Indian J Physiol Pharmacol 2012; 56(2) : 117–124

IMPEDANCE CARDIOGRAPHY FOR MONITORING CHANGES IN

RACHNA PARASHAR1, MANISH BAJPAI2, MANISH GOYAL3, SHRADDHA SINGH2, SUNITA TIWARI2* AND V. S. NARAYAN2

1 Department of Physiology, LLRM Medical College, Meerut U.P., India 2 Department of Physiology, CSM Medical University, Lucknow – 226003, U.P., India and 3 Department of Physiology, All India Institute of Medical Science, New Delhi–110029, India

( Received on February 20, 2011 ) Abstract : Impedance Cardiography (ICG) is a non invasive method useful for continuous monitoring of cardiac output but, it still has not found wide usage for measuring cardiac output in clinics and research. Most studies focused on comparing the cardiac output measured at rest with reference methods. In the present study we evaluated the validity of ICG against Doppler (DE) in measuring cardiac output changes that occur during static exercise. Cardiac output of 30 healthy males between 18-26 yrs of age was measured during supine rest, during and 5 min after completion of 3 minute static exercise by ICG and DE. The increase in cardiac output during exercise measured with ICG and DE does not differ significantly (1.04±0.72 L/min and 1.05±1.24 L/min respectively) and has significantly high correlation (r=0.76, P<0.001). The bias and limits of agreement are (–0.01±0.83) in acceptable limits. The pooled means of cardiac output measured by ICG and DE do not differ significantly and bears a significant correlation (r=0.812, P<0.001). The bias (d±s) calculated is 0.15±0.64 L/min. ICG could provide valid information regarding the relative changes in cardiac output.

Key words : cardiac output echocardiography monitoring

INTRODUCTION as blood is pumped into the (2). Cardiac output and stroke volume represents the Impedance Plethysmography (IP), a functional expression of cardiovascular noninvasive technique, originally was performance and can be used to monitor described for measurement of blood flow (1). changes in a patient’s hemodynamic status. Application of this technique, has been Impedance Cardiography (ICG) have been extended for determining cardiac output by found potentially useful in monitoring recording changes in impedance that occur cardiac output of patient in inpatient settings

*Corresponding Author : Sunita Tiwari, Professor & Head, Department of Physiology, CSM Medical University, Lucknow, Email: [email protected] 118 Parashar et al Indian J Physiol Pharmacol 2012; 56(2) as the changes in impedance and thus stroke volume in healthy subjects. The change in volume can be recorded and displayed cardiac output, which takes place during continuously on screen. In critical care isotonic exercise, has been measured with settings, monitoring of hemodynamic ICG and DE. parameters by catheterization though useful but have MATERIAL AND METHODS received several criticism because of higher risk of infections and increase mortality Subjects owing to the invasive nature of the technique (3, 4). Recently, Silver et. al. (2004) have 30 healthy males, between 18-26 yrs of pointed out that owing to high cost involved age, were recruited for this study. Brief in pulmonary artery catheterization this history and clinical examination was done procedure may be replaced by noninvasive to rule out cardiovascular and respiratory ICG (5). There are reports suggesting ICG is diseases. Informed consent was obtained fairly comparable to the dye dilution, after explaining the nature of study to the thermodilution techniques and also to subjects and ethical clearance was taken (DE), for from the institutional ethics committee. measurement of cardiac output at rest Subjects were called 2 hrs after light break (6,7,8). Results from studies comparing the fast in morning. absolute values of cardiac output at rest may not be extrapolated to situations where Devices gauging the fluctuations in cardiac output rather than absolute values were needed for Doppler echocardiography example in exercise physiology research and in intensive care setups. The diameter of the left ventricular outflow tract was used to calculate the cross In the present study we evaluated the sectional area, assuming a circular profile. validity of ICG against DE,. as a reference Measurements were made by cross sectional method for comparison, in measuring the echocardiography in the parasternal long changes in cardiac output that occurs during axis view (Hewlett-Packard Sonos 5500® static exercise. DE has shown a good Echocardiography System). Stroke volume correlation with invasive methods for and cardiac output were measured using measurement of cardiac output and is the protocols as described earlier (12). most accepted non invasive method for determining cardiac output (9, 10). DE is Impedance cardiography particularly advantageous to be used in this exercise paradigm as it is capable of The Impedance Plethysmograph determining the cardiac output quickly and (NICOMON, Larsen & Turbo India Ltd.) was easily (8, 11). connected to the subject via four pairs of button electrodes as described previously In the present study, hand grip isometric (13). The lower thoracic voltage sensing exercise paradigm has been employed to electrodes were placed at the level of the register changes in cardiac output/stroke xiphisternum in the mid-axillary lines and Indian J Physiol Pharmacol 2012; 56(2) Impedance Cardiography for Monitoring Changes 119 the cervical sensing electrodes were and were compared using correlation positioned laterally at the base of the neck analysis. As the correlation analysis may be as close as possible to the clavicles. The misleading for assessing validity, agreement “current” electrodes, delivering an alternating was also calculated using Bland & Altman current of 4 mA at 48 kHz, were placed with plots (14). All three pairs of data were pooled one pair 5 cm above the cervical sensing and correlation analysis was done between electrodes and the other pair 5 cm below cardiac output measured by ICG and DE and the thoracic sensing electrodes. The recorded limits of agreement, P value less than 0.05 waveforms of changes in impedance are was taken as significant. analyzed as a function of time using Kubicek’s equation and area under the curve RESULTS represents maximum change in blood flow during systole i.e. stroke volume. The A total of 30 subjects who were evaluated NICOMON automatically averages stroke were healthy from clinical point of view and volume over ten cardiac cycles and displays their baseline characteristics are presented the cardiac output using corresponding R-R in Table I. intervals which are simultaneously recorded by it. Cardiac outputs, measured before exercise during rest, during exercise and 5 Protocol minute of rest after exercise by Impedance Cardiography and Doppler Echocardiography, Baseline cardiac output was measured in are presented in Table II. The mean cardiac supine position after 30 min of rest by Impedance Cardiography. Simultaneously trained cardiac physician recorded cardiac TABLE I : Baseline characteristics of the subjects. Data are expressed as Mean±SD. output by echocardiography. The subject was asked to perform the static exercise for 3 No. of subjects (n) 30 minutes using hand grip dynamometer with Sex (M:F) 18:12 Age (in yrs) 21.5±2.4 sub-maximal static effort at 30% of maximum Height (in cms) 171.1±5.3 voluntary contraction. The cardiac output Weight (in kgs) 61.2±7.5 2 was again determined during and 5 minutes BMI (kg/cm ) 20.97±2.7 Systolic blood pressure (in mmHg) 120.9±6.5 after completion of exercise. Diastolic blood pressure (in mmHg) 75.2±5.9

Statistical analysis TABLE II : Cardiac output obtained during rest, during exercise and 5 minute after rest Paired ‘t’-test was used to compare the as measured by Impedance Cardiography and Doppler Echocardiography. N=30, pairs of cardiac output measured by Values represents Mean±SD of cardiac Impedance Cardiography and Doppler output in L/MIN). Echocardiography measured three times i.e. Cardiac Output At During After at rest, during exercise and 5 min after Rest Exercise Exercise exercise. The changes in cardiac output measured as both increase during exercise Impedance cardiography 4.87±0.83 6.01±0.98 5.07±0.87 Doppler echocardiography 4.98±0.88 6.17±1.01 5.26±0.95 and decrease after exercise, were calculated 120 Parashar et al Indian J Physiol Pharmacol 2012; 56(2) output values measured with two techniques TABLE III : Limits of agreement between cardiac output measured measured with do not differ significantly (P>0.05) either Impedance Cardiography and Doppler during rest, during exercise or after 5 Echocardiography. d=bias, s=standard deviation. minutes rest. Cardiac output measured d s d+1.96s d–1.96s with ICG and DE The pooled mean of cardiac output Pooled data before 0.15 0.64 1.41 –1.10 measured by Impedance Cardiography and exercise, during exercise Doppler echocardiography are 5.32±1.01 L/ and rest after exercise min. and 5.47±1.07 L/min respectively, which Change in cardiac output –0.01 0.83 1.61 –1.64 during exercise and again do not differ significantly (P>0.1). before exercise

Correlation analysis revealed a significant correlation (r=0.812, P<0.001) between the graphs are presented (Fig. 2) The bias pooled cardiac outputs measured with (expressed as d ± s) for cardiac output Impedance Cardiography and Doppler measurement by Impedance Cardiography echocardiography (Scatter plot Shown in and Doppler Echocardiography was 0.15±0.64 Fig. 1). L/min.

To test the validity of Impedance The changes in cardiac output (Mean±SD) Cardiography for measuring cardiac output, during and after exercise measured with mean difference (d) and standard deviation Impedance Cardiography and Doppler (s) was determined and limits of agreement echocardiography was 1.04±0.72 L/min and were calculated as d + 1.96 s and d – 1.96 s 1.05±1.24 L/min respectively which did not (Table III). And for graphical representation differ significantly (P>0.05). The detected of limits of agreement Bland & Airman

Fig. 3: Bland & Altman plot showing limits of Fig. 1: Correlation between cardiac out measured agreement [solid reference line represents by Doppler Echocardiography (DE) and mean difference (d) and dotted reference Impedance Plethysmography (IP). lines represent d±1.96s]. Indian J Physiol Pharmacol 2012; 56(2) Impedance Cardiography for Monitoring Changes 121 change in cardiac output with two techniques method is time consuming and invasive, DE have significantly high correlation (r=0.76, is particularly advantageous in this exercise P<0.001). The bias and limits of agreement paradigm as it is quicker and easier (8, 11). have been presented in Table III and In the present study we evaluated the validity graphical representation in Fig. 3. of ICG using Doppler Echocardiography as a reference method for measuring the changes in cardiac output that occurs during static exercise.

The cardiac output by Impedance Cardiography did not differ from that by Doppler Echocardiography measured at rest, during static exercise and after static exercise. The pooled Cardiac outputs at rest, during and after static exercise also did not differ significantly measured by Impedance Cardiography and Doppler echocardiography. The correlation coefficient for the pooled data was also quite high signifying the fact that Impedance Cardiography is as good as Doppler echocardiography for measuring Fig. 3: Bland & Altman plot showing limits of agreement [solid reference line represents cardiac output over a wide range (2.8 L – mean difference (d) and dotted reference 8.3 L). The high correlation levels are in lines represent d±1.96s]. accordance with the previous studies over a limited range of cardiac output measurement DISCUSSION (10, 17, 18) Moreover, the limits of agreement calculated for pooled data is After introduction of Impedance within acceptable limits and are in Cardiography for cardiac output in 1985 by accordance with previous studies (17-20). Muji et. al., (2) the validity of this technique Thus high correlation and acceptable limits has been in question despite a large no. of of agreement signify the reliability of published reports comparing it with Impedance Cardiography in measuring Thermodilution and Doppler Echocardiographic cardiac output over wide range. methods (10, 15, 16). Most of the studies focused on comparing the absolute values The reliability of Impedance Cardiography of cardiac output at rest therefore their for gauging the changes in cardiac output results may not be extrapolated to situations has also been evaluated in this study and where gauging the fluctuations in cardiac again the changes measured with Impedance output is important as in intensive care Cardiography correlates well with the results setups. with Doppler Echocardiography. The limits of agreement are favorable in this case too. For cardiac output measurement, as Pick Previous reports have established excellent 122 Parashar et al Indian J Physiol Pharmacol 2012; 56(2) preciseness and reproducibility of Impedance with aging would be prominent with Cardiography (15, 21) therefore; the increased blood flow and pressure during Impedance Cardiography can be expected to exercise. Furthermore, the influence of an become the standard modality for monitoring enhanced sympathetic activity during cardiac output in intensive care setups as intense exercise on the propagation of a well as exercise physiology research and pressure pulse might be increased in aging similar applications. therefore, further investigation in various age groups are required. In addition, more Recently two multicentre studies data regarding its application in patients PREDICT study (Prospective Evaluation with different clinical conditions with and identification of Decompensation by altered states of cardiac output needs to be Impedance Cardiography Test) and BIG study explored. (Bioimpedance cardiography) have been published and show encouraging results In conclusion summary, we tested the for application of ICG in the management validity of non invasive cardiovascular of failure patients (22, 23). In parameters measured by Impedance addition, the data from the critically ill Cardiography during sub maximal static patients with altered states of cardiac output exercise in healthy young males. The good needs to be explored to establish the use of agreement between Impedance Cardiography Impedance Cardiography in critical care and Doppler Echocardiography both for settings. absolute values and for gauging changes in cardiac output suggest that Impedance Doppler Echocardiography itself may not Cardiography could provide valid information be used for monitoring purposes as it is regarding the relative changes in cardiac operator dependent technique which needs output during sub maximal static exercise expertise. On other hand ICG does not need in healthy young humans. This conclusion expertise and is a cost effective method which can be extrapolated to critical care setup may be used for continuous monitoring where beat to measurement of cardiac output purpose. may provide crucial information regarding the hemodynamic status of the patient. More data from patients may be helpful in judging In this investigation, the study the clinical utility of the impedance population was limited to healthy young Cardiography for continuous monitoring of subjects and as the Impedance Cardiography cardiac output non-invasively. depends on the pressure area relationship, age related changes in the arterial system e.g. arteriosclerosis, may influence the ACKNOWLEDGEMENTS results Although the aortic compliance did not depend on the degree of sclerosis for Authors wish to thank Dr. V.S. Narayan, pressure in the physiological range (24) the Dr. Payal and Dr. Manish for their help in influence of increased arterial wall stiffness Doppler Echocardigraphy. Indian J Physiol Pharmacol 2012; 56(2) Impedance Cardiography for Monitoring Changes 123

REFERENCES

1. Young DG, Cox RH, Stones EK. Evaluation of mode echocardiography. Am J Cardiol 1988; 62: quantitative impedance plethysmography for 131–135. continuous flow measurement: III, blood flow determination in-vivo. Am J Phys Med 1967; 46: 10. Christie J, Sheldahl LM, Tristani FE, Sagar 1450. KB, Ptacin MJ, Wann S. Determination of stroke volume and cardiac output during 2. Muzi M, Ebert TJ, Tristani FE, Jeutter DC, exercise: comparison of two-dimensional and Barney JA, Smith JJ. Determination of cardiac Doppler echocardiography, Pick oximetry and output using ensemble averaged impedance thermodilution. Circulation 1987; 76: 539–547. cardiogram. J Appl Physiol 1985; 50: 200–205. 11. Parrot CW, Kenneth MB, Quale C, Lewis DL. 3. Connors AF Jr, Speroff T, Dawson NV, Thomas Comparison of changes in ejection fraction to C, Harrell FE Jr, Wagner D, Desbiens N, changes in impedance cardiography cardiac index Goldman L, Wu AW, Califf RM, Fulkerson WJ and systolic time ratio. Congest Heart Fail 2004; Jr, Vidaillet H, Broste S, Bellamy P, Lynn J, 10(S): 11–13. Knaus WA. The effectiveness of right heart catheterization in the initial care of critically ill 12. Lewis JF, Kuo LC, Nelson JG, Limacher MC, patients. SUPPORT Investigators. JAMA 1996; Quinones MA. Pulsed Doppler echocardiographic 18: 889–897. determination of stroke volume and cardiac output: clinical validation of two methods using 4. Sandham JD, Hull RD, Brant RF, Knox L, Pineo the apical window. Circulation 1984; 70: 425– GF, Doig CJ, Laporta DP, Viner S, Passerini L, 431. Devitt H, Kirby A, Jacka M; Canadian Critical Care Clinical Trials Group. A randomized, 13. Barde PB, Jindal G, Singh R, Deepak KK. New controlled trial of the use of pulmonary-artery method of electrode placement for determination catheters in high-risk surgical patients. N Engl of cardiac output using impedance cardiography. J Med 2003; 348: 5–14. Ind J Physiol & Pharmacol 2006; 50: 234–240.

5. Silver MA, Cianci P, Brennan S, Longeran- 14. Bland JM, Altman DG. Statistical methods for Thomas H, Ahmad F. Evaluation of impedance assessing agreement between two methods cardiography as an alternative to pulmonary of clinical measurement. Lancet 1986; 8: 307– artery catheterization in critically ill patients. 310. Congest Heart Fail 2004; 10(S): 17–21. 15. Ng HW, Walley T, Tsao Y, Breckenridge AM. 6. Albert NM, Hail MD, Li J, Young JB. Equivalence Comparison and reproducibility of transthoracic of the bioimpedance and thermodilution in bioimpedance and dual beam Doppler ultrasound measuring cardiac output and index in patients measurement of cardiac function in healthy with advanced, decompensated chronic heart volunteers. Br J Clin Pharmacol 1991; 32: 275– failure hospitalized in critical care. Am J Crit 282. Care 2004; 13: 469–479. 16. Doering L, Lum E, Dracup K, Friedman A. 7. Northridge DB, Findlay IN, Wilson J, Henderson Predictors of between-method differences in E, Dargie HJ. Non-invasive determination of cardiac-output measurement using thoracic cardiac output by Doppler echocardiography and electrical bioimpedance and thermodilution. Crit electrical bioimpedance. Br Heart J 1990; 63: Care Med 1995; 23: 1667–1673. 93–97. 17. Kravtsova EL, Voronkov Iul. Use of the method 8. Yung GL, Fedullo PF, Kinninger K, Johnson W, of thoracic electric bioimpedance in medical Channick R. Comparison of impedance evaluation of central in man. cardiography to direct Pick and thermodilution Aviakosm Ekolog Med 1995; 29(4): 52–54. cardiac output determination in pulmonary 18. Wong DH, Tremper KK, Stemmer EA, O’Connor arterial hypertension. Congest Heart Fail 2004; D, Wilbur S, Zaccari J, Reeves C, Weidoff P, 10(S): 7–10. Trujillo RJ. Noninvasive cardiac output: 9. Hoit BD, Rashwan M, Watt C, Sahn DJ, simultaneous comparison of two different Bhargava V. Calculating cardiac output from methods with thermodilution. Anesthesiology transmitral volume flow using Doppler and M- 1990; 72: 784–792. 124 Parashar et al Indian J Physiol Pharmacol 2012; 56(2)

19. Schmidt C, Theilmeier G, Van Aken H, BH, Young JB, Fishbein DP, Hauptman PJ, Korsmeier P, Wirtz SP, Berendes E, Hoffmeier Bourge RC, Strobeck JE, Murali S, Schocken D, A, Meissner A. Comparison of electrical Teerlink JR, Levy WC, Trupp RJ, Silver MA; velocimetry and transoesophageal doppler Prospective Evaluation and Identification of echocardiography for measuring stroke volume Cardiac Decompensation by ICG Test (PREDICT) and cardiac output. Br J Anaesth 2005; 95: Study Investigators and Coordinators. Utility of 603–610. impedance cardiography for the identification of short-term risk of clinical decompensation in 20. Karakitsos DN, Patrianakos AP, Paraskevopoulos stable patients with chronic heart failure. J Am A, Parthenakis FI, Tzenakis N, Fourtounas C, Coll Cardiol 2006; 47: 2245–2252. Daphnis EK, Vardas PE. Impedance cardiography derived cardiac output in hemodialysis patients: 23. Binanay C, Califf RM, Hasselblad V, O’Connor a study of reproducibility and comparison with CM, Shah MR, Sopko G, Stevenson LW, Francis echocardiography. Int J Artif Organs 2006; 29: GS, Leier CV, Miller LW; ESCAPE Investigators 564–572. and ESCAPE Study Coordinators. Evaluation 21. Rowland TW, Melanson EL, Popowski BE, study of congestive heart failure and pulmonary Ferrpne LC. Test-retest reproducibility artery catheterization effectiveness: the ESCAPE of maximum cardiac output by Doppler trial. JAMA 2005; 294: 1625–1633. echocardiography. Am J Cardiol 1998; 81: 1228– 24. Langewouters GJ, Wesseling KH, Goedhard WJ. 1230. The static elastic properties of 45 human thoracic 22. Packer M, Abraham WT, Mehra MR, Yancy CW, and 20 abdominal in vitro and the Lawless CE, Mitchell JE, Smart FW, Bijou R, parameters of a new model. J Biomech 1984; 17: O’Connor CM, Massie BM, Pina IL, Greenberg 425–435.