Despite an increasing awareness regarding pain management in neonates and the availability of published guidelines for the treatment of procedural pain, preterm neonates experience pain leading to short- and long-term detrimental effects.
To assess the frequency of use of analgesics in invasive procedures in neonates and the associated pain burden in this population.
For 151 neonates, we prospectively recorded all painful procedures, including the number of attempts required, and analgesic therapy used during the first 14 days of neonatal intensive care unit admission. These data were linked to estimates of the pain of each procedure, obtained from the opinions of experienced clinicians.
On average, each neonate was subjected to a mean ± SD of 14 ± 4 procedures per day. The highest exposure to painful procedures occurred during the first day of admission, and most procedures (63.6%) consisted of suctioning. Many procedures (26 of 31 listed on a questionnaire) were estimated to be painful (pain scores >4 on a 10-point scale). Preemptive analgesic therapy was provided to fewer than 35% of neonates per study day, while 39.7% of the neonates did not receive any analgesic therapy in the neonatal intensive care unit.
Clinicians estimated that most neonatal intensive care unit procedures are painful, but only a third of the neonates received appropriate analgesic therapy. Despite the accumulating evidence that neonatal procedural pain is harmful, analgesic treatment for painful procedures is limited. Systematic approaches are required to reduce the occurrence of pain and to improve the analgesic treatment of repetitive pain in neonates.
ADVANCES IN perinatal care have increased the survival of preterm neonates in neonatal intensive care units (NICUs) worldwide. The physiological instability and underlying diseases of these infants necessitate various invasive procedures, such as endotracheal intubation, heelsticks, insertion of indwelling venous and arterial catheters, as well as nasal, tracheal, and gastric suctioning. Although previous studies1-4 have reported the frequency of daily procedures in the NICU (Table 1), many procedures such as nasopharyngeal tube insertions and nasal suctioning were not included, and the number of failed procedures has never been evaluated, to our knowledge.
Overview of Studies Investigating the Number of Procedures in the Neonatal Intensive Care Unit (NICU)
Despite the current knowledge that low birth weight infants are able to experience pain, many daily procedures are still performed without pharmacological or nonpharmacological analgesic therapy.2,5-9 Indeed, preterm neonates are highly sensitive to pain10-12 and amass acute responses to painful procedures.13,14 Short- and long-term effects of painful procedures in neonates15,16 occur as a consequence of their immature and vulnerable nervous systems, reflected in an altered pain response,17 possibly leading to changes in neural development. Studies in animals indicate developmental changes in the brain18,19 and in the spinal dorsal horn20 that are associated with neonatal pain. A higher frequency of painful invasive procedures in low birth weight infants has been associated with a greater pain response at 32 weeks compared with controls.21 Although it is unknown whether these changes in pain response persist until older age,22,23 more cognitive24 and psychopathological25-32 problems have been reported in children who are born preterm.33,34
A growing awareness in the past several years of the importance of adequate neonatal pain management led to the discussion whether analgesia should be given to all ventilated newborns.35,36 Although there is controversy about the risks and benefits of continuous opioid administration (and as a consequence neonatologists are reluctant to prescribe them), recently published international consensus statements37,38 have provided neonatal pain management guidelines, including those for procedural interventions. Therefore, we hypothesized that analgesic therapy is frequently used and that procedural pain is minimal in neonates admitted to the NICU. Prospectively, we collected bedside data on the number and type of daily painful procedures performed in 151 neonates during their stay in a tertiary care NICU, including failed procedures (eg, multiple attempts to insert peripheral venous catheters). These data were integrated with the results of a questionnaire evaluating the opinions of experienced clinicians about the pain of different procedures. Furthermore, the analgesic therapy used was evaluated and compared with current guidelines.
A pain research team (including a neonatologist [D.R.], pediatric intensivist [D.T.], research nurse, and psychologist [M. van D.]) with extensive research39-50 and clinical experience compiled a checklist containing all daily painful procedures, invasive and noninvasive, including multiple attempts. The checklist designed for this study was tested and refined during a pilot study in December 2000 and January 2001, and 34 procedures were included in the final version.
From February 1, 2001, to July 31, 2001, we documented all procedures performed in all neonates during the first 14 days of admission in a tertiary care NICU. The unit is part of the biggest children's hospital in the Netherlands, which comprises a perinatal center and pediatric surgery setting. Patients older than 3 days at admission and infants discharged or transferred within 24 hours after admission were excluded from the study. Nurses and physicians noted all procedures performed in real time each day, including the number of attempts for each procedure. The recorded procedures were cross-checked by the researchers every day to ensure accuracy. Background variables and mode of respiratory support were noted daily, and the Clinical Risk Index for Babies (CRIB)51 was scored as a measure of severity of illness.
In addition, all administered analgesics were noted during the study days. Standardized pharmacological analgesic therapy for procedural pain in our NICU is limited to a morphine loading dose (100 µg/kg of body weight) before tracheal intubation (unless intubation occurs in the delivery room) and before the insertion of thoracostomy tubes. Infants with indwelling thoracostomy tubes receive a continuous morphine infusion (10 µg/kg per hour) until the tube is removed.
To estimate the pain of NICU procedures, we developed a questionnaire listing all invasive procedures from our checklist, with 2 noninvasive procedures (diaper change and cranial ultrasound) included as control variables. The questionnaire was distributed among the nurses and physicians of (1) the NICU, where the procedures were also counted, (2) a pediatric surgical intensive care unit present in the same hospital where newborns with major congenital anomalies are admitted, and (3) the level III NICU of another, nonuniversity, hospital (Isala Clinics). Participants were asked to estimate a rating from 0 (not painful) to 10 (most painful) for each procedure, without taking specific circumstances into account.
Procedures were counted per calendar day. Because the first and last study days are usually shorter than 24 hours, the numbers of procedures were corrected for the actual length of stay on these days.
Random regression modeling (PROC MIXED; SAS Institute, Cary, NC) was used to simultaneously estimate the effect of the time-varying covariates respiratory support (no support, nasal oxygen, continuous positive airway pressure, and mechanical ventilation), postnatal age, gestational age, and length of study on the number of procedures performed. Because a procedural volume difference on the first study day compared with the other days was expected, the study days were dichotomized into 2 variables: 0 (1 day) and 1 (2-14 days). The outcome variable, ie, frequency of procedures, was log-transformed (base 10) to achieve a normal distribution. The model incorporated random intercepts and random slopes.
Multiple regression analysis was performed to estimate the effect of background variables (profession, sex, age, unit, hospital [Erasmus MC–Sophia Children's Hospital or Isala Clinics], parent [yes or no], and years of NICU experience) on the pain scoring of the participants.
Data are presented as mean ± SD.
One hundred fifty-one neonates were included in the study; 89 other newborns who were discharged within 24 hours were excluded. Table 2 lists the study subjects' background characteristics and primary diagnoses. Most neonates (n = 129) were admitted and enrolled on the first postnatal day. Gestational ages ranged from 25.3 to 42.0 weeks, with a mean age of 32.4 ± 4.5 weeks. The CRIB51 scores ranged from 0 to 16, with a mean score of 3.9 ± 3.3. Study subjects required respiratory support consisting of mechanical ventilation on 49.6% of study days (one third high-frequency oscillation and two thirds conventional ventilation), continuous positive airway pressure on 22.5% of study days, nasal oxygen on 15.6% of study days, and subjects required no respiratory support on 12.2% of study days. During 55.2% of the study days, patients had an arterial line (42.8% peripheral arterial lines and 12.3% umbilical arterial lines).
Background Characteristics and Primary Diagnoses in 151 Infants
During 1375 patient-days, 19 674 procedures were performed, with a mean NICU stay of 9.1 ± 4.4 days per patient. Table 3 gives the procedures rank-ordered by their frequency. Suctioning of nasal, endotracheal, and nasopharyngeal tubes constituted 63.6% of the performed procedures. The mean number of procedures per neonate per day was 14.3 ± 4, with a range of 0 to 53 procedures per day. Almost one third (30.9%) of the 1076 insertions of intravenous cannulae were not successful. Procedures for placement of central venous catheters, peripheral arterial catheters, and umbilical catheters were not successful in 45.6%, 37.5%, and 34.6% of attempts, respectively. Failure rates for venipunctures and lumbar punctures were 21.0% and 17.5%, respectively (Figure 1).
Incidence of Procedures, With Frequencies per Infant per Day
Percentages of failed and successful procedures.
Random regression modeling (Table 4) showed significantly higher frequencies of procedures during the first study day compared with days 2 to 14 (P<.001). The frequency of procedures was not predicted by gestational age (P = .51), day of admission (P = .50), or postnatal age (P = .72).
Random Regression Model for Predicting the Frequency of Procedures
Procedures were performed with significantly higher frequencies in patients receiving nasal oxygen, continuous positive airway pressure, and ventilation (P<.001 for all) compared with those without respiratory support.
Two hundred forty-seven questionnaires were distributed, with a response rate of 59.9% (n = 148), which was similar for nurses and physicians (Table 5 gives their background characteristics). The mean pain score, across all respondents and all procedures, on the 10-point scale was 5.2 ± 1.3. The mean pain score per procedure varied from 1.7 ± 1.6 for diaper change to 8.9 ± 1.4 for intubation. Pain ratings given by the nurses and physicians are shown in Figure 2.
Background Characteristics of 148 Questionnaire Respondents
Questionnaire results per procedure are shown by median scores of physicians (triangles) and nurses (squares) and their interquartile ranges.
Because procedures were scored on a 10-point scale, results of this questionnaire can be compared with a frequently used pain score, the visual analog scale. A visual analog scale score above 4 is generally used as a criterion for administration of extra analgesic therapy.45 Therefore, procedures with pain scores above 4 can be considered as moderately painful. Procedures that received lower scores included the control variables, diaper change and cranial ultrasound, as well as insertion of nasal cannulae, X-rays, and the removal of nasogastric tubes. All other procedures were considered moderately to severely painful.
Multiple regression analyses (Table 6) showed that nurses scored various procedures as more painful than physicians (β = .30, P = .001), caregivers who were parents themselves scored procedures lower than persons without children (β = .27, P = .002), and ratings between the 2 hospitals were somewhat different (β = .20, P = .03). The multiple regression coefficient (R = 0.43) explained about 15% of the variance in the model.
Multiple Regression Model for Pain Scores of 148 Respondents
Pharmacological analgesic treatment was studied in 126 patients (Figure 3); 25 neonates were excluded because they were enrolled in a double-blind, randomized controlled trial comparing morphine with placebo. Fifty patients (39.7%) did not receive any analgesic therapy during the study. Analgesic treatment regimens consisted of a morphine loading dose (given before intubation in 67.8% of administrations), continuous morphine infusion, or morphine in combination with vecuronium bromide, midazolam hydrochloride, or acetaminophen supplements. Fewer than 35% of newborns received analgesic therapy each day. Most therapy was administered during the first day, when 41 infants (32.5%) received analgesics, and decreased within 2 weeks to 12.2% and 14.6% of infants receiving analgesics on days 13 and 14, respectively.
Pharmacological analgesic treatment per day of admission.
Analgesic therapy, pharmacological and nonpharmacological, was rarely applied before invasive procedures. Analgesics were generally used only before tracheal intubation or insertion of a chest tube and usually were not given in association with other procedures with high pain scores. Although some nurses used pacifiers and tried to comfort infants during and after procedures by handling, nonpharmacological analgesic treatment was not given routinely for any of the procedures.
In sharp contrast to the accumulating evidence that repetitive pain is harmful in newborns, and despite major clinical advances over the past 10 years, neonates experience up to 14 painful procedures per day, and, remarkably, more than 65% of the patients in this study did not receive appropriate analgesic therapy.
We report a higher number of painful procedures in this study compared with previous studies, perhaps because we used a more extensive list of procedures and accounted for failed procedures (Table 1). For instance, Benis and Suresh3 reported a mean of 6 procedures per day during the entire NICU stay of 15 neonates. Barker and Rutter1 reported an increased number of procedures in infants younger than 31 weeks' gestation compared with older infants, but did not mention the length of NICU stay, which may explain the exposure to a greater number of invasive procedures. In the present study, however, the frequency of procedures per day was not related to gestational age, perhaps because term neonates admitted to the NICU may have a severity of illness comparable to that of preterm neonates.
To our knowledge, we are the first to report prospective data on a substantial number of failed procedures in the NICU. Although attempts by even the most experienced clinicians may be unsuccessful, the relative inexperience of trainees may partly explain the high proportion of failed procedures in this study. These findings may, therefore, be applicable to NICUs located in other academic centers.
Efforts should be aimed at minimizing the number of invasive procedures, as stated in recent consensus guidelines.37,38 Our study showed that the number of procedures is significantly higher in neonates requiring nasal oxygen, continuous positive airway pressure, or ventilation and that procedures occur mostly on the first day of admission, because of initial stabilization, monitoring, and diagnostic evaluation. Furthermore, the number of heelsticks and venipunctures is subsequently decreased in patients having arterial lines. Our nursing protocols require tracheal suctioning every 4 hours and as needed for ventilated neonates, whereas a recent study52 showed comparable ventilatory outcomes when the frequency of routine suctioning was decreased to every 8 hours.
Nurses and physicians agreed that most neonatal procedures cause moderate or severe pain,45 with pain scores above 4 estimated for 26 of the 31 procedures. Physicians ascribed lower pain scores; because they are mostly responsible for prescribing analgesics, this may contribute to the limited use of analgesic therapy in neonates.2 Others have reported comparable significant differences in pain scores of invasive procedures between nurses and physicians.53,54 The clinicians not returning the questionnaire might have been unmotivated and not interested in pain management in the NICU. However, a formal analysis of their motives was not undertaken. This lack of interest is probably related to their belief that these daily procedures are not that painful. As the response rate was not very high, the results of our questionnaire might show some overestimation of clinicians' overall opinion about the pain level of these procedures.
In our NICU, nurses tried to cluster procedures during routine nursing care, after which they comforted patients by touch, pacifiers, or positioning (eg, swaddling), whereas other behavioral and environmental approaches were used irregularly. Similar to our results, a multicenter study55 in France also showed minimal use of analgesics and a lack of standardization in the pharmacological regimens used in the NICU. Kahn et al56 reported a 28.6-fold variation in the use of opioids among 6 NICUs. Variations in attitude toward pain may limit the generalizability of our study findings to other centers around the world.
Restrained use of opioids by neonatologists can be explained by the fact that there is wide disagreement as to whether the evidence base is sufficient to justify prolonged exposure to opioids in this vulnerable population. There is some evidence, from studies in rats, that neonatal morphine exposure causes specific long-term behavioral effects57 and might cause retarded growth and motor development.58 Underlying pathologic mechanisms have been demonstrated by morphine-induced apoptosis in human fetal cell cultures59 and by µ-opiate receptor down-regulation following morphine treatment in neonatal rat brain.60 Prolonged use of high doses of opioids in animal and in vitro models complicates extrapolation of these findings to daily NICU practice. The only study61 investigating long-term effects of human neonatal morphine treatment showed no effects in 5- to 6-year-olds. Anand and colleagues62,63 reported decreased mortality with the use of postoperative analgesia in premature infants. However, a similar benefit of routine use of morphine has not been reported in this population without a surgical operation. Grunau et al21 found that altered pain responses in preterm neonates were predicted by the number of previous painful procedures and were normalized by the early use of morphine analgesia. Although accumulating data suggest that analgesic therapy with morphine might be useful to prevent some of the long-term effects of repetitive neonatal pain, further evidence about the safety of prolonged use of opioids is needed.
We recommend that a continuous intravenous infusion of opioids should be considered for infants requiring respiratory support during the first 24 hours of admission, in combination with well validated pain scores as part of routine nursing care. Use of an algorithm would enable caregivers to respond immediately and in a structured way when they observe pain in these infants. Although this recommendation is supported by preliminary studies,39,64-67 results from larger randomized controlled trials are needed to decide if ventilated neonates should be routinely treated with continuous opioids.68 As treatment regimens evolve, consistent practices in the NICU should be developed to minimize invasive procedures that continue to hurt our newborns.
Corresponding author: Dick Tibboel, MD, PhD, Department of Pediatric Surgery, Erasmus MC–Sophia Children's Hospital, Dr Molewaterplein 60, 3015 GJ Rotterdam, the Netherlands (e-mail: firstname.lastname@example.org).
Accepted for publication June 19, 2003.
This work was supported by grant MW-NWO 940-31-048 from the Netherlands Organization for Scientific Research, The Hague.
We thank N. Jongeneel, RN, C. Bunkers, RN, and E. Smink, RN, for distributing the surveys among physicians and nurses; C. Hoving, MSc, for helping to collect the data from nurses and physicians who completed the survey; H. J. Duivenvoorden, PhD, for statistical advice, and J. Hagoort for his help in preparing the manuscript.
As evidence about the short- and long-term harmful effects of pain in neonates accumulates, efforts should be made to decrease the number of pain experiences and to improve analgesic therapy. This study reports the number of painful procedures that neonates experience during an NICU stay. In contrast to the high incidence of painful procedures, we show the limited use of analgesic treatment, despite the awareness of nurses and physicians that most procedures are indeed painful.
N Exposure to invasive procedures in neonatal intensive care unit admissions. Arch Dis Child Fetal Neonatal Ed.
1995;72F47- F48PubMedGoogle ScholarCrossref
KJS A cross-sectional survey of pain and pharmacological analgesia in Canadian neonatal intensive care units. Clin J Pain.
1997;13308- 312PubMedGoogle ScholarCrossref
GK Frequency of invasive procedures in very low birth weight (VLBW) infants in the neonatal intensive care unit (NICU) [abstract]. Pediatric Res.
2001;49392AAbstract 2253Google Scholar
KJS Epidemiology of pain in neonates. Res Clin Forums.
1998;209- 18Google Scholar
LS A national survey of the assessment and treatment of pain and agitation in the neonatal intensive care unit. J Obstet Gynecol Neonatal Nurs.
1987;16387- 393PubMedGoogle ScholarCrossref
E Use of analgesic agents for invasive medical procedures in pediatric and neonatal intensive care units. J Pediatr.
1992;121647- 649PubMedGoogle ScholarCrossref
EP Pain management in Canadian level 3 neonatal intensive care units. CMAJ.
1994;150499- 504PubMedGoogle Scholar
V Human fetal and maternal noradrenaline responses to invasive procedures. Pediatr Res.
494- 499PubMedGoogle ScholarCrossref
HD Pain in the preterm neonate: behavioural and physiological indices. Pain.
1993;52287- 299PubMedGoogle ScholarCrossref
L Developmental changes in response to heelstick in preterm infants. Dev Med Child Neurol.
1996;38438- 445PubMedGoogle ScholarCrossref
PJ, ed.ed Neurophysiological and neurobiological correlates of supraspinal pain processing measurement techniques. Progress in Pain Research and Management
Seattle, Wash IASP Press1998;21- 46Google Scholar
FM Can adverse neonatal experiences alter brain development and subsequent behavior? Biol Neonate.
2000;7769- 82PubMedGoogle ScholarCrossref
PM Long-term behavioral effects of repetitive pain in neonatal rat pups. Physiol Behav.
1999;66627- 637PubMedGoogle ScholarCrossref
T Altered nociceptive neuronal circuits after neonatal peripheral inflammation. Science.
2000;289628- 631PubMedGoogle ScholarCrossref
SK Demographic and therapeutic determinants of pain reactivity in very low birth weight neonates at 32 weeks' postconceptional age. Pediatrics.
2001;107105- 112PubMedGoogle ScholarCrossref
RE Behavior, pain perception, and the extremely low-birth weight survivor. Clin Perinatol.
2000;27363- 379PubMedGoogle ScholarCrossref
KJS Cognitive and behavioral outcomes of school-aged children who were born preterm: a meta-analysis. JAMA.
2002;288728- 737PubMedGoogle ScholarCrossref
EL Early pain experience, child and family factors, as precursors of somatization. Pain.
1994;56353- 359PubMedGoogle ScholarCrossref
et al. Regional brain volume abnormalities and long-term cognitive outcome in preterm infants. JAMA.
2000;2841939- 1947PubMedGoogle ScholarCrossref
RD Brief report: birth status, medical complications, and social environment. J Pediatr Psychol.
2000;25353- 358PubMedGoogle ScholarCrossref
M Learning disability, attention-deficit disorder, and language impairment as outcomes of prematurity: a longitudinal descriptive study. J Learn Disabil.
1998;31294- 306PubMedGoogle ScholarCrossref
P Mediators of behavioral problems in 7-year-old children born after 24 to 28 weeks of gestation. J Dev Behav Pediatr.
2001;221- 10PubMedGoogle ScholarCrossref
et al. Neuropsychological outcome at adolescence of very preterm birth and its relation to brain structure. Dev Med Child Neurol.
2001;43226- 233PubMedGoogle ScholarCrossref
N Attention deficit hyperactivity disorders and other psychiatric outcomes in very low birthweight children at 12 years. J Child Psychol Psychiatry.
1997;38931- 941PubMedGoogle ScholarCrossref
M School-age consequences of birth weight less than 750 g: a review and update. Dev Neuropsychol.
2000;17289- 321PubMedGoogle ScholarCrossref
M Middle-school–age outcomes in children with very low birthweight. Child Dev.
2000;711495- 1511PubMedGoogle ScholarCrossref
JE Narcotic analgesia for ventilated newborns: are placebo-controlled trials ethical and necessary? J Pediatr.
1999;134127- 129PubMedGoogle ScholarCrossref
KJSInternational Evidence-Based Group for Neonatal Pain, Consensus statement for the prevention and management of pain in the newborn. Arch Pediatr Adolesc Med.
2001;155173- 180PubMedGoogle ScholarCrossref
Committee on Fetus and Newborn, Committee on Drugs, Section on Anesthesiology, Section on Surgery, Fetus and Newborn Committee, Prevention and management of pain and stress in the neonate. Pediatrics.
2000;105454- 461Google ScholarCrossref
D Hormonal and metabolic stress responses after major surgery in children aged 0-3 years. Br J Anaesth.
2001;87390- 399PubMedGoogle ScholarCrossref
D Multiple-dose pharmacokinetics of rectally administered acetaminophen in term infants. Clin Pharmacol Ther.
1999;66509- 515PubMedGoogle ScholarCrossref
et al. Pharmacokinetics and metabolism of rectally administered paracetamol in preterm neonates. Arch Dis Child Fetal Neonatal Ed.
1999;80F59- F63PubMedGoogle ScholarCrossref
et al. Effects of rectally administered paracetamol on infants delivered by vacuum extraction. Eur J Obstet Gynecol Reprod Biol.
2001;9473- 78PubMedGoogle ScholarCrossref
et al. Efficacy of continuous versus intermittent morphine administration after major surgery in 0–3-year-old infants: a double-blind randomized controlled trial. Pain.
2002;98305- 3PubMedGoogle ScholarCrossref
HJ The reliability and validity of the COMFORT scale as a postoperative pain instrument in 0 to 3-year-old infants. Pain.
2000;84367- 377PubMedGoogle ScholarCrossref
J Observational visual analog scale in pediatric pain assessment. Clin J Pain.
2002;18310- 316PubMedGoogle ScholarCrossref
D Review of pain assessment measures for postoperative pain in vulnerable infants. Clin Perinatol.
2002;29469- 491PubMedGoogle ScholarCrossref
D The effects of analgesia in the vulnerable infant during the perinatal period. Clin Perinatol.
2002;29511- 534PubMedGoogle ScholarCrossref
van der Marel
et al. Analgesic efficacy of rectal versus oral acetaminophen in children after major craniofacial surgery. Clin Pharmacol Ther.
2001;7082- 90PubMedGoogle ScholarCrossref
D Are postoperative pain instruments useful for specific groups of vulnerable infants? Clin Perinatol.
2002;29469- 491PubMedGoogle ScholarCrossref
et al. Major surgery within the first 3 months of life and subsequent biobehavioral pain responses to immunization at later age: a case comparison study. Pediatrics.
2003;111129- 135PubMedGoogle ScholarCrossref
LW A comparison of two airway suctioning frequencies in mechanically ventilated, very-low-birthweight infants. Respir Care.
2001;46783- 788PubMedGoogle Scholar
JP Pain and pain management in newborn infants: a survey of physicians and nurses. Pediatrics.
1997;100626- 632PubMedGoogle ScholarCrossref
J Staff perception of pain on a neonatal intensive care unit [letter]. Arch Dis Child Fetal Neonatal Ed.
R Pain management in French neonatal intensive care units. Acta Paediatr.
2002;91822- 826PubMedGoogle ScholarCrossref
et al. Variation among neonatal intensive care units in narcotic administration. Arch Pediatr Adolesc Med.
1998;152844- 851PubMedGoogle ScholarCrossref
KJ Interactions of inflammatory pain and morphine in infant rats: long-term behavioral effects. Physiol Behav.
2001;7351- 58PubMedGoogle ScholarCrossref
PK Morphine induces apoptosis of human microglia and neurons. Neuropharmacology.
2002;42829- 836PubMedGoogle ScholarCrossref
A Visualization of µ-opiate receptor downregulation following morphine treatment in neonatal rat brain. Brain Res Dev Brain Res.
1991;6419- 26PubMedGoogle ScholarCrossref
M Outcome at 5-6 years of prematurely born children who received morphine as neonates. Arch Dis Child Fetal Neonatal Ed.
1998;79F40- F43PubMedGoogle ScholarCrossref
A Randomised trial of fentanyl anaesthesia in preterm babies undergoing surgery. Lancet.
1987;162- 66PubMedGoogle ScholarCrossref
PR Halothane-morphine compared with high-dose sufentanil for anesthesia and postoperative analgesia in neonatal cardiac surgery. N Engl J Med.
1992;3261- 9PubMedGoogle ScholarCrossref
et al. Analgesia and sedation in preterm neonates who require ventilatory support . Arch Pediatr Adolesc Med.
1999;153331- 338[published correction appears in Arch Pediatr Adolesc Med
S Morphine increases synchronous ventilation in preterm infants. J Paediatr Child Health.
1995;31176- 179PubMedGoogle ScholarCrossref
JE Narcotic sedation stabilizes arterial blood pressure fluctuations in sick premature infants. J Perinatol.
1991;11365- 371PubMedGoogle Scholar
V Advantages of fentanyl over morphine in analgesia for ventilated newborn infants after birth: a randomized trial. J Pediatr.
1999;134144- 150PubMedGoogle ScholarCrossref
NEOPAIN Multi-center Group, Effects of morphine therapy on neurologic outcomes in ventilated preterm neonates. Pediatric Res.
2002;51361Abstract 2101.Google ScholarCrossref