Protocol
The patients will enter a Spontaneous Breathing Trial (S.B.T.) for sixty minutes. During the
S.B.T. the desirable measurements will be made: transdiaphragmatic pressure, diaphragm
thickening, Tdi,pi , Tdi,ee , TFdi, Di,e, Maximal Inspiratory Pressure (M.I.P.) , Rapid
Shallow Breathing Index (R.S.B.I.). If the S.B.T. is successful then the patient will be
removed from the mechanical ventilator. For the next 48 hours the patient will be monitored
for distress signs and thus be reintubated or if the weaning of the patient was successful
extubation.
Flow chart of the study
The patient meets the S.B.T. criteria.
The patient is placed in a semi recumbent position.
A nasogastric tube attached with two balloons, which are connected with a pressure
transducer, one placed in the stomach and the other one in the lower third of the
esophagus.
The balloons will be connected with a Hans Rudolph pneumotachograph for the measurement
of the tidal volume, the flow and the esophageal pressure.
An occlusion test will be performed to make sure the esophageal balloon is correctly
placed.
Measurement of Pes, Pgas and simultaneous sonographic measurements (Diaphragm
thickening, Tdipi, Tdiee, Dex, TFdi) during the S.B.T. with the patient being in t-tube.
Performance of a MIP maneuver.
Measurement of the same parameters during an S.B.T. with an airway of reduced diameter
(resistive breathing).
Performance of lung ultrasound for B-lines estimation at the end of SBT, MIP and the
resistive breathing)
If the S.B.T. is successful according to the R.S.B.I. the patient will be disconnected
from the mechanical ventilation.
Monitoring of the patient for the next 48 hours, for the extubation respiratory distress
syndrome or reintubation for another reason.
Further monitoring for 30 days in order to measure the time spent in mechanical
ventilation
Definitions
Transdiaphragmatic pressure
Pdi will be measured with the use of a nasogastric catheter which has 2 balloons attached at
25 cm and 10 cm from it's distal end. The balloons, of 7 cm length, will be connected with a
pressure transducer and with the use of a linear pneumotachometer Hans Rudolph RSS 100HR the
graphic representation of the pressures will be displayed in a computer monitor. The balloons
will contain 0,5-1 ml of air and will be placed in the stomach and the lower third of the
esophagus. To ensure the correct positioning of the balloons an occlusion test (Baydur test)
will be performed ,where the similar fluctuations between the esophageal and the airway
pressure will be affirmative. As long as the stomach balloon is concerned the positive
pressures during the inspiration will be affirmative of the correct positioning.
Tension Time Diaphragm Index
Ttdi is used as an index of diaphragmatic exhaustion. Measurements above 0,15 are indicative
of the diaphragmatic incapability to maintain spontaneous breathing for more than 45 minutes,
whence the naming of Ttdi critical
Ttdi = (Pdi/Pdi,max)x(TixTtot)
Rapid Shallow Breathing Index
Rapid Shallow Breathing Index (R.S.B.I.) is a method used for predicting the weaning outcome.
It is measured during spontaneous breathing and equates to the ratio of respiratory rate per
minute (RR) to the tidal volume (Vt) expressed in liters.
R.S.B.I. = RR/Vt
It has been shown that an R.S.B.I. values below 105 breaths/min/L can predict a successful
weaning.
Maximal Inspiratory Pressure
Maximal Inspiratory Pressure (M.I.P.) is used as a predictive factor of the weaning outcome.
It is measured with a manometer placed in a fully occluded airway, while the patient tries to
breathe through it for 20 seconds. The highest pressure value is named M.I.P. It has been
shown that values below -25 cmH2O are indicative of a positive weaning outcome.
Pressure-Time Product of the Esophageal pressure
PTPes was calculated as the area enclosed within the Pes trace, the superimposed recoil of
the chest wall and onset and end of inspiratory flow. The chest wall recoil pressure was
calculated as 4% of predicted vital capacity.
Diaphragm Ultrasound
The diaphragmatic function is assessed during spontaneous breathing and during breathing with
an airway of reduced diameter. The diaphragmatic movement is assessed with ultrasound
(Phillips iE33) using a Linear 3-11 MHz head. The ultrasound probe is placed in the mid
axillary line perpendicular to the zone of apposition between the 8th-10th intercostal
spaces. In this position, the diaphragm is shown as a non-echogenic layer between two
echogenic layers which represent the diaphragmatic pleura and the abdominal peritoneum. With
this technique the diaphragmatic thickening (Tdi,pi - Tdi,ee), diaphragmatic thickness at
end-expiration (Tdi,ee) and end-inspiration (Tdi,pi), diaphragmatic excursion Di,e and
diaphragmatic thickening fraction TFdi are measured. All the measurements are made using
M-Mode Sonography.
Lung Ultrasound
Lung ultrasound (LUS) was performed in the anterior chest to estimate the number of B-Lines
at the end of each phase. A restrictive two-region LUS test, conducted in the anterior chest
region, was considered to assess EVLW during these maneuvers. Lung ultrasound was conducted
at the end of SBT, MIP and RBT to evaluate for the generation of B-lines during each of these
phases. Afterwards patients were categorized, according to the number of B-lines present, in
four categories depending on the total number of B-lines 0: 0 B-lines, 1: 1-2 B-lines, 2: 3-7
B-lines, 3: >7 B-lines.