Ultrasonographic assessment of parasternal intercostal muscles during mechanical ventilation

Abstract

Although mechanical ventilation is a lifesaving treatment, abundant evidence indicates that its prolonged use (1 week or more) promotes respiratory muscle weakness due to both contractile dysfunction and atrophy. Along with the diaphragm, the intercostal muscles are one of the most important groups of respiratory muscles. In recent years, muscular ultrasound has become a useful bedside tool for the clinician to identify patients with respiratory muscle dysfunction related to critical illness and/or invasive mechanical ventilation. Images obtained over the course of illness can document changes in muscle dimension and can be used to estimate changes in function. Recent evidence suggests the clinical usefulness of ultrasound imaging in the assessment of intercostal muscle function. In this narrative review, we summarize the current literature on ultrasound imaging of the parasternal intercostal muscles as used to assess the extent of muscle activation and muscle weakness and its potential impact during discontinuation of mechanical ventilation. In addition, we proposed a practical flowchart based on recent evidence and experience of our group that can be applied during the weaning phase. This approach integrates multiple predictive parameters of weaning success with respiratory muscle ultrasound.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1

Intercostal muscles anatomy. A schematic picture of anatomy and function of internal and external intercostal muscles. Panel A: the figure represents the anatomical distribution of intercostal muscles. Panel B: the figure represents the schematic functional activity of intercostal muscles. In particular, the external intercostal connects the ribs in such a way that contraction of the muscles lift the ribs and the rib cage allowing to expand the anterior–posterior dimension of the chest wall. On the other hand, the contraction of the internal intercostal (oriented opposite to the external) determines the opposite effect such as lowering of the ribs and reducing the anterior–posterior dimension

Fig. 2

Intercostal muscle ultrasound. The figure depicts an ultrasound image of parasternal intercostal muscle ultrasound. a The ultrasound probe position at the parasternal space. b The ultrasound image depicted by ultrasound using a B-mode setting in which the intercostal muscle lies between two ribs (R), cranial to the pleural line, behind the pectoral muscle. The left side is a schematic view of the ultrasound image. ICM intercostal muscle

Fig. 3

A simplified diagram of respiratory muscular ultrasound during weaning trial from mechanical ventilation. Once a pressure support (PS) trial or a spontaneous breathing trial (SBT) is started and patients failed a termination of the weaning process within 24 h a diaphragm evaluation exploring the diaphragmatic thickening fraction (TFdi) should be performed. At the same time, the parasternal intercostal muscle should be evaluated. The simplest method is to evaluate the thickness and the thickening fraction (TFic) as calculated for the diaphragm (TF = ((TH end inspiration – TH end expiration)/TH end expiration)) * 100. A value of TFic less than 10%, associated with a TFdi greater than 20% indicates a pattern of breathing in which respiratory muscles are not recruited, and is then suggestive of a successful weaning trial. All these measurements should be integrated with other routinely used predictive parameters of weaning failure, such as rapid shallow breathing index (RSBI), negative inspiratory function (NIF), forced vital capacity (VFC) and the tidal change in esophageal pressure (Δpes). In any case, consider optimizing muscular load acting on pharmacological intervention, metabolic supply, and respiratory muscular in/activity