Congestive heart failure (CHF) is one of the leading causes of frequent visits to the emergency department (ED) with a prevalence of 1-2% in the general population and a five year mortality rate after diagnosis reported at 60% in males and 45% in females 1. Several methods are used to assess and monitor CHF patients during therapy including clinical symptoms, physical examination, echocardiography, brain natriuretic peptide (BNP) and chest radiography 2. Since the advent of the stethoscope, clinicians have routinely listened to the sounds produced by a patient's internal organs, such as the heart and lungs, as a means of assessment and to diagnose pathology. Lung sounds (lung vibrations) are produced by airflow in and out of the lungs. In the past decade, there have been attempts to refine noninvasive acoustic data to better detect and monitor pulmonary abnormalities through the use of computerized lung sound analysis 3. The theory behind using this type of analysis is that diseases affecting the lungs would result in alterations of lung vibration energy that may be too subtle to be detected on the skin surface using conventional methods. These altered vibrations may be due to changes in amount of vibration created due to increase or decrease in airflow, changes in the transmission of vibrations through the diseased lung parenchyma, or pleural space and heterogeneity of disease throughout the lung 34567.

Computerized vibration imaging technology is able to record lung vibrations (energy) and convert the signals to a dynamic image of the lung in near real time. This technology has been studied recently for the detection of pleural effusion, and graft function in single lung transplant recipients 45. To our knowledge, the relationship between vibration energy measured at the chest surface of the thorax of untreated and treated CHF has never been reported.

The aim of this pilot study is to document in detail the differences in respiratory sound patterns between normal individuals, CHF patients during acute exacerbations, and those same patients after clinical improvement.

In this initial exploratory study, we evaluated the visual display of respiratory sound patterns in patients with acute CHF exacerbation and after clinical improvement. Decreased geographical area of the vibration energy images were observed in CHF patients at presentation. With clinical improvement of CHF symptoms, the areas of these images increased. The decrease in geographic area was more pronounced in the presence of REPE. In CHF patients with REPE, total lung vibration energy decreased with clinical improvement.

The maldistribution of vibration energy of respiratory sounds in acute CHF exacerbations is likely produced by the distribution of pulmonary edema (which may or may not be radiographically evident). The vibration energy image represents the relative distribution of vibration energy, not the absolute energy. A larger image indicates a more homogeneous distribution of vibration intensity throughout the lung and a smaller image a more focal distribution. Because of how the vibration energy image is created, an increase in the size of the image after clinical improvement reflects a more homogeneous distribution of vibration intensity with decrease in lung water. The lack of homogeneity of lung vibration intensity throughout the chest in the presence of acute CHF might be explained by several mechanisms. Gravity-driven maldistribution of pulmonary edema may play a role 910111213 and heterogeneity of narrowed airways may also contribute to the heterogeneity of lung vibration images 14151617181920. Due to the effects of gravity on lung water in patient who are in a seated position, it is expected that pulmonary edema would have more pronounced effects in the dependent lower lung regions causing vibration energy image to be smaller. This was the case in patients with pulmonary edema on erect chest radiographs (with or without mechanical ventilation).

The reason for the decreased area (distribution of vibration) seen peripherally in CHF patients is likely due to decreased transmission of breath sounds to peripheral lung tissue in the presence of pulmonary edema. This would in turn result in a smaller image due to decreased homogeneity of vibration intensity (less vibration peripherally and increased centrally) 45. Lower airflow peripherally due to edema or narrowed airways could also result in a smaller image.

The increase in total vibration energy in patients with REPE is likely caused by more significant airflow obstruction resulting in narrowed airways and more turbulence. Although airflow obstruction in the setting of pulmonary edema has long been acknowledged by clinicians 12, the mechanisms responsible for this observation remain obscure. The elevation of pulmonary or bronchial vascular pressure likely results in reflex bronchoconstriction 13. Other potential causes of airway narrowing include a geometric decrease in airway size from reduced lung volume, obstruction from intraluminal edema fluid, and bronchial mucosal swelling 13. Some investigators 1415, but not all 16, have found an increase in bronchial responsiveness to methacholine in patients with left ventricular dysfunction or mitral valve disease. The significance of this finding is not clear. Contrary to earlier reasoning, there is no evidence that engorged bronchovascular bundles directly compress small airways 171819. CHF associated airway edema may also cause larger airways to be narrowed producing increased turbulence and noise 12131415161718.

The edema may additionally cause smaller airways to close and reopen during inspiration; thereby, increasing noise 19212223. Therapies that reduce interstitial pulmonary edema and pleural effusion result in reversal of the effects described above and would be expected to increase image area, as demonstrated in this investigation.

Another manifestation of CHF is cardiomegaly. Since the pulmonary system and the heart share one thoracic space, an enlarged heart would decrease the space available for the lungs to expand 242526. This may be an additional explanation for the decreased vibration energy areas. Cardiomegaly may cause decreased lung volume particularly in the lower lung fields.

In healthy volunteers, it has been shown that total vibration energy increases with increasing tidal volume 5. Although the tidal volumes of the non-ventilated CHF patients in this study were not measured, the decrease in vibration energy seen with treatment is not likely due to decreased tidal volume because: 1) patients were instructed to take deep breaths for all recordings and larger volumes would be expected after clinical improvement 27 and 2) the increased work of breathing in CHF would likely cause respiratory muscle fatigue and shallow breathing (lower tidal volume) before therapy 2829.

Nevertheless, to remove the confounding effects of tidal volume changes before and after clinical improvements, mechanically ventilated CHF patients were studied as a control group for same tidal volumes (not practical to quantitate tidal volume in the non-mechanically ventilated acutely dyspneic CHF patients), when REPE was present and again when the CHF had improved and REPE was reduced. In these patients, the geographical areas of the vibration energy images were also diminished, and increased with clinical improvements. The total vibration energy of respiratory sound was high and decreased following therapy. This group of patients had the same tidal volumes, ventilation modes and settings before and after clinically improvements. Therefore, these changes were not due to alterations in tidal volumes.

There were several limitations to this investigation. Because this was a preliminary study our sample size was small and we may not have been adequately powered for all of our analyses. We did not objectively measure pulmonary edema (e.g. via a pulmonary catheter). In a trial of critically ill patients, the use of a pulmonary artery catheter (PAC) was associated with increased mortality rates in patients with a lower severity of illness score while reducing mortality in the most severely ill 28. A recently published analysis of 53,312 trauma patients revealed no beneficial effects of PAC monitoring on mortality in low-risk patients 303132. Since low-risk patients are unlikely to derive benefits from PAC monitoring, the use of such invasive monitoring would have placed participants in our investigation at increased and unjustifiable risk. As an alternative to invasive monitoring, we did note negative fluid balances for all CHF participants. Finally, inclusion of participants with pleural effusions may have led to confounding. Since patients with pulmonary effusion as well as pulmonary edema were pooled with patients who only had pulmonary edema, the possible confounding effects of effusion on the area and vibration energy is not known, although the CHF patients with and without pleural effusion showed similar trends. In most patients, a repeat chest radiograph was not deemed necessary by the physicians. A pre-study power calculation was not possible due to unknown size of effect of pulmonary edema and treatment on amount and distribution of vibration energy.

In conclusion, with clinical improvement of acute exacerbation of CHF, there is a more homogenous distribution of vibration energy of respiratory sound. Respiratory sound analysis is non-invasive and also does not expose the patient to ionizing radiation. It can be conducted at the bedside and repeat measures are easily obtained. Based on our preliminary findings, this modality may be a useful adjunct to current methods in assessing improvement in acute CHF exacerbations.

CHF: congestive heart failure; ED: emergency department; IQR: interquartile range; PAC: pulmonary artery catheter; REPE: radiographically evident pulmonary edema; SD: standard deviation; VRI: vibration response imaging.

The authors declare that they have no competing interests.

ZW and SJ carried out the VRI recordings. ZW, SJ and BB worked on the calculations of recordings and data analysis. ZW and SJ drafted the manuscript. BB, KS and KNG revised the manuscript critically for important intellectual content. All authors edited and approved the final manuscript.