Respiratory Muscle Weakness, a Major Contributor to Pediatric Extubation Failure: Does Low Serum Phosphorus Contribute to Muscle Weakness?

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We read with great enthusiasm the article published by Khemani et al (1) in a recent issue of Critical Care Medicine about the association between respiratory muscle strength and extubation failure in children on mechanical ventilator. Extubation failure is associated with longer ICU length of stay and higher mortality and morbidity (2). Thus, identification of risk factors for extubation failure and trying to address them are essential to improve patient outcomes. In this context, authors identified a dose-response relationship between lower maximum airway pressure during occlusion (aPiMax) and reintubation. Children with diminished respiratory muscle strength (aPiMax ≤ 30 cm H2O) at time of extubation were nearly three times more likely to end up in reintubation than those with preserved respiratory muscle strength (aPiMax > 30 cm H2O). Our recent work on hypophosphatemia in critically ill children make us believe that low serum phosphorus may be one of the causes of low respiratory muscle excursion leading to extubation failure. In our cohort, 10 out 112 children were reintubated within 48 hour. Out of these 10 children, nine had hypophosphatemia (3). Serum phosphorus is one of the main constituents of adenosine triphosphate (ATP), the energy currency of the cell. Studies have shown that low serum phosphorous lead to decreased muscle ATP synthesis which may lead to diaphragmatic weakness or any other respiratory muscle weakness (4, 5). We believe that level of serum phosphate may be one among the various other metabolic abnormalities correlating with aPiMaX. Postcardiac surgery patients with cardiopulmonary bypass are prone to develop hypophosphatemia (6). The authors report that nearly half of their patients were status post cardiac surgery in their study cohort (1). The authors have presented detailed assessment of respiratory muscle functioning. It would be interesting to know if there were any differences in the nutritional status of patients in the three groups. Given the important role of phosphorus, did the authors assess the relationship with serum phosphate levels and also other electrolytes such as potassium? It would be worthwhile to explore the contribution of these in multivariable analysis.
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