Discontinuing Mechanical Ventilation

This chapter describes the process of removing patients from mechanical ventilation (also called weaning from mechanical ventilation), and the difficulties that can occur during the transition to unassisted breathing (1,2,3,4).

I. Readiness Evaluation

The management of ventilator-dependent patients should include a daily evaluation for signs that ventilatory support may no longer be necessary. A checklist of the items in this evaluation is shown in Table 22.1.

A. Weaning Parameters

When the conditions in Table 22.1 are all present, the patient is removed from the ventilator briefly (for 1–2 minutes) to obtain the measurements listed in Table 22.2. These are called “weaning parameters”, and they are used to predict the likelihood of success or failure in the transition to unassisted breathing.
Note the wide range of likelihood ratios in Table 22.1, which indicates that each of the weaning parameters can have a poor predictor value in individual patients. As a result, the emerging consensus is that weaning parameters are not necessary, and trials of spontaneous, unassisted breathing can begin when the readiness criteria in Table 22.1 are satisfied.

Table 22.1 Checklist for a Trial of Spontaneous Breathing

Table 22.2 Measurements Used to Predict a Successful Trial of Spontaneous Breathing

Measurement	Threshold for Success	Likelihood Ratios^ζ
Tidal Volume (V_T)	4–6 mL/kg	0.7–3.8
Respiratory Rate (RR)	30–38 bpm	1.0–3.8
RR/V_T Ratio	60–105 bpm/L	0.8–4.7
Maximum Inspiratory Pressure (PI_max)	−15 to −30 cm H₂O	0–3.2
^ζ The likelihood ratio is the likelihood that the measurement will predict success, divided by the likelihood that the measurement will predict failure. From Reference 2.

II. Spontaneous Breathing Trial

The traditional approach to discontinuing mechanical ventilation emphasizes a gradual reduction in ventilatory support (over hours to days), and this creates unnecessary delays in removing ventilatory support for patients who are capable of unassisted breathing. (This delayed approach is evident in the practice of placing patients back on a ventilator at night to “rest them”.) In contrast, spontaneous breathing trials (SBTs) are conducted with no ventilatory support, so that patients capable of unassisted breathing can be identified quickly. There are two methods for conducting an SBT, as described next.

A. Using the Ventilator Circuit

SBTs are often conducted while the patient breathes through the ventilator circuit.

The advantage of this method is the ability to monitor the patient’s tidal volume (V_T) and respiratory rate (RR), which allows for the early detection of rapid, shallow breathing (indicated by an increase in the RR/V_T ratio), which is a sign of ventilatory failure (5).
The drawback of this method is the resistance to breathing through the ventilator tubing, and the work involved in opening a valve on the ventilator to receive inhaled O₂.
Low-level pressure support ventilation (5 cm H₂O) is used to counteract the resistance to breathing through the ventilator circuit, but does not augment the patient’s tidal volume. (For a description of pressure support ventilation, see Chapter 19, Section IV.)

B. Disconnecting the Ventilator

SBTs can also be conducted when the patient is disconnected from the ventilator.

This method employs a simple circuit design, which is illustrated in Figure 22.1. A source of O₂ (usually from a wall outlet) is delivered to the patient at a high flow rate (higher than the patient’s inspiratory flow rate).
The high flow rate in this circuit achieves 3 goals: (a) it promotes comfortable breathing in patients with increased ventilatory demands, (b) it prevents the patient from inhaling low O₂ gas from the expiratory limb of the circuit, and (c) it carries exhaled CO₂ away from the patient, and thereby prevents CO₂ rebreathing.
Because the breathing circuit employs a T-shaped adapter, this type of SBT is also known as a T-piece weaning trial.
The major disadvantage of the T-piece weaning trials is the inability to monitor the patient’s tidal volume and respiratory rate.

C. Which Method is Preferred?

There is no evidence of superiority for either method of spontaneous breathing (3). However, T-piece weaning trials are favored because they more closely approximate the conditions after extubation (6).

FIGURE 22.1 The design of the breathing circuit for spontaneous breathing trials while totally disconnected from the ventilator (also called T-piece weaning because of the T-shaped adapter in the circuit). See text for further explanation.

D. Success vs. Failure

A majority of patients (∼ 80%) who tolerate SBTs for 2 hours can be permanently removed from the ventilator (1,2). Failure to tolerate spontaneous breathing is usually signaled by one or more of the following:

Signs of respiratory distress; e.g., agitation, rapid breathing, and use of accessory muscles of respiration.
Signs of respiratory muscle weakness; e.g., paradoxical inward movement of the abdominal wall during inspiration.
Progressive hypoxemia or hypercapnia.

E. Rapid Breathing

Rapid breathing during SBTs can be the result of anxiety rather than ventilatory failure (7). This is an important distinction because it is possible to manage anxiety without terminating the SBT trial (see later).

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