Interpretation of signals: commonly used medications in the ICU

Narcotic analgesics
In lower doses, narcotics in general tend to increase the amplitude of both alpha and beta frequency bands. Clinically, patients have reported their mood during these changes as associated with a sense of well being. In higher doses, theta and delta frequencies develop, heralding the onset of sedation.

Fentanyl and morphine
The effect on the EEG of intravenous fentanyl in doses of 30–70 μg/kg, following premedication with oral lorazepam 4–5 mg or 10 mg intramuscular morphine, was studied in cardiac surgery patients [6]. Within 1 min, the normal alpha rhythm was replaced by diffuse theta waves, followed quickly by predominant delta activity consistent with anesthesia. Lower doses of fentanyl resulted in slowed alpha activity with an increase in theta bands. In the cat, very high levels of fentanyl, in the range of 40–80 μg/kg produced seizure-like activity [7]. The EEG response after administration of morphine is similar to that of fentanyl, and resembles the pattern demonstrated by the centrally acting serotonin uptake inhibitors and alpha-2 agonists such as clonidine [8].

Meperidine
Administration of meperidine in usual analgesic intramuscular doses, produced a picture of alpha activity voltage reduction and the appearance of slow waves of intermediate voltage, consistent with mild hypnosis [9].

Benzodiazepines
In low doses, the anxiolytic action of benzodiazepines tends to decrease the percentage of alpha activity while increasing the prevalence of beta waves over 18 Hz [8]. At high doses, theta and delta activity occurs, expressing the sedative activity of the drug.

Midazolam
In one study of ICU patients receiving midazolam, alpha frequency decreased and beta activity became prominent. In higher doses, these changes disappeared to be replaced by delta/theta activity [10]. The onset of action and recovery following intravenous administration of midazolam is rapid. When administered to counteract sedative effect, the benzodiazepine antagonist flumazinil quickly normalizes alpha wave activity [11].

Lorazepam
EEG effects were relatively slow in onset, reaching base-line over 30 min after infusion, and with a prolonged duration of action compared to diazepam. Increased activity in the 13–30 Hz range was prolonged, followed by increased sedative activity [12].

Non-benzodiazepine hypnotics

Propofol
Administration of propofol for anesthesia initially produced an increase in the amplitude of the alpha rhythm, followed by an increase in delta and theta activity and burst suppression in some patients, consistent with anesthesia [13]. There is a very rapid onset of action and recovery following intravenous administration of propofol.

Barbiturates
Intravenous administration of short acting barbiturates initially increases the amplitude of beta activity. Increasing doses of barbiturates can produce a pattern of coincider delta and beta activity [14]. Large doses create a sedative pattern of delta/theta activity.

Adjuncts to sedation
An alpha-2 adrenoreceptor agonist is capable of potentiating narcotic actions. EEG effects of clonidine are similar to narcotics.

Clonidine
The sedative action of the alpha-2 agonist clonidine is commonly appreciated in the ICU setting [15]. The EEG response to clonidine is similar to that of morphine and thought to have a similar central mode of action [16]. Clonidine increased the time rats spent in the drowsy stage of wakefulness, which corresponds to behavioral sedation and inhibited REM sleep for 6–9 h after administration [17].