Significance of the endogenous digoxin-like substance in infants and mothers

Digoxin serum concentrations were measured by a routine radioimmunoassay in 30 neonates not receiving digoxin; nonetheless, digoxin levels were between 0.17 nM and 1.64nM (means = 0.64nM +/- 0.27 nM). There was a negative correlation between gestational age and concentration of an endogenous digoxin-like substance (EDLS). Neonates less than or equal to 32 wk gestational age had higher levels of EDLS than neonates greater than 32 wk old. EDLS concentrations were compared in 22 mothers and their 24 offspring and were higher in all newborn infants (0.34nM +/- 0.09nM and 0.15nM +/- 0.08nM). EDLS was shown to inhibit Na+-K+-adenosinetriphosphatase activity by measurement of 86Rb uptake in erythrocytes exposed to sera samples from 30 infants in the study. EDLS levels greater than 0.6 ng/ml were associated with lesser 86Rb uptake. Simulation kinetics suggest that the presence of 0.6nM EDLS would lengthen the digoxin t1/2 by 64%, reduce the volume of distribution by 23%, and lower clearance by 53% if the peak "true" digoxin level were 2 ng/ml. EDLS concentrations of 1.5 ng/ml would increase the t1/2 by 207% while reducing the volume of distribution by 43% and clearance by 81%. These considerations cast serious doubts on the validity of currently accepted digoxin kinetics and dosing in preterm infants.

Aminoglycoside-related nephrotoxicity in the premature newborn

The nephrotoxicity of gentamicin and amikacin was compared during presumed sepsis in 107 premature neonates. To examine the possibility that nephrotoxicity was directly associated with the clinical conditions of "sepsis," a control group of 26 chloramphenicol-treated newborns was also studied. Two markers of proximal renal tubular injury, N-acetyl-beta-glucosaminidase (NAG) and beta 2-microglobulin, were measured in 6-hr aliquots of urine. Because urine creatinine excretion increased with postconception age, markers were expressed in terms of excretion rate rather than per milligram of creatinine. The NAG excretion rate was significantly higher in gentamicin-treated patients (138 +/- 10 U/min, mean +/- SE) than in amikacin-treated patients (85 +/- 7 U/min) but did not differ between patients treated with amikacin and those treated with chloramphenicol (81 +/- 11 U/min). Excretion of beta 2-microglobulin did not differ among the three patient groups. We conclude that amikacin may be less nephrotoxic than gentamicin in the premature newborn.

Assessment of immediate post-anaesthetic recovery in young children following intravenous morphine infusions, halothane, and isoflurane

Within 15 minutes of terminating general anaesthesia, progressive recovery of consciousness, spontaneous ventilation and cough, and limb movements were assessed in 60 young children (age range 0-5 years, mean +/- SEM; 2.83 +/- 0.34; weight 13.86 +/- 0.41 kg). All patients were ASA physical status class I-III, received a standard intravenous induction (atropine 0.02 mg X kg-1, thiopental sodium 5 mg X kg-1, diazepam 0.2 mg X kg-1), were intubated with an orotracheal tube following the administration of metocurine, 0.4 mg X kg-1, and were maintained under general anaesthesia with nitrous oxide and oxygen in a 70:30 mixture administered by a T-piece circuit. They were ventilated mechanically to maintain normal blood-oxygen tension and normocarbia. The patients were assessed in three equal groups according to the anaesthetic supplement they received. Group I received intravenous infusions of morphine sulfate (loading dose 60 micrograms X kg-1 administered over 5 minutes followed by a continuous intravenous infusion of 2 micrograms X kg-1 X min-1. Patients in Groups II and III had 0.5 per cent halothane and 1.0 per cent isoflurane respectively added to the nitrous oxide/oxygen fresh gas mixture rather than morphine sulphate infusions. By the end of the study period, there was no significant difference in the degree of recovery between the morphine and the isoflurane groups but the patients in the halothane group had recovered to a lesser degree. Generally, the patients in the morphine group were awake but not crying, while those in the other two groups were less sedated.

Theophylline disposition in cystic fibrosis

The disposition of intravenously and orally administered theophylline was compared in 10 patients with cystic fibrosis (CF) and 10 healthy volunteers. The mean total body clearance of the patients with CF was twice that of the control subjects (p less than 0.001). The volume of distribution of theophylline was also greater in the patients with CF (p less than 0.05). These observations indicate that if theophylline is used in patients with CF, doses larger than those commonly given to asthmatics may be required to maintain therapeutic serum concentrations.

Chloramphenicol pharmacokinetics in the newborn

We studied pharmacokinetics of chloramphenicol in 9 neonates having a mean gestational age of 31.2 +/- 1.9 weeks (mean +/- SEM). The studied dose was the final dose of treatment in 8 of these and the first dose in 2 of these. 1 neonate was studied twice. Concentrations of chloramphenicol and its 3-monosuccinate and 1-monosuccinate esters were measured in serum by high performance liquid chromatography. Apparent total body clearance of chloramphenicol correlated with postnatal age (r = 0.81, p less than 0.01). Mean apparent clearance was 1.1 ml X min-1 X kg-1. Serum concentrations of succinate esters were below assay sensitivity after 6 h postdose. Factors leading to excessive chloramphenicol concentrations (greater than 25.0 mg/l) were evaluated in another 44 newborns. Instability of the patient's clinical condition was an important cause of excessive serum concentrations during ongoing therapy.

Initiation of chloramphenicol therapy in the newborn infant

To evaluate the ability of a loading dose of chloramphenicol succinate to rapidly, achieve adequate serum concentrations of chloramphenicol, we compared two intravenously administered dosages of chloramphenicol succinate given to initiate treatment. Thirteen premature neonates received an initial dose of 12.5 mg/kg; 26 received a loading dose of 20 mg/kg. Capillary blood samples were obtained at two, four, and 12 hours after the first dose. After the dose of 12.5 mg/kg, 45% of the neonates did not achieve serum concentrations greater than 10 mg/L. After the loading dose of 20 mg/kg, all neonates achieved concentrations greater than 10 mg/L. The peak chloramphenicol concentrations after the 12.5 mg/kg dose was 8.8 +/- 11.2 mg/L (+/- SEM) and after the 20 mg/kg loading dose, 15.9 +/- 0.7 mg/L. The disposition of chloramphenicol was age dependent. Chloramphenicol concentration peaked at four hours in neonates less than or equal to 2 days postnatal age and at two hours in neonates 3 to 55 days postnatal age. Chloramphenicol succinate concentrations were greater in younger than in older neonates at both two and four hours after the dose. We conclude that a loading dose is appropriate when using chloramphenicol succinate in neonates.

Performance on a reciprocal tapping task with variations in intertapping interval

Fitts' law was investigated in a study of the effect of the index of difficulty (ID) and intertapping interval upon reaction time (RT) and movement time (MT) for a reciprocal tapping task. ID showed its well-established relationship with MT as described by Fitts' law: MT = aID + b. Improvement in the linearity of this relationship was, however, demonstrated by expressing MT in logarithmic units. While ID had an unsubstantial (though significant) effect on RT, increases in intertapping interval from zero to any level of discrete tapping led to significant increases in RT of about 135 msec. The results are interpreted as lending support to Fitts' thesis that RT and MT reflect independent phases of information processing.

Ethanol and spinal presynaptic inhibition in man

Ethanol, 0.65 gm per kilogram of body weight, was administered orally to 6 normal subjects. The mean blood ethanol levels ranged from 0.74 to 0.99 gm per liter over a subsequent 40-minute testing period. The proportion of the soleus motoneuron pool activated by the Achilles tendon reflex was reduced. Vibratory inhibition of the monosynaptic reflex was used as an estimate of spinal presynaptic inhibition. It was unaffected by ethanol.

Diazepam effect on reflex activity in patients with complete spinal lesions and in those with other causes of spasticity

The effects of diazepam on reflex pathways in patients having complete spinal lesions and in patients having incomplete lesions of the spinal cord or multiple sclerosis are compared to determine whether diazepam has an action at spinal level. The drug produced no significant alteration in the excitability of the monosynaptic arc in the patients with complete spinal lesions. In contrast, in the group with incomplete spinal lesions or multiple sclerosis, diazepam reduced the excitability of the monosynaptic arc. This action did not appear to result from a reduction in fusimotor drive. Diazepam also reduced the tonic vibration reflex in this group. It is postulated that these effects may be due to a supraspinal action of the drug.

The effect of diazepam on presynaptic inhibition in patients with complete and incomplete spinal cord lesions

The effect of diazepam on presynaptic inhibition in man has been examined in 5 patients with complete spinal transections and 7 patients with incomplete lesions. The inhibition of the H reflex by vibration applied to the tendo Achilles was used to assess presynaptic inhibition of the Ia monosynaptic pathway. Diazepam increased this inhibition in the patients with incomplete lesions, but had no significant effect on the inhibition in the patients with complete spinal transections. Evidently diazepam can enhance presynaptic inhibition in man. The effect, however, cannot be demonstrated in patients with longstanding complete spinal lesions possibly because of some alteration in the segmental presynaptic inhibitory mechanism in this group.