A mechanically ventilated postoperative patient is assessed for extubation. During a 30-second spontaneous breathing trial on pressure support of 5 cmH2O she takes 32 breaths per minute with an exhaled tidal volume averaging 220 mL. Which statement is correct?

• A Rapid shallow breathing index is approximately 68, predicting successful extubation
• B The index cannot be calculated unless measured over a full minute
• C Tidal volume below 300 mL alone contraindicates extubation
• D Rapid shallow breathing index is approximately 145, predicting failed weaning ✓

Explanation

The rapid shallow breathing index is respiratory rate divided by tidal volume expressed in litres: 32 divided by 0.22 gives about 145. A value above 105 breaths per minute per litre predicts weaning failure, while a value at or below 105 favours successful extubation. A 30-second measurement multiplied by two may be used in place of a full minute. Option A reflects the common error of dividing by volume in millilitres.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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