We found that a "Dynamic PEEP" CT protocol can provide essential information to assist in the ventilator management of patients with severe BPD and suspected tracheobronchomalacia. In particular, it provides a way to optimize PEEP, promote patient safety by minimizing the need for bronchoscopy and procedural sedation, and allows for assessment of the pulmonary, cardiac, and vascular anatomy.
This approach may be particularly suitable for patients unable to undergo flexible bronchoscopy due to clinical instability or small tracheal tube size. In all four cases, we demonstrated that information from a "Dynamic PEEP" study was able to identify optimal distending pressures in patients with TBM. Importantly, each study led to ventilator changes, two in which PEEP was increased, and two where it was decreased.
In skilled hands, flexible bronchoscopy is an excellent tool that can be used to identify tracheobronchomalacia and assess optimal PEEP7, 11,13. While performing bronchoscopy, one can set varying levels of PEEP on the ventilator in order to determine the minimal appropriate distending pressure to overcome airway collapse and reduce the chance of barotrauma. Despite this, bronchoscopy in the neonatal population is limited by multiple factors. In infants, the relative size of the bronchoscope to the airway and the difficulty maintaining respiratory stability during the procedure, particularly in those diagnosed with BPD and pulmonary hypertension, may limit the quality of information obtained. Moreover, bronchoscopy often requires sedation and does not provide information regarding lung parenchymal disease and its response to varying PEEP levels10 –11.
The "Dynamic PEEP" CT approach to diagnose TBM eliminates the subjectivity of a visual evaluation during bronchoscopy related to the operator experience and mitigates the effect sedatives have on airway dynamics. Prior studies have shown the utility of CT scans to evaluate airway malacia and PEEP levels in patients with a history of tracheoesophageal fistula and bronchopulmonary dysplasia 12,14−15. A significant limitation of CT, in general, is the concern over the higher doses of radiation relative to other imaging modalities, particularly in the pediatric population. We were able to minimize the radiation dose by decreasing the field of view and amperage of the CT tube while maintaining a high level of diagnostic confidence in measuring airway lumen changes in response to various PEEP levels (Table 2). Previous studies showed no changes in diagnostic accurary of CT scans after reducing the radiation dose. Lee et al 16, showed comparable diagnostic confidence in patients evaluated for tracheobronchomalacia when CT images were acquired with a reduced dose technique of 50% from standard dose technique in pediatric patients.
Table 2
Diagnostic dose is the dose of routine diagnostic quality CT performed with the "limited PEEP" scan. Total PEEP scan dose includes CTDIvol and sum of DLP of all the "limited" PEEP CT scans performed during the study.
|
Patient 1
|
Patient 2
|
Patient 3
|
Patient 4
|
Dose
|
Diagnostic
|
Total PEEP
|
Diagnostic
|
Total PEEP
|
Diagnostic
|
Total PEEP
|
Diagnostic
|
Total PEEP
|
CTDI vol (mGy)
|
3.37
|
0.58
|
2.89
|
1.16
|
2.79
|
0.34
|
2.89
|
0.58
|
DLP (mGy-cm)
|
35.37
|
41.73
|
44.53
|
78.48
|
54.91
|
23.7
|
54.83
|
18.27
|
Another critical aspect of a CT scan is that it can provide information regarding the lung parenchyma and other airway abnormalities. A higher PEEP level demonstrated improvement in atelectatic areas and showed pulmonary vein stenosis with collaterals in our first case. Identifying airway and pulmonary vascular problems is crucial since it can help to predict the clinical course and long-term management. For example, pulmonary vein stenosis is a relatively common complication of extreme prematurity and was identified in 4.7% of infants with severe BPD.11 Infants with pulmonary vein stenosis in association with BPD have worse outcomes than those with BPD alone, including longer time on mechanical ventilation, longer duration of hospital stay, and increased mortality10. A retrospective study of infants with severe bronchopulmonary dysplasia and pulmonary hypertension showed a 35% and 27% incidence of aortopulmonary collaterals and pulmonary vein stenosis, often missed during echocardiographic studies17, which highlights the role of CT imaging in a selective high-risk population.
Recent developments in imaging, particularly ultrashort echo-time MRI (UTE MRI), are promising. Hyssinger et al18 evaluated the correlation between flexible bronchoscopy and MRI for the assessment of tracheomalacia. UTE-MRI showed a moderate correlation with tracheomalacia severity and high specificity with a strict definition of tracheomalacia (> 40% change in cross-sectional area). UTE-MRI can decrease the exposure to radiation that infants and children with underlying respiratory disorders are at risk for.