Measuring diaphragm thickness with ultrasound in mechanically ventilated patients: feasibility, reproducibility and validity

Ultrasound measurements of diaphragm thickness (T (di)) and thickening (TFdi) may be useful to monitor diaphragm activity and detect diaphragm atrophy in mechanically ventilated patients. We aimed to establish the reproducibility of measurements in ventilated patients and determine whether passive inflation by the ventilator might cause thickening apart from inspiratory effort. Five observers measured T (di) and TFdi in 96 mechanically ventilated patients. The probe site was marked in 66 of the 96 patients. TFdi was measured at peak and end-inspiration (airway occluded and diaphragm relaxed) in nine healthy volunteers inhaling to varying lung volumes. The association with diaphragm electrical activity was quantified. Right hemidiaphragm thickness was obtained on 95 % of attempts; left hemidiaphragm measurements could not be obtained consistently. Right hemidiaphragm thickness measurements were highly reproducible (mean +/- A SD 2.4 +/- A 0.8 mm, repeatability coefficient 0.2 mm, reproducibility coefficient 0.4 mm), particularly after marking the location of the probe. TFdi measurements were only moderately reproducible (median 11 %, IQR 3-17 %, repeatability coefficient 17 %, reproducibility coefficient 16 %). TFdi and diaphragm electrical activity were positively correlated, r (2) = 0.32, p < 0.01). At inspiratory volumes below 50 % of inspiratory capacity, passive inflation did not cause diaphragm thickening. TFdi was considerably lower in patients on either partially assisted or controlled ventilation compared to healthy subjects (median 11 vs. 35 %, p < 0.001). Ultrasound measurements of right hemidiaphragm thickness are feasible and highly reproducible in ventilated patients. At clinically relevant inspiratory volumes, diaphragm thickening reflects muscular contraction and not passive inflation. This technique can be reliably employed to monitor diaphragm thickness, activity, and function during mechanical ventilation.