Propofol Causes Higher RSBI in Weaning Assessment
Abstract & Commentary
Synopsis: Propofol causes changes in breathing pattern that could lead the caregiver to erroneously conclude that a patient is not ready to begin a spontaneous breathing trial.
Source: Khamiess S, et al. Respir Care. 2002;47: 150-153.
The purpose of this study was to determine whether propofol affects spontaneous breathing pattern in critically ill patients recovering from respiratory failure during initial attempts at liberation from mechanical ventilation. Ten critically ill patients were studied in the medical intensive care unit of a community teaching hospital. The rapid-shallow breathing index (RSBI) was recorded while patients breathed on 5 cm H2O of continuous positive airway pressure. RSBI was determined before and again 20-30 min after cessation of propofol infusion.
During propofol infusion, patients breathed with a higher RSBI (156 vs 115 breaths/min/L; P = .004). In 3 of 10 patients, the RSBI was unfavorable during propofol infusion, but became favorable after the cessation of propofol infusion. The weight-adjusted dose of propofol did not correlate with the changes in RSBI. Khamiess and colleagues conclude that propofol changes breathing pattern that could lead the caregiver to erroneously conclude that a patient is not ready to begin a spontaneous breathing trial (SBT).
Comment by Dean R. Hess, PhD, RRT
Initially described more than 10 years ago,1 the RSBI is commonly used to assess patients’ readiness for a SBT. Although the specific cut-point used to predict weanability varies among investigators, it is generally accepted that a higher RSBI is associated with a higher likelihood of SBT failure. It is not unusual to determine a RSBI to screen patients for weaning readiness before complete withdrawal of sedation. In fact, a high RSBI—particularly if viewed in isolation of other issues related to weaning—might prompt the clinician to forgo a SBT. The results of this study suggest caution when interpreting the RSBI in patients receiving propofol infusion. It is of interest to note that similar results have been reported for patients receiving benzodiazepine infusions.2,3
It is not unusual to make decisions regarding the potential for liberation from mechanical ventilation while patients continue to receive sedative infusions. The results of this study suggest that such decisions should be made after discontinuation of sedative infusions. In fact, assessment for ventilator weaning readiness while the patient is continued on sedative infusions may lead to the incorrect decision to continue ventilatory assistance. These results may explain, in part, the observation that the duration of mechanical ventilation is significantly shortened with daily interruption of sedative infustions.4
In recent years, the role of "weaning parameters" for predicting readiness for discontinuation of ventilatory support has been called into question. Although the RSBI is perhaps the most accurate weaning parameter, it is far from perfect.5 One of the messages from this study is that the RSBI must be interpreted cautiously. The RSBI is associated with both false positives and false negatives, and must always be interpreted in the context of all other clinical issues pertinent to the patient. If the RSBI is used in the assessment of weaning readiness, it should be recorded after the cessation of sedative infusions. Even then, there may be a significant number of patients in whom the RSBI is high, but who nonetheless complete a SBT and are successfully extubated.
I will use this "bully pulpit" to make a point that I feel cannot be over-emphasized. That is, the best way to determine whether a patient is ready to assume spontaneous breathing is to discontinue ventilator support and closely observe the patient’s response (being prepared to re-initiate support if needed). The patient’s ability to complete a SBT of 30 to 120 min indicates that the patient is ready to be liberated from mechanical ventilation. Weaning parameters, including the RSBI, are seldom helpful and often wrong. The reader is referred to recently published evidence-based guidelines on weaning from mechanical ventilation,6 (discussed in the April issue of Critical Care Alert) where these issues are addressed in more detail.
Dr. Hess is Assistant Professor of Anesthesia, Harvard Medical School, Assistant Director of Respiratory care, Massachusetts General Hospital, Cambridge, MA.
References
1. Yang KL, Tobin MJ. N Engl J Med. 1991;324: 1445-1450.
2. Gautier H, Gaudy JH. J Appl Physiol. 1978;45: 171-176.
3. Berggren L, et al. Acta Anesthesiol Scand. 1987;31: 667-672.
4. Kress JP, et al. N Engl J Med. 2000;342:1471-1477.
5. Meade MO, et al. Respir Care. 2001;46:1408-1415.
6. MacIntyre NR, et al. Chest. 2001;120(6 Suppl): 375S-395S.