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

Abstract

Background: Spirometry is regarded as the gold standard for the diagnosis of COPD, yet the condition is widely underdiagnosed. Therefore, additional screening methods that are easy to perform and to interpret are needed. Recently, we demonstrated that low frequency ultrasound (LFU) may be helpful for monitoring lung diseases. The objective of this study was to evaluate whether LFU can be used to detect air trapping in COPD. In addition, we evaluated the ability of LFU to detect the effects of short-acting bronchodilator medication.

Methods: Seventeen patients with COPD and 9 healthy subjects were examined by body plethysmography and LFU. Ultrasound frequencies ranging from 1 to 40 kHz were transmitted to the sternum and received at the back during inspiration and expiration. The high pass frequency was determined from the inspiratory and the expiratory signals and their difference termed ΔF. Measurements were repeated after inhalation of salbutamol.

Results: We found significant differences in ΔF between COPD subjects and healthy subjects. These differences were already significant at GOLD stage 1 and increased with the severity of COPD. Sensitivity for detection of GOLD stage 1 was 83% and for GOLD stages worse than 1 it was 91%. Bronchodilator effects could not be detected reliably.

Conclusions: We conclude that low frequency ultrasound is cost-effective, easy to perform and suitable for detecting air trapping. It might be useful in screening for COPD.

Trial registration: ClinicalTrials.gov: NCT01080924.

Figures

Figure 1

Measurement setup. Ultrasound pulses were generated and transmitted to the sternum. After being received at the back by two sensors, the signal was amplified and digitalized.

Figure 2

Frequency spectra at inspiration and expiration. The lowest frequency of the first strong amplitude signal was measured during inspiration and expiration and the difference between these frequencies was termed ΔF.

Figure 3

Frequency shift ΔF at maximum inspiration and expiration. Frequency shifts ΔF were compared between healthy subjects (n = 9) and COPD subjects classified by GOLD (GOLD 1: n = 6, GOLD 2: n = 4, GOLD 3: n = 7). a) During non-forced maximum breathing, one-way ANOVA and Dunnett's post-hoc test showed significant differences between healthy subjects and each GOLD stage. Significance increased from GOLD 1 to GOLD 3 (GOLD 1: p = 0.023, GOLD 2: p = 0.008, GOLD 3: p = 0.0007). b) During forced maximum breathing, there was a significant difference between healthy subjects and GOLD stage 3 at p = 0.0008.

Figure 4

Lung function and ultrasound before and after inhalation of Salbutamol. a) FEV1 increased significantly in healthy and COPD subjects (healthy: p = 0.01, GOLD 1: p = 0.011, GOLD 2: p = 0.015, GOLD 3: p = 0.012). b) Airway resistance decreased significantly in healthy subjects (p = 0.0002) as well as in GOLD stage 1 (p = 0.005) and GOLD stage 3 (p = 0.032). c) Residual volume decreased significantly in healthy subjects (p = 0.033) as well as in GOLD stage 1 (p = 0.016) and GOLD stage 3 (p = 0.02). d) There was no significant difference of frequency shift during non-forced maximum breathing in any group.