Heart Failure: Acoustic Cardiography Improves Clinicians’ Confidence for Making the Diagnosis when BNP is Ambiguous

August 26, 2007 By Grendel Burrell [mailto:grendel.burrell@gmail.com]

Despite improvement in diagnosis and treatment, as many as 18% or more of patients with suspected acute heart failure in the emergency department may be misdiagnosed (PMID: 16387212). 80% of patients who present to the emergency department for heart failure are admitted to the hospital. (PMID: 1049994) For most cardiologists and emergency physicians heart failure is a small percentage of their practice, and these patients may present a diagnostic challenge. The diagnostic utility of the S3 heart sound is well accepted, and new technological advances help overcome the challenges of detection by auscultation.

Despite improvement in diagnosis and treatment, up to 46% of patients with acute dyspnea caused by heart failure currently are misdiagnosed (PMID: 1590605) (PMID: 11962564) (PMID: 12135939). Chest X-ray is a component of the ED assessment of patients with dyspnea. Collins et al previously found that 1 of every 5 patients admitted from the ED with acute decompensated heart failure had no signs of congestion on chest radiography. Patients lacking signs of congestion on chest x-ray during the emergency department assessment were more likely to have an ED non-heart failure diagnosis than patients with signs of congestion. However, clinicians should not rule out heart failure in patients with no radiographic signs of congestion. (PMID: 16387212)

Echocardiography is a critical component in the detection of underlying LV dysfunction, but the procedure is expensive, can be time consuming, and is not available 24/7 in many hospitals. Physicians have adopted other tests for heart failure, especially the measurement of B-type natriuretic peptide (BNP). However, BNP values in the so-called “gray zone” from 100 to 500 pg/ml can make the diagnosis more challenging (PMID: 14960741). The third heart sound (S3) has been shown to be highly specific for heart failure, but its detection using auscultation, especially in places like noisy emergency departments, presents another challenge (PMID: 15886379) (PMID: 11529211).

Acoustic cardiography records and algorithmically interprets simultaneous diagnostic digital 12- lead electrocardiographic and acoustic data. In the September 1 issue of the American Journal of Cardiology, authors Michel Zuber, MD, Peter Kipfer, MD, and Christine Attenhofer Jost, MD describe their results when they tested the hypothesis that acoustic cardiography can augment the use of BNP in the detection of LV dysfunction (Am J Cardiol 2007;100:66-869).

164 ambulatory patients (mean age 64) referred for Doppler echocardiography for possible heart failure were included. Within an hour of Doppler echo, each patient had blood drawn for BNP and serum creatinine and also was evaluated with acoustic cardiography using AUDICOR® (Inovise Medical, Inc., Portland, Oregon). Of this group, the patients with BNP 100-500 pg/ml were selected as subjects for additional study.

These patients underwent a complete 2-dimensional and Doppler echo. The Doppler echo results were interpreted by the investigators who were blinded to both BNP (Biosite Triage test) and the acoustic cardiographic data. Doppler echo was used to identify patients with and without increased LV filling pressures. Diastolic function was assessed using the LV filling pattern and tissue Doppler exam of the lateral mitral annulus, including E/Z ratio, deceleration time of the E wave, E/E/ ratio, and pulmonary venous flow pattern. LV dysfunction was defined as LV ejection fraction < 50%.

Acoustic cardiography is accomplished in the same time it takes to record a 12-lead ECG. Acoustic cardiography records and simultaneously interprets ECG and acoustic data, including S3 using sensors at V3 and V4. For purposes of this study, a 10-second AUDICOR® recording was obtained for each patient. The system, as used in this study, evaluated S3 and the percent EMAT. Percent EMAT (electromechanical activation time) quantifies the time required for ventricular contraction to generate sufficient force to close the mitral valve (Q-S1).

In this study, 83/164 patients had BNP <100 pg/ml. Of the 69 patients with BNP 100-500, 74% (51 patients) had LV dysfunction. Of the 51 patients, 49% had Doppler echo evidence of systolic LV dysfunction with normal filling pressures. 37% had systolic LV dysfunction in increased filling pressures, and 14% had isolated diastolic LV dysfunction.

For each subgroup of LV dysfunction, BNP in the range of 100-500 pg/ml had poor diagnostic specificity. However, regardless of the level of filling pressures, the addition of acoustic cardiography to elucidate the role of BNP in this range provided excellent rule-in power for systolic LV dysfunction. The diagnostic impact was lower in patients with isolated diastolic LV dysfunction than for those with systolic LV dysfunction.

The authors state that this study confirms “the pathologic significance of BNP in the gray zone of 100 to 500 pg/ml“ as “uncertain”. And with 42% of patients in this study having BNP is that range, this appears to be a common situation. Sean Collins, MD, Assistant Professor, University of Cincinnati, Department of Emergency Medicine told WikiDoc, “The best use of BNP is to exclude heart failure. When there is a low or intermediate likelihood of acute heart failure and BNP values are <100 pg/ml (high sensitivity), the patient is unlikely to have acute heart failure.”

In this study, acoustic cardiography using S3 and EMAT provided an “accurate and fast tool for outpatients”. The data showed that acoustic cardiography increased the positive likelihood ratios for detecting LV dysfunction by a factor >30. Using a BNP threshold of > 500 pg/ml produced a very high specificity but would have identified only 12% of the truly positive patients with LV dysfunction.

Results of this study provide information on the diagnostic utility of BNP for detecting LV dysfunction in the ranges of > 100, 100-500, and >500 pg/ml and confirm that in the range of 100-500 pg/ml BNP had neither good rule-in or rule-out power for LV dysfunction. When acoustic cardiographic parameters were added to the values of BNP in the range of 100-500 pg/ml for detecting LV dysfunction, the percentage of EMAT alone and EMAT in combination with S3 provided excellent rule-in power for LV dysfunction with high positive likelihood ratios.

The authors’ findings on the utility of acoustical cardiography data were consistent with those from a study showing that the electronically recorded S3 is highly specific for heart failure (PMID: 15886379). Lok et al observed that in contrast to electronic detection of S3, auscultation was not consistently reliable. They executed a prospective blinded study in the cardiology and internal medicine units of a university-affiliated teaching hospital. They studied forty patients with a cardiac diagnosis and 6 patients without. Two cardiologists, one general internist, three senior and two junior postgraduate internal medicine trainees, blinded to the patients' characteristics, examined the patients and documented their findings. Computerized phonocardiogram was obtained in all patients as a gold standard and was interpreted by a blinded, independent cardiologist. In this study, the agreement between observers and the phonocardiography gold standard in the correct identification of S4 and S3 was poor and the lack of agreement did not appear to be a function of the experience of the observers (PMID: 9824002).

WikiDoc asked Dr. Collins how the results of Zuber et al could be extrapolated to the ED. Collins replied, “The benefit of acoustic cardiography is that it has fairly high specificity for acute heart failure. In the study by Zuber, acoustic cardiography was helpful in those patients with indeterminate BNP values. The combination of an S3 and EMAT had a specificity of 100%-virtually diagnostic for acute heart failure when present. However, because of its intermediate sensitivity, it was not present in about 1/3 of patients. These are important findings and could facilitate earlier diagnosis and treatment of acute heart failure especially when there is clinical uncertainty and the BNP value is indeterminate. However, these results need to be validated in a cohort of ED patients before this can be implemented in clinical practice.”

In the current study by Zuber et al, two non-invasive diagnostic modalities were used. While BNP is often used to evaluate acutely dyspneic patients in the emergency department, Zuber et al showed that both BNP and acoustic cardiography could be used effectively in the office or outpatient setting. Their data showed that percentage of EMAT, whether used alone or in conjunction with the electronically detected S3 was additive for detecting LV dysfunction. The authors conclude, “Easily obtainable acoustic cardiographic data substantially improved the diagnostic evaluation of patients with nondiagnostic BNP values and therefore can increase the confidence with which physicians diagnose and treat LV dysfunction.”