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Patients with drug overdose are intubated to facilitate mechanical ventilation and for airway protection. Mechanical ventilation is usually initiated due to apnea or acute ventilatory failure. Oxygenation is often not a concern with these patients unless aspiration has occurred.
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These patients are not difficult to ventilate unless aspiration has occurred. They tend to be young and otherwise healthy without underlying lung disease. The ventilatory mode of choice is A/C (continuous mandatory ventilation [CMV]) provided with either pressure or volume ventilation (Table 26-2 and Figure 26-1). Any mode with a backup rate is acceptable. In spite of the fact that the lungs are normal, VT and airway pressures should always be lung-protective. As a result a VT of 6 to 8 mL/kg ideal body weight is appropriate with a rate of about 15 to 20/min, dependent on Paco2. If volume-controlled ventilation is selected, an inspiratory time of 1 second is appropriate. With pressure-controlled ventilation, the pressure control level should be set to provide the desired VT of 6 to 8 mL/kg with an inspiratory time of 1 second. Plateau pressure should be kept less than 30 cm H2O. Since oxygenation is not a concern unless the patient has aspirated, Fio2 ≤ 0.40 is usually adequate to maintain normal Pao2 (> 80 mm Hg). The use of 5 cm H2O positive end-expiratory pressure (PEEP) to maintain functional residual capacity is encouraged, provided cardiovascular function is stable and the addition of PEEP does not adversely affect cardiac output. Since many ingested drugs produce peripheral vasodilation, concern regarding airway pressures is warranted.
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Regurgitation and aspiration are the primary concerns with overdose patients, and precautions should be taken until the patient is ready for extubation. The cuff on the endotracheal tube should be adequately inflated. Hemodynamic stability is a concern with many overdose patients since arrhythmias may occur. Monitoring of electrocardiogram and systemic arterial blood pressure is indicated. Since underlying lung disease is not usually an issue, arterial blood gases are monitored infrequently, but frequent evaluation of acid-base balance may be necessary with some ingested drugs (eg, salicylates). In some cases alkalosis may be indicated to facilitate clearance of the ingested drug. Since mechanical ventilation is indicated for respiratory depression, careful monitoring of the level of consciousness and patient-ventilator synchrony are necessary. Many patients become agitated and combative as their level of neurologic depression decreases (Table 26-3).
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Discontinuation of ventilatory support is indicated when the drug is sufficiently cleared to allow spontaneous ventilation. Once the patient is awake and there is no concern regarding neurologic relapse, mechanical ventilation can be discontinued. Of concern are sedative overdoses where the drug is highly lipid-soluble and slowly released into the systemic circulation. These patients may fluctuate between periods of wakefulness and sedation. Premature ventilator discontinuance in this setting could be disastrous.