Table 1 Principal studies regarding intercostal muscle ultrasound

Cala et al. 1998 [33]

Four healthy subjects

7.5-MHz curvilinear phased array transducer to obtain ultrasound image of the second right and left interspace in the sagittal plane during tidal breathing and at residual volume, functional residual capacity, and total lung capacity. Inter-rib distance, parasternal intercostal thickness, and motion of the midpoint were measured

During inspiration, the parasternal intercostal muscle moves ventrally and straightens, and lung volume influences its shape and motion. The findings support an intercostal stabilizing function of these muscles

Diab et al. 1998 [31]

Experimental and one healthy volunteer trial

Measurements of the area of the intercostal muscles in normal man during maximal inhalation and exhalation. The 5-MHz probe was placed perpendicular to the chest wall and parallel to the longitudinal axis of the body. The probe was passed over a line at the medial border of the left and right scapulae. Measurements started in an upward direction from the intercostal spaces between the 11th and 12th ribs to those between the 5th and 6th ribs

There is symmetrical expansion of the intercostal muscle area and indirectly of muscular activity at maximal inhalation and maximal exhalation. This finding emphasizes the role of the intercostal muscles in keeping the chest wall in balance during breathing. Measurements of the intercostal muscle area between the 5th and 12th ribs approximately at the medial scapular line computed from the perpendiculars of the area at maximal inhalation give more reliable results

Yoshida et al. 2019 [35]

Twelve healthy subjects

Intercostal muscle thickness measured using ultrasound at rest and at maximal breathing in the anterior, lateral, and posterior parts of the right intercostal spaces

The thickness of the intercostal muscle showed significant increases in the first, second, third, fourth, and sixth intercostal spaces of the anterior portions. There were no significant differences in the lateral or posterior portions between rest and maximal breathing

Nakanishi et al. 2019 [47]

80 ICU MV patients

Ultrasound intercostal muscle thickness was measured on days 1, 3, 5, and 7

Intercostal muscle thickness was associated with prolonged mechanical ventilation and length of ICU stay

Wallbridge et al. 2019 [34]

20 COPD patients

Ultrasound measurement of thickness and echogenicity of 2nd and 3rd parasternal intercostal muscles, dominant pectoralis major and quadriceps, and diaphragm thickness; spirometry; and chest CT

Ultrasound intercostal thickness moderately correlated with FEV1% predicted and quadriceps thickness. Echogenicity correlated negatively with FEV1% predicted; CT-measured lateral intercostal mass correlate negatively with parasternal ultrasound intercostal thickness. Changes in muscle quantity and quality reflected spirometric disease severity

Dres et al. 2020 [36]

Twenty healthy subjects; 16 patients in PSV. Fifty-four patients during SBT

Ultrasound of parasternal intercostal muscle at the level of 2nd right intercostal space; thickness of the parasternal intercostal muscle measured at end expiration and at peak inspiration. Change in thickness determined the thickening fraction of the parasternal intercostal muscle

There was a progressive decrease in parasternal muscle thickening fraction with increasing levels of PS and an inverse correlation between parasternal muscle thickening fraction and the pressure generating capacity of the diaphragm. The parasternal muscle thickening fraction was higher (17%) in patients with diaphragm. The pressure generating capacity of the diaphragm, the diaphragm thickening fraction and the parasternal thickening fraction similarly predicted failure or the spontaneous breathing trial

Umbrello et al. 2020 [38]

Twenty-one ICU MV patients in PSV

Parasternal intercostal ultrasound (thickness and thickening fraction) at three-level of PS (baseline, 25% and 50% reduction). Concomitant recording of the esophageal and transdiaphragmatic pressure–time products, work of breathing, and diaphragm ultrasound

Additional measurement of parasternal intercostal thickening may discriminate a low inspiratory effort or a high effort in the presence of a dysfunctional diaphragm