Chapter 26: Drug Overdose

Patients with overdose are a small percentage of those mechanically ventilated. Many of these patients require immediate intubation and mechanical ventilation—often by prehospital personnel. Ventilation of these patients is usually straightforward. However, complications can complicate the course of mechanical ventilation if not managed correctly.

The patient presenting with a drug overdose is frequently obtunded and unable to effectively maintain spontaneous breathing. However, with some classes of drugs (eg, tricyclic antidepressants) central nervous system hyperactivity may be the initial clinical presentation. If ingested in sufficient quantity, all drugs can result in respiratory depression and necessitate intubation and mechanical ventilation (Table 26-1). In addition, cardiovascular compromise commonly occurs with many types of drug overdoses. Narcotics and sedatives frequently result in hypotension, while tricyclic antidepressants and cocaine can cause life-threatening arrhythmias. The length of ventilatory support may be short or prolonged depending on the drug ingested, the quantity ingested, and the presence of underlying lung disease or complications. Patients may have periods of wakefulness followed by periods of profound respiratory depression. Even when the quantity of ingested drug is insufficient to depress spontaneous breathing, risk of aspiration may still be a primary concern necessitating close observation or intubation for airway protection.

Indications

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.

Ventilator Settings

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, V_T and airway pressures should always be lung-protective. As a result a V_T 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 V_T 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.

Monitoring

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).

Liberation

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.