This was a prospective observational study performed between June 2015 and February 2017 in the medical intensive care units (ICU) of North Shore University Hospital, Manhasset, New York and Long Island Jewish Medical Center, New Hyde Park, New York, both belonging to Northwell Health. The institutional review board of the Northwell Health approved the study (IRB protocol #15-049). Study subjects or their appropriate surrogates gave informed consent for participation in the study.
Inclusion Criteria
The ICU team decided about the readiness of a patient to be weaned from MV, the timing of the initiation of the SBT, and extubation based on their clinical assessment. If the ICU team decided to extubate a patient, the primary investigator (AP) was informed.
Exclusion Criteria
Exclusion criteria were age < 18 years, pregnancy, lack of informed consent, MV for < 24 h, unilateral or bilateral diaphragm paralysis, use of a paralytic agent during ICU stay, and do not re-intubate status post extubation.
Study Design
The investigator (AP) performed diaphragm ultrasonography at three separate times:
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1.
During assist control (A/C) mode of MV while patients were consistently triggering the ventilator as they were being weaned off sedation prior to initiation of the SBT.
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2.
After completing 30 min of the SBT on PSV with inspiratory pressure of 5 cm of water and positive end expiratory pressure of 5 cm of water. There were at least 30 min between the two measurements during A/C and PSV. At this time, the rapid shallow breathing index (RSBI) was calculated from the tidal volume delivered by the ventilator on PSV mode and respiratory rate as follows: [RSBI = respiratory rate/tidal volume (in liters)].
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3.
Following discontinuation of MV. The ultrasonography was repeated between 4 and 24 h after extubation.
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4.
The ICU team was blinded to the ultrasonography measurements. The decision to re-intubate or to use non-invasive ventilation (NIV) after extubation was left up to the discretion of ICU team.
Definition of Weaning Failure
Weaning failure was defined as the inability to maintain spontaneous breathing for at least 48 h, without any form of ventilatory support [9]. Patients who required re-intubation or NIV within 48 h of extubation were included in the ‘failure’ group.
Diaphragm Ultrasonography Measurements
Patients were studied in a semi-recumbent position between 20° and 40°. The right hemidiaphragm was examined using 2-dimensional and M-mode ultrasonography to record diaphragm excursion and contraction velocity during tidal breathing.
Measurement of Diaphragm Excursion
Diaphragm excursion was measured using a 3.5 MHz convex phase array probe (M turbo P21 probe, SonoSite, Bothell, WA) as previously described (Fig. 1) [10]. With liver serving as an acoustic window, the probe was placed immediately below the right costal margin in the mid-clavicular line in longitudinal scanning plane with the tomographic plane angled in the cephalad direction such that the ultrasound beam was perpendicular to the posterior third of the right hemidiaphragm (Fig. 2) [10]. When the diaphragm was identified with 2-dimensional imaging, the M-mode interrogation line was adjusted to be perpendicular to the movement of the posterior one-third of the right hemidiaphragm. The diaphragm excursion was measured on the vertical axis of the M-mode tracing (cm) from the beginning to end of tidal inspiration from the leading edge of the right hemidiaphragm.
Measurement of Diaphragm Contraction Velocity
From the M-mode tracing, diaphragm contraction velocity was calculated from the excursion and inspiratory time (Fig. 1) as follows:
$${\text{Contraction velocity}}\,{\text{(cm/s)}}={\text{Diaphragm excursion}}\,{\text{(cm)/inspiratory time}}\,{\text{(s)}}.$$
The investigator performed diaphragm ultrasonography measurements when there was a pattern of consistent tidal breathing pattern in order to exclude variability related to tidal volume. Three measurements each of diaphragm excursion and contraction velocity were performed during tidal breathing and their mean was calculated. The duration of ultrasonography examination was < 5 min.
Statistical Analysis
Mann–Whitney U Test was used to compare continuous variables while Chi-square tests or Fisher’s exact tests were used to compare categorical variables. Percent change of diaphragm excursion and contraction velocity between A/C, SBT, and extubation were calculated separately as
$$\left[ {({\text{post}} - {\text{pre}}){\text{/pre}}} \right] \times 100.$$
A separate logistic regression model predicting “success” of extubation was performed for each of the above measures. A receiver operating characteristic (ROC) area under curve (AUC) was constructed to look at the model’s ability to predict “success.” The level of significance was fixed at p < 0.05. All analyses were performed using SAS version 9.4 (SAS Institute Inc., Cary, NC).