Cerebrospinal Fluid Morphine Concentrations Research Articles

An observational study examining the effects of a surgically induced inflammatory response on the distribution of morphine and its metabolites into cerebrospinal fluid

Morphine is administered intravenously for pain management in the perioperative period. The effect of the inflammatory response to surgery on morphine distribution across the blood-brain barrier (BBB) in humans was investigated. We hypothesized that a graded surgically induced, systemic inflammatory response alters cerebrospinal fluid (CSF) levels of morphine, morphine-3-glucuronide (M3G), and morphine-6-glucuronide (M6G) through a temporary reduction in BBB drug efflux transporter function. We conducted a prospective pharmacokinetic study of the plasma and CSF distribution of the P-glycoprotein (PGP) substrate morphine in 33 patients undergoing open thoracic (n=18) or endovascular (n=15) aortic aneurysm repair. Morphine was administered with induction of anesthesia and in the intensive care unit. Plasma and CSF concentrations of interleukin (IL)-6, morphine, M3G, M6G, and albumin were measured prior to surgery (baseline), during surgery, and postoperatively every six hours until removal of the CSF drain. The area under the curve (AUC) was determined for plasma and CSF IL-6, morphine, M3G, and M6G concentrations vs time. The primary endpoint measures were the correlations between the morphine, M6G, and M3G AUC CSF/plasma ratios and systemic inflammation as quantified by the time-normalized IL-6 exposure, which was calculated for each individual by dividing the total exposure (AUC) by time (t). A Bonferroni corrected P<0.017 indicated a significant correlation. Plasma and CSF IL-6 concentrations increased postoperatively. The median [interquartile range] IL-6 exposures were significantly higher in the open vs endovascular surgical group for plasma (105 [40-256] pg·mL-1 vs 29 [16-70] pg·mL-1, respectively; P=0.013) and CSF (79 [26-133] pg·mL-1 vs 16 [9-80] pg·mL-1, respectively; P=0.013). For the primary endpoint, the plasma IL-6 AUC/t did not correlate with the CSF accumulation of morphine (r=-0.009; P=0.96) or M3G (r=0.37; P=0.04) when corrected for surgical procedure, age, and sex. There were insufficient data on CSF concentration to complete the primary analysis for M6G. Morphine distribution into the CSF was not significantly altered in patients undergoing thoracic aortic aneurysm repair. This suggests that BBB PGP function may not be affected by the perioperative inflammatory response. www.clinicaltrials.gov , NCT 00878371. Registered 7 April 2009.

Morphine reduces spinal c-fos expression dose-dependently during experimental laparotomy in pigs: A combined pharmacokinetic and surgical study

The pharmacokinetics of intravenous morphine 2.5 mg/kg ( n = 4) and 10 mg/kg ( n = 4) in plasma and cerebrospinal fluid (CSF) of pigs was studied. Plasma half-life was 1.0 ± 0.1 h and the main metabolite was morphine-3-glucuronide, whereas morphine-6-glucuronide was negligible. CSF morphine concentration peaked after 20–30 min (2.5 mg/kg) and 60–120 min (10 mg/kg), and elimination half-life was 3.5 ± 0.3 h. Subsequently, the effect of morphine on surgery-induced spinal nociception in pigs subjected to unilateral laparotomy was evaluated by stereological quantification of the total number of Fos-like-immunoreactive (Fos-LI) spinal neurons of the dorsal horn. Surgery ( n = 4) induced 91,680 ± 14,974 Fos-LI neurons ipsilaterally and morphine reduced this number to 45,771 ± 8755 following the 2.5 mg/kg dose ( p < 0.01; n = 6) and 14,981 ± 2327 following the 10 mg/kg dose ( p < 0.001; n = 6). These results indicate that morphine dose-dependently reduces the number of surgery-induced Fos-LI neurons in the spinal cord. As even a high dose of morphine does not reduce spinal c-fos expression to basal level, it may be appropriate to use other analgesics simultaneously with morphine during surgery.

Serum and cerebrospinal fluid morphine pharmacokinetics after single doses of intravenous and intramuscular morphine after hip replacement surgery

To compare the time course of morphine and metabolite concentrations in serum and cerebrospinal fluid (CSF) after intravenous and intramuscular administration after surgery. This was a randomized double-blind, double-dummy study in patients who had undergone hip replacement surgery. Morphine (M, 10 mg) was administered intravenously (IV) or intramuscularly (IM). Arterial blood and CSF samples (from a spinal catheter) were drawn simultaneously at 10, 30, 60, and 120 min after administration. Morphine and metabolites [morphine-3-glucuronide (M-3-G), morphine-6-glucuronide (M-6-G), and normorphine (NM)] were determined by a validated liquid chromatography-tandem mass spectrometry method. Thirty-eight patients were included: 13 men and 25 women, 20 in the IV, 18 in the IM group. Serum concentrations of M after 10 min were consistently higher after IM than IV, concentrations of M-3-G and M-6-G after IM surpassed those of IV after 45 min. NM was not found. None of the metabolites was found in CSF. CSF morphine concentrations and CSF/serum concentration ratios were consistently higher after IV compared to IM. The mean AUC(CSF)/AUC(serum) (0-120 min) concentration ratios were 0.18 and 0.09 after IV and IM, respectively. The uptake of morphine to the CSF was consistently higher after IV administration than after IM already after 10 min. The higher CSF concentration may be caused by an initially higher morphine blood/CSF gradient following IV morphine injection. The pharmacokinetic findings are compatible with a more rapid and extensive initial effect of IV morphine compared with IM.

Postoperative nausea and vomiting in regional anesthesia: a review.

ANESTHESIA has become remarkably safe, and while death and permanent damage have become rare occurrences, other sequelae of anesthesia are gaining more importance. Postoperative nausea and vomiting (PONV) still is the most troublesome adverse event encountered in the recovery room, despite advances in prevention and treatment. The incidence of PONV has remained high and has a major negative impact on patient satisfaction about the overall surgical experience. Furthermore, the ongoing trend toward ambulatory procedures has increased the focus on PONV as its occurrence may delay discharge or cause unanticipated hospital admission. General anesthesia has long been considered as causing a greater frequency and severity of PONV than regional anesthetic techniques. Recent studies investigating this time-honored dictum in a controlled manner mostly, but not unanimously, confirmed it. Accordingly, considerable effort has been invested to examine etiology, define patients at risk, and outline preventive and therapeutic strategies in patients undergoing general anesthesia. Reviews dealing with PONV have discussed almost exclusively general anesthesia and largely ignored regional anesthesia. This contrasts with the increasing popularity of regional anesthesia. A survey in Europe showed that one third of patients are undergoing regional anesthesia for their operative procedure. In France, the proportion of regional anesthesia increased from 15 to 25% of all anesthetics administered from 1980 to 1996. The number of local anesthetic and analgesic agents available for regional anesthesia has increased over the last two decades. Since the introduction of intrathecal and epidural morphine in 1979, a multitude of medications, such as synthetic opioids, 2-agonists, and cholinesterase inhibitors, have been introduced in an attempt to enhance the action of local anesthetics. The decision about their usefulness will not only rely on their effects on nerve blockade and pain relief, but also on their influence on side effects such as PONV. This review focuses on PONV in the setting of perioperative regional anesthesia. General aspects of PONV, such as physiology, patient, and perioperative factors involved are discussed. Few studies regarding these issues have been specifically devoted to regional anesthesia. Therefore, much information must be derived from investigations of general anesthesia. Specific regional anesthetic techniques and the influence of adjunctive medications on PONV are also presented. Combined general–regional anesthesia is purposefully excluded, avoiding the many variables introduced by general anesthesia. A final section is devoted to continuous peripheral nerve blocks and their possible impact on PONV.

Cerebrospinal Fluid and Plasma Concentrations of Morphine, Morphine-3-Glucuronide, and Morphine-6-Glucuronide in Patients Before and After Initiation of Intracerebroventricular Morphine for Cancer Pain Management

Twenty-three patients treated with intracerebroventricular (ICV) morphine in this study not only obtained excellent pain relief without rapid increases in dose, but also experienced a reduction in morphine-related side effects. By 24 h after initiation of ICV morphine, the mean trough cerebrospinal fluid (CSF) morphine concentration (approximately 20 [micro sign]M) was 50-fold higher than the baseline concentration (approximately 0.4 [micro sign]M), and the CSF concentration of morphine-6-glucuronide (M6G) was undetectable (<0.01 [micro sign]M). The mean CSF concentration of morphine-3-glucuronide (M3G) decreased 90%, from a baseline concentration of 1 [micro sign]M to 0.1 [micro sign]M by Day 7 postventriculostomy. Thereafter, the mean trough CSF M3G concentration remained relatively constant while ICV morphine was continued, although the concomitant M3G plasma concentrations were undetectable (<0.01 [micro sign]M). The large increase in the CSF morphine concentration in patients receiving ICV morphine strongly suggests that increased CSF morphine levels are unlikely to be the primary cause of analgesic tolerance or undesirable excitatory side effects (hyperalgesia, myoclonus, seizures) experienced by some patients receiving chronic large-dose systemic morphine. Implications: After initiation of intracerebroventricular morphine, cancer patients experienced excellent pain relief. Although the mean morphine concentration in cerebrospinal fluid increased 50-fold relative to preventriculostomy levels, rapid dose increases did not occur, which suggests that increased cerebrospinal fluid morphine levels are unlikely to be the main cause of analgesic tolerance. (Anesth Analg 1999;88:109-16)

Cerebrospinal Fluid and Plasma Concentrations of Morphine, Morphine-3-Glucuronide, and Morphine-6-Glucuronide in Patients Before and After Initiation of Intracerebroventricular Morphine for Cancer Pain Management

Twenty-three patients treated with intracerebroventricular (ICV) morphine in this study not only obtained excellent pain relief without rapid increases in dose, but also experienced a reduction in morphine-related side effects. By 24 h after initiation of ICV morphine, the mean trough cerebrospinal fluid (CSF) morphine concentration (approximately 20 microM) was 50-fold higher than the baseline concentration (approximately 0.4 microM), and the CSF concentration of morphine-6-glucuronide (M6G) was undetectable (<0.01 microM). The mean CSF concentration of morphine-3-glucuronide (M3G) decreased 90%, from a baseline concentration of 1 microM to 0.1 microM by Day 7 postventriculostomy. Thereafter, the mean trough CSF M3G concentration remained relatively constant while ICV morphine was continued, although the concomitant M3G plasma concentrations were undetectable (<0.01 microM). The large increase in the CSF morphine concentration in patients receiving ICV morphine strongly suggests that increased CSF morphine levels are unlikely to be the primary cause of analgesic tolerance or undesirable excitatory side effects (hyperalgesia, myoclonus, seizures) experienced by some patients receiving chronic large-dose systemic morphine. After initiation of intracerebroventricular morphine, cancer patients experienced excellent pain relief. Although the mean morphine concentration in cerebrospinal fluid increased 50-fold relative to preventriculostomy levels, rapid dose increases did not occur, which suggests that increased cerebrospinal fluid morphine levels are unlikely to be the main cause of analgesic tolerance.

Morphine, morphine-6-glucuronide, and morphine-3-glucuronide in cerebrospinal fluid and plasma after epidural administration of morphine

It has been suggested that the potency of epidural morphine might be explained by spinal metabolism to the active and potent metabolite morphine-6-glucuronide (M6G). The main objective of this study was to describe the early pharmacokinetics of epidurally administered, morphine with special attention to the appearance of the glucuronated metabolites in cerebrospinal fluid (CSF). Morphine was administered epidurally to eight patients scheduled for major abdominal surgery. The concentrations of morphine and its 6-glucuronide and 3-glucuronide metabolites were monitored in blood and CSF at 10, 30, 60, and 120 minutes and 10 and 24 hours. Postoperative pain was estimated on a visual analog scale, and analgesia requirements (administered by a patient-controlled technique) were recorded. Only traces of the metabolites were found in CSF and in only two patients throughout the 24 hours. Both metabolites appeared rapidly (within 30 minutes) in plasma in all patients and were found in plasma throughout the study period. Morphine concentration peaked in CSF within 30 minutes at a very high level; in plasma, it peaked at 10 minutes. No correlation was seen between initial or later concentrations of morphine in CSF and postoperative pain or morphine requirements. No evidence of spinal metabolism of morphine could be found. Rapid distribution of morphine to CSF and plasma occurred after epidural administration. No value of initial CSF morphine concentrations for prediction of analgesic requirements could be demonstrated.

Morphine, Morphine-6-Glucuronide, and Morphine-3-Glucuronide in Cerebrospinal Fluid and Plasma After Epidural Administration of Morphine

Background and ObjectivesIt has been suggested that the potency of epidural morphine might be explained by spinal metabolism to the active and potent metabolite morphine-6-glucuronide (M6G). The main objective of this study was to describe the early pharmacokinetics of epidurally administered, morphine with special attention to the appearance of the glucuronated metabolites in cerebrospinal fluid (CSF).MethodsMorphine was administered epidurally to eight patients scheduled for major abdominal surgery. The concentrations of morphine and its 6-glucuronide and 3-glucuronide metabolites were monitored in blood and CSF at 10, 30, 60, and 120 minutes and 10 and 24 hours. Postoperative pain was estimated on a visual analog scale, and analgesia requirements (administered by a patient-controlled techique) were recorded.ResultsOnly traces of the metabolites were found in CSF and in only two patients throughout the 24 hours. Both metabolites appeared rapidly (within 30 minutes) in plasma in all patients and were found in plasma throughout the study period. Morphine concentration peaked in CSF within 30 minutes at a very high level; in plasma, it peaked at 10 minutes. No correlation was seen between initial or later concentrations of morphine in CSF and postoperative pain or morphine requirements.ConclusionsNo evidence of spinal metabolism of morphine could be found. Rapid distribution of morphine to CSF and plasma occurred after epidural administration. No value of initial CSF morphine concentrations for prediction of analgesic requirements could be demonstrated.

The effect of prior dural puncture on cisternal cerebrospinal fluid morphine concentrations in sheep after administration of lumbar epidural morphine.

Combined spinal epidural anesthesia has become increasingly popular as a method of providing rapid onset of analgesia or surgical block with access for further administration of analgesics or anesthetics. No in vivo studies have evaluated the relationship between dural puncture and drug transfer from the epidural space to the cerebrospinal fluid (CSF). To determine whether morphine administered in the epidural space adjacent to a dural puncture results in increased CSF concentrations at the cisterna magna (CM), 12 adult ewes were studied. Each animal was assigned to one of three groups. Animals in Group 1 served as a control and received no dural puncture. Animals in Group 2 received a dural puncture with a 25-gauge (G) Whitacre needle, while Group 3 animals received a dural puncture with an 18-G Tuohy needle. One hour after dural puncture, each animal was given epidural morphine, 0.2 mg/kg. Six hours after the administration of epidural morphine, CSF from the CM was sampled and analyzed by gas chromatography-mass spectrometry for morphine concentration. The mean morphine concentration at the CM for Group 1 (control) was 22 +/- 12 ng/mL, whereas animals with 25-G and 18-G dural punctures had concentrations of 154 +/- 32 ng/mL and 405 +/- 53 ng/mL, respectively (P = 0.0005). These data demonstrate that a significant increase in CSF morphine concentration at the brainstem will occur when lumbar epidural morphine is administered adjacent to a dural puncture. Furthermore, the increase in CSF morphine concentration is positively correlated with the size of the needle producing the dural puncture. These findings highlight the potential for delayed respiratory depression when epidural opiate administration follows a dural puncture.

The Effect of Prior Dural Puncture on Cisternal Cerebrospinal Fluid Morphine Concentrations in Sheep After Administration of Lumbar Epidural Morphine

The Effect of Prior Dural Puncture on Cisternal Cerebrospinal Fluid Morphine Concentrations in Sheep After Administration of Lumbar Epidural Morphine

Morphine and morphine‐glucuronide concentrations in plasma and CSF during long‐term administration of oral morphine.

Concentrations of morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) were measured by h.p.l.c. in plasma and cerebrospinal fluid (CSF) samples from 16 patients with cancer receiving oral (controlled-release) morphine. There was a close correlation between plasma and CSF morphine concentrations (r = 0.94, P = 0.0001) and both correlated with drug dosage (r = 0.61, P = 0.013 and r = 0.74, P = 0.0001, respectively). M3G and M6G in plasma and CSF were correlated (r = 0.81 and r = 0.82, both P = 0.0001). No relationship was apparent between M plus M6G concentrations in the CSF and pain scores.

Disposition and Respiratory Effects of Intrathecal Morphine in Children

The extent and duration of respiratory depression after opioid administration are poorly defined in infants and children. The disposition and respiratory effects of intrathecal morphine were studied in ten patients (ages 4 months-15 yr) after repair of craniofacial defects. Morphine, 0.02 mg/kg, was administered intrathecally before the end of surgery. Postoperatively, we determined the minute ventilation (VE) in response to increasing partial pressure of end-tidal carbon dioxide (PETCO2) during carbon dioxide rebreathing. The slope (VE/PETCO2) and intercept (VE at PETCO2 60 mmHg, VE 60) of the carbon dioxide response curve were calculated at 6, 12, and 18 h after morphine administration. Cerebrospinal fluid (CSF) and blood were analyzed for morphine concentration by radioimmunoassay. Mean VE/PETCO2 decreased from a preoperative value of 35.1 +/- 3.7 to 16.3 +/- 2.8 ml.kg-1 x min-1 x mmHg-1 at 6 h after morphine, and remained depressed to 23.4 +/- 2.9 and 23.5 +/- 3.3 ml.kg-1 x min-1 x mmHg-1 at 12 h and 18 h, respectively, compared to preoperatively). The infants' (n = 3) VE/PETCO2 at 6 h were 21, 4, and 27 ml.kg-1 x min-1 x mmHg-1. Mean VE 60 decreased from 874 +/- 125 to 276 +/- 32 ml x kg-1 x min-1 at 6 h, but then recovered at 12 and 18 h to 491 +/- 68 and 567 +/- 82 ml.kg-1 x min-1, respectively. The infants' VE 60 at 6 h were 350, 142, and 245 ml.kg-1 x min-1. Mean CSF morphine concentration was 2,860 +/- 540 ng/ml at 6 h, and decreased to 640 +/- 220 and 220 +/- 150 ng/ml at 12 and 18 h, respectively. Intrathecal morphine, 0.02 mg/kg, depressed the ventilatory response to carbon dioxide for up to 18 h concomitant with increased CSF morphine concentrations. Infants (4-12 months of age) did not exhibit greater ventilatory depression than did children (2-15 yr of age).

Morphine pharmacokinetics and pain assessment after intracerebroventricular administration in patients with terminal cancer

Morphine pharmacokinetics and pain relief were evaluated after intracerebroventricular administration of morphine (0.4 +/- 0.11 mg) in seven patients with cancer suffering from intractable pain. Ventricular cerebrospinal fluid (CSF), lumbar CSF, and plasma morphine concentrations were analyzed by a specific morphine radioimmunoassay. A two-compartment model was sufficient to describe the kinetics of morphine in ventricular CSF. Morphine diffuses to the lumbar level, and the mean maximum concentration was 192 +/- 105 ng/ml at 4.5 +/- 1.3 hours. Ventricular and lumbar CSF morphine kinetics showed a similar decline during the elimination phase, with terminal half-lives of 3.8 +/- 0.6 hours and 4.2 +/- 1.6 hours, respectively. Pain relief was evaluated by a visual analog scale: the test showed a rapid onset of analgesia (less than 10 minutes). Analgesic effectiveness reached a maximum between 6 and 10 hours. The relationship between pharmacologic effect and morphine concentrations in ventricular CSF resulted in an anticlockwise hysteresis curve. The presence of morphine in lumbar CSF suggested an additive spinal action of morphine, which probably plays a role in the duration of analgesia.

EXTRADURAL MORPHINE: INFLUENCE OF ADRENALINE ADMIXTURE

The influence of the addition of adrenaline on extradural morphine analgesia and pharmacokinetics was investigated in a double-blind study. Morphine 2 mg was administered to 14 patients undergoing thoracotomy. In addition, adrenaline 50 micrograms added to the extradural solution, was administered to half of the patients, selected randomly. Morphine concentrations in serial plasma and cerebrospinal fluid (CSF) samples were measured. Postoperative analgesia was estimated by determining the requirement for additional analgesics. Following extradural administration of plain morphine, the peak morphine concentrations in CSF were 22 +/- 5 (SEM) times those in plasma; during the elimination phase the CSF concentrations exceeded those in plasma by about 150 times. The area under the concentration v. time curve (AUC) was 162 +/- 27 (SEM) times larger in CSF than in plasma. The admixture of adrenaline with the extradural morphine increased the individual variability in CSF and plasma concentrations. However, compared with the plain morphine group, adrenaline did not significantly increase the concentrations of morphine in CSF, nor were the morphine concentrations in plasma significantly decreased. The duration of analgesia was related to the amount of morphine in CSF, that is AUC (P less than 0.05) and peak concentrations of morphine in CSF (P less than 0.05).

Epidural mass associated with lack of efficacy of epidural morphine and undetectable CSF morphine concentrations

A case is presented in which epidurally administered morphine failed to provide the expected extent and duration of pain relief. The patient had severe back pain in the upper lumbar and low thoracic regions and epigastric pain which was presumed to be caused by widespread metastatic deposits in the vertebral column from a prostatic carcinoma. The lack of clinical effect of epidural morphine was shown to be related to the unexpected presence of an epidural mass which was located below the dermatomal region for the patient's pain. Myelography and a CT scan indicated that the mass effectively compressed the subarachnoid space, thereby preventing the rostral spread of morphine in the cerebrospinal fluid (CSF) following lumbar epidural administration, resulting in inadequate pain relief.

Neuropathologic lesions and CSF morphine concentrations during chronic continuous intraspinal morphine infusion. A clinical and post-mortem study

Seven patients with chronic intractable pain due to cancer were given chronic intraspinal narcotic administration (CINA) and subsequently underwent postmortem examination. All deaths were unrelated to CINA. Two of these patients were found to have clinically unsuspected posterior column degeneration. Both patients had had epidural catheters placed and one had received prior radiotherapy to ports which included parts of the spinal cord. In retrospect, it is impossible to ascertain whether the degeneration occurred before or after infusion of morphine began. Review of the potential causes for posterior column degeneration suggests that neuropathy associated with malignant disease is more likely the cause of the degeneration rather than intraspinal infusion of morphine. However, continued vigilance at autopsy is recommended. In addition, utilizing a new method for measuring cerebrospinal fluid (CSF) concentrations of morphine via high-pressure liquid chromatography, CSF morphine levels at steady state were measured in 5 patients. These levels were much lower than peak levels previously reported following bolus intraspinal administration. The ability of these measurements to contribute to knowledge of efficacy, toxicity. lumbar-cisternal concentration gradients and differentiation of tolerance from drug delivery problems is discussed.

Kinetics of morphine in cerebrospinal fluid after epidural administration.

Forty patients undergoing arthroscopy were given an epidural dose of 0.05 mg morphine-HCl in 0.1 ml saline/kg body weight to study the disposition of morphine in the cerebrospinal fluid (CSF). In each patient one to three CSF samples were collected (86 samples in total). A mean peak concentration of 13 890 nmol/l was achieved 75 min after morphine administration. The compiled data show an elimination half-life of 162 min (r = 0.98). Individual half-lives in seven patients with three samples ranged from 61-172 min. Large interindividual variations were found in CSF-concentrations of morphine, 9- and 8-fold at 3 and 8 h, respectively, after the dose. However, 16 h after administration no patient had a concentration less than 81 nmol/l. At 8 h after the dose, CSF concentrations of morphine were significantly higher (P less than 0.05) in a group of patients (n = 5) kept uptilted (80 degrees), as compared to those in the supine position (n = 5). Such a difference was not observed 3 h after the dose. The sampling procedure and age also seemed to influence CSF concentrations of morphine. There was no correlation between the dose given in mg and the CSF concentrations achieved. Strict standardization is thus mandatory when studying the disposition of opiates in CSF after epidural or intrathecal administration. Since our calculated half-lives of morphine in CSF were similar to those reported in plasma, the long-lasting effect is probably related to the high initial morphine concentrations in CSF.

Pharmacokinetics of Morphine Injected Intravenously into the Anesthetized Dog

This study determined the disposition of morphine in plasma and cerebrospinal fluid (CSF) and its relationship to ventilatory depression. 14C-Morphine sulfate (0.3 mg/kg or 2.0 mg/kg as the base) was injected intravenously into dogs anesthetized with enflurane-N2O-O2. Arterial plasma, cisternal CSF, and urine were analyzed for unchanged morphine and morphine metabolites. By 10 min after injection, 97 per cent of the injected morphine had been cleared from plasma. The terminal elimination half-times for morphine in plasma were not significantly different for the two doses and averaged 75 ± 5 (SEM) min. The volumes of distribution were significantly greater for 0.3 mg/kg than for 2.0 mg/kg morphine; this difference appeared to be related to the lesser hypotension following the smaller dose. Metabolism of morphine was rapid and efficient; metabolites were apparent in plasma by 1.5 min. Morphine conjugates, primarily morphine glucuronide, accounted for more than 90 per cent of the radioactivity in plasma at two hours. By six hours, 66 ± 3 per cent of the administered 14C radioactivity had been recovered in the urine, but only 13 ± 2 per cent had been excreted as unchanged morphine. Morphine concentrations in CSF did not peak until 15–30 min after injection, and the terminal half-time was significantly greater than that of morphine in plasma. Ventilatory depression was prolonged, and its abatement did not correlate with the decline of morphine concentrations in either plasma or CSF. It is concluded that morphine was rapidly cleared from the plasma by its uptake into tissues and by its efficient conjugation with glucuronic acid. Ultimate elimination of the drug was primarily by renal excretion of conjugated morphine. Uptake of morphine into CSF was delayed and its elimination from CSF was prolonged in comparison with its clearance from plasma. The ventilatory depressant effects of morphine were not directly related to its concentration in either CSF or plasma.

Pharmacokinetics of morphine injected intravenously into the anesthetized dog.

This study determined the disposition of morphine in plasma and cerebrospinal fluid (CSF) and its relationship to ventilatory depression. 14C-Morphine sulfate (0.3 mg/kg or 2.0 mg/kg as the base) was injected intravenously into dogs anesthetized with enflurane-N2O-O2. Arterial plasma, cisternal CSF, and urine were analyzed for unchanged morphine and morphine metabolites. By 10 min after injection, 97 per cent of the injected morphine had been cleared from plasma. The terminal elimination half-times for morphine in plasma were not significantly different for the two doses and averaged 75 ± 5 (SEM) min. The volumes of distribution were significantly greater for 0.3 mg/kg than for 2.0 mg/kg morphine; this difference appeared to be related to the lesser hypotension following the smaller dose. Metabolism of morphine was rapid and efficient; metabolites were apparent in plasma by 1.5 min. Morphine conjugates, primarily morphine glucuronide, accounted for more than 90 per cent of the radioactivity in plasma at two hours. By six hours, 66 ± 3 per cent of the administered 14C radioactivity had been recovered in the urine, but only 13 ± 2 per cent had been excreted as unchanged morphine. Morphine concentrations in CSF did not peak until 15–30 min after injection, and the terminal half-time was significantly greater than that of morphine in plasma. Ventilatory depression was prolonged, and its abatement did not correlate with the decline of morphine concentrations in either plasma or CSF. It is concluded that morphine was rapidly cleared from the plasma by its uptake into tissues and by its efficient conjugation with glucuronic acid. Ultimate elimination of the drug was primarily by renal excretion of conjugated morphine. Uptake of morphine into CSF was delayed and its elimination from CSF was prolonged in comparison with its clearance from plasma. The ventilatory depressant effects of morphine were not directly related to its concentration in either CSF or plasma.