Detection of air trapping in chronic obstructive pulmonary disease by low frequency ultrasound

Chronic obstructive pulmonary disease (COPD) poses a challenge to current and future health care systems. As a result of increased tobacco consumption and demographic development, COPD is expected to become the third leading cause of death worldwide by the year 2020 [1]. Early diagnosis and intervention is necessary to prevent a further decline of lung function in these patients. The Global Initiative for Chronic Obstructive Lung Disease (GOLD) recommends spirometry as the gold standard for the diagnosis of COPD, since it is the most reproducible, standardized and objective way of measuring airflow limitation [2]. However, to perform spirometry, experienced and regularly trained medical assistants are needed as well as physicians for interpreting the results. Possibly due to these problems, spirometry is not frequently used by general practitioners and underdiagnosis of COPD is widespread [3‐8]. Therefore, an additional screening method that is easier to perform and to interpret is needed.

Conventional ultrasound with frequencies ranging from 2 to 10 MHz is increasingly used for the diagnosis of pulmonary diseases including pneumothorax, pleural effusion, alveolar-interstitial syndrome and lung consolidation. However, its application is restricted to superficial examination and to abnormally dense lungs [9‐12]. The healthy lung cannot be visualized, because differences in acoustic impedances between aerated lung tissue and pleural cavity cause total internal reflection [13].

A novel approach to non-invasive monitoring of the lungs is low frequency ultrasound spectroscopy [14]. Earlier, Goncharoff et al. described the sound transmission between 5 and 20 kHz from the mouth to the back [15]. More recently, Rüter et al. applying frequencies between 5 and 40 kHz to the sternum demonstrated that the signals received at the back differed between inspiration and expiration in healthy human subjects. The sound spectra changed dependent on the lung aeration: higher aeration resulted in a weaker signal and in a shift of the high pass filter towards higher frequencies. In contrast, in COPD patients the sound spectra during inspiration and expiration remained unchanged [14]. During expiration the signal of COPD patients was similar to the inspiratory signal of healthy subjects, suggesting that this method may be useful for detecting air trapping. In that study, the area under the curve of the sound spectra between 10 and 20 kHz correlated with the forced expiratory volume in 1 sec (FEV₁) and the intrathoracic gas volume (ITGV) and differed significantly between COPD and non-COPD subjects [14].

Air trapping is a critical clinical feature of COPD. The objective of our study was to evaluate whether air trapping in COPD patients can be detected reliably using low frequency ultrasound (LFU). COPD patients were classified into severity stages GOLD 1-3 [2] and were examined by both body plethysmography and low frequency ultrasound. The amplitude and the high pass frequencies of the sound spectra were compared between COPD patients and healthy subjects. Furthermore we analyzed the bronchodilator effect of salbutamol by body plethysmography and frequency content.

Despite the fact that the physics of low frequency ultrasound are not fully understood, this study shows that air trapping in COPD can be detected by LFU. We found significant differences in ΔF between healthy subjects and COPD subjects during maximum inspiration and expiration. These differences were already significant at GOLD stage 1 and increased with the severity of COPD. The sensitivity for detecting GOLD stage 1 was 83.3%, for detecting moderate to severe COPD (GOLD 2 and 3) it was 90.9%. Our sensitivity analysis (Table 2) showed that these measurements should be obtained during unforced maximum breathing.

Spirometry is required to make the diagnosis of COPD; the presence of a post-bronchodilator FEV₁/FVC < 0.70 confirms the presence of persistent airflow reduction and thus of COPD according to the GOLD guideline recommendations [2]. Although there is currently insufficient evidence to recommend routine use of low frequency ultrasound, this additional measurement may help to disclose the degree of air trapping in patients with COPD. The method may also be useful in cases where body plethysmography or comparable techniques are unavailable.

In contrast to previous work we found no significant differences in the signal amplitude between COPD patients and healthy subjects [14]. This may be explained by the subject selection, because Rüter et al. examined a group of patients with mainly very severe COPD (GOLD stage 4) [14], while we examined COPD patients with GOLD stages 1 to 3. In principle, however, the present study confirms the previous findings [14]. As expected, ΔF increased in response to bronchodilation, but these results were not significant. In contrast, bronchodilation as measured by spirometry and by body plethysmography was associated with a significant increase of FEV₁ and a significant decrease of airway resistance, respectively. Thus, these conventional measures are more suitable than LFU for assessing bronchodilator responses.

As mentioned above, it is still unclear how low frequency ultrasound is transmitted through the human thorax. Rüter et al. showed that with increasing air content of the lungs during inspiration the high pass frequency increased while the signal amplitude decreased. It was suggested that sound traveling through the lungs was influenced by differences in lung density [14]. An important finding in our study was that the typical expiratory signal was received above the abdominal tissue of a healthy person. When placing the sensor above the lower lung border, the signal changed from an expiratory signal with high amplitude and low high-pass frequency during expiration, to an inspiratory signal with lower amplitude and higher high-pass frequency during inspiration. These results can be explained if we assume that the signal is traveling through the abdominal tissue and is attenuated and filtered by the lung, moving into the signal pathway during inspiration. Such a pathway would also explain the dependency on the BMI (Additional file 1; Suppl. Figure 1), because abdominal fat tissue is expected to transmit sound signals well. Thus, the movement of the diaphragm may be the reason for the frequency variability. In COPD hyperinflation of the lungs leads to lowering of the diaphragm and to straightening of the diaphragmatic domes while diaphragmatic movement is reduced [17‐19]. Rüter et al. described the expiratory signal of a COPD subject as resembling the inspiratory signal of a healthy person [14]. Airway obstruction of COPD patients limits their ability to exhale. This would explain why these patients are not able to achieve the expiratory diaphragmatic position of healthy subjects, resulting in an attenuated signal containing higher frequencies. Accordingly, the frequency variability in COPD patients was less pronounced than in healthy subjects and decreased with air trapping.

At present, the LFU method has a number of limitations that need further clarification. It remains uncertain whether the method can be applied to obese patients, as sound transmission through the subcutaneous tissue cannot be excluded. Our studies showed that the signal of subjects with increased body mass index was of higher amplitude than in patients with normal weight. Age, however, does not appear to affect the signal, because a covariate analysis failed to detect age as a factor influencing the LFU signals. However, further studies to define the effect of age and gender are needed. It appears that the necessary degree of cooperation may be less than with conventional FEV1 measurements because meaningful measurements are possible with non-forced maximum expiration. Why the LFU method was even more sensitive with non-forced as opposed to forced expiration breathing is unclear at present; whether this might be explained by the speed and distortion of the chest wall displacement needs to be determined.

In summary, we have shown that low frequency ultrasound is able to detect air trapping in COPD patients of GOLD stage 1-3 and appears to be a useful additional tool in the screening for COPD. It is inexpensive, easy to perform and noninvasive, so it could be applied during routine checks in general practice medicine to monitor air trapping. In contrast to spirometry and body plethysmography, no special training is required. We conclude that low frequency ultrasound might be helpful in identifying air trapping and in deciding which patients should undergo more specialized lung function testing. We suggest that further studies with more study subjects should be performed to define reference values and to further standardize the measuring procedure.