Diaphragmatic ultrasound: a review of its methodological aspects and clinical uses

Abstract

The diaphragm is the main muscle of respiration, acting continuously and uninterruptedly to sustain the task of breathing. Diaphragmatic dysfunction can occur secondary to numerous pathological conditions and is usually underdiagnosed in clinical practice because of its nonspecific presentation. Although several techniques have been used in evaluating diaphragmatic function, the diagnosis of diaphragmatic dysfunction is still problematic. Diaphragmatic ultrasound has gained importance because of its many advantages, including the fact that it is noninvasive, does not expose patients to radiation, is widely available, provides immediate results, is highly accurate, and is repeatable at the bedside. Various authors have described ultrasound techniques to assess diaphragmatic excursion and diaphragm thickening in the zone of apposition. Recent studies have proposed standardization of the methods. This article reviews the usefulness of ultrasound for the evaluation of diaphragmatic function, addressing the details of the technique, the main findings, and the clinical applications.

Figure 1. A convex transducer (A) uses a lower frequency, allowing a deep penetration and a wide field of view. In a convex transducer, the crystals are embedded along a curved shape (A). The ultrasound beams emitted from the lateral aspects of the transducer lead to decreased lateral resolution and a pie-shaped image on the screen (B and top of C). Convex transducers are primarily used for abdominal scans due to their wider and deeper view. A linear transducer (D) emits a beam with a high frequency (6-12 MHz), providing better resolution and less penetration, making it ideal for imaging superficial structures. The crystals are aligned in a linear fashion within a flat head and produce sound waves in a straight line. The image produced is rectangular in shape (E) with high lateral resolution. The imaging modes are demonstrated in B, C, E, and F. The diaphragm is seen in B mode, also known as real-time imaging (B and E). B-mode ultrasound presents a two-dimensional slice of a three-dimensional structure, rendering a cross-sectional view. The diaphragm is seen in M mode (C and F), which displays the motion of a given structure over time through the placement of a vertical (exploratory, M-mode) line in the directed plane of the transducer, during quiet breathing (QB), deep breathing (DB), and voluntary sniff (VS). The M-mode line is anchored at the top and center of the screen, although its orientation and direction can be adjusted laterally. On the screen, the motion of the structure is plotted along the y-axis, and time is plotted along the x-axis, in seconds. M-mode ultrasound allows high time resolution.

Figure 2. In A, measuring the excursion of right hemidiaphragm using the anterior subcostal view with the convex probe positioned below the costal margin between the midclavicular line (MCL) and anterior axillary line (AAL). In B, ultrasound appearance of the right hemidiaphragm in the subcostal region between the MCL and AAL. In C, schematic representation of the measurement of diaphragmatic excursion: on the left, placement of the probe in the subcostal region to display the diaphragm in B mode and placement of the exploratory line demonstrating excursion from expiration to inspiration (points A-B). In D, measurement of diaphragmatic excursion in M mode. The top of the figure depicts the normal right diaphragm in B mode, and the bottom portion depicts M-mode ultrasound of the diaphragmatic excursion during quiet breathing (QB), deep breathing (DB), and voluntary sniff (VS).

Figure 3. Measurement of diaphragmatic excursion. At the top of all of the panels, we can see images in B mode showing the position of the probe, whereas at the bottom of each panel, the M-mode images show the diaphragmatic excursion (A and B), lack of excursion (C), and paradoxical excursion (D). Panel A depicts diaphragmatic excursion during quiet breathing (QB), and panel B shows diaphragmatic excursion during a voluntary sniff (VS). Panels C and D depict the trace of a paralyzed diaphragm. In C, diaphragmatic excursion is absent during QB. Panel D shows paradoxical motion during VS.

Figure 4. In A, measuring the thickness of right hemidiaphragm through the placement of the linear transducer over the zone of apposition (ZOA) at the ninth intercostal space, between the anterior axillary and midaxillary lines. In B, ultrasound appearance of the left hemidiaphragm at the ZOA between the ninth and tenth intercostal spaces, during quiet breathing, at functional residual capacity. In C, measurement of diaphragm thickness: the top of the figure displays the ZOA of a normal diaphragm, in B mode; and the bottom portion shows, in M mode, the diaphragm thickness at end-expiration (exp), or distance A-A, and diaphragm thickness at end-inspiration (insp), or distance B-B.