By Betty Tran, MD, MSc

Assistant Professor of Medicine, Pulmonary and Critical Care Medicine, Rush University Medical Center, Chicago

Dr. Tran reports no financial relationships relevant to this field of study.

SYNOPSIS: The authors of a multicenter, prospective cohort study did not find an association between the presence of moderate or large pleural effusion and ventilator weaning failure.

SOURCE: Dres M, et al. Prevalence and impact on weaning of pleural effusion at the time of liberation from mechanical ventilation. A multicenter prospective observational study. Anesthesiology 2017;126:1107-1115.

Rates of ventilator weaning failure after a single spontaneous breathing trial (SBT) are reported between 26-42%.1 There may be multiple contributors, but weaning failure ultimately is the result of excessive load on the respiratory system that exceeds the capacity of the respiratory muscles and/or poor gas exchange. To the extent that pleural effusion can both increase the load on the respiratory system by impairing lung mechanics and disrupt gas exchange, theoretically, it can affect weaning outcome.

To determine whether the presence of pleural effusion affects ventilator weaning success or failure, Dres et al prospectively enrolled intubated patients on mechanical ventilation for at least 24 hours who met criteria for a first SBT in three ICUs (one medical, two medical/surgical) in Paris. SBTs were performed on pressure support ventilation at 7 cm H2O and zero positive end-expiratory pressure or a T-piece for 30 minutes. SBT failure was predefined and present if any one of the following were met: pulse oximetry (SpO2) < 90% on an FiO2 of > 50%, respiratory distress (respiratory rate > 35 or increased by > 50%, cyanosis, agitation), systolic blood pressure > 180 mmHg, cardiac arrhythmia, or respiratory acidosis with pH < 7.32 with PaCO2 > 50 mmHg. Weaning failure was defined as a failed SBT or if the patient required reintubation or noninvasive ventilation (NIV) within 48 hours of extubation. NIV used prophylactically for patients at risk for extubation failure (e.g., hypercapnia during SBT) was not counted as weaning failure. Pleural effusions were assessed via bedside ultrasound after completion of the SBT and classified according to the British Thoracic Society classification as small, moderate, or large and quantified by volume.2,3 Ultrasound images from additional patients were performed separately to assess for interobserver and intraobserver reproducibility. The primary endpoint was the prevalence of pleural effusion (none to small vs. moderate to large) in patients with weaning failure vs. success.

A total of 136 patients were enrolled in the study; most were receiving mechanical ventilation for acute respiratory failure or shock for a median of six days (interquartile range, 3-11 days). Pleural effusion was present in 51 patients. Overall, 18 patients had a moderate to large pleural effusion and 118 had a small or no pleural effusion. Overall, interobserver agreement for visual estimation of pleural effusion was found to range from kappa = 0.70-0.89, and intraobserver agreement ranged from kappa = 0.69-0.70. Not surprisingly, moderate to large pleural effusions were more frequent in patients with chronic renal failure, with septic or hemorrhagic shock as the primary reason for mechanical ventilation, and in those presenting with higher weight gain between ICU admission and first SBT.

Among the 136 patients enrolled, 91 had a successful SBT and were extubated; 10 of these patients ultimately required reintubation within 48 hours. Overall, 57 patients were considered weaning failures. The prevalence and volume of pleural effusion were similar in both the weaning success and weaning failure groups. In addition, the extubation failure rate, total duration of mechanical ventilation, and ICU length of stay were similar in both the group of patients who had none to small pleural effusion and the group with moderate to large pleural effusion.

COMMENTARY

When patients fail SBTs, we are forced to contemplate what interventions could be performed to improve their chances of weaning success. Pleural effusions are common in critically ill patients; although the current study reports a prevalence of 37%, some series cite rates of > 60%.4 In an effort to do something, we consider a thoracentesis in the hopes of improving a patient’s ability to wean from the ventilator. However, results from this study suggest that even moderate to large pleural effusions are not associated significantly with weaning success or failure. Although small in scope with few patients exhibiting moderate to large pleural effusions, this is the first multicenter, observational study on this topic. Its use of bedside ultrasound to detect and quantify pleural effusion is more accurate than use of chest radiographs, as seen in prior studies, and reproducibility between and within observers was moderate. Overall, this study highlights an important theme that is often seen in medicine: Even though biologic mechanisms exist and appear feasible, they may not produce a significant clinical effect. Although pleural effusion can affect respiratory dynamics by decreasing compliance, induce hypoxia via ventilation-perfusion mismatch and shunting, and increase cardiac filling pressures, these mechanisms do not appear to affect weaning failure. When reviewing possible causes for ventilator weaning failure, other etiologies besides pleural effusion probably should be considered first.

REFERENCES

  1. Boles JM, et al. Weaning from mechanical ventilation. Eur Respir J 2007;29:1033-1056.
  2. Havelock T, et al. Pleural procedures and thoracic ultrasound: British Thoracic Society Pleural Disease Guideline 2010. Thorax 2010;65(Suppl 2):ii61-76.
  3. Balik M, et al. Ultrasound estimation of volume of pleural fluid in mechanically ventilated patients. Intensive Care Med 2006;32:318-321.
  4. Mattison LE, et al. Pleural effusions in the medical ICU: Prevalence, causes, and clinical implications. Chest 1997;111:1018-1023.