Article

Thoracic impedance vs chest radiograph to diagnose acute pulmonary edema in the ED

Original Contribution

Thoracic impedance vs chest radiograph to diagnose acute pulmonary edema in the ED?,??

Dave Milzman MDa,b, Anthony Napoli MDc,?, Christopher Hogan MDd,

Alex Zlidenny MDe, Tim Janchar MDf

aDepartment of Emergency Medicine, Georgetown University Medical Center and Washington Hospital Center,

Washington, DC 20010, USA

bGeorgetown University School of Medicine, Washington, DC 20010, USA

cDepartment of Emergency Medicine, Brown University Medical School, Rhode Island, Hospital, Providence, RI 02903, USA

dDepartment of Emergency Medicine, MCV Medical School, MCV Hospital, Richmond, VA 23298, USA eDepartment of Emergency Medicine, University of California at Irvine, Orange, CA 92868, USA fDepartment of Emergency Medicine, Harbor-UCLA Medical Center, Torrance, CA 90502, USA

Received 25 January 2008; revised 30 May 2008; accepted 7 June 2008

Abstract

Objective: We sought to investigate the relationship between thoracic impedance (Zo) and pulmonary edema on chest radiography in patients presenting to the emergency department (ED) with signs and symptoms of acute decompensated heart failure (ADHF).

Design: This was a prospective, blinded convenience sample of patients with signs and symptoms of ADHF who underwent measurement of Zo with concomitant chest radiography. Attending physicians blinded to the Zo values interpreted the radiographs, categorizing the severity of pulmonary edema as normal (NL), cephalization (CZ), interstitial edema (IE), or alveolar edema (AE). Intergroup comparisons were analyzed with a 2-way analysis of variance (ANOVA), with P b .05 considered statistically significant and reported using 95% confidence intervals (CIs).

Setting: We enrolled patients (>=18 years) presenting to a tertiary care medical center ED with signs and

symptoms consistent with ADHF.

Results: A total of 203 patients were enrolled, with 27 (14%) excluded because of coexisting pulmonary diseases. The mean Zo values were inversely related to the 4 varying degrees of radiographic pulmonary vascular congestion as follows: NL, 25.6 (95% CI, 22.9-28.3); CZ, 20.8 (95%

CI, 18.1-23.5); IE, 18.0 (95% CI, 16.3-19.7); and with AE, 14.5 (95% CI, 12.8-16.2) (ANOVA, P b

.04). A Zo less than 19.0 ohms had 90% sensitivity and 94% specificity (likelihood ratio [LR], – 0.1; LR + 15) for identifying radiographic findings consistent with pulmonary edema. Females had an increased mean Zo value compared to males (P b .03).

? DPM,CH conceived the study, designed the trial, enrolled and treated patients and obtained research funding. DPM, CH, AZ, and TJ undertook recruitment of patients and management of data. DPM and AMN managed data analysis. DPM is responsible for statistical analysis. DPM, CH, and AMN drafted the manuscript and contributed substantially to its revision. DPM takes responsibility for the article as a whole.

?? Support for this project was provided in part from Research Grants from Emergency Medicine Foundation and American Heart Association with

additional support from Renaissance Technology (Newtown, Pa) by providing the monitor and technical support for this research work.

* Corresponding author.

E-mail address: [email protected] (A. Napoli).

0735-6757/$ – see front matter (C) 2009 doi:10.1016/j.ajem.2008.06.002

Conclusion: The Zo value obtained via thoracic bioimpedance monitoring accurately predicts the presence and severity of pulmonary edema found on initial chest radiograph in patients suspected of ADHF.

(C) 2009

Introduction

Acute decompensated heart failure is a common and costly emergency department (ED) presentation, with ED admissions for ADHF increasing for the past 20 years [1-4]. The survival of patients after an episode of ADHF remains poor, with annual mortality rates of 10% and exceeding 50% for those with New York Heart Association class IV disease [3,4].

Successful management of ADHF in the ED requires an accurate assessment of the patient’s cardiac function and pulmonary fluid status. Traditionally, physicians have relied on vital signs, oxygen saturation, and physical findings, but these fail to accurately differentiate treatment classes of ADHF or guide treatment [5,6]. B-type Natriuretic Peptide measurement is now available in the ED, but there is some controversy regarding its contribution to clinical diagnosis and management [7-12]. None of these auxiliary tests continuously assess the degree of pulmonary vascular congestion, much less cardiac output and cardiac function during treatment. In addition, adjunct Laboratory analysis, such as BNP, is often only helpful as a diagnostic marker 30 to 60 minutes after Treatment decisions have been made and is therefore not useful in real time. B-type natriuretic peptide also does not correlate well with pulmonary capillary wedge pressure (PCWP) [13].

Thoracic impedance (Zo), a measure of the biologic resistance of current flow across the chest cavity, has been evaluated in heart failure in both chronic [14] and acute heart failure and demonstrates significant Prognostic ability [15]. It improves clinical decision making in the ED by helping identify subtle ADHF [16]. Thoracic impedance also has the potential, unlike the chest radiograph and BNP, to be a real- time reflection of the volume status of the patient during short-term management. The largest prior study to date [17] found significant differences between normal and abnormal chest radiographs but did not demonstrate a relationship between impedance and radiographic findings. To show that Zo can be used as a real-time measure of volume, one must first show there is a correlation between the two, which can be done indirectly in the ED by correlation with chest radiograph. Our goal was to compare the Zo measured from thoracic electrical bioimpedance (TEB) with the degree of radiographic pulmonary congestion and edema on initial chest radiograph during immediate presentation to the ED. We hypothesized that in patients with signs and symptoms of ADHF, thoracic electrical bioimpedance, as measured by Zo, would accurately predict the degree of pulmonary venous congestion as measured by previously validated findings on chest radiograph.

Methods

Study design

This was an institutional review board-approved, pro- spective, observational study enrolling a convenience sample of patients with signs and symptoms of ADHF.

Study setting and population

This study was performed in the ED of a tertiary care medical center in patients with signs and symptoms consistent with ADHF.

Study protocol

A bioimpedance cardiac output monitor (IQ Monitor, Model 2001; Renaissance Technologies, Newtown, PA) was placed on study subjects upon arrival after informed consent was obtained. Monitor and supporting data were collected during the ED stay.

All treating physicians were blinded to Zo values. The inclusion criteria were adults (age N18 years) who agreed to study participation and had isolated signs and symptoms of ADHF. Exclusion criteria were the presence of Pleural effusions (noncardiogenic), atelectasis not associated with ADHF, isolated infiltrates consistent with pneumonia, focal disease findings on radiograph or on ED diagnosis, the use of any ventilatory support, or endotracheal intubation before chest radiograph.

Two sets of ECG-like leads were placed on the patient. The first set was placed bilaterally at the base of the neck and on the thorax at the xiphoid level in the midaxillary line. The second set was an additional set of 3 Standard ECG leads. The monitor measured and recorded continuous cardiovas- cular parameters (cardiac output/index, thoracic impedance, stroke volume, and heart rate) with new measurements reported every 30 seconds. Each set of leads contains a transmitting and sensing electrode. A high frequency current is transmitted caudally over the thorax reflecting changes in thoracic aorta flow. Alternating this current, combined with monitoring the R-R interval, allows for the calculation of cardiac output and other central hemodynamic data via impedance [18]. Total thoracic fluid status (Zo) measures the base impedance or amount of resistance that the signal meets as it traverses through air and fluid through the thoracic cavity. Fluid in the chest (ie, pulmonary edema) decreases impedance and is represented by a lower Zo value. Clear lung fields with air have increased impedance (Zo). Any

change in thoracic fluid status is rapidly demonstrated by a corresponding inverse change in Zo, that is, less pulmonary congestion causes a higher Zo.

All chest radiographs were portable, imaged in an upright anterior-posterior fashion. Five-minute averages of Zo measurement before and after the radiograph were used for analysis. All chest radiographs were interpreted by 2 attending radiologists and 2 Attending emergency physicians with greater than 10 years experience and who were blinded to the Zo values. Delineation of radiographic classification was based upon agreement by at least 3 of the 4 reviewers. These radiographs were placed into 1 of 4 treatment categories: (1) normal (NL), no immediate sign of pulmonary congestion or cardiomegaly; (2) cephalization (CZ), reversal in the size difference between the normally smaller upper lobe and larger lower lobe vessels; (3) interstitial edema (IE), a loss of distinct vascular margins with interlobular edema (Kerley B lines were not required for inclusion); and (4) alveolar edema (AE), vascular redistribution of the upper zones and fluid concentration in the inner two-thirds of the lung with obvious opacification of the air spaces and cardiomegaly (defined as a cardiothoracic ratio N0.5, greater on an anterior-posterior film) [12].

Data analysis

Data were collected by trained research assistants and entered into an Excel database (MS Corp, Redmond, WA). Sample size calculation estimated 49 patients in each group to detect a 25% difference between Zo values based on prior literature. This would provide a power of 80% to demonstrate a statistically significant difference (2-tail significance level of P b .05). All hemodynamic data obtained were downloaded and maintained on an Excel database. Analysis of variance (ANOVA) was used to assess the relationship between chest radiograph and the Zo.

Primary outcome

Our primary outcome was to demonstrate a relationship between thoracic bioimpedance, as measured by Zo, and radiographic findings of pulmonary edema in patients with signs and symptoms of ADHF.

Secondary outcome

Our secondary outcome was to examine the relationship of Zo with Disposition decision and location.

Results

Of 203 enrolled patients, 27 (14%) met exclusion criteria. The mean patient age was 67.7 +- 18.3 years; 57.1% of

Chest radiograph category

n

Zo

95% CI

Normal/no evidence of CHF (NL)

53

25.6 ?

22.9-28.3 ?

Cephalization/intravascular fullness

46

20.8 ?

18.1-23.5 ?

(CZ)

Interstitial edema (IE)

52

18.0 ?

16.3-19.7 ?

Alveolar edema with cardiomegaly

25

14.5 ?

12.8-16.2 ?

(AE)

A significant intergroup difference exists between each chest radiograph category and Zo value (ANOVA, P b .04). There was no intergroup difference based on age or sex.

the subjects were male. Of the subjects, 82% required hospital admission; 14% of these were admitted to the medical intensive care unit, 56% to the coronary care unit (CCU), 30% to general medical floors, and 18% were discharged home.

Table 1 Thoracic impedance vs chest radiograph to diagnose acute pulmonary edema in the ED

The mean Zo values on the initial chest radiographs demonstrated intergroup differences, with mean Zo values inversely related to 4 varying degrees of radiographic pulmonary congestion (95% confidence interval [CI])– NL, 25.6 ohms (22.9-28.3); CZ, 20.8 ohms (18.1-23.5); IE,

18.0 ohms (16.3-19.7); and AE, 14.5 ohms (12.8-16.2)

(ANOVA differences, P b .04) (Table 1). A thoracic impedance value (Zo) of less than 19 ohms had a 90% sensitivity, a 94% specificity, a positive likelihood ratio of 15, and a negative likelihood ratio of 0.1 for identifying chest radiographic findings of pulmonary edema (ie, all groups except “normal”).

The mean thoracic impedance (Zo) for patients requiring admission to the cardiac (CCU) or intensive care unit (ICU) was lower (18.3 ohms [95% CI, 16.9-19.7]) compared to those admitted to the medical floor (22.7 ohms [95% CI, 18.1-27.3]) (P b .03). Female patients had an increased mean Zo value compared to males–22.8 ohms vs 20.2 ohms, respectively (Student t test, P b .03). The impedance value was elevated in females with respect to males within each radiographic class, but the difference within any single, radiographic class was not statistically significant.

Discussion

The need for a quantitative assessment of ADHF to diagnose and guide treatment has never been greater as the acuity and frequency of ADHF continues to increase [19,20]. The literature demonstrating the poor accuracy of physical examination findings [6,21-26] is well known and has prompted the search for other parameters with which to evaluate ADHF. As the population becomes more ill, intravenous vasoactive agents such as nitroglycerin are playing a larger part in ADHF management in the ED; thus, ongoing assessment of the cardiopulmonary status becomes

even more important in this setting. In this pilot study, we demonstrate that thoracic bioimpedance accurately predicts chest radiographic findings of pulmonary edema in patients with signs and symptoms of ADHF.

There are few viable options for measuring cardiopul- monary function in the ED. For instance, the use of pulmonary artery catheters has been shown to have minimal impact on long-term survival of acutely ill patients [27-29], particularly in the ADHF setting [30], and is not practical in the ED. Although BNP offers insight into the ventricular volume, there are no data suggesting how quickly it changes once treatment is initiated, and it would require serial Blood draws. Instead, BNP appears to be mostly diagnostic and prognostic [7,21,31-34]. This literature tends to support increasing levels of BNP as indicative of more severe disease state [8]. Correlation of BNP with impedance values may better define severity of presentation. However, although BNP appears to be a measure of ventricular volume, studies have shown that it does not correlate well with PCWP (r = 0.32) [13,35]. In fact, the highest published correlation with PCWP to date is the correlation of the PCWP with the O/C ratio, as measured by impedance cardiography (r = 0.92) [36]. However, the O/C ratio remains difficult to repeatedly measure and continuously report to be of use in real time. The Zo value, or direct impedance across the chest wall, does not have this limitation.

Clinical assessment of the degree of pulmonary conges- tion and cardiac function in ADHF is difficult and is often inaccurate [3-5,37]. Radiographic detection of pulmonary congestion, however, has demonstrated a diagnostic accu- racy of 85% for a PCWP greater than 18 mm Hg [37]. Perihilar haze has been associated with a PCWP of 18 to 25 mm Hg and a PCWP more than 25 is associated with alveolar edema [12]. The pulmonary artery catheter (PAC) determination of PCWP remains the “gold standard” and most used method for assessing central hemodynamic status [38]. However, the placement of the PAC for ADHF patients is reserved almost exclusively for use in ICUs [38,54]. The TEB technology provides the clinician in a non-ICU setting with a valuable tool to assess the severity of pulmonary edema in the ED with less resource use and complications than invasive means [22,38,54]. Like the PAC, it provides continuous objective data that reflect changes in pulmonary vascular congestion. The electro- physiologic and technical basis behind noninvasive tech- nology has been described previously [39]. Impedance cardiography was introduced as a means of estimating cardiac output in prior studies [38,40-42,54] and shown to be useful in the ED [43], but here, it is used as a noninvasive adjunct for assessing the severity of pulmonary edema. In conjunction with previous research that has shown the use of impedance cardiography in measuring cardiac output in the ADHF patient [43], this technology may help better distinguish the New York Heart Association functional classification of the individual patient. Although a study demonstrating real-time trending of Zo in response

to therapy is attractive, it is first necessary to demonstrate the relationship of Zo with chest radiographic findings of pulmonary edema; we feel this physician-blinded pilot study of more than 200 patients establishes this relation.

Prior reports of thoracic bioimpedance monitoring have focused on cardiac output and cardiac index measured by the PAC method, with a high correlation between bioimpedance derived and PAC thermodilution-derived cardiac output (CO) values (r = 0.85) in smaller studies and in a metanalysis of more than 1500 total patients [39,44,45]. However, there are only a limited number of reports on the value of Zo and its clinical applications [46- 49]. The accuracy of Zo has been reported in patients undergoing hemodialysis with positive changes inversely proportional to the actual thoracic fluid volumes [40]. Case reports by other authors have also found use in impedance measurement for monitoring patient response to treatment [50]. Conners et al described ADHF and Central venous oxygen saturation values relating to Treatment outcome but did not compare values to radiographic findings of pulmonary edema [51]. Previous studies to date and small case reports discuss the potential use of thoracic impedance as a consistent measure of thoracic fluid volume and as an adjunct to chest radiography. The outcome of this study confirms our earlier findings as well as the findings of a smaller study by other investigators [17,46]. The largest prior study reported in the literature [17] demon- strated a significant difference between normal and abnormal chest radiographs but did not demonstrate a relationship between impedance and radiographic findings of pulmonary edema. More recent work has demonstrated a significant difference in pulmonary venous congestion in patients with ADHF as compared to chronic obstructive pulmonary disease and other causes of dyspnea [43], but this study was not specifically designed to compare chest radiography with pulmonary venous wedge pressure. Our study is the largest to date to demonstrate a relationship between TEB and radiographic findings of pulmonary edema and is a necessary first step to demonstrate efficacy as a real-time adjunct to monitoring therapeutic response. We demonstrate an inverse relationship between Zo and the presence of radiologic findings consistent with ADHF (Table 1). Patients without evidence of ADHF on the initial ED chest radiograph (ie, normal) had a mean Zo of 25.1 ohms, whereas patients with ample radiographic evidence of ADHF had a mean Zo of 14.4 ohms.

Treatment of ADHF and especially cardiogenic shock in the ED presents the treating physician with a great therapeutic challenge. Clinicians are often incompletely aware of the intravascular status and may even underdiurese patients [52,53]. Continuous readings of thoracic impedance and Cardiac functions may allow for more informed decisions on drugs or fluid to be titrated by providing early optimization of cardiac output and fluid balance. Our investigation has demonstrated a number of findings regarding the use of thoracic bioimpedance in the ED.

First and foremost, we report the largest study to date to correlate thoracic impedance with radiographic evidence of pulmonary edema. Second, we demonstrate a high sensi- tivity and specificity of Zo for radiograhic evidence of pulmonary edema in patients with signs and symptoms of ADHF. Third, our secondary outcomes demonstrate a significant relationship between disposition location and sex.

Limitations and future questions

This study using TEB to assess pulmonary edema severity by chest radiography has limitations. One concern is the accuracy of cardiac output monitoring in the presence of arrhythmias, most notably in patients with frequent pre- mature ventricular contractions and some paced rhythms. However, this small variability in cardiac output does not affect Zo because impedance variations secondary to aortic pulse waves is a negligible component of overall thoracic impedance. This measurement is also therefore not likely to be manufacturer dependent. All chest radiographs were anteroposterior, and patients with mechanical ventilation were excluded, making it unclear how well these results would translate to intubated patients or those patients who get a 2-view chest radiograph. Because the underlying technology uses thoracic fluid content, any force (such as positive pressure ventilation) that displaces fluid from the chest will impact measurement. The ED correlation of chest radiograph findings with Zo and PCWP (which is the current gold standard) would be optimal. However, our study design was based on the previously demonstrated correlation between PCWP and chest radiographs. Routine ED treatment rarely uses invasive monitoring but instead uses clinical assessment, chest radiographs, and BNP in the continuous management of the patient with ADHF.

The difference in Zo between sexes is noteworthy, with a 10% increase on normal Zo values in females compared to males. This likely has more to do with thoracic cAge SIze, with the male usually possessing a larger cavity cross- sectional area and greater tissue water content for muscle- to-fat ratios. Adipose tissue is known to be one of the tissues with the highest impedance [54]. No study has been performed on obese patients to assess the additional impact that the body mass index or body habitus may have on Zo. The fairly homogeneous nature of heart failure patients with similar body habitus and little capacity to maintain muscular chest walls may explain the small confidence intervals found in each of the 4 heart failure groups [55,56]. Our population consisted of almost exclusively Geriatric patients with reduced heterogeneity among body habitus. The patient population with ADHF may be predisposed to abnormal findings. However, this homogeneity likely adds to the accuracy of the results and the potential when further large scale studies are done with isolation of particular subset populations.

Conclusions

The bioimpedance-derived Zo accurately predicts the presence of pulmonary edema found on initial chest radiograph and may serve as an important adjunct in the management of patients with signs and symptoms of ADHF.

Acknowledgments

The authors would like to thank Drs Sangeeta Desai and Brad Wood for their work in reviewing radiographs and the manuscript.

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