Midazolam is a short-acting benzodiazepine central nervous system (CNS) depressant.
The effects of midazolam on the CNS are dependent on the dose administered, the route of administration, and the presence or absence of other medications. Onset time of sedative effects after IM administration in adults is 15 minutes, with peak sedation occurring 30 to 60 minutes following injection. In one adult study, when tested the following day, 73% of the patients who received midazolam intramuscularly had no recall of memory cards shown 30 minutes following drug administration; 40% had no recall of the memory cards shown 60 minutes following drug administration. Onset time of sedative effects in the pediatric population begins within 5 minutes and peaks at 15 to 30 minutes depending upon the dose administered. In pediatric patients, up to 85% had no recall of pictures shown after receiving intramuscular midazolam compared with 5% of the placebo controls.
Sedation in adult and pediatric patients is achieved within 3 to 5 minutes after intravenous (IV) injection; the time of onset is affected by total dose administered and the concurrent administration of narcotic premedication. Seventy-one percent of the patients in endoscopy studies had no recall of introduction of the endoscope; 82% of the patients had no recall of withdrawal of the endoscope. In one study of pediatric patients undergoing lumbar puncture or bone marrow aspiration, 88% of patients had impaired recall vs 9% of the placebo controls. In another pediatric oncology study, 91% of midazolam treated patients were amnestic compared with 35% of patients who had received fentanyl alone.
When midazolam is given IV as an anesthetic induction agent, induction of anesthesia occurs in approximately 1.5 minutes when narcotic premedication has been administered and in 2 to 2.5 minutes without narcotic premedication or other sedative premedication. Some impairment in a test of memory was noted in 90% of the patients studied. A dose response study of pediatric patients premedicated with 1.0 mg/kg intramuscular (IM) meperidine found that only 4 out of 6 pediatric patients who received 600 mcg/kg IV midazolam lost consciousness, with eye closing at 108 ± 140 seconds. This group was compared with pediatric patients who were given thiopental 5 mg/kg IV; 6 out of 6 closed their eyes at 20 ± 3.2 seconds. Midazolam did not dependably induce anesthesia at this dose despite concomitant opioid administration in pediatric patients.
Midazolam, used as directed, does not delay awakening from general anesthesia in adults. Gross tests of recovery after awakening (orientation, ability to stand and walk, suitability for discharge from the recovery room, return to baseline Trieger competency) usually indicate recovery within 2 hours but recovery may take up to 6 hours in some cases. When compared with patients who received thiopental, patients who received midazolam generally recovered at a slightly slower rate. Recovery from anesthesia or sedation for procedures in pediatric patients depends on the dose of midazolam administered, coadministration of other medications causing CNS depression and duration of the procedure.
In patients without intracranial lesions, induction of general anesthesia with IV midazolam is associated with a moderate decrease in cerebrospinal fluid pressure (lumbar puncture measurements), similar to that observed following IV thiopental. Preliminary data in neurosurgical patients with normal intracranial pressure but decreased compliance (subarachnoid screw measurements) show comparable elevations of intracranial pressure with midazolam and with thiopental during intubation. No similar studies have been reported in pediatric patients.
The usual recommended intramuscular premedicating doses of midazolam do not depress the ventilatory response to carbon dioxide stimulation to a clinically significant extent in adults. Intravenous induction doses of midazolam depress the ventilatory response to carbon dioxide stimulation for 15 minutes or more beyond the duration of ventilatory depression following administration of thiopental in adults. Impairment of ventilatory response to carbon dioxide is more marked in adult patients with chronic obstructive pulmonary disease (COPD). Sedation with IV midazolam does not adversely affect the mechanics of respiration (resistance, static recoil, most lung volume measurements); total lung capacity and peak expiratory flow decrease significantly but static compliance and maximum expiratory flow at 50% of awake total lung capacity (Vmax) increase. In one study of pediatric patients under general anesthesia, intramuscular midazolam (100 or 200 mcg/kg) was shown to depress the response to carbon dioxide in a dose-related manner.
In cardiac hemodynamic studies in adults, IV induction of general anesthesia with midazolam was associated with a slight to moderate decrease in mean arterial pressure, cardiac output, stroke volume and systemic vascular resistance. Slow heart rates (less than 65/minute), particularly in patients taking propranolol for angina, tended to rise slightly; faster heart rates (eg, 85/minute) tended to slow slightly. In pediatric patients, a comparison of IV midazolam (500 mcg/kg) with propofol (2.5 mg/kg) revealed a mean 15% decrease in systolic blood pressure in patients who had received IV midazolam vs a mean 25% decrease in systolic blood pressure following propofol.
Midazolam's activity is primarily due to the parent drug. Elimination of the parent drug takes place via hepatic metabolism of midazolam to hydroxylated metabolites that are conjugated and excreted in the urine. Six single-dose pharmacokinetic studies involving healthy adults yield pharmacokinetic parameters for midazolam in the following ranges: volume of distribution (Vd), 1.0 to 3.1 L/kg; elimination half-life, 1.8 to 6.4 hours (mean approximately 3 hours); total clearance (Cl), 0.25 to 0.54 L/hr/kg. In a parallel group study, there was no difference in the clearance in subjects administered 0.15 mg/kg (n=4) and 0.30 mg/kg (n=4) IV doses indicating linear kinetics. The clearance was successively reduced by approximately 30% at doses of 0.45 mg/kg (n=4) and 0.6 mg/kg (n=5) indicating non-linear kinetics in this dose range.
Absorption: The absolute bioavailability of the intramuscular
route was greater than 90% in a crossover study in which healthy subjects (n=17)
were administered a 7.5 mg IV or IM dose. The mean peak concentration (Cmax)
and time to peak (Tmax) following the IM dose was 90 ng/mL (20% CV) and 0.5
hr (50% CV). Cmax for the 1-hydroxy metabolite following the IM dose was 8 ng/mL
(Tmax = 1.0 hr).
Following IM administration, Cmax for midazolam and its 1-hydroxy metabolite were approximately one-half of those achieved after intravenous injection.
Distribution: The volume of distribution (Vd) determined from
six single-dose pharmacokinetic studies involving healthy adults ranged from
1.0 to 3.1 L/kg. Female gender, old age, and obesity are associated with increased
values of midazolam Vd. In humans, midazolam has been shown to cross the placenta
and enter into fetal circulation and has been detected in human milk and CSF
(see Special Populations).
In adults and pediatric patients older than 1 year, midazolam is approximately 97% bound to plasma protein, principally albumin.
Metabolism: In vitro studies with human liver microsomes
indicate that the biotransformation of midazolam is mediated by cytochrome P450
3A4. This cytochrome also appears to be present in gastrointestinal tract mucosa
as well as liver. Sixty to seventy percent of the biotransformation products
is 1-hydroxy-midazolam (also termed alpha-hydroxy- midazolam) while 4-hydroxy-midazolam
constitutes 5% or less. Small amounts of a dihydroxy derivative have also been
detected but not quantified. The principal urinary excretion products are glucuronide
conjugates of the hydroxylated derivatives
Drugs that inhibit the activity of cytochrome P450 3A4 may inhibit midazolam clearance and elevate steady-state midazolam concentrations.
Studies of the intravenous administration of 1-hydroxy-midazolam in humans
suggest that 1-hydroxy-midazolam is at least as potent as the parent compound
and may contribute to the net pharmacologic activity of midazolam. In vitro
studies have demonstrated that the affinities of 1- and 4-hydroxy-midazolam
for the benzodiazepine receptor are approximately 20% and 7% , respectively,
relative to midazolam.
Excretion: Clearance of midazolam is reduced in association with
old age, congestive heart failure, liver disease (cirrhosis) or conditions which
diminish cardiac output and hepatic blood flow.
The principal urinary excretion product is 1-hydroxy-midazolam in the form of a glucuronide conjugate; smaller amounts of the glucuronide conjugates of 4-hydroxy- and dihydroxy-midazolam are detected as well. The amount of midazolam excreted unchanged in the urine after a single IV dose is less than 0.5% (n=5). Following a single IV infusion in 5 healthy volunteers, 45% to 57% of the dose was excreted in the urine as 1-hydroxymethyl midazolam conjugate.
Pharmacokinetics - Continuous Infusion: The pharmacokinetic profile
of midazolam following continuous infusion, based on 282 adult subjects, has
been shown to be similar to that following single-dose administration for subjects
of comparable age, gender, body habitus and health status. However, midazolam
can accumulate in peripheral tissues with continuous infusion. The effects of
accumulation are greater after long-term infusions than after short-term infusions.
The effects of accumulation can be reduced by maintaining the lowest midazolam
infusion rate that produces satisfactory sedation.
Infrequent hypotensive episodes have occurred during continuous infusion; however, neither the time to onset nor the duration of the episode appeared to be related to plasma concentrations of midazolam or alpha-hydroxy-midazolam. Further, there does not appear to be an increased chance of occurrence of a hypotensive episode with increased loading doses.
Patients with renal impairment may have longer elimination half-lives for midazolam
(see Special Populations: Renal Failure).
Changes in the pharmacokinetic profile of midazolam due to drug interactions,
physiological variables, etc., may result in changes in the plasma concentration-time
profile and pharmacological response to midazolam in these patients. For example,
patients with acute renal failure appear to have a longer elimination half-life
for midazolam and may experience delayed recovery (see Special Populations:
Renal Failure). In other groups, the relationship between prolonged half-
life and duration of effect has not been established.
Pediatrics and Neonates: In pediatric patients aged 1 year and
older, the pharmacokinetic properties following a single dose of midazolam reported
in 10 separate studies of midazolam are similar to those in adults. Weight-normalized
clearance is similar or higher (0.19 to 0.80 L/hr/kg) than in adults and the
terminal elimination half-life (0.78 to 3.3 hours) is similar to or shorter
than in adults. The pharmacokinetic properties during and following continuous
intravenous infusion in pediatric patients in the operating room as an adjunct
to general anesthesia and in the intensive care environment are similar to those
In seriously ill neonates, however, the terminal elimination half-life of midazolam is substantially prolonged (6.5 to 12.0 hours) and the clearance reduced (0.07 to 0.12 L/hr/kg) compared to healthy adults or other groups of pediatric patients.
It cannot be determined if these differences are due to age, immature organ function or metabolic pathways, underlying illness or debility.
Obese: In a study comparing normals (n=20) and obese patients
(n=20) the mean half-life was greater in the obese group (5.9 vs 2.3 hours).
This was due to an increase of approximately 50% in the Vd corrected for total
body weight. The clearance was not significantly different between groups.
Geriatric: In three parallel group studies, the pharmacokinetics
of midazolam administered IV or IM were compared in young (mean age 29, n=52)
and healthy elderly subjects (mean age 73, n=53). Plasma half-life was approximately
two- fold higher in the elderly. The mean Vd based on total body weight increased
consistently between 15% to 100% in the elderly. The mean Cl decreased approximately
25% in the elderly in two studies and was similar to that of the younger patients
in the other.
Congestive Heart Failure: In patients suffering from congestive
heart failure, there appeared to be a two-fold increase in the elimination half-life,
a 25% decrease in the plasma clearance and a 40% increase in the volume of distribution
Hepatic Insufficiency: Midazolam pharmacokinetics were studied
after an IV single dose (0.075 mg/kg) was administered to 7 patients with biopsy
proven alcoholic cirrhosis and 8 control patients. The mean half-life of midazolam
increased 2.5-fold in the alcoholic patients. Clearance was reduced by 50% and
the Vd increased by 20%. In another study in 21 male patients with cirrhosis,
without ascites and with normal kidney function as determined by creatinine
clearance, no changes in the pharmacokinetics of midazolam or 1-hydroxy-midazolam
were observed when compared to healthy individuals.
Renal Failure: Patients with renal impairment may have longer
elimination half-lives for midazolam and its metabolites which may result in
Midazolam and 1-hydroxy-midazolam pharmacokinetics in 6 ICU patients who developed acute renal failure (ARF) were compared with a normal renal function control group. Midazolam was administered as an infusion (5 to 15 mg/hr). Midazolam clearance was reduced (1.9 vs 2.8 mL/min/kg) and the half-life was prolonged (7.6 vs 13 hours) in the ARF patients. The renal clearance of the 1-hydroxy-midazolam glucuronide was prolonged in the ARF group (4 vs 136 mL/min) and the half-life was prolonged (12 vs > 25 hours). Plasma levels accumulated in all ARF patients to about ten times that of the parent drug. The relationship between accumulating metabolite levels and prolonged sedation is unclear.
In a study of chronic renal failure patients (n=15) receiving a single IV dose, there was a two-fold increase in the clearance and volume of distribution but the half-life remained unchanged. Metabolite levels were not studied.
Plasma Concentration-Effect Relationship: Concentration-effect
relationships (after an IV dose) have been demonstrated for a variety of pharmacodynamic
measures (eg, reaction time, eye movement, sedation) and are associated with
extensive intersubject variability. Logistic regression analysis of sedation
scores and steady-state plasma concentration indicated that at plasma concentrations
greater than 100 ng/mL there was at least a 50% probability that patients would
be sedated, but respond to verbal commands (sedation score = 3). At 200 ng/mL
there was at least a 50% probability that patients would be asleep, but respond
to glabellar tap (sedation score = 4).
Drug Interactions: For information concerning pharmacokinetic
drug interactions with midazolam (see PRECAUTIONS: DRUG