RSBI (Rapid Shallow Breathing Index)

The Rapid Shallow Breathing Index (RSBI) Calculator computes the ratio of respiratory rate to tidal volume (f/VT), the most widely used predictor of mechanical ventilation weaning success. Developed by Yang and Tobin in 1991, RSBI quantifies the breathing pattern during a brief period of unassisted breathing: rapid, shallow breathing (high RSBI) predicts weaning failure, while slower, deeper breathing (low RSBI) predicts successful extubation.

The calculation is simple: RSBI = Respiratory Rate (breaths/min) / Tidal Volume (liters). The threshold of 105 breaths/min/L was established as the optimal cutoff in the original study: RSBI below 105 predicted weaning success with 97% sensitivity and 64% specificity (positive predictive value 78%, negative predictive value 95%). Values below 80 are particularly reassuring for successful weaning.

RSBI is typically measured during a 1-3 minute trial of spontaneous breathing on T-piece or minimal pressure support (CPAP 5 cmH2O) while the patient breathes without ventilator assistance. The measurement should be obtained within the first 1-3 minutes because prolonged spontaneous breathing may fatigue the patient, and serial measurements during a spontaneous breathing trial provide more information than a single value.

The physiological rationale: respiratory muscles under stress adopt a rapid, shallow pattern to minimize work per breath. This protective reflex reduces diaphragmatic fatigue but at the cost of increased dead space ventilation and inadequate alveolar ventilation. RSBI captures this maladaptive pattern quantitatively, integrating neural drive, respiratory muscle strength, respiratory mechanics, and gas exchange efficiency into a single measurable ratio.

RSBI above 105 should not automatically preclude weaning attempts but should prompt investigation of reversible causes: excessive secretions, fluid overload, inadequate pain control, anxiety, electrolyte abnormalities (particularly phosphate, magnesium, calcium), critical illness myopathy, unresolved infection, and inadequate nutrition. Addressing these factors may convert a failing RSBI to a passing one.

Despite its widespread use and strong negative predictive value, RSBI has moderate positive predictive value, meaning some patients with RSBI below 105 still fail extubation. Clinical judgment, the overall trajectory of illness, cough strength, mental status, and ability to protect the airway remain essential complementary assessments for the extubation decision.