Medicine Section - Noninvasive Measurements of Cardiac Hemodynamics in the Outpatient Setting

IMPORTANT REMINDER

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PLEASE NOTE: Contracts exclude from coverage, among other things, services or procedures that are considered investigational or cosmetic. Providers may bill members for services or procedures that are considered investigational or cosmetic. Providers are encouraged to inform members before rendering such services that the members are likely to be financially responsible for the cost of these services.

Description

In the intensive care unit, hemodynamic monitoring using a pulmonary artery catheter (also referred to as right heart catheterization) is commonly used to provide prognostic information and guide treatment decisions. Cardiac output is commonly measured as part of such monitoring in patients with heart failure, shock syndromes, and after coronary artery bypass graft surgery. Techniques include thermodilution, dye dilution, or the Fick method, although thermodilution is most often used.  Thoracic electrical bioimpedance and inert gas rebreathing are two techniques that have been investigated for many years as a noninvasive alternative for measuring cardiac output.

Thoracic Electrical Bioimpedance

Thoracic electrical bioimpedance, a form of plethysmography, is defined as the electrical resistance of tissue to the flow of current. For example, when small electrical signals are transmitted through the thorax, the current travels along blood-filled aorta, which is the most conductive area. Changes in bioimpedance, resulting from the pulsatile changes in volume and velocity of blood in the aorta, are inversely proportional to the stroke volume (cardiac output equals the stroke volume times heart rate).

Inert Gas Rebreathing

Inert gas rebreathing is based on the observation that the absorption and disappearance of a blood soluble gas is proportional to cardiac blood flow. The patient is asked to breathe and rebreathe from a rubber bag filled with oxygen mixed with foreign gases; typically nitrous oxide and sulphur hexafluoride. The nitrous oxide is soluble in blood and is therefore absorbed during the blood’s passage through the lungs at a rate that is proportional to the blood flow. The sulphur hexafluoride is insoluble in blood and therefore stays in the gas phase and is used to determine the lung volume from which the soluble gas is removed. These gases and CO-2 are measured continuously and simultaneously at the mouthpiece.

Development of a noninvasive measurement would permit more convenient and safer monitoring in the intensive care unit, and could be used for monitoring in other settings, such as the emergency room, on the general medical floor, or in the outpatient clinic. In the outpatient clinic thoracic bioimpedance has been investigated as a technique to optimize drug therapy in patients with congestive heart failure. Echocardiography, transesophageal echocardiography (TEE), and Doppler ultrasound are other noninvasive methods for monitoring cardiac output.

The BioZ™ is a device approved by the U.S. Food and Drug Administration (FDA) that measures thoracic bioimpedance.

Innocur (Innovision, Denmark) is an inert gas rebreathing device. It has not yet been reviewed by the FDA.

Policy/Criteria

In the outpatient setting, thoracic electrical bioimpedance and inert gas rebreathing are considered investigational.

Scientific Background

Thoracic Electrical Bioimpedance

A variety of small case series have reported inconsistent results regarding the relationship between measurements of cardiac output (CO) determined by thoracic electric bioimpedance and thermodilution techniques. For example, Belardinelli and colleagues compared the use of thoracic electrical bioimpedance, thermodilution, and the Fick method to estimate cardiac output in 25 patients with documented coronary artery disease and a previous myocardial infarction. (2) There was a high degree of correlation between cardiac output as measured by thoracic bioimpedance and other invasive measures. Shoemaker and colleagues reported on a multicenter trial of thoracic bioimpedance compared to thermodilution in 68 critically ill patients. (3) Again, the changes in cardiac output as measured by thoracic electrical bioimpedance closely tracked those measured by thermodilution. In contrast, Sageman and colleagues did not recommend the use of bioimpedance as a postoperative monitoring technique for patients who had undergone coronary artery bypass. (4) In this study of 50 patients, only a poor correlation was found between thermodilution and bioimpedance, due primarily to the postoperative distortion of the patient's anatomy and the presence of endotracheal, mediastinal and chest tubes. In a study of 34 patients undergoing cardiac surgery, Doering and colleagues also found that there was poor agreement between thoracic electrical bioimpedance and thermodilution in the immediate postoperative period. (5) The largest case series, the COST Study, has been published in abstract form only. (6) In this case series, estimations of cardiac output using thermodilution methods and thoracic electrical bioimpedance were performed in 191 patients who underwent right heart catheterization for a variety of clinical indications. Linear regression analysis revealed an overall correlation of r=0.73. The authors concluded that cardiac output can be reliably measured with either thermodilution or thoracic electrical bioimpedance, and that thoracic electrical bioimpedance has the additional value of being noninvasive.

Since this policy was first published there has been minimal additional literature focusing on the potential applications of thoracic electrical bioimpedance in the outpatient setting, and no literature specifically focusing on the improved health outcomes in patients undergoing thoracic electrical bioimpedance.  As noted in a 2000 editorial, thoracic bioimpedance may have an important role in the outpatient management of congestive heart failure, but "earlier studies have not sought to evaluate the clinical importance of the data generated by impedance cardiography. They have not determined whether evaluation of the status of the central circulation by impedance cardiography can predict clinical events and, thus, be used to alter the treatment of patients. Obtaining such information is critical if the use of impedance cardiography is to expand from its present application where it has excelled, in short term management of acutely ill hospitalized patients, to the long term outpatient management of recently ill or hospitalized patients with severe chronic disorders." (7)

In 2002, the Agency for Health Care Research and Quality published a technology assessment of thoracic electrical bioimpedance, which concluded that limitations in available studies did not allow the Agency to draw meaningful conclusions to determine whether the accuracy of thoracic electrical bioimpedance compared to other hemodynamic parameters. (9) The Agency also found a lack of studies focusing on clinical outcomes and little evidence to draw conclusions on patient outcomes for the following clinical areas:

• Acute dyspnea
• Cardiac patients with need for fluid management
• Hypertension
• Inotropic therapy
• Monitoring in patients with suspected or known cardiovascular disease
• Pacemakers
• Post heart transplant evaluation

Evaluation of Dyspnea

Peacock reported findings on a “convenience” sample of 89 patients age 65 and older who presented to an emergency department with dyspnea. (17) The final diagnosis was heart failure in 48% and obstructive lung disease in 22%. After receiving the results of ICG, such as vascular resistance and cardiac index, the 31 practitioners in this study changed their working diagnosis in 13% of cases and changed medications administered in 39%. Findings from this study are limited due to small sample size, approach to patient selection, and unclear effects on relevant patient outcomes. Lo reported on a study of 52 patients who presented with acute dyspnea to emergency departments in Taiwan. (18) Compared to standard care (history, examination, and laboratory testing), they reported that addition of ICG improved sensitivity (75% vs. 60%) and specificity (88% vs. 66%) for determining cardiac cause of dyspnea. Again, implications of these findings are limited by the small sample size and the uncertain impact on outcomes.

Hypertension

Taler and colleagues, in a study published in 2002, randomized 117 subjects with refractory hypertension to a drug protocol established by either a hypertension specialist or an algorithm based on serial hemodynamic measurements derived from bioimpedance monitoring. (8) After three months of therapy blood pressure was lowered by intensified drug therapy in both groups. However, blood pressures were lowest in the bioimpedance-controlled group. Although the number of patients taking diuretics did not differ between groups, final diuretic dosage was higher in the bioimpedance-controlled group. Whether bioimpedance may be of value in making treatment decisions for patients with refractory hypertension in a community primary care practice is not yet known.

Smith reported on the use of impedance cardiography (ICG) as a method to improve blood pressure control in 164 patients with hypertension whose blood pressure was not controlled on 1 to 3 medications. (16) The study was conducted in 11 primary care centers. Results were reported (this was not an intention-to-treat analysis) on blood pressure control at 3 months for 95 patients in the standard treatment arm and 69 in the hemodynamic (ICG) arm. At the end of 3 months, 77% of the ICG group achieved blood pressure control (< 140/90) compared to 55% in the standard treatment group; reductions in diastolic blood pressure were also greater in the ICG group (12 mm Hg vs. 5 mm, respectively). This study had very strict exclusion criteria which makes it difficult to apply to the general population. While these results are interesting, larger studies with longer follow-up will be needed to determine if the improvements in blood pressure are sustained and of sufficient magnitude to potentially lead to improved outcomes. The use of the technology in a broader spectrum of patients may also need further evaluation.

Heart Failure

Packer reported on use of ICG to predict risk of decompensation in patients with chronic heart failure. (19) In this study, 212 stable patients with heart failure and a recent episode of decompensation underwent serial evaluation and blinded ICG testing every 2 weeks for 26 weeks and were followed up for the occurrence of death or worsening heart failure requiring hospitalization or emergent care. During the study, 59 patients experienced 104 episodes of decompensated heart failure: 16 deaths, 78 hospitalizations, and 10 emergency visits. A composite score of 3 ICG parameters was a strong predictor of an event during the next 14 days (p = 0.0002). Patients noted to have a high-risk composite score at a visit had a 2.5 times greater likelihood of a near-term event and those with a low-risk score had a 70% lower likelihood when compared to ones at intermediate risk. However, the impact of use of these results on clinical outcomes is not known.

In 2001, the American College of Cardiology/American Heart Association issued guidelines for chronic heart failure. (10) These guidelines indicate: "Although hemodynamic measurements can also be performed by non-invasive methods, such as transthoracic bioimpedance, routine use of this technology cannot be recommended at the present time because the accuracy of bioelectrical parameters has not been defined in patients with chronic heart failure and it has not been shown to be more valuable than routine tests, including the physical examination. Moreover, it is not clear whether serial noninvasive hemodynamic measurements can be used to gauge the efficacy of treatment or to identify patients most likely to deteriorate symptomatically during long-term follow-up."  The 2005 updated ACC/AHA chronic heart failure guidelines did not alter these recommendations. (11)

As noted in a review article, the issue continues to be how the use of thoracic electrical bioimpedance can be used in the outpatient to improve patient management, either in terms of diagnosis, risk stratification and monitoring patients with cardiovascular conditions. (12) A number of studies have been published since the last update describing the use of thoracic bioimpedance (also referred to as impedance cardiography) in a various clinical situations. While results of more studies of ICG are being published, many studies are limited by small populations and uncertainty about the impact on clinical outcomes. In addition, not all studies have evaluated additional novel markers such as BNP (brain natriuretic peptide). In a 2006 review article, Wang comments that there are limited data concerning improved outcomes using ICG in the clinical setting and that, given the data, ICG use should be limited to the research setting. (20) Therefore, the policy statement is unchanged.

Inert Gas Rebreathing

In contrast to thoracic electric bioimpedance, there is relatively little published literature on inert gas rebreathing, although a literature search suggests that this technique has been used as a research tool for many years. (13-15)  An updated literature review found only two phase I clinical trials regarding the impact of inert gas rebreathing on clinical management. The published literature does not change this policy statement.

References

1. BlueCross BlueShield Association, Medical Policy Reference Manual, Policy No. 2.02.12
2. Belardinelli R, Ciampani N, Costantini C et al. Comparison of impedance cardiography with thermodilution and direct Fick methods of noninvasive measurement of stroke volume and cardiac output during incremental exercise in patients with ischemic cardiomyopathy. Am J Cardiol 1996;77:1293-301
3. Shoemaker WC, Woo CC, Bishop MH, et al. Multicenter trial of a new thoracic electrical bioimpedance device for cardiac output estimation. Crit Care Med 1994;22:1907-12
4. Sageman WS, Amundson DE. Thoracic electrical bioimpedance measurement of cardiac output in post-aortocoronary bypass patients. Crit Care Med 1993;21:1139-42
5. Doering L, Lum E, Dracup K, Friedman A. Predictors of between-method difference in cardiac output measurement using thoracic electrical bioimpedance and thermodilution. Crit Care Med 1995;23:1667-1673
6. Raisinghani A, Diaco NV, Sageman SW et al. The COST Study: A multicenter trial comparing measurement of cardiac output by thoracic electrical bioimpedance with thermodilution. American College of Cardiology 47th Annual Scientific Sessions, April 1, 1998
7. Strobeck JE, Silver M, Ventura H. Impedance cardiography: Noninvasive measurement of cardiac stroke volume and thoracic fluid content. Congestive Heart Failure 2000;6:3-6
8. Taler SJ, Textor SC, Augustine J. Resistant Hypertension; comparing hemodynamic Management to Specialist Care. Hypertension 2002 May; 39(5)
9. Agency for Health Care Research and Quality technology assessment. Thoracic Electrical Bioimpedance.  November 2002.  http://www.cms.hhs.gov/mcd/viewtechassess.asp?id=23  (Verified  07/17/08)
10. Hunt SA, Baker DW, Chin DH, et al. ACC/AHA Guidelines for the evaluation and management of chronic congestive heart failure in the adult: a report of the American College of Cardiology/ American Heart Association Task Force on Practice Guidelines. J Circulation 2001;104(24)2996-3007. Complete guidelines available at: http://www.acc.org/qualityandscience/clinical/topic/topic.htm#H  (Verified  07/17/08)
11. Hunt SA. ACC/AHA 2005 guideline update for the diagnosis and management of chronic heart failure in the adult: A report of the American College of Cardiology/ American Heart Association Task Force on Practice Guidelines (Writing committee to update the 2001 guidelines for evaluation and management of chronic heart failure). J Am Coll Cardiol 2005;46:1-82.  Complete guidelines available at http://www.acc.org/qualityandscience/clinical/topic/topic.htm#H   (Verified  07/17/08)
12. Moshkovitz Y, Kaluski E, Milo O et al. Recent developments in cardiac output determination by bioimpedance: Comparison with invasive cardiac output and potential cardiovascular applications. Curr Opin Cardiol 2004;19(3):229-37
13. Christensen P, Clemensen P, Andersen PK et al. Thermodilution versus inert gas rebreathing for estimation of effective pulmonary blood flow. Crit Care Med 2000;28(1):51-6
14. Durkin RJ, Evans TW, Winter SM et al. Noninvasive estimation of pulmonary vascular resistance by stroke index measurement with an inert gas rebreathing technique. Chest 1994;106(1):59-66
15. Stok WJ, Baisch F, Hillebrecht A et al. Noninvasive cardiac output measurement by arterial pulse analysis compared with inert gas rebreathing. J Applied Physiol 1993;74(6):2687-93
16. Smith RD, Levy P, Ferrario CM et al. Value of noninvasive hemodynamics to achieve blood pressure control in hypertensive subjects. Hypertension 2006: 47(4):771-7
17. Peacock WF, Summers RL, Vogel J et al. Impact of impedance cardiography on diagnosis and therapy of emergent dyspnea: the ED-IMPACT trial. Acad Emerg Med 2006; 13(4):365-71
18. Lo HY, Liao SC, Ng CJ et al. Utility of impedance cardiography for dyspneic patients in the ED. Am J Emerg Med 2007; 25(4):437-41
19. Packer M, Abraham WT, Mehra MR et al. Utility of impedance cardiography for the identification of short-term risk of clinical decompensation in stable patients with chronic heart failure. J Am Coll Cardiol 2006; 47(11):2245-52
20. Wang DJ, Gottlieb SS. Impedance cardiography: more questions than answers. Curr Cardiol Rep 2006; 8(3):180-6

Cross References

Non-invasive Measurement of Left Ventricular End Diastolic Pressure (LVEDP) in the Outpatient Setting, Regence Medical Policy Manual, Medicine, Policy No. 118

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