When taking blood pressure the diaphragm of the stethoscope is placed over the?

Introduction

Manual auscultatory blood pressure [BP] measurement is widely recommended as the gold standard for noninvasive clinical BP measurement. It is the most accurate technique for routine BP measurement 1–3; it requires a cuff, a stethoscope, and a cuff pressure display. A trained observer uses a stethoscope to listen for the Korotkoff sounds associated with blood flow through the brachial artery as a BP cuff encircling the upper arm is deflated 4. The appearance and disappearance of Korotkoff sounds is associated with systolic and diastolic BPs [SBP and DBP], respectively, and the BPs at these times are read from a cuff pressure display.

A stethoscope usually consists of a bell, a diaphragm, a tube, and earpieces. Either the stethoscope bell or the stethoscope diaphragm is used for capturing the appearance and disappearance of Korotkoff sounds during cuff deflation. The common viewpoint is that the stethoscope bell would perform better in recording Korotkoff sounds with a low frequency range, whereas the stethoscope diaphragm would perform better with a high frequency range 5,6. The study published by Abella et al. 6 showed that the stethoscope bell provided a louder output than the diaphragm at the low frequency range, and thus they suggested that the stethoscope bell outperforms the diaphragm in recording heart sounds. The stethoscope diaphragm was recommended when high-frequency components of heart or Korotkoff sounds were required 5. However, ambiguous recommendations for selecting the stethoscope bell or diaphragm side for BP measurement have been provided in different textbooks. Among the recommendations published over the past 20 years, two recommended the stethoscope bell 7,8, three the stethoscope diaphragm 9–11, and three the bell and/or diaphragm 12–14. International BP measurement guidelines also gave diverse recommendations. Both the 1997 Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure 15 and the 1999 WHO – International Society of Hypertension guidelines 16 recommended the stethoscope bell, because it is believed that the Korotkoff sounds mainly contain low-frequency components. The 2003 Seventh Report of the Joint National Committee 17 did not state specifically which stethoscope side should be used. However, the 2003 European Society of Hypertension guidelines 18 recommended the use of the diaphragm side because it is easier to hold and covers a greater skin contact area.

The importance of accurate BP measurement in clinical practice is without doubt, and small inaccuracies in BP measurement can have considerable consequences 19. It has been reported by population studies that overestimating or underestimating BP by even 5 mmHg can seriously compromise diagnosis, resulting in millions of people being wrongly diagnosed as hypertensive, with attendant exposure to adverse drug effects, or being denied treatment, leading to associated cardiovascular conditions, including fatal stroke and fatal myocardial infarction 20,21. Therefore, any potential small BP difference caused by stethoscope characteristics is clinically important and worth further investigation.

In real clinical practice, both bell and diaphragm sides of the stethoscope are commonly used. Obtaining the sounds from different stethoscope sides may generate different ways for enabling Korotkoff sounds to be heard, resulting in different interpretations by observers and hence different BP readings. In addition, stethoscope tube lengths vary from 55 to 80 cm, but they usually have a length of 70 cm, which is simply referred to as ‘standard tube’ in this study. However, the influence of stethoscope length on BP readings has not been quantitatively investigated.

Thus, the aims of the current study were to quantify the BP difference between the measurements undertaken using the stethoscope bell and diaphragm sides, and between those undertaken using different tube lengths.

Methods

Participants

The required sample size was estimated from a power calculation allowing a 5 mmHg mean BP difference to be detected with a typical 8 mmHg SD of BP measurement; 21 participants were required to achieve a confidence level of 95% and a statistical power of 80%. Thirty-two healthy participants [19 male and 13 female] were recruited from May to July 2014, with ages ranging from 24 to 68 years. They were mainly from among the staff, students, and visitors of Freeman Hospital and Newcastle University. Exclusion criteria for this study were age under 18 years or over 70 years, known cardiovascular disease including atrial fibrillation or other irregular heart rhythms, and pregnancy.

This study received ethical approval from the Newcastle & North Tyneside Research Ethics Committee. The investigation conformed to the principles of the Declaration of Helsinki. All participants gave their written informed consent to participate in the study. Table 1 briefly summarizes the demographic information of the participants, including sex, age, height, weight, and arm circumference.

Korotkoff sound recording

All BP measurements were performed in a quiet and temperature-controlled clinical measurement room by a trained operator at the Freeman Hospital, Newcastle upon Tyne, UK. Before the formal recording, each participant was asked to rest on a chair for 5 min. BP measurements were performed with the participant in a sitting position, with his/her feet placed on the floor and the arm supported at the level of the heart. For signal recording, we located the stethoscope head at the position with the maximum pulse beat obtained with moderate applied pressure. The analog sound signals were then recorded to a computer from an audio amplifier with a constant gain for all recordings. This gain had been set in a preliminary study. No recorded signal saturated the recording range. The participants were also asked to breathe gently during the measurement. The whole procedure followed the guidelines recommended by the British Hypertension Society and American Heart Association 2,22.

Figure 1 shows a diagram of the BP measurement system with four different combinations of stethoscope characteristics: bell plus standard tube [70 cm], bell plus short tube [5 cm], diaphragm plus standard tube, and diaphragm plus short tube. The tubes used were rubber tubes with an inner diameter of 2.4 mm and a thickness of about 0.25 mm. For each participant, there were two repeat sessions with four measurements for each, giving a total of eight recordings. There was a time interval of at least 1 min between the four measurements within a session and at least 4 min between the two sessions, allowing recovery of cardiovascular hemodynamics. The order of the four measurements within the sessions for each participant was randomized. During cuff deflation, the cuff pressure and Korotkoff sounds were digitally recorded at a sample rate of 2000 Hz. The cuff pressure was linearly deflated at a standard rate of 2–3 mmHg/s. The deflation rate was automatically controlled.

Blood pressure determination

For each participant, eight recordings of Korotkoff sounds [from two repeat sessions and four stethoscope combinations] were converted into .wav files using Matlab 2011a [MathWork Inc., Natick, Massachusetts, USA]. Because of the potential BP measurement bias from repeat BP determinations 23–25, all Korotkoff sounds recorded in this study were replayed twice [on two different days] to one trained listener to test the repeatability of BP determination. The observer was well trained and certified using the British Hypertension Society’s Blood Pressure Measurement educational tool and supporting material. The order of replaying all the 256 Korotkoff sound recordings [from eight Korotkoff recordings for each participant×32 participants] was randomized, and the listener was unaware of any participant or combination information. All BP measurements and microphone signal playbacks were performed in a quiet, temperature-controlled clinical measurement room. We recorded the background noise level in this room, and the noise level was usually below 30 dB when performing the signal playback. The same microphone amplifier and computer settings were used throughout the study to ensure that the playback was exactly the same for every participant on all days. The listener identified the pressure associated with the appearance and disappearance of the sounds for the determination of SBP and DBP by reading the cuff pressure display, similar to a mercury column. BP values were determined to an accuracy of 2 mmHg.

Data and statistical analysis

In total, 16 values were obtained from each participant [from two stethoscope sides, two tube lengths, two repeat measure recordings, and two BP determinations on separate days] for both SBP and DBP. The overall mean and SD of the BPs were calculated across all participants, as well as separately for the two stethoscope sides, two tube lengths, and their combinations.

The SPSS Statistics 19 software package [SPSS Inc., Chicago, Illinois, USA] was used to carry out analysis of variance to study the repeatability of measurements between the two repeat sessions and between the two BP determinations on separate days, as well as the effects of stethoscope side and tube length. The mean BP differences between the above factors were also analyzed. All differences were paired values, and a P-value less than 0.05 was considered as a statistically significant difference.

Results

Blood pressures

Among all 32 participants, the mean±SD BP calculated from all data was 109.9±12.3 mmHg for SBP and 71.2±9.3 mmHg for DBP. The results for the separate stethoscope characteristics are shown in Table 2.

Repeat measurements

There was only one significant paired difference between the repeat measurement sessions for SBP and DBP [all P-values>0.10, except the SBP difference measured using the short tube and diaphragm 1.7±3.5 mmHg, P=0.01]. The overall SBP and DBP changes between the two measurement sessions were 0.91±4.72 and −0.07±3.74 mmHg.

Repeat listening

Figure 2 shows histogram of within-subject SBP and DBP differences between the repeat determinations on separate days. There was no significant paired difference between the repeat listening results for both SBP [0.16±2.12 mmHg, P=0.24] and DBP [0.23±2.20 mmHg, P=0.11]. As the order of the repeat listening was randomized and the repeat listening was on another day, our results confirmed the accuracy of BP determination for the data in this study.

Effect of bell or diaphragm

All results showed a tendency toward higher BP values with the bell in comparison with the diaphragm, and this was statistically significant for DBP [mean difference 0.66 mmHg, 95% confidence interval 0.18–1.15 mmHg, P=0.007; Fig. 3].