Bouachour and colleagues carried out a study to determine whether gastric intramucosal pH (pHim) and/or gastric intramucosal carbon dioxide tension (PCO2im), measured by tonometry, can be used to predict the success of weaning in chronic obstructive pulmonary disease (COPD) patients. The authors studied 26 consecutive COPD patients undergoing mechanical ventilation for acute respiratory failure caused by pulmonary infection who were considered by their attending physicians to be weanable from mechanical ventilation. All the patients underwent a weaning trial of spontaneous breathing via a T-piece for 2 h. Arterial blood gas values and PCO2im were measured 24 h before, just before, and after 20 min of the weaning trial with the T-piece.
Six patients failed the weaning trial and required the reinstitution of mechanical ventilation without having been extubated. The other 20 patients were weaned successfully. No differences in weaning parameters and blood gas analysis readings were observed between the two groups before the weaning period. Moreover, the two groups did not differ in age, Simplified Acute Physiology Score (SAPS) on admission, SAPS on the day of the weaning trial, and ventilation duration. However, pHim was significantly lower or the gradient between PCO2im and arterial PCO2 (DPCO2) was significantly higher in the six patients who failed the weaning trial. All of them had a pHim less than 7.30, while the 20 patients who were successfully weaned had a pHim greater than 7.30. Notably, in this study, DPCO2 remained unchanged after 20 min of spontaneous breathing on T-piece. (Bouachour G, et al. Eur Respir J 1996; 9:1868-1873.)
COMMENT BY FRANCISCO BAIGORRI, MD, PhD
Weaning patients from mechanical ventilation is still a major problem in intensive care units. The decision to wean is based on clinical and gasometric criteria showing an improvement in, or resolution of the underlying cause of acute respiratory failure and adequate gas exchange. Many different indices have been proposed to predict the outcome of weaning from mechanical ventilation once the clinical improvement occurs. Vital capacity, minute ventilation, maximal inspiratory pressure, ratio of respiratory frequency to tidal volume, and airway occlusion pressure are some of them.
However, it is well known that the above "weaning parameters" do not reliably predict weaning failure. Moshenifar and colleagues were the first to evaluate pHim as a predictor of success or failure of weaning from mechanical ventilation, testing the hypothesis that gastric pHi can be used as a rapid indicator of blood-flow diversion from the splanchnic bed in patients in whom the demands of the respiratory pump during weaning trials are excessive or who have inadequate oxygen delivery to meet these demands (Ann Intern Med 1993;119:794-798). They confirmed that pHim significantly decreased (in absence of arterial PCO2 increment) during the weaning trial in those patients who could not be weaned from mechanical ventilation, while it remained stable in those patients who were successfully weaned from mechanical ventilation.
However, the study by Bouachour et al adds another view to the problem, suggesting that pHim may also identify patients who are still too severely ill to be weaned from mechanical ventilation despite the improvement in clinical and gasometric parameters. In the study by Bouachour et al, just before the weaning trial, patients who failed had a higher heart rate and a lower systolic blood pressure than patients who were successfully weaned. None of these patients had acute left ventricular dysfunction. Thus, we may speculate about the presence of inadequate resuscitation and/or ongoing sepsis in these patients only revealed by the presence of gastric intramucosal acidosis. If these findings are confirmed in further studies, it may provide another interesting approach to the problem of failure of weaning from mechanical ventilation in patients who fulfill conventional criteria for successful extubation.