Weaning is the act of reducing ventilatory support in preparation for liberation from mechanical ventilation. Now that specific, objective criteria (RSBI ≤105) have been described, attention must be given to the methods to bring the patient to that state, specifically, to the ventilatory and adjunct strategies that can be employed to improve a patient's respiratory status to one with a RSBI ≤105, when the patient does not require any additional ventilatory assistance.
First consider, independently, the FiO2, and when, after intubation, it may be reduced. Until the mid-1990s, the variables commonly available to evaluate artificial ventilation were those obtained by an arterial blood gas (ABG): pH, PaO2, and Paco2. When considering how much oxygen was being delivered to the systemic arteries and whether or not FiO2 can be reduced, the salient value was the PaO2. But with the widespread use of pulse oximetry, it is now possible to know how much oxygen is being delivered to the systemic arteries by the continuous, noninvasive, bedside measurement of the oxygen saturation of arterial hemoglobin (SaO2). Recall the equation of oxygen content of arterial blood16:
CaO2 = [1.34 × Hgb × SaO2] + [0.003 × PaO2]
where 1.34 is the oxygen binding capacity of hemoglobin (mL O2 /g Hgb), hgb the concentration of hemoglobin in blood (g/dL), SaO2 the percent hemoglobin saturated with oxygen, 0.003 the solubility coefficient of oxygen in water at PO2 of 1 mm Hg (mL O2/100 mL water/mm Hg), PaO2 the partial pressure of oxygen (mm Hg), and CaO2 the oxygen content of arterial blood (g O2/dL plasma).
From the above equation, it can be seen that, except in cases of hyperbaric oxygen therapy, the partial pressure of oxygen is only a minimal contributor to the oxygen content of arterial blood and the term, therefore, may be removed from the equation. The equation then simplifies to:
CaO2 = 1.34 × hemoglobin × SaO2
From the above equation, it can be seen that for a given hemoglobin concentration, the amount of oxygen in arterial blood is directly and completely measured by pulse oximetry. It also shows that SaO2 (as determined by a pulse oximeter) is, in fact, a better measure of oxygen content than PaO2 (as reported on a blood gas result). For blood gas instrumentation that reports oxygen saturation results, it must be kept in mind that frequently the saturation number is calculated from the PaO2, and not measured directly as is done by a pulse oximeter.
Since SaO2 is a dynamic measure of the oxygen content of arterial blood, the ventilator's FiO2 can be titrated by monitoring pulse oximetry. In fact, given the dangers of hyperoxia17,18 and the ease, safety, and rapidity with which SaO2 is measured, FiO2 should be reduced as soon as possible after intubation, as tolerated, by monitoring the pulse oximeter.
The remaining parameters to wean will be the amount and frequency of ventilator support as well as how the ventilator delivers that support (i.e., the mode). The first two parameters determine minute ventilation, the adequacy of which can only be objectively ascertained by the ABG: specifically, the pH and Paco2. Still, unlike a continuous bedside pulse oximeter, ABGs are only snapshots in time. Fortunately, there is a monitor capable of continually measuring the patient's pH and, thereby, Paco2: the patient's own respiratory center.
It follows, then, as soon as a patient's condition sufficiently improves, sedation should be lightened enough to ensure comfort and cooperation with the ventilator (ventilatory synchrony) yet allow normal ventilatory drive. All that remains is to use a ventilatory mode that allows the patient to regulate his or her minute volume. This suggests that modes that allow the patient to both breathe spontaneously and regulate the volume of his or her spontaneous breaths will facilitate weaning.
Once a patient has achieved an adequate and comfortable respiratory status on the ventilator, parameters can then be reduced (e.g., fewer mandatory breaths per minute), to allow the patient to take over more and more of his or her own breathing. Finally, the patient can be moved to an entirely patient-controlled ventilatory mode, providing just enough support to overcome the resistance of the ETT and ventilator support.12,13 To achieve this goal is to achieve the maximal level of ventilatory weaning, proving the patient's pulmonary, cardiovascular, and other organ systems can tolerate extubation. Of course, this goal says nothing about extubation as a function of the patient's airway—itself reliant on airway anatomy and physiology (e.g., no swelling)—or the patient's ability to protect the airway, but it is the end point of weaning after which no further decrease in ventilatory support is either meaningful or appropriate.
Finding the optimal way to move a patient from initial ventilator settings to a state where the patient is ready for extubation is not trivial. Indeed, much work has been done on the topic.15
It would seem that the logical way to eventually remove support of any kind would be to decrease the amount of support and evaluate whether the subject tolerates the loss. With regards to ventilatory support, it is possible to decrease either, or both, the amount or frequency, of support, as well as additional parameters. Some approaches to weaning have involved simply removing the patient from the ventilator for a short period of time and, if tolerated, gradually increasing the length of time. One then establishes some value for time off the ventilator that, once reached, is assumed to indicate that the patient can function off the ventilator permanently. Often a cutoff of 2 hours has been used, but periods as long as 24 hours have been reported. This process, however, involves removing the patient from the ventilator, which, as discussed above, is not desirable. By using the ventilator for assistance, support can be gradually reduced or removed to facilitate a faster weaning.
The first consideration when using the ventilator to assist in weaning is the mode of ventilation. In 1994, Brochard et al were the first to examine the question of the optimal mode for weaning in a prospective randomized controlled trial (RCT).14 After excluding all patients who passed a single 2-hour T-piece SBT and were successfully extubated, they randomized patients to be weaned via gradually increasing SBTs on T-piece, synchronized intermittent mandatory ventilation (SIMV), or pressure support ventilation (PSV). They found that patients weaned fastest and most successfully using PSV. Furthermore, PSV yielded a shorter length of stay in the ICU and a trend toward lower all-cause mortality.
Conversely, in 1995, Esteban et al found that PSV actually increased weaning time,15 with SIMV being better, and once-daily SBTs on T-piece the best. However, unlike Brochard who only studied patient's who failed their first SBT or extubation attempt, Esteban studied all comers. This means Esteban's results are colored by the fact that 76% of his patient population was successfully extubated on the first attempt, suggesting a majority of his patients did not require weaning, per se, merely identification of those patients ready to be extubated on their first attempt. This crucial point was proven in a 1996 study by Ely et al that demonstrated that many patients who can be successfully extubated are not identified by the intensivist as such,2 and thus require objective evaluation and action based on positive performance on the evaluative measures. The only firm conclusion that can be drawn is that SBT is not the best weaning method, but rather is an indispensable test to identify patients as soon as they are ready for extubation.
Although denied in the discussion, Brochard's approach to SIMV potentially blunted the performance of SIMV or T-piece by using a protocol that limited how fast patients could be weaned on various modes. In other words, if SIMV did, in fact, lead to faster weaning in this study, it would not be apparent since the speed with which a patient could be weaned was limited by the study's protocol.
Finally, in both studies continuous positive airway pressure (CPAP) was allowed, but not required, which may confound the results of both studies. Since positive end-expiratory pressure (PEEP) of 5 cm H2O is generally considered physiologic, it would seem a possible methodological bias to have not included PEEP on all patients, or at least have it set and titrated based on protocol. Brochard does note that CPAP was set in most patients with chronic obstructive pulmonary disease (COPD) to facilitate patient triggering of the ventilator by ensuring pressure in the circuit equaled the patient's intrinsic PEEP, which decreases the inspiratory effort needed to drop the pressure sufficiently to trigger the ventilator.
For other less traditional modes, the mode itself may either not lend itself to weaning via eventual conversion to CPAP and PS or lend itself to a distinctive weaning methodology. As opposed to SIMV, which is usually programmed with volumes (e.g., volumes of 6–8 mL/kg), pressure-regulated modes function by lessening the pressures applied before lowering the mandatory rate. This can be done by lowering the mandatory-breath pressure and monitoring to ensure that adequate volumes are maintained. As pulmonary compliance, and therefore resultant volumes, increases, the pressure may be again reduced. At lower pressures, the mandatory rate can eventually be lowered.
In airway pressure release ventilation (APRV), it seems prudent to first lower Phigh until it is in a safe range below 20 mm Hg. At that point, Thigh may be increased, therefore increasing the time the patient remains at Phigh, which also decreases the number of releases, or mandatory breaths, per minute.19 This method is commonly known as the “drop and stretch” method, referring to dropping, or lowering, Phigh and stretching, or increasing, Thigh. In general, Phigh can be dropped in increments of 2 mm Hg, while Thigh can be increased in 0.5- to 2-second increments. Eventually Phigh will end up at levels of 5–12 mm Hg and, as Thigh is increased, CPAP is approximated. Of course, PS or ATC should also be applied, assisting the patient's spontaneous breaths in overcoming the resistance of the circuit, as in more traditional modes. Once these settings are achieved, the patient may be extubated directly from APRV or may be converted to CPAP/PS to calculate a RSBI.