The use of invasive mechanical ventilation is a life-saving and essential component of pediatric intensive care. However, prolonged intubation can be associated with an increased risk of ventilator-induced lung injury, nosocomial infections, airway irritation or trauma, and sedative dependence.1,2 A spontaneous breathing trial (SBT) is typically performed in children to gauge readiness for extubation because gradual weaning may not be required to be successfully liberated from the ventilator.3 Although SBTs are utilized in the vast majority of pediatric ICUs, the optimal formulation and interpretation of results has yet to emerge in the literature.
In the current issue of Respiratory Care, Mortamet et al4 report observations made in a cohort of mechanically ventilated children during an SBT. This is an important area of pediatric mechanical ventilation, and the authors should be applauded for focusing their work on this topic. They collected a number of important respiratory and hemodynamic factors, including but not limited to esophageal pressure (Pes), electrical activity of the diaphragm (EAdi), and oxygen consumption (V̇O2). These data were recorded over the course of 90 min: 30 min before, 30 min during, and 30 min after an SBT in CPAP. None of the variables recorded in the study reached statistical significance. The minute-to-minute pressure-time product of Pes increased from 23 during baseline ventilation to 83 cm H2O × s during the SBT, but this was not statistically significant (P = .75). A trend was noted for peak EAdi, which increased from 7.5 to 15.9 μV (P = .059) before and during the SBT. V̇O2 was 3.6, 3.8, and 3.8 mL/kg/min before, during, and after the SBT, respectively (P = .98).
In general, the study is adequately designed from a methodological perspective to report changes in parameters during the peri-SBT time period. However, the study is underpowered to detect statistically significant changes among the main outcome variables of Pes, EAdi, and V̇O2. The pressure-time product of Pes more than tripled, but the P value did not approach significance. Rightfully so, the authors anticipated only modest increases in V̇O2 because the diaphragm has been reported to account for only 1–3% of the total V̇O2 during breathing.5 Interestingly, the authors report a 5.5% increase in V̇O2 before and during the SBT, which may be associated primarily with increased work of the diaphragm during CPAP/SBT. This may be one of the most interesting findings of the study. The authors also point out that the absolute values for V̇O2 and CO2 elimination (V̇CO2) are very low compared to data published by my group.6 Also of concern are the values for the respiratory quotient (RQ = V̇CO2/V̇O2) obtained during the study. The average RQ of a cohort of children in the PICU is typically 0.89–0.91,6–8 and the physiologic range for RQ is 0.67–1.3.9 An RQ that is very low, as in the findings by Mortamet et al4 (ie, approximately 0.71), is most likely to occur from either the nearly exclusive metabolism of lipid by the child or by measurement error from a leak, gas sensor, or flow sensor on the indirect calorimeter itself. Given either of these scenarios, it is difficult to say whether the findings for V̇O2 can be extrapolated to other populations.
Based on these comments, it is difficult to make any specific generalizations about how these data can be incorporated into practice at this point in time. Ignoring P values and V̇O2 for a moment, however, I believe the work by Mortamet et al4 is both thought-provoking and hypothesis-generating. If one is able to accurately measure factors during baseline ventilation, and then during an SBT, what is the acceptable magnitude of a change in Pes, EAdi, or V̇O2? At what threshold shall we set these variables to adequately detect those who are likely to fail extubation? Can we use other factors that are readily obtained on the ventilator as a surrogate for the more difficult to obtain measurements of Pes and V̇O2?
In the future, findings from the study by Mortamet et al may serve as pilot data to design a larger study powered to detect changes in Pes, EAdi, and V̇O2. Importantly, this work should be expanded to detect differences in the data between those subjects who were successfully extubated and those who failed or required noninvasive respiratory support.
Footnotes
- Correspondence: Craig D Smallwood PhD RRT, 300 Longwood Ave., Bader Building 634, Boston, MA 02115. E-mail: craig.smallwood{at}childrens.harvard.edu.
Dr Smallwood has disclosed relationships with Mallinckrodt Pharmaceuticals, Bernoulli Health, and Ventec Life Systems.
See the Original Study on Page 509
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