• Weaning criteria emerged in the late 1970s in an attempt to identify patient potential for successful extubation. Although these “standard weaning criteria,” which included negative inspiratory force or pressure (NIF or NIP), positive expiratory pressure (PEP), spontaneous tidal volume (SVt), and vital capacity (VC), were used widely over the years to test weaning readiness, they gradually grew out of favor because they did not perform well as predictors, especially in disparate categories of patient conditions.² Two systematic reviews evaluated the weaning process and concluded that weaning criteria (also known as predictors or indices) did not predict weaning.^4,8 They were found to be good negative predictors (i.e., that the weaning attempt would be unsuccessful) but poor positive predictors (i.e., that the weaning attempt would be successful).^1,9,11,12 Regardless, the criteria do provide information about respiratory muscle strength and endurance and may be especially helpful in following trends in gains in strength and endurance in patients with debilitated weak conditions or in patients with myopathies. The criteria also may help in evaluation of respiratory muscle fatigue (see Procedure 37).

• Negative inspiratory force (NIF) also is called negative inspiratory pressure (NIP) or sometimes maximal inspiratory pressure (MIP). The measurement of NIF is effort independent (the patient does not have to actively cooperate) and is considered the most reliable of the standard weaning criteria (SWC). NIF is a measure of inspiratory respiratory muscle strength. It is a strong negative predictor but a poor positive predictor.1,4,12 The most common threshold cited for NIF is less than or equal to –20 cm H₂O. Because this measurement is non–effort dependent, with good technique (see the procedure), the value is reliable unless central drive is impaired. For example, with sedation, a cuff leak, or respiratory muscle fatigue, the value may be adversely affected.

• Positive expiratory pressure (PEP) is effort dependent and requires that the patient cooperate fully to obtain a reliable value. PEP is a measure of expiratory muscle strength and ability to cough. The threshold for PEP is greater than or equal to +30 cm H₂O.

• Spontaneous tidal volume (SVt) is a measure of respiratory muscle endurance. The threshold for SVt is greater than or equal to 5 mL/kg of body weight. When muscles fatigue, the compensatory breathing pattern is rapid and shallow. As a result, investigators have combined SVt and spontaneous respiratory rate (fx) in a ratio called the rapid shallow breathing index (fx/Vt).¹³

• The fx/Vt index threshold associated with success is less than or equal to 105. This threshold is calculated by obtaining the spontaneous respiratory rate and dividing it by the Vt in liters.¹³ In elderly medical patients, the threshold is less than or equal to 130.⁷

• Vital capacity (VC) is also a measure of respiratory muscle endurance or reserve or both. A fatigued patient is unable to triple or even double the size of a breath. The threshold for VC is greater than or equal to 10 to 15 mL/kg (at least two to three times SVt).

• Vital capacity may be especially helpful in patients with neurologic conditions like myasthenia or Guillain-Barré syndrome. In these patients, a decrease in the VC suggests loss of reserve and impending respiratory muscle failure.³

• All SWC are best used in combination with other assessment data to determine the appropriateness of weaning trials or extubation.2,4,8,11,12

• Randomized controlled trials (RCTs) were conducted to determine when and how best to wean patients from mechanical ventilatory support. The studies showed the efficacy and safety of multidisciplinary protocols with use of a “wean screen” (a set of discrete criteria that suggest stability, such as a fraction of inspired oxygen [FiO₂] less than 0.50, positive end-expiratory pressure [PEEP] less than 8 cm H₂O, no vasopressor use, etc.) followed by a carefully monitored spontaneous breathing trial (SBT) in attaining positive outcomes.5,6,10