Discontinuation of Mechanical Ventilation

Published on 07/03/2015 by admin

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Chapter 10 Discontinuation of Mechanical Ventilation

Theodore W. Marcy, MD, MPH , Jenny L. Martino, MD, MSPH

1 What proportion of patients can be readily removed from mechanical ventilation?

The majority of patients (75%) supported with mechanical ventilation are able to resume unsupported breathing within 7 days of intubation if the illness that resulted in respiratory failure resolves or improves. One of the clinician’s challenging tasks is to determine when the patient is ready for ventilator discontinuation. Continuing mechanical ventilation beyond the time that is necessary exposes the patient to risks for nosocomial infection and ventilator-induced lung injury. Conversely, removing ventilatory support from a patient prematurely can lead to severe stress from respiratory and cardiovascular decompensation and exposes the patient to the risks associated with reintubation including increased mortality rate, increased time in the intensive care unit (ICU), and need for long-term care in a rehabilitation facility.

2 When should patients receiving mechanical ventilation be assessed for ventilator discontinuation?

Every patient receiving mechanical ventilation should be assessed for ventilator discontinuation on a daily basis as long as his or her medical status meets the following criteria:

image Lung injury stable or resolving

image Adequate gas exchange with low positive end-expiratory pressure (PEEP) and fraction of inspired oxygen (FiO2) requirements (e.g., PEEP < 5-8 cm H2O, FiO2 < 0.4-0.5)

image Hemodynamic stability (e.g., not requiring pressors, no serious arrhythmias)

image Patient capable of initiating inspiratory efforts

Evidence indicates that systematic daily weaning assessments improve patient outcomes, reduce the number of days patients are dependent on the ventilator, and reduce the number of patients who require tracheostomies.

3 How, exactly, should this assessment be done?

As of yet, no systematic weaning protocol has been agreed on. However, most protocols have a stepwise assessment that varies in the details. The above criteria should be assessed daily as a wean screen. For the patients who pass the daily wean screen, there is first an initial brief trial or readiness assessment during which patients are closely observed for 1 to 5 minutes while receiving minimal or no support (continuous positive airway pressure [CPAP] ≤ 5 cm H2O, T-piece trial, 5 to 7 cm H2O pressure support from the ventilator, or automatic tube compensation) to assess their ability to undergo a formal spontaneous breathing trial (SBT). If the patient does well during the readiness assessment, an SBT is performed for between 30 and 120 minutes. During this time patients are closely monitored for signs of respiratory insufficiency, hemodynamic deterioration, problems with gas exchange, or patient discomfort. Full ventilatory support is promptly reinitiated if problems develop. Successfully completing an SBT is highly predictive of successful ventilator discontinuation. These steps are illustrated in Figure 10-1.

image

Figure 10-1 A protocol for daily assessment for extubation. RSBI, Rapid shallow breathing index calculated by frequency of breaths per minute divided by tidal volume in liters; SBT, spontaneous breathing trial.

4 What is the rapid shallow breathing index (RSBI)? What does it predict?

The RSBI is calculated by using the following formula:

image

Higher values for the RSBI signify a pattern of breathing often seen in patients with respiratory muscle fatigue who tend to have weak inspiratory efforts and consequently higher respiratory rates. Values greater than 105 to 120 predict patients who may not tolerate an SBT. However, the observed RSBI value can be increased by recent suctioning, anxiety, fever, and the size of the endotracheal tube. Clinicians should be aware that the RSBI will generally be lower during use of minimal support modes (pressure support, CPAP) in comparison with when patients are just using a T-piece. The RSBI is most often measured during the readiness assessment to identify patients who may go forward with an SBT. The RSBI does not predict the ability for the patient to have the endotracheal tube removed on its own.

5 To which mode should the ventilator be set during the SBT?

The specific ventilatory mode during the SBT is not critical. The choices usually include one of the following:

image T-piece circuit that provides a constant flow of oxygen past the endotracheal tube with an extension downstream to prevent entrainment of room air

image Low levels of pressure support ventilation of 5 to 8 cm H2O

image CPAP

A disadvantage of T-piece trials is that the patient is not connected to the ventilator’s alarm systems that supplement clinician monitoring for apnea or tachypnea. The choice among these options will depend on each provider’s experience and preferences.

6 What are the traditional weaning parameters, and how are they used?

Traditional weaning parameters include maximal inspiratory pressure, minute volume, vital capacity, maximum voluntary ventilation, thoracic compliance, and respiratory resistance. In the past, they were used to predict the likelihood of success with weaning trials. It is now known that they do not discriminate well between patients who will have success and those who will have failure after extubation. Assessment during a carefully monitored SBT appears to provide the most clinically useful information regarding ventilator discontinuation. Measurement of traditional weaning parameters is generally not necessary.

7 Describe what to do about sedation and analgesia with such a patient

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