , V is the aortic volume just before the onset of the systole, ΔR is the systolic electrical resistance (also termed Z or impedance variation), R (or Z) is the baseline resistance (or impedance) of the electrical field Principles

• From Mann H and Kedrov et al. first attempt to exploit the impedance electrical change for calculating the cardiac output:

• the creator of the R (or Z) is primarily the aortic systolic volume change

Mann H. Proc Soc Exp Biol Med 1937; Kedrov AA. Klin Med 1941, 1948. Bonjer et al. Circulation 1952. Principles

• Tischenko by ballistocardiogram, was • Kubicek et al. , in 1966 able to provide additional proof that the origin of the Z is to be found in the systolic dilatation of the and its tributaries.  man :

• Kubicek et al. 1974,

 woman:

Where:

i) L is the distance between the sensing electrodes ii) Z0 is baseline impedance

Tischenko MI Sechenov Physiol J 1973; Bernste DP.Ttextbook of critical care medicine 1989.

Bioimpedance analysis

Typical thoracic impedance cardiogram; a) the conventional electrode arrangement.

CO with ICG and TD Limits and applicability of ICG

• Patient-related limitations (obesity, emphysema, pulmonary edema, mechanical ventilation etc.) • Inaccuracies related to motion artifact and electrode position • Costs sometimes prohibitive • The impedance signal is of very low magnitude (1/1000 of the magnitude of an EKG signal) and therefore its sensitivity to interferences and artifacts is high. • The Z0 parameter, which is key in the classic equations, is inaccurate when the patient is not normal or near normal, not at rest or when electrode placement is sub-optimal • New devices for impedance cardiology were made to be independent of baseline thoracic impedance (Z0). PhysioFlow®

• Physio Flow PF-07; Manatec Biome´dical; Macheren,France) emits an alternating electrical current of 1.8 mA and 75 kHz via electrodes (Ag/AgCl, Blue Sensor VL; Medicotest; Oelstykke, Denmark).

• Two sets of two electrodes, one transmitting and one sensing, are applied above the supraclavicular fossa at the left base of the neck and along the xiphoid, respectively. Another set of two electrodes is used to monitor a single ECG signal. PhysioFlow® PF07 EnduroTM Methodology

• fc is the HR (beats per minute) based on the R-R interval determined on the ECG first derivative • SVi is SV index (milliliters per meters squared of BSA • BSA is the body surface area (meters squared). • BM is the body mass (kilograms), and H is height (centimeters) (Haycock formula)

• SVical, and is based on 30 consecutive beats, the patient being immobile and relaxed sitting on the cyle ergometer. • SVical records the largest Z variation during ventricular systole (maximum Z minimum Z) and the largest variation rate of the Z signal, called contractility index (dZ/dtmax) • Thoracic flow inversion time (TFIT) [in milliseconds • CTI =dZ/dtmax Signals

k is a constant, and SVical represents the baseline reference.

Waveforms obtained with the PhysioFlow device. Z =bioimpedance signal; Zmax = maximum bioimpedance signal. How to improve the robustness/stability of the signal?

• SM-ICGTM uses a first level optimized input filter finely adapted to the high frequency current transmitted, so that the Impedance signal measured is mostly free from noise coming from other electronic or physiologic emitters.

• SM-ICGTM also uses a second level highly sophisticated filter, called HD-ZTM (for High Definition Impedance) to eliminate all artifacts from the chest impedance signals that are not correlated with the heart cycle and the generation of cardiac flow. With HD-ZTM the relative amplitude of the noise to the amplitude of the cardiac signal does not matter. It has been developed to cancel severe artifacts in the most demanding situations like a running race (marathon) or in a very noisy critical care environment. Examples of how the filter works on "difficult" dZ signals

Example 1: HD-ZTM applied to the dZ signal of a marathon runner (the green signal above is before filtering and the green signal below is after HD-ZTM filtering) Results

Comparison between CO values obtanined using bioimpedance (PhysioFlow®) and direct Fick method during incremental exercise test. Advantages of Physioflow References

• Moshkovitz Y, Kaluski E, Milo O, Vered Z, Cotter G. Recent developments in cardiac output determination by bioimpedance; comparison with invasive cardiac output and potential cardiovascular applications. Curr Opin Cardiol 19: 229–237, 2004. • Charloux A, Lonsdorfer-Wolf E, Richard R, et al. A new impedance cardiograph device for the non- invasive evaluation of cardiac output at rest and during exercise: Comparison with the “direct” Fick method. Eur J Appl Physiol :82:313–320, 2000. • Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet; (I):307–310. 1986. • H. H. Woltjer, H. J. Bogaard, P. M. J. M. de Vries. The technique of impedance cardiography. European Heart Journal 18, 1396-1403, 1997 • Paul E. Marik, Paul G. Barash, Giovanni Landoni. Noninvasive Cardiac Output Monitors: A State-of the- Art Review Journal of Cardiothoracic and Vascular Anesthesia, Vol xx, No x (Month), 2012. in press • Andrew Sherwood, Michael T. Allen, Jochen Fahrenberg, Robert M. Kelsey, William R. Lovallo, Lorenz J.P. Van Doornen. Methodological Guidelines for Impedance Cardiography. Psychophysiology; Vol 27, No.1, 1990. • Hanan Keren, Daniel Burkhoff, Pierre Squara. Evaluation of a noninvasive continuous cardiac output monitoring system based on thoracic bioreactance. Am J Physiol Heart Circ Physiol 293: H583–H589, 2007. • A. Scherhag, J.J. Kaden, E. Kentschke, T. Sueselbeck, and M. Borggrefe. Comparison of Impedance Cardiography and Thermodilution-Derived Measurements of Stroke Volume and Cardiac Output at Rest and During Exercise Testing. Cardiovascular Drugs and Therapy 19 141–147, 2005.