Discussion
This study demonstrated the ability of point of care LUS to predict
success of extubation in mechanically ventilated premature infants.
Infants with higher LUS were supported with higher venitlatory pressure.
LVEI did not differ among infants who succeeded or failed extubation.
However, it correlated with PAP.
Infants with higher LUS were supported with significantly higher
pressure of conventional mechanical ventilation. The use of higher PIP
is indicative of decreased compliance in infants with severe lung
disease. A previous study of CPAP-supported infants demonstrated the
need for higher pressure in infants with higher LUS12. Oxygen saturation index (OSI) was also increased
in infants with severe lung disease who had higher LUS. This finding
agreed with previous studies that showed correlation of LUS with OSI and
with the ratio of arterial oxygen saturation to FiO212,15.
This study is the first to use LUS to predict success of extubation from
mechanical ventilation in premature infants. Many lung ultrasound scores
have been developed to assess lung aeration and guide respiratory care
in adults especially those with restrictive lung disorders; LUS in this
population is strongly recommended (level of evidence A)18. Compared to adult literature, lung ultrasound has
not been adequately addressed in neonates although it is easier owing to
the small patients’ size and the absence of obesity or heavy musculature19. A few studies described the ultrasound patterns in
common respiratory conditions in neonates, such as meconium aspiration
syndrome, hyaline membrane disease, transient tachypnea of the neonate,
and pneumothorax 21-23. Other studies described the
usefulness of LUS in predicting the need for invasive mechanical
ventilation in infants supported with noninvasive ventilation24,25 and the need for surfactant replacement in
CPAP-supported extremely preterm neonates 26. However,
there is no study to predict success of extubation from mechanical
ventilation. Giving the ease of its use at the bedside and the
non-invasive nature of LUS, it is helpful to obtain LUS before
extubation to avoid the exposure of infants to re-intubation should the
extubation attempt fail. One of the main barriers to the more extensive
use of the ultrasound technology in premature infants is the lack of
efficient training solutions and the need to have structured
quality-check assurance 27. Once training is
established the use of LUS can be the first-line imaging technique in
preterm infants 28.
Evaluating an infant for extubation readiness is clinically a challenge.
There are significant variations in the decision making process of
extubation; that lacks objective evidence-based criteria and is often
contingent on caregiver experience. A recent international survey showed
extubation readiness subject to caregiver personal interpretations of
blood gas parameters and overall feeling of clinical stability of
infants 29. Some physicians advocate conducting apnea
test before extubation, although this test is often conducted variably30. Therefore, it is important to have an objective
method and/or a scoring system with a calculated prediction of success
rate accordingly. LUS fulfils this unmet gap in neonatal practice. In
this study, when LUS was ≤11extubation was successful in
>90% of infants.
Pulmonary hypertension in newborns (PPHN) is triggered by multiple
etiologies including hypoxemia and underlying parenchymal lung diseases
and can potentially hinder extubation 28. Performing a
full echocardiographic studies on all mechanically ventilated preterm
infants would be exhaustive and may not consistently correlate with PPHN14. LVEI is a quantifiable measure of the amount of
distortion of ventricular septal geometry due to elevated right
ventricular systolic or diastolic pressures and/or volumes. Greater
degrees of LVEI have been associated with PPHN in children and adults,
but have not been studied thoroughly in premature infants. LVEI has the
added benefit of being easily measured from any short axis view of the
mid-left ventricle 13. Increased end-diastolic LVEI
would indicate volume overload as in hemodynamically significant PDA and
increased end-systolic LVEI may indicate right ventricular volume
overload as in PPHN 31. In the current study, systolic
LVEI correlated with PAP that was measured concomitantly by
echocardiography. It correlated with the presence of PDA as well.
However, end-systolic LVEI did not correlate with OSI and did not prove
to be valuable in predicting readiness for extubation. Of note, in this
study we used LVEI only as a surrogate for pulmonary hypertension
although a full assessment of PAP may have shown benefit in predicting
extubation failure.
This study has the strength of addressing the success of extubation
trials that is a real challenge in premature infants. The study has some
limitations including the lack of comparison of LUS with findings of MRI
or CT scan of the chest. To ensure consistency and reliability of the
scores, all LUS studies were performed by the same investigator who was
an experienced pediatric radiologist. However, these studies were well
within the scope of POCUS and as suggested by international guidelines
would be practically performed by neonatal caregivers with minimal
experience with US 7,32. Similarly, we have identified
the US machine and proble models that were used for this research,
although other devices with reasonable resolution will suffice32. LUS addresses failure of extubation related to
lung parenchymal disease. Other factors involved in extubation failure
such as airway edema and apnea of prematurity cannot be predicted with
LUS. Respiratory management in this center allows mechanical ventilation
for several days, therefore the investigators advise using caution when
extrapolating the current findings in settings that have different
respiratory practices. In fact, being ventilated for several days, the
respiratory failure for these enrolled infants would be better defined
as an early stage of chronic pulmonary insufficiency rather than the
classical respiratory distress syndrome 33.