Respiratory Therapy Blog

Mechanical ventilation is associated with a number of risks and recognizing when the patient has adequately recovered from their illness that caused their intubation is key to minimizing their time on the ventilator and these risks. It has become common practice throughout the past number of years to use some bedside physiologic measurements (weaning predictors) to decide if the patient is ready to breathe spontaneously.

Some weaning protocols measure the ratio of frequency to tidal volume (f/VT) during 1 min of spontaneous breathing as a final step before beginning a prolonged spontaneous breathing trial (SBT). The f/VT ratio is used to predict the success of weaning for that day and if a patient has a poor f/VT they may be spared having to perform a SBT. These patients are spared performing a SBT because of the possible risks associated with a failed SBT such as excessive anxiety, hemodynamic instability and it may require greater than 24 hours for muscle strength to recover.

In this month’s Critical Care Medicine (Oct 2006), Tanios, Nevin, Hendra et al. (1) looked at the impact weaning predictors (f/TV) had in weaning protocols to assess a patient’s readiness to advance to a SBT. They looked at 304 patients that had been ventilated for greater than 24 hrs, every patient under went a weaning screen daily at 7 am, half had the f/TV measured but not included in the weaning readiness assessment while the rest had the f/TV measured and included in the weaning assessment.

The criteria to pass the daily screening:

• PaO2/FiO2 ratio ≥ 150 or SaO2 > 90% at FiO2 ≤ 40%
• PEEP ≤ 5 cm H2O
• mean arterial pressure ≥ 60 mmHg without vasopressors (low-dose dobutamine or dopamine allowed)
• awake or easily arousable
• adequate cough and not requiring suctioning more than every 2 hours
• for patients randomized to the f/VT group, the f/VT could not exceed 105 breaths/min/L

If any of these are not met the patient would not go on to a SBT and would be continued to be screened daily. The SBT consisted of a 2 hour trial on a continuous positive airway pressure (CPAP) of 5 cmH2O and a pressure support (PS) of up to 7 cmH2O after which, if successful, the patient was extubated.

The average weaning time for the group that had f/VT included in the weaning protocol was 3 days, for the group that had f/VT omitted the weaning time was 2 days. The patients that had weaning decisions made without f/VT did not have a higher reintubation or mortality rate.

While the purpose of weaning protocols is to assess when patients are ready for a SBT and to avoid premature SBTs that lead to failure, they may actually lead to a longer weaning period. Failed SBTs may cause respiratory muscle fatigue that can take >24 hours to recover but there is no definitive evidence that a failed SBT adversely affects weaning outcome.

The role of the weaning predictors such as f/VT may need to be moved further along down the weaning process. At our institution we typically now use the f/VT ratio at the end of a SBT to help with deciding whether to extubate, instead of using it as a tool to see if the patient is ready to progress to a SBT. We may need to begin to reevaluate the use of weaning predictors such as f/VT and the role they play in the weaning process.

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(1) Tanios MA, Nevins ML, Hendra P et al. A randomized, controlled trial of the role of weaning predictors in clinical decision making. Crit Care Med. 2006 Oct;34(10):2530-35.