Discontinuing Mechanical Ventilation
There are two phases to discontinuing mechanical ventilation. In the first, “readiness testing,” so-called weaning parameters and other subjective and objec-tive assessments are used to determine whether the patient can sustain progressive withdrawal of mechanical ventilator support. The second phase, “weaning” or “liberation,” describes the way in which mechanical support is removed.
Readiness testing should include determining whether the process that necessitated mechanical ventilation has been reversed or controlled. Com-plicating factors such as bronchospasm, heart fail-ure, infection, malnutrition, metabolic acidosis or alkalosis, anemia, increased CO 2 production due to increased carbohydrate loads, altered mental status, and sleep deprivation should be adequately treated. Underlying lung disease and respiratory muscle wasting from prolonged disuse often complicate weaning. Patients who fail to wean despite apparent readiness often have COPD or chronic heart failure.
Weaning (or liberation) from mechanical venti-lation should be considered when patients no longer meet general criteria for mechanical ventilation (see Table 57–4). In general, this occurs when patients have a pH greater than 7.25, show adequate arte-rial oxygen saturation while receiving FIO2 less than 0.5, are able to spontaneously breathe, are hemo-dynamically stable, and have no current signs of myocardial ischemia. Additional mechanical indi-ces have also been suggested (Table 57–6). Useful weaning parameters include arterial blood gas ten-sions, respiratory rate, and rapid shallow breathing index (RSBI, see). Intact airway reflexes and a cooperative patient are also mandatory prior to completion of the weaning and extubation unless the patient will retain a cuffed tracheostomy tube. Similarly, adequate oxygenation (arterial oxygen saturation >90% on 40–50% O2 with <5 cm H2O of PEEP) is imperative prior to extubation. When the patient is weaned from mechanical ventilation and extubation is planned, the RSBI is frequently used to help predict who can be successfully weaned from mechanical ventilation and extubated. With the patient breathing spontaneously on a Τ-piece, the VT (in liters) and respiratory rate (f) are measured:
Patients with an RSBI less than 100 can be success-fully extubated. Those with an RSBI greater than 120 should retain some degree of mechanical ventilator support.
The common techniques to wean a patient from the ventilator include SIMV, pressure sup-port, or periods of spontaneous breathing alone on a Τ-piece or on low levels of CPAP. Mandatory minute ventilation has also been suggested as an ideal weaning technique, but experience with it is limited. Finally, many institutions use “automated tube compensation” to provide just enough pres-sure support to compensate for the resistance of breathing through an endotracheal tube. Newer mechanical ventilators have a setting that will auto-matically adjust gas flows to make this adjustment. In practice in adults breathing through conven-tionally sized tubes (7.5–8.5), the adjustment will typically amount to pressure support of 5 cm H2O and PEEP of 5 cm H2O.
With SIMV the number of mechanical breaths is progressively decreased (by 1–2 breaths/min) as long as the arterial CO2 tension and respiratory rate remain acceptable (generally <45–50 mm Hg and <30 breaths/min, respectively). If pressure sup-port is concomitantly used, it should generally be reduced to 5–8 cm H 2O. In patients with acid–base disturbances or chronic CO2 retention, arterial blood pH (>7.35) is more useful than CO 2 tension. Blood gas measurements can be checked after a minimum of 15–30 min at each setting. When an IMV of 2–4 breaths is reached, mechanical ventila-tion is discontinued if arterial oxygenation remains acceptable.
Weaning with PSV alone is accomplished by gradu-ally decreasing the pressure support level by 2–3 cm H2O while VT, arterial blood gas tensions, and respi-ratory rate are monitored (using the same criteria as for IMV). The goal is to try to ensure a VT of 4–6 mL/ kg and an f of less than 30 with acceptable PaO2 and PaCO2. When a pressure support level of 5–8 cm H2O is reached, the patient is considered weaned.
Τ-piece trials allow observation while the patient breathes spontaneously without any mechanical breaths. The Τ-piece attaches directly to the tracheal tube or tracheostomy tube and has corrugated tub-ing on the other two limbs. A humidified oxygen–air mixture flows into the proximal limb and exits from the distal limb. Sufficient gas flow must be given in the proximal limb to prevent the mist from being completely drawn back at the distal limb during inspiration; this ensures that the patient is receiving the desired oxygen concentration. The patient is observed closely during this period; obvious new signs of fatigue, chest retractions, tachypnea, tachy-cardia, arrhythmias, or hypertension or hypotension should terminate the trial. If the patient appears to tolerate the trial period and the RSBI is less than 100, mechanical ventilation can be discontinued perma-nently. If the patient can also protect and clear the airway, the tracheal tube can be removed.
If the patient has been intubated for a pro-longed period or has severe underlying lung disease, sequential Τ-piece trials may be necessary. Periodic trials of 10–30 min are initiated and progressively increased by 5–10 min or longer per trial as long as the patient appears comfortable and maintains acceptable arterial blood gas measurements.
Many patients develop progressive atelectasis during prolonged Τ-piece trials. This may reflect the absence of a normal “physiological” PEEP when the larynx is bypassed by a tracheal tube. If this is a concern, spontaneous breathing trials on low levels (5 cm H2O) of CPAP can be tried. The CPAP helps maintain FRC and prevent atelectasis.
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