Is My Patient Too Blue? Who Can Benefit From Early Intervention After a Bidirectional Cavopulmonary Anastomosis?*

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The bidirectional cavopulmonary anastomosis (BCPA) is the intermediate step in single ventricle palliation. The operation involves an end-to-side anastomosis between the superior vena cava (SVC) and right pulmonary artery. Physiologically, it is characterized by a passive source of pulmonary blood flow, reduces the volume load of the ventricle, and resolves the issue of parallel pulmonary and systemic circulations. In general, surgical mortality for the BCPA is low, and the resulting physiology is more robust than that of a parallel single ventricle circulation (1, 2).
When postoperative problems occur, they are most often manifest by significant cyanosis, high SVC pressure, or both. Preoperative risk factors predicting BCPA failure are well established (3–5). Once these problems are manifest, the contrasting approaches of “watch and wait” and aggressive reintervention can vary from patient to patient, with little data to support either approach. Needed interventions can be delayed while waiting for spontaneous resolution of symptoms as postoperative healing progresses. Thus, identification of predictive factors that are evident in the early postoperative period and indicate the need for intervention could be of great help to clinicians.
In this issue of Pediatric Critical Care Medicine, Cigarroa et al (6) present their study examining the association between alveolar dead space fraction (AVDSf) and postoperative intervention and outcomes in patients undergoing BCPA. In this retrospective case-control study of 191 patients, the authors examined associations between AVDSf ([PaCO2 – end-tidal CO2 (PetCO2)]/PaCO2), arterial oxyhemoglobin saturation, and transpulmonary gradient and the occurrence of a surgical or catheter-based intervention, death or transplant prior to hospital discharge.
The authors have identified that both admission AVDSf and oxygen saturations have significance for reintervention and poor outcome. Although no single biomarker will be a perfect predictor of BCPA failure, combining markers can optimize their predictive value. The potential value of AVDSf is that despite statistically significant differences in averages, the absolute oxygen saturation and transpulmonary gradient for many of those who became cases is not that different from what is seen in controls on an individual basis, as is often the situation when translating population statistics to personalized decision-making. By adding AVDSf to both these postoperative variables as well as known preoperative risk factors, the ability to assess who might benefit from early catheterization and intervention is improved.
With regard to the choice of AVDSf as the primary measure of dead space ventilation (VD), multiple studies have demonstrated that higher markers of VD are associated with poorer clinical outcome in various patient populations (7–9). Considerable controversy exists over the method to estimate VD, and whether or not AVDSf can be used as an appropriate surrogate for volumetric capnography calculation of physiologic VD and its relationship to tidal volume (VT). Because volumetric capnography is not used routinely in mechanically ventilated children, it becomes important to carefully consider how necessary volumetric capnography is over time-based capnography. Bhalla et al (10) sought to investigate the correlation between volumetric capnography and time-based capnography dead space measurements in a heterogeneous group of mechanically ventilated children. They demonstrated that the correlation between physiologic VD/VT and AVDSf was weaker in children with low exhaled VT (< 100 mL), a pulmonary reason for mechanical ventilation, large airway VD (> 3 mL/kg), or low PaO2/FIO2 (< 200 mm Hg). Given that the majority of patients included in the current study would fall into the latter category, AVDSf may poorly correlate with physiologic VD/VT, and the accuracy of substituting PetCO2 for volumetric capnography could be challenged in this patient subgroup.

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