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Standard weaning criteria (SWC) evaluate respiratory muscle strength and endurance by using negative inspiratory force (NIF) and positive expiratory pressure (PEP). Another index, the rapid shallow breathing index or ratio of respiratory frequency to tidal volume (f/VT) identifies a breathing pattern associated with unsuccessful weaning.
These criteria may help determine the need for intubation, the patient's ability to tolerate weaning trials, the presence of respiratory muscle fatigue, and extubation potential.
SWC are good negative predictors that the weaning attempt will be unsuccessful but poor positive predictors that the weaning attempt will be successful.1 Regardless, SWC provide information about respiratory muscle strength and endurance. SWC are especially helpful in following trends in strength and endurance in patients who are debilitated or weak or patients with myopathies.
NIF also is called negative inspiratory pressure (NIP) or maximal inspiratory pressure (MIP). The measurement of NIF is effort independent (meaning that the patient does not have to cooperate), and it is a reliable SWC predictor but a poor positive predictor.1 The most common threshold cited for NIF is greater than or equal to −20 cm H2O.3 Because this is an effort-independent measurement, the value is reliable with good technique, unless the central respiratory drive is impaired. For example, sedation, a cuff leak, or respiratory muscle fatigue adversely affects the value.
Spontaneous tidal volume (VTS) is a measure of respiratory muscle endurance. The threshold for VTS is greater than or equal to 5 ml/kg of body weight.3 When muscles fatigue, the compensatory breathing pattern is rapid and shallow. VTS and f are combined in a ratio called the f/VT to gauge respiratory muscle fatigue.
The rapid shallow breathing index or f/VT, where f is the number of breaths, or the spontaneous respiratory rate, and VT is the tidal volume, has been developed to identify a breathing pattern associated with unsuccessful weaning. The f/VT index threshold associated with success is less than or equal to 105.1 It is calculated by obtaining the spontaneous respiratory rate and dividing it by the VT in liters. In elderly medical patients, the threshold is slightly higher. 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 VTS).1
Measurement of VC may be especially helpful in patients with neurologic conditions, such as myasthenia gravis or Guillain-Barré syndrome. In these patients, a decrease in the VC suggests loss of reserve and impending respiratory muscle failure.
All SWC are best used in combination with other assessment data to determine the appropriateness of weaning trials or extubation.
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Rationale: The respirometer is used to measure V
TS and VC. Volume and pressure may be measured while the patient is on the ventilator.
Ensure no large cuff leak exists because a large leak adversely affects measurements.
If the patient's oxygen saturation decreases (this may vary with the individual patient and thresholds set by the health care team) or if other signs of intolerance of the procedure emerge, abort the test or perform it for a shorter interval, as tolerated.
A good VC effort requires a maximum inspiration followed by a maximum expiration.
Rationale: A pressure manometer is used to measure NIF.
In most cases, the pressure manometer is attached to the airway via one-way valves (
Figure 1). The valves (one is for inspiration and one is for expiration) are capped as necessary to ensure a closed system and a clean measurement device for attachment to the patient's artificial airway.
Do not perform the NIF when the patient's central respiratory drive is absent because of sedation or neurologic injury.
Rationale: The goal is to obtain the patient's best effort.
Rationale: Multiple attempts ensure that the patient's best effort is recorded.
Abort the test if signs of deterioration occur, including increased respiratory rate, increased heart rate, or decreased oxygen saturation.
Rationale: If the measurements do not meet anticipated levels, the patient may need an initiation of PPV or a continuance of mechanical ventilation. If the measurements equal or exceed the goals, initiation of weaning or extubation may be indicated.
Rationale: Decisions related to weaning trials, intubation, or extubation are made using the results of these tests in conjunction with others.
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