Considerations on Impedance Cardiography

CONSIDERATIONS ON IMPEDANCE CARDIOGRAPHY

PIERRE CASTHI~LY, SIVAM RAMANATHAN AND JACK CHALON

ABSTRACI Impedance cardiography is an electronic plethysmographic technique which provides infor- mation on cardiac stroke index, myocardial performance, thoracic fluid content and peripheral circulation. The method has gained popularity in recent years because it is not invasive. While less precise in absolute terms than invasive methods the results are repro- ducible, and the technique accurately assesses variations in measurements. It may be used in most anaesthetized patients without the possibility of any of the complications which some- time accompany the use of more precise invasive methods. This paper describes impedance cardiography and methods for its use.

Description FIRST dtt IMPEDANCE CARDIOGRAPHY is an electronic DERIVATIVE plethysmographic technique which provides in- OF THE ZCG formation on cardiac stroke index, myocardial performance, thoracic fluid content, and peripheral circulation. The method has gained popu- r larity in recent years because it is not invasive. Although the technique was introduced as early O, I Ohm ZCG A/2 ...... as 1930 ~'2 it was only perfected in the late sixties A to meet the requirements of the Apollo space flights. In 1966, Kubicek, et al) had derived a ECG formula to calculate cardiac stroke volume through changes in thoracic impedance. The impedance (z) offered by mammalian tis- PHONO- CARDIOGRAM sue to an alternating electric current of 100 kHz and 4 ma is resistive in nature. The waves derived FIGURE 1 Simultaneous tracings of first derivative from impedance variations, occurring during the of the impedance cardiogram, impedance cardiogram. cardiac cycle, form the Impedance Cardiogram ECG and phonocardiogram. The first derivative tracing or ZCG (Figure 1). Kubicek, et al. 3 investigated and the impedance cardiogram show recordings of calibration signals on the right. Points A, B, X, Y, O and the origin of the impedance wave in dogs with Z of the dz/dt wave and A, C and V of the ZCG are electromagnetic flowmeters during the selectiv~ explained in the text. The phonoeardiogram displays obstruction of right and left ventricular outflows. the first and second heart sounds. Note that the point X They found that the major component of the pul- of the dz/dt wave occurs at the same time as the second heart sound. satile z wave was related to aortic outflow and a minor fraction to right ventricular outflow. Res- period. The first derivative of the impedance piratory impedance variations are electrically cycle (ZCG) includes points A, B, X, Y, O and Z dampened. (Figure 1). 6 They are respectively related to atrial A typical ZCG (Figure 1) consists of 3 seg- contraction, maximal first sound vibration, aortic ments: z'4's (1) wave A corresponds to the P wave and pulmonary valve closure, mitral valve open- and the PR interval of the ECG, (2) wave C to the ing, and the onset of the third sound. At the QRS complex and, thus, ventricular ejection, and beginning of left ventricular ejection is a point (J) (3) wave V is synchronous with the protodiastolic which follows B and is well defined in Figure 2. Pierre Casth~ly, M.D., Instructor; Sivam Ramanathan, The differential at the peak between B and X M.D., Associate Professor; Jack Chalon, M.D., Pro- (dz/dt)m~, denotes maximal ventricular ejection. fessor. Department of Anesthesiology, New York Uni- versity Medical Center, 550 First Avenue, New York, Utilization New York 10016. Address reprint requests to: Pierre Casthrly, M.D., Impedance cardiography is used to calculate New York University Medical Center, Anesthesiology stroke volume. One approximates the thorax to a H-623,550 First Avenue, New York, New York 10016. cylinder containing a homogeneous conduction 481 Canad. Anaesth. Soc. J., vol. 27, no. 5, September 1980 482 CANADIAN ANAESTHETISTS" SOCIETY JOURNAL

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FIGURE 3 Electrode placement for usingimpedance "' JlrllJlil " cardiograph. Exact locations are described in the text. IT ~ Electrodes I and 4 are used to pass a constant current through the chest and also detect ECG signals. The FIGURE 2 First derivative impedance cardiogram impedance cardiogram, its first derivative and the basic recorded from an adult patient. Points A, B, J, X, Y, O thoracic impedance [Zo) are detected by electrodes 2 and Z are explained in the text. Point J corresponds to and 3. onset of rapid ejection from the left ventricle. Ven- tricular ejection time T (in seconds) is the time interval mylar strip electrodes 2.5 cm wide (Figure 3), between the points J and X. T is 0.32 seconds in this recording. Strips 2 and 3, respectively, encircle the root of the neck and the thorax at the level of the xiphoid medium.~ Human chest anatomy introduces er- process. Strips 1 and 4 are 3 cm above and below rors which vary among individuals, Therefore, strips 2 and 3 respectively. Adequate tension en- the technique is more useful in following trends sures skin contact. Patient electrodes are num- than in calculating absolute values. bered sequentially and securely clipped to corre- According to Scott, ~ Bonjer established a re- sponding leads. The system incorporates a basic lationship between impedance changes (Z) and circuit which sends a 4 ma and 100 kHz alternat- changes in volume (V) in a symmetrical homo- ing current between electrodes I and 4. The geneous cylinder with a basic resistivity of product of this current (I) and the thoracic impe- f ohm/centimeters, which he expressed as: dance (Z0) generates a voltage (E) equal to I x Z0 Formula A V = .f(l/z)2Z during its passage between electrodes 2 and 3. 6 Amplification of this signal produces the impe- in which l was the length of the cylinder, z the dance cardiogram (ZCG). A phonocardiograph impedance of the cylinder and Z impedance circuit and an ECG signal from electrodes 1 and 4 changes. The equation may be used to calculate are included (Figure 1). ~ ZCG, (dz[dt)mi., ECG stroke volume by substituting Z by peak ven- and basic thoracic impedance (Z0) can be ob- tricular ejection rate multiplied by ventricular tained through output jacks connected to a re- ejection time in seconds (T), or: corder3 A warming up time of 20 minutes is re- Formula B V = f(l/z)Z(dz/dt)minXT quired for efficient operation. The phonocardiograph identifies point X (or second heart sound); Both haematocrit and temperature affect body however, since this point is usually well defined, resistivity. 7 An average resistivity value of the addition of phonocardiography is often an 150ohms/cm has thus been used to minimize unnecessary additional refinement. Patient these effects, g If the haematocrit has been calcu- movements (including breathing) are dampened lated, the resistivity becomes: by a trace stabilizing control which eliminates baseline shifts. f = 56 x 8e ~176 Optimal results are obtained by starting the in which H is the haematocrit, and e the natural tracing during voluntary apnoea. To simplify logarithm. stroke volume calculations the cardiograph is Most commercially available impedance cardio- switched to calibration and the gain adjusted to graphs are portable units weighing around 8 kg. 1 cm for a signal of I ohm. The (dz/dt)min is re- A facultative computer incorporating an oscillo- corded during at least 6 heart beats. The Z0 con- scopic display may be used but is an unneces- trol is then adjusted to the "hold" position for sary expense. It can be replaced by a two-channel recall. Each set of recordings is followed by a recorder with sweeps of 25 and 50 mm/sec using calibration tracing signal to estimate (dz/dt)min timed chart paper. amplitude and ventricular ejection time. The (dz/dt),,in and stroke volume calculation are Methodology shown in Figure 2. Vertical lines indicate 0.04 sec To obtain an impedance cardiogram, the pa- intervals. Ventricular ejection time is measured tient wears a set of four adhesive aluminized from J to X (0.32secs). The amplitude of CASTHELY, el al.: IMPEDANCE CARDIOGRAPHY 483

(dz/dt)m,,, is 1.4 cm which, when divided by the assess myocardial isometric time-tension re- amplitude of the calibration signal (I cm), pro- lationships. vides the first derivative of impedance during Accumulation of fluid in the chest will alter ventricular ejection. With a Zo of 22 ohms, a basic thoracic impedance (Zo). Pomerantz, et haematocrit of 44 per cent, resistivity factor of al. 2~'22 noted that as little as 50 ml of fluid in the 150 ohm/cm, electrodes 2 and 3, 24 cm apart, and pleural cavity produced significant impedance a heart rate of 65/min, the stroke volume can be changes. Therefore, these measurements can be calculated from Formula B, previously given; used both for the diagnosis of pleural effusions which becomes: and early pulmonary edema and to follow their course. Luepper, et at. 23 found that changes in Z0 Stroke Volume = 150(24/22) 2 x 1.4 could quantitate changes in central blood vol- • 0.32 = 80 ml ume. Keller and Blumberg z4 noted impedance changes during haemodialysis. Pulmonary em- and the cardiac output, 80 x 65 = (5,200 ml/min, boli, pneumothorax and pulmonary atelectasis or) 5.2 I/rain. have been diagnosed by Berman, et al. 25 by the Many authors have investigated the reliability impedance method. Impedance cardiography can of the measurement of cardiac output by the im- also assess peripheral circulatory adequacy if en- pedance technique, t~ The general opinion is circling electrodes are placed around the affected that overestimations are frequent but reproduci- extremity.9 Finally, all these manoeuvres can be bility accurate (when compared with thermo- carried out in most anaesthetized patients, espe- dilutional techniques). Results can be consid- cially when invasive methods are considered to erably improved by assessing blood resistivity be contraindicated. The technique is especially rather than using a fixed mean value of useful when unexpected circulatory failure or in- 150 ohms/cm. ~0 However, in the very obese, re- creased pulmonary fluid content occur during sistivity must be raised to 175) 2 Pate, et al., ~3 surgery. who compared impedance cardiac outputs against dye dilution technique, the radioisotope method, pulse contour assessments and electro- REFERENCES magnetic flowmetry, confirmed that the plethys- 1. SCOTT, N. Bio-impedance measurement. In mographic technique erred towards overestima- Non-invasive Clinical Measurement. Tayler, tion but accurately assessed variations in mea- D.E.M., Whamond. J., Eds., Baltimore: Univer- surements. Inaccuracies have been ascribed to sity Park Press (1977). right to left shunts, inequalities in velocity during 2. MILLER, J.C. & HORV^TH, S.M. Impedance cardiography. Psychophys 15:80 (1978). left ventricular ejection, pectoral assymetry and 3. KUalCEK,W.G., KO'rTKE, F.J., RAMOS, M.U., et influence of right ventricular ejection on pulsatile al. The Minnesota impedance cardiography-theory 2. 14 and applications. Biochem. Eng. 9:410 (1974). 4. VAN DEWATER, J.M., MOUNT, B.E., BARELA. J.R., et al. Monitoring the chest with impedance. Additional considerations Chest 64:597 (1973). 5. KARNEGIS,J.N. & KonlCEg, W.G. Physiological The first derivative of the impedance cardio- correlates of the cardiac thoracic impedance gram can be used to assess other indices of waveform. Am. Heart J. 79: 519(1970). myocardial performance. Among these are: 6. L^BABIOI,Z., EHMKE, D.A., DURNIN, R.D.,etaL 1. The Heather Index obtained by dividing The first derivative thoracic impedance cardio- maximum dz/dt by the time elapsed since the gram. Circul. 4l: 651 (1970). 7. GEDOES, L.A. & SADLER, C. The specific resis- preceding R wave of the ECG (or RZ interval in tance of blood at body temperature. Med. Bio. ohms/sec2) which approximates the ratio of the Eng. I1: 336(1973). pre-ejection phrase by the ventricular ejection 8. DENNISTON, J.C., MAILER, J.T., REEVES, J.T., et time, ~9 al, Measurement of cardiac output by electrical impedance at rest and during exercise. J, Appl. 2. Left Ventricular Ejection Time measured Physiol. 40:91 (1976). from the dz/dt wave form which yields results 9. IFM/Impedance Cardiography Instruction Manual similar to those obtained by the study of the pulse (1975). wave tracingfl 9 and 10. SECHER, N.J., THOMSEN, A. & ARNSBO. P. Mea- 3. The Siegel Index 2~ which claims that the surement of rapid changes in cardiac stroke volume. An evaluation of the impedence cardiography point B of the dz/dt wave form correlates well method. Acta Anaesth. Scand. 21:353 (1977). with the isovolumetric period of ventricular con- II. RASMUSSEN, J.P., SORENSEN, B. ~/. KANN, T. traction (dz/dt)max and can, therefore, be used to Evaluation of impedance cardiography as a non- 484 CANADIAN ANAESTHETISTS' SOCIETYJOURNAL

invasive means of measuring systolic time intervals dance cardiac output system. Aerospace Med. 37: and cardiac output. Acta Anaesth. Scand. 19:210 1208 (1966). (1975). 19. HILL, D.W. & MERRIFIELD, A.J. Left ventricular 12. RASMUSSEN, J.P., ERIKSEN, J. & ANDERSEN, J. ejection and the Heather Index measured by non- Evaluation of impedance cardiography during invasive methods during postural changes in man. anesthesia in extremely obese patients. Acta Acta Anaesth. Scand. 20:313 (1976). Anaesth Scand 21:342 (1977). 20. SIEGEL, J.H., FABIAN, M., LANKAU, C., et al. 13. PATE, T.D., BAKER, L.E. & ROSBOROUGH, J.P. Clinical and experimental use of thoracic impe- The simultaneous comparison of the electrical im- dance plethysmography in quantifying myocardial pedance method of measuring stroke volume and contractility. Surgery 67:907 (1970). cardiac output with four other methods. Cardiovas. 21. POMERANTZ, M., BAUMGARTNER, R., LAURID- Res. Cen. Bull. 14:39 (1975). SON, J., et al. Transthoracic electrical impedance 14. LABABIDI,Z. EHMKE, D.A., DUaNIN, R.E., etal. for the early detection of pulmonary edema. Evaluation of impedance cardiac output in chil- Surgery 66:260 (1969). dren. Pediatrics 47:870 ( 1971). 22. POMERANTZ, M., DELGADO, F, & EISEMAN, B. t5. GABRIEL, S.. ATTERHOG, J.H., OR/5, L., et al. Clinical evaluation of Iransthoracic electrical im- Measurement of cardiac output by impedance pedance as a guide to intrathoracic fluid volumes. cardiography in patients with myocardial infarc- Ann. Surg. 171: 686 (1970). tion. Scand. J. Clin. Lab. Invest. 36: 29(1976). 23. LUEPKER, R.V,, MICHAEL J.R. & WARBASSE, J.R. 16. JUDY, W.V., LANGLEY, F.M., McCOWEN, K.D., Transthoraeic electrical impedance: quantitative et al. Comparative evaluation of the thoracic impe- evaluation of a non-invasive measure of thoracic dance and isotope dilution methods for measuring fluid vohnme. Amer. Heart J. 85:83 (1973). cardiac output, Aerospace Med. 40:532 (1969). 24. KELLER, G. & BLUMBERG. A. Monitoring of pul- 17. NAOGAR, C.Z., DOBNIK, D.B., FLESSAS, A.P., et monary fluid volume and stroke volume by impe- al. Accuracy of the stroke index as determined by dance cardiography in patients on hemodialysis. the transthoracic electrical impedance method. Chest 72: 56(1977). Anesthesiology 42:201 (1975). 25. BERMAN, I.R., SCHEETZ, W.L., JENKINS, E.B., et 18. KUBICEK. W.B., KARNEGIS, J.N., PATTFRSON, al. Transthoracic electrical impedance as a guide to R.P., et al. Development of evaluation of an impe- intravascular overload. Arch. Surg. 102:61 (1971).

R~'SUM~: La recherche de mrthodes non-invasives pour la mesure du d~bit cardiaque, du travail myocarde, du contenu fluide du thorax et de l'rtat de la circulation p~riph6rique, a gagn6 du terrain durant la deraibre drcennie. La cardiographie par imp6dance est un exemple des travaux qui out port6 fruit h ce sujet, drcouverte d~s 1930, elle ne fut perfectionnre et utilis~e h fond que trente ans plus tard, au cours des explorations lunaires Appolon. II s'agit d'une mrthode plrthysmographique 61ectronique qui mesure I'imprdance thoracique durant les phases du cycle cardiaque. Une formule drrivre par Kubicek et coll. permet le calcul du volume d'~jection, et done du drbit cardiaque, en fonction de changements d'imprdance thoracique. La m&hode permet aussi de mesurer le rapport des durres respectives de la contraction isovolrmique ventriculaire et de la phase d'rjection (index Heather). Hill et coll. ont 6tabli une formule par laquelle il est possible de mesurer la dur~e d'~jection ventriculaire gauche et Siegel et coll. pour leur part out con~:u une m~thode de calcul de la cofitraetion isomrtrique du myocarde en fonction de changements d'imprdance thoracique. Le cardiogramme par impr s'obtient par l'encerclement de la base du cou et de la poitrine du malade par quatre rubans (2.5 cm de large) en mylar aluminisr. Un courant 61ectrique de 4 milliamprres et de 100 kilohertz esl introduit entre la seconde etla troisi~me dectrode; son voltage amplifi~ permet d'obtenir la courbe des variations d'imp~dance thoraeique par rapport au temps. La plrthysmographie des membres s'obtient par le placement des 61ectrodes autour de I'extrrmit6 choisie dont elle mesure la circulation sanguine. Plusieurs auteurs out soulign6 le manque de prrcision de cette technique: on reconnalt de far grn~:rale, qu'elle surestime les valeurs mesurres. Cependant, la reproductibilit~ des rrsultats rend la technique utilisable pour mesurer des changements plutrt que des valeurs absolues. Les erreurs au drbut sont attribuables au fait qu'on a assum6 que le thorax 6tait cylindrique et b. contenu homog~ne; la morphologie individuelle des patients introduit des variations particulibres qui se retrouvent dans la mesure de leur imprdance thoraclque. La cardiographle par imprdance poss~:de deux avantages importants: la simplicit6 relative des appareils de mesure et leur prix 6conomique. De plus, elle peut s'employer durant I'anesthrsie sans aucune crainte des complications qui accompagnent parfois l'emploi de mrthodes invasives plus prrcises. La recherche du m~thodes de mesures non-invasives demeure une priorit~ et nous croyons que la cardiographie par imprdance reprrsente une avance importante dans ce sens.