Table 4. Parameters, reference values and comments of ultrasound assessment of breathing effort.

Technique	Parameter	Reference values	Comments
Diaphragm thickness (Tdi)	Diaphragm thickness at end-expiration	1.5–2.4 mm (55,56) at end-expiration	Non-invasive and useful to assess development of atrophy. Not a measurement of effort per se. Left hemidiaphragm may be difficult to visualize
Thickening fraction (TFdi)	TFdi = (Tdi,ei − Tdi,ee)/Tdi,ee	24–53% during quiet breathing (57), up to 157% during vigorous effort (58)	Does not allow for direct quantification of muscle pressures
Caudal displacement	M-mode of diaphragm displacement during tidal breathing	1.6–1.8 cm during quiet breathing, up to 7.5 cm during deep breathing (59)	Cannot distinguish patient work from ventilator work during partially supported ventilation

US, ultrasound; Tdi (ee/ei), diaphragm thickness (at end-expiration/end-inspiration); TFdi, thickening fraction of the diaphragm.