3.4 Predictors of prolonged mechanical ventilation duration and
weaning failure (Figure 4, 5, 6)
Age < 1.5 years can predict weaning failure with a sensitivity
of 71% and specificity of 68%. The LUS scores and DE did not have any
significant correlation with weaning failure or mechanical ventilation
duration and ICU stay days. We found the baseline right diaphragm
thickness value of < 24 mm could predict MV duration of
> 13 hours with (70 % sensitivity and 70% specificity)
and AUC of 0.68, p=0.026.. The left diaphragm thickening fraction on PSV
with a cut-off value of 17.15% was found as a predictor for the weaning
failure (sensitivity 85%, specificity 49%, AUC ROC 0.75, p= 0.032).
DISCUSSION
In this observational study we tried to identify the bedside ultrasonic
predictors of weaning failure from mechanical ventilation in pediatric
patients who underwent cardiac surgery under cardiopulmonary bypass. The
pulmonary complications of CPB can be manifested by the presence of
B-lines and diaphragm dysfunction. 24-29
We found the median LUS score to increase from baseline to PSV (post
operatively) in concurrence with the previous literatures21,22,29,41.The significant elevation of the scores in
the postextubation period in group 1 may be the reason for the weaning
failure, whereas in the group 2 the scores remained the same
postextubation. However, we did not find any correlation between LUS
scores and the events of weaning failure. This can be attributed to the
fact that the post cardiac surgery lung edema is of multifactorial
origin. 12-14 The similar intraoperative management
with a negative fluid balance after CPB and before extubation (table 1)
and cardiac functions maybe the reason for the similar lung profiles for
both the groups. The cardiogenic pulmonary edema did not contribute
significantly to the LUS scores of our patients, and the LVEF and E/e’
did not differ between the two groups of patients. The fluid balance
measured before extubation corelated significantly with MV duration
(r2 =0.471, p=0.001) and ICU stay days
(r2 =0.297, p=0.038), suggesting the patients with
negative fluid balance have favorable outcomes.
We could establish the relevance of TDF value of less than 17.15 %
(sensitivity of 85%) of the left side diaphragm during the PSV in
predicting the weaning failure .The diaphragm contributes to 75% of
respiratory effort in children, as evidenced by a DTF less than 17.15%
with increased rates of reintubation. The DTF less than 17%- 21 %
during SBT in children have been shown to be associated with extubation
failure. 42 This study was conducted in medical ICU
patients with longer times of intubation prior to giving an extubation
trial , which could have resulted in greater diaphragm remodeling as
compared to our subset of patients who were intubated for a short
duration. These studies measured the diaphragm only on the right side
which was easier to visualize. In our study, we found significant
changes of the diaphragm on the left side only. This could be due to the
fact that there is frequent handling of the phrenic nerve on the left
side.
The diaphragm atrophy rate is 3.4% per day in children.43 There was a decrease of thickness values during the
pressure support mode of ventilation in our patients. There are no
defined reference values of normal diaphragm thickness in children, so
we used the pre-operative baseline values as our reference. These values
were lower than the values measured by Glau et al of 2cm (1.8-2.5)
measured in children with acute respiratory failure , as our patients
were critically ill cardiac patients who might have had compensated
respiratory problems prior to surgery.
The diaphragm excursion is decreased after mechanical ventilation even
four hours after extubation (group 1 :p-value -0.022;group 2 : p-value
-0.00), suggesting the detrimental effects of neuromuscular blockers and
mandatory ventilation on diaphragm functions. However, the diaphragm
excursion has limited predictive ability for weaning outcomes.32, 40, 42 The excursion may be influenced by the
pressure provided by the ventilator leading to a similar degree of
excursion despite significantly different levels of muscle efforts ,
whereas thickness is influenced only by active contraction.
The DE and TDI have been used to assess the feasibility of extubation
during SBT in the adult patients on prolonged mechanical ventilation;
however its suitability to be used during the weaning process in
pediatric patients after cardiac surgery has not been clearly
established. 31-35
CONCLUSION
LUS scores cannot predict weaning failure whereas diaphragmatic
thickening fraction during SBT <17.15% was found to be a
predictor of weaning failure in pediatric post cardiac surgical
patients.
Our study was limited by the fact that we had a short time course and we
did not follow the patients in the post extubation period beyond 4 hours
in non-reintubated patients. Our study had heterogeneous patient
population undergoing variety of procedures having very different
surgical procedures and their effect on the kids. So the cut –off
values may not be applied generalized to all pediatric patients. We had
a small sample size, which could have made the significant results
appear non-significant. We did not evaluate the concurrent effects of
lung compliance on the extubation outcomes. We did not evaluate EVLW or
diaphragm dysfunction by other known methods like Trans diaphragmatic
pressure movements, Fluoroscopy, Phrenic nerve conduction studies. In
future, the study can be expanded to the learning of the long-term
effects of CPB on mechanical ventilation. The study can be extended onto
non-CPB surgery patients.