Formula | Miscellaneous notes | |
Measurements of oxygenation and gas exchange | ||
PaO2/FiO2 | Poor predictor when used alone but is useful as part of the clinical assessment.[1,2] | |
PaO2/PAO2 | Poor predictor when used alone. | |
A-a oxygen gradient | Poor predictor when used alone. | |
Dead space (VD/VT) | Poor predictor when used alone. | |
Simple measurements of respiratory system load and respiratory muscle capacity | ||
NIF or MIP* | MIP can be measured by attaching an aneroid manometer to the opening of the endotracheal tube and asking the patient to maximally inspire against an occluded airway. Although it is a poor predictor of weaning outcome, it may be useful as supplementary information in patients with neuromuscular weakness.[3] | |
Respiratory system compliance (dynamic, static) | Cst,rs = VT / (plateau pressure – PEEP) | Inconsistent predictive capacity.[4,5] |
Respiratory system resistance | ||
Total minute ventilation* | RR × VT | Weaning is improbable among patients whose minute ventilation is too high or too low.[3] |
Vital capacity | ||
Respiratory frequency* | ||
Tidal volume* | ||
Integrative indices | ||
RSBI* | f/VT | Refer to UpToDate text for details on measurement and performance. |
Dynamic CROP index* | [Cdyn × MIP × (PaO2/PAO2)] / RR | Positive and negative predictive value of 71 and 70%, respectively.[4] |
CORE index | [Cdyn × (P0.1/MIP) × (PaO2/PAO2)] / RR | Accurate predictor of SBT success/failure but is complex and difficult to obtain.[6] |
IWI | [(Cst,rs) × SaO2] / [f/VT] | More accurate than other weaning predictors but Cst,rs is difficult to measure in a spontaneously breathing patient.[5] |
IEQ | [(0.75 VT/Cdyn) × (TI/TTOT)] / MIP | An IEQ >0.15 has been suggested as the fatiguing threshold that predicts weaning failure.[7] |
Complex measurements (may require special equipment) | ||
P0.1 | P0.1 is the airway pressure generated in the first 0.1 second during an inspiratory effort against an occluded airway and can be measured by some ventilators. P0.1 >3.2 to 6 cm H2O has been associated with weaning failure.[8] | |
P0.1/MIP* | Predictive capacity of P0.1 is better when normalized for MIP.[9] | |
Pes/Pes max | Measured by esophageal balloon. Pes is the esophageal pressure generated during a spontaneous tidal breath and Pes max is the maximal pressure that can be generated. Values >0.4 suggest a fatiguing load that is not sustainable.[3] | |
O2COB | O2COB is the difference between total oxygen consumption during spontaneous breathing and relaxed mechanical ventilation. O2COB is <5% of the total oxygen consumption in most healthy individuals. No threshold has been identified. Changes in non-respiratory oxygen consumption can confound the measurement.[10] | |
Mechanical work of breathing | VT × ITp | ITp has to be measured using an esophageal balloon. A threshold value has not been identified.[11] |
Diaphragmatic ultrasound | Diaphragmatic excursion can be directly measured during inspiration in cm. DTF = [diaphragmatic thickness at TLC – RV] / diaphragmatic thickness at RV | Weaning is longer in those with a diaphragmatic descent <100 mm during inspiration.[12] A thickening fraction of >30% was associated with sensitivity 0.88, specificity 0.71, positive predictive value 0.91, and negative predictive value 0.63 for predicting extubation success.[13] Combining diaphragmatic ultrasound with the rapid shallow breathing index may further increase predictive accuracy.[14] |
Pdi/Pdi max | Pdi = Pg – Pes | Pdi (diaphragmatic pressure during a spontaneous tidal breath) and Pdi max (maximal diaphragmatic pressure) are measured using esophageal and gastric balloons where Pdi = Pg – Pes. Values >0.4 suggest a fatiguing load that is not sustainable.[3] |
TTdi | TTdi = (Pdi/Pdi max) × (TI/TTOT) | TTdi = (Pdi/Pdi max) × (TI/TTOT). Values of >0.15 to 0.18 indicate a load on the respiratory muscles that is not sustainable.[3] |
Gastric intramucosal pH or PgCO2 | Gastric acidosis results from blood flow that is diverted from the splanchnic vascular bed to the respiratory muscles during weaning in order to meet the oxygen demands of the respiratory muscles.[15] |
A-a: Alveolar-arterial; Cdyn: dynamic compliance; CORE: Compliance, Oxygenation, Respiration, Effort; CROP: Compliance, Respiratory rate, Oxygenation, maximal inspiratory Pressure; Cst,rs: static respiratory system compliance; DTF: diaphragmatic thickening fraction; f: frequency of breathing; FiO2: fraction of inspired oxygen; IEQ: inspiratory effort quotient; ITp: the intrathoracic pressure generated by respiratory muscle contraction; IWI: integrative weaning index; MIP: maximal inspiratory pressure; NIF: negative inspiratory force; O2COB: oxygen cost of breathing; Pg: gastric pressure; PgCO2: gastric carbon dioxide tension; P0.1: airway occlusion pressure 0.1 sec after the start of inspiratory flow; PAO2: alveolar oxygen tension; PaO2: arterial oxygen tension; Pdi: diaphragmatic pressure; PEEP: positive end-expiratory pressure; Pes: esophageal pressure; RR: respiratory rate; RSBI: rapid shallow breathing index; RV: residual volume; SaO2: arterial oxygen saturation; SBT: spontaneous breathing trial; TI: inspiratory time; TLC: total lung capactiy; TTdi: tension-time index; TTOT: total breath duration; VD: dead space ventilation; VT: tidal volume.
* Predictors found to be most accurate in a systematic review.[3]Adapted from: Epstein SK. Weaning from ventilatory support. In: Textbook of Pulmonary Diseases, 7th ed, Crapo JD, Glassroth J, Karlinsky J, King TE (Eds), Lippincott Williams & Wilkins 2003.
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