Diaphragm ultrasound as indicator of respiratory effort in critically ill patients undergoing assisted mechanical ventilation: a pilot clinical study

Abstract

Introduction: Pressure-support ventilation, is widely used in critically ill patients; however, the relative contribution of patient's effort during assisted breathing is difficult to measure in clinical conditions. Aim of the present study was to evaluate the performance of ultrasonographic indices of diaphragm contractile activity (respiratory excursion and thickening) in comparison to traditional indices of inspiratory muscle effort during assisted mechanical ventilation.

Method: Consecutive patients admitted to the ICU after major elective surgery who met criteria for a spontaneous breathing trial with pressure support ventilation were enrolled. Patients with airflow obstruction or after thoracic/gastric/esophageal surgery were excluded. Variable levels of inspiratory muscle effort were achieved by delivery of different levels of ventilatory assistance by random application of pressure support (0, 5 and 15 cmH2O). The right hemidiaphragm was evaluated by B- and M-mode ultrasonography to record respiratory excursion and thickening. Airway, gastric and oesophageal pressures, and airflow were recorded to calculate indices of respiratory effort (diaphragm and esophageal pressure-time product).

Results: 25 patients were enrolled. With increasing levels of pressure support, parallel reductions were found between diaphragm thickening and both diaphragm and esophageal pressure-time product (respectively, R = 0.701, p < 0.001 and R = 0.801, p < 0.001) during tidal breathing. No correlation was found between either diaphragm or esophageal pressure-time product and diaphragm excursion (respectively, R = -0.081, p = 0.506 and R = 0.003, p = 0.981), nor was diaphragm excursion correlated to diaphragm thickening (R = 0.093, p = 0.450) during tidal breathing.

Conclusions: In patients undergoing in assisted mechanical ventilation, diaphragm thickening is a reliable indicator of respiratory effort, whereas diaphragm excursion should not be used to quantitatively assess diaphragm contractile activity.

Figure 1

Example of respiratory tracing and ultrasonographic measurements during the 0 cmH ₂ O pressure support step (PS0) . (A) Sample recording of respiratory tracings during PS0: P_es, esophageal pressure; P_ga, gastric pressure; Paw, airway pressure; Vi, respiratory flow; P_di, transdiaphragmatic pressure. White area represents inspiration and gray area depicts expiration. (B) Ultrasonographic view of diaphragmatic excursion during breathing in B-mode (upper) and M-mode (lower). (C) Ultrasonographic view of diaphragm in the zone of apposition during inspiration (upper) and expiration (lower); the diaphragm is identified as a three-layer structure (non-echogenic central layer bordered by two echogenic layers, the peritoneum and the diaphragmatic pleurae) as indicated by yellow crosses.

Figure 2

Individual patient data for diaphragm excursion and thickening fraction in the different steps of the study. PS, pressure support.

Figure 3

Correlation between excursion and esophageal and diaphragmatic pressure-time product. PTPes, esophageal pressure-time product per breath; PTPdi, diaphragmatic pressure-time product per breath; PSV, pressure support ventilation.

Figure 4

Correlation between thickening fraction and esophageal and diaphragmatic pressure-time product. PTPes, esophageal pressure-time product per breath; PTPdi, diaphragmatic pressure-time product per breath; TF, thickening fraction; PSV, pressure support ventilation.