Key Point

The RSBI rate, a new measure of change in the Rapid Shallow Breathing Index, appears to be an accurate predictor of weaning outcomes.

SAN FRANCISCO—The Rapid Shallow Breathing Index (RSBI), calculated by dividing the tidal volume into the respiratory rate, was designed as a weaning parameter but has shown poor specificity. The problem may be that the RSBI is not dynamic, making it inadequate to predict respiratory failure during weaning from mechanical ventilation.

Movement in the RSBI may better forecast weaning outcomes, Leopoldo N. Segal, MD, and colleagues have hypothesized. "To study this, we developed a new parameter which we call the RSBI rate," said Dr. Segal at the Society of Critical Care Medicine’s 35th Critical Care Congress.¹ "This is simply the rate of change in the RSBI during a weaning trial."

The results of his recent study supported the use of the RSBI rate in managing the withdrawal of ventilatory support. "The RSBI rate is a very accurate and useful tool in predicting weaning outcome," pointed out Dr. Segal, who is a resident in the Internal Medicine Department at Morristown Memorial Hospital in New Jersey.

His group prospectively evaluated the RSBI rate in 30 patients requiring more than 48 hours of mechanical ventilation through an endotracheal tube with an inner diameter of at least 7.5 mm. These patients were enrolled because they tolerated a spontaneous breathing trial and were determined by an independent intensivist to be appropriate candidates for weaning in accordance with American Thoracic Society guidelines.

Thus, their weaning protocol consisted of a T-piece trial with oxygen to maintain spontaneous breathing for up to two hours. To measure the RSBI rate, the investigators calculated the RSBI multiple times, at the beginning of the weaning protocol and periodically during the two-hour protocol. They arrived at the RSBI rate with the following formula: (RSBI 2 – RSBI 1)/RSBI 1 x 100.

Twenty-one patients were successfully extubated, three were extubated but required reintubation within 24 hours, and six could not be extubated because of intolerance of the T-piece trial. The extubated group had an RSBI rate of 67.5, versus 40.2 in the patients who could not be permanently extubated.

Of the patients who were weaned, only two (9.5%) had an RSBI rate greater than 20% at some point during the weaning protocol. By contrast, all of the patients who could not be weaned had an RSBI rate above 20%.

In a statistical analysis, an RSBI rate of less than 20% was 90.4% sensitive and 100% specific for predicting weaning success. It had a positive predictive value of 100% and a negative predictive value of 81.8%.

—Timothy Begany

Reference
1. Segal LN, Fiel SB, Ruggiero S, et al. Use of the rate of change of the RSBI during spontaneous breathing trial as an accurate predictor of weaning outcome. Presented at: Society of Critical Care Medicine’s 35th Critical Care Congress; January 10, 2006; San Francisco, Calif.

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