Which position is most favorable position for performing the assessment of breath sounds?

Abstract

Auscultation is defined as “the act of listening for sounds within the body, chiefly for ascertaining the condition of the lungs, heart, pleura, abdomen, and other organs.”1[p139] Through auscultation, physical therapists look for signs of excessive secretions [presence of added sounds] or obtain evidence of satisfactory lung inflation [satisfactory “air entry”]. In clinical circumstances, lung sound intensity is often related to lung volume, and an increase in lung sound intensity on auscultation is considered indicative of lung expansion.

In a side-lying position, the weight of the abdominal contents pushes the dome of the lower diaphragm farther into the thorax than does the dome of the upper diaphragm.2[p102] Despite a diminished lung volume, the lower diaphragm contracts more effectively than the upper diaphragm during inspiration and, therefore, ventilation distributes preferentially to the dependent lung.2[p141] There is evidence that breath sound intensity correlates with pulmonary ventilation.3 Based on this evidence, lung sound intensity should be higher in the dependent lung than in the nondependent lung. There are relatively few reports, however, examining the effect of positioning on lung sound intensity.

We investigated the intensity and spectral characteristics of lung sounds recorded electronically in the sitting and side-lying positions in young adults without pulmonary dysfunction. We hypothesized that there would be no differences in the data recorded in corresponding regions between [1] the left and right lungs in the sitting position, [2] the dependent and nondependent lungs in the side-lying position [in side-lying positions, the upper hemithorax is “nondependent,” and the side in contact with the bed is “dependent”], [3] the sitting position and the dependent position, or [4] the sitting position and the nondependent position.

Method

Subjects

Physical therapist students were invited [by an open letter displayed on the students' bulletin board] to participate in a session in which lung sounds would be recorded electronically. At the session, before examination, students were asked whether they were free from any known cardiorespiratory disease. Each subject was then asked to perform spirometry testing, using a Microspiro HI-298 spirometer.* Subjects with any one of the following lung function variables that were less than 85% of the predicted value were excluded from the study: forced vital capacity, forced expiratory volume in 1 second, and peak expiratory flow. Thirteen students responded to the invitation letter. Two respondents [one who had asthma and one who had a cold with audible crackles] were excluded from the study. Data from the remaining 11 subjects [5 male, 6 female] were used in the study. The subjects had a mean age of 20.4 years [SD=2.7, range=19–26], a mean height of 166.3 cm [SD=6.5, range=154–175], and a mean weight of 57.5 kg [SD=10, range=45–75].

Instrumentation

Three identical microphones [Realistic Electret 33-1052†] were attached to 3 stethoscope chest pieces [Littmann Classic II‡] via 5-cm-long rubber tubing. Signals from the microphone-stethoscope chest pieces were samples at a frequency of 5 kHz and amplified by 3 separate amplifiers [Bruel & Kjaer type 2609§]. Output from the amplifier was converted to a digital signal by aPCMCIA A/D PC card [DT7102 PC Card-EZ‖] via a multichannel connector panel [DT784‖]. The PC card was installed in a notebook computer [Compaq Presario 1082 with Intel Pentium 166-MHz CPU, 48MB RAM#] [Fig. 1]. Signals from a pneumotachograph** were directed simultaneously to an oscilloscope [Philips PM3350A††] and to one of the channels of the connector panel via a pressure transducer [Bio Precision PT-010‡‡]. The pneumotachograph was calibrated before each sound recording session with a 2-L syringe, such that a flow rate of 1 L/s was equivalent to 0.1 V.].

Figure 1

Diagrammatic presentation of the equipment setup for the study.

Equipment Calibration

All microphones were calibrated using a sound level [Bruel & Kjaer type 4231§] and had a sensitivity of 3 mV/Pa. The performance of each microphone-stethoscope chest piece was compared with that of a reference standard microphone [Bruel & Kjaer type4166§], based on US Bureau of National Standards data. The calibration showed a coherence function value between 0.9 and 1 [frequency range=31.5–500 Hz] and a negligible, in terms of this study, performance variation of 1% to 3%.

Experimental Procedure

The 3 chest-piece diaphragms were attached to the left thigh [background sound] and the left and right posterior chest walls with three 4- × 10-cm Hypafix adhesive dressing retention sheets,§§ reinforced by three 1.5- × 10-cm strips of Micropore surgical tape‡ [Fig. 2]. The posterior chest wall attachment was at the eighth intercostal space in the midscapular line.

Figure 2

Stethoscope chest piece attachment.

Each subject was seated in front of an oscilloscope with his or her forearms supported and both hands holding the pneumotachograph [Fig. 3]. A noseclip was fitted to ensure mouth breathing during sound recording. The oscilloscope displayed signals on-line from the pneumotachograph to provide visual feedback of the pattern of each breath to the subject. The subject was asked to practice 6-second breathing cycles with a peak inspiratory flow rate of approximately 1.5 to 2 L/s [a flow rate also used by other researchers4]. A standardized flow rate has been used as lung sound spectra vary at different airflows.4 Our experience shows that tidal breathing in young subjects falls within this range. Two horizontal grid lines [±0.2 V] were highlighted on the oscilloscope screen. The subject was then given the following instruction: “Take a breath in and out through your mouth, using 3 seconds for breathing in and 3 seconds for breathing out, trying to keep your breathing curve close to, but within, the 2 identified lines on this screen.”

Figure 3

Subject during lung sound recording in the sitting position.

Lung sounds for 5 breaths were recorded with the HPVEE for Windows software program.‖‖ This procedure was repeated with the subject in the right side-lying position and then in the left side-lying position. In the side-lying positions, the subject lay on a plinth with a pillow under the head and with a pillow between the knees, which were slightly flexed. The angle of hip and knee flexion was not standardized, but the subjects were reminded that they should be comfortable and able to relax in the position. The elbows were flexed, with the hands holding the pneumotachograph as in the sitting position. The oscilloscope was positioned so that the subject could view the screen.

To minimize data variability as a consequence of the recording apparatus, particularly in an attempt to limit any difference in data recorded being due to a difference in performance of the 2 chest pieces, the left and right chest pieces were interchanged after recordings from 5 subjects were obtained.

Data Processing

The raw data were visually examined before processing. Data were discarded if the peak inspiratory flow signal was less than 0.1 V or greater than 0.2 V or the data appeared clipped [ie, a recorded sound intensity beyond the processing capabilities of the hardware]. The inspiratory and expiratory phases of each breath were identified from the highest and lowest points in the flow signal curve using the positive and negative flow signals from the pneumotachograph. A signal window that included 512 data points to the left and right of the highest data point was subjected to fast Fourier transform analysis [a numerical algorithm for fast computation of discrete signals from time domain to frequency domain]. The inspiratory frequency spectrum obtained from the “background” chest piece was subtracted from the inspiratory spectrum recorded from the left or right chest piece. The spectra from 5 breaths were averaged to provide a resultant inspiratory or expiratory spectrum. Positional data in the same subject were analyzed using paired t tests with the SPSS for Windows, version 7.5, statistical package.## The P