Physiologic effects of doxapram in idiopathic apnea of prematurity

Doxapram for apnoea of prematurity and neurodevelopmental outcomes at age 5-6 years

OBJECTIVE

To assess the long-term neurodevelopmental impact of doxapram for treating apnoea of prematurity.

DESIGN

Secondary analysis of the French national cohort study EPIPAGE-2. Recruitment took place in 2011. A standardised neurodevelopmental assessment was performed at age 5-6 years. A 2:1 propensity score matching was used to control for the non-randomised assignment of doxapram treatment.

SETTING

Population-based cohort study.

PATIENTS

All children born before 32 weeks' gestation alive at age 5-6 years.

INTERVENTIONS

Blind and standardised assessment by trained neuropsychologists and paediatricians at age 5-6 years.

MAIN OUTCOME MEASURES

Neurodevelopmental outcomes at age 5-6 years assessed by trained paediatricians and neuropsychologists: cerebral palsy, developmental coordination disorders, IQ and behavioural difficulties. A composite criterion for overall neurodevelopmental disabilities was built.

RESULTS

The population consisted of 2950 children; 275 (8.6%) received doxapram. Median (IQR) gestational age was 29.4 (27.6-30.9) weeks. At age 5-6 years, complete neurodevelopmental assessment was available for 60.3% (1780 of 2950) of children and partial assessment for 10.6% (314 of 2950). In the initial sample, children receiving doxapram had evidence of greater clinical severity than those not treated. Doxapram treatment was associated with overall neurodevelopmental disabilities of any severity (OR 1.43, 95% CI 1.07 to 1.92, p=0.02). Eight hundred and twenty-one children were included in the 2:1 matched sample. In this sample, perinatal characteristics of both groups were similar and doxapram treatment was not associated with overall neurodevelopmental disabilities (OR 1.09, 95% CI 0.76 to 1.57, p=0.63).

CONCLUSIONS

In children born before 32 weeks' gestation, doxapram treatment for apnoea of prematurity was not associated with neurodevelopmental disabilities.

Doxapram for the prevention and treatment of apnea in preterm infants

BACKGROUND

Apnea of prematurity is a common problem in preterm infants that may have significant consequences on their development. Methylxanthines (aminophylline, theophylline, and caffeine) are effective in the treatment of apnea of prematurity. Doxapram is used as a respiratory stimulant in cases refractory to the methylxanthine treatment.

OBJECTIVES

To evaluate the benefits and harms of doxapram administration on the incidence of apnea and other short-term and longer-term clinical outcomes in preterm infants.

SEARCH METHODS

We used standard, extensive Cochrane search methods. The latest search date was March 2023.

SELECTION CRITERIA

We included randomized controlled trials (RCTs) assessing the role of doxapram in prevention and treatment of apnea of prematurity and prevention of reintubation in preterm infants (less than 37 weeks' gestation). We included studies comparing doxapram with either placebo or methylxanthines as a control group, or when doxapram was used as an adjunct to methylxanthines and compared to methylxanthines alone as a control group. We included studies of doxapram at any dose and route.

DATA COLLECTION AND ANALYSIS

We used standard Cochrane methods. Our primary outcomes were clinical apnea, need for positive pressure ventilation after initiation of treatment, failed apnea reduction after two to seven days, and failed extubation (defined as unable to wean from invasive intermittent positive pressure ventilation [IPPV] and extubate or reintubation for IPPV within one week). We used GRADE to assess the certainty of evidence for each outcome.

MAIN RESULTS

We included eight RCTs enrolling 248 infants. Seven studies (214 participants) provided data for meta-analysis. Five studied doxapram for treatment of apnea in preterm infants. Three studied doxapram to prevent reintubation in preterm infants. None studied doxapram in preventing apnea in preterm infants. All studies administered doxapram intravenously as continuous infusions. Two studies used doxapram as an adjunct to aminophylline compared to aminophylline alone and one study as an adjunct to caffeine compared to caffeine alone. When used to treat apnea, compared to no treatment, doxapram may result in a slight reduction in failed apnea reduction (risk ratio [RR] 0.45, 95% confidence interval [CI] 0.20 to 1.05; 1 study, 21 participants; low-certainty evidence). The evidence is very uncertain about the effect of doxapram on need for positive pressure ventilation after initiation of treatment (RR 0.31, 95% CI 0.01 to 6.74; 1 study, 21 participants; very low-certainty evidence). Doxapram may result in little to no difference in side effects causing cessation of therapy (0 events in both groups; risk difference [RD] 0.00, 95% CI -0.17 to 0.17; 1 study, 21 participants; low-certainty evidence). Compared to alternative treatment, the evidence is very uncertain about the effect of doxapram on failed apnea reduction (RR 1.35, 95% CI 0.53 to 3.45; 4 studies, 84 participants; very low-certainty evidence). The evidence is very uncertain about the effect of doxapram on need for positive pressure ventilation after initiation of treatment (RR 2.40, 95% CI 0.11 to 51.32; 2 studies, 37 participants; very-low certainty evidence; note 1 study recorded 0 events in both groups. Thus, the RR and CIs were calculated from 1 study rather than 2). Doxapram may result in little to no difference in side effects causing cessation of therapy (0 events in all groups; RD 0.00, 95% CI -0.15 to 0.15; 37 participants; 2 studies; low-certainty evidence). As adjunct therapy to methylxanthine, the evidence is very uncertain about the effect of doxapram on failed apnea reduction after two to seven days (RR 0.08, 95% CI 0.01 to 1.17; 1 study, 10 participants; very low-certainty evidence). No studies reported on clinical apnea, chronic lung disease at 36 weeks' postmenstrual age (PMA), death at any time during initial hospitalization, long-term neurodevelopmental outcomes in the three comparisons, and need for positive pressure ventilation and side effects when used as adjunct therapy to methylxanthine. In studies to prevent reintubation, when compared to alternative treatment, the evidence is very uncertain about the effect of doxapram on failed extubation (RR 0.43, 95% CI 0.10 to 1.83; 1 study, 25 participants; very low-certainty evidence). As adjunct therapy to methylxanthine, doxapram may result in a slight reduction in 'clinical apnea' after initiation of treatment (RR 0.36, 95% CI 0.13 to 0.98; 1 study, 56 participants; low-certainty evidence). Doxapram may result in little to no difference in failed extubation (RR 0.92, 95% CI 0.52 to 1.62; 1 study, 56 participants; low-certainty evidence). The evidence is very uncertain about the effect of doxapram on side effects causing cessation of therapy (RR 6.42, 95% CI 0.80 to 51.26; 2 studies, 85 participants; very low-certainty evidence). No studies reported need for positive pressure ventilation, chronic lung disease at 36 weeks' PMA, long-term neurodevelopmental outcomes in the three comparisons; failed extubation when compared to no treatment; and clinical apnea, death at any time during initial hospitalization, and side effects when compared to no treatment or alternative treatment. We identified two ongoing studies, one conducted in Germany and one in multiple centers in the Netherlands and Belgium.

AUTHORS' CONCLUSIONS

In treating apnea of prematurity, doxapram may slightly reduce failure in apnea reduction when compared to no treatment and there may be little to no difference in side effects against both no treatment and alternative treatment. The evidence is very uncertain about the need for positive pressure ventilation when compared to no treatment or alternative treatment and about failed apnea reduction when used as alternative or adjunct therapy to methylxanthine. For use to prevent reintubation, doxapram may reduce apnea episodes when administered in adjunct to methylxanthine, but with little to no difference in failed extubation. The evidence is very uncertain about doxapram's effect on death when used as adjunct therapy to methylxanthine and about failed extubation when used as alternative or adjunct therapy to methylxanthine. There is a knowledge gap about the use of doxapram as a therapy to prevent apnea. More studies are needed to clarify the role of doxapram in the treatment of apnea of prematurity, addressing concerns about long-term outcomes. The ongoing studies may provide useful data.

The bioavailability and maturing clearance of doxapram in preterm infants

BACKGROUND

Doxapram is used for the treatment of apnea of prematurity in dosing regimens only based on bodyweight, as pharmacokinetic data are limited. This study describes the pharmacokinetics of doxapram and keto-doxapram in preterm infants.

METHODS

Data (302 samples) from 75 neonates were included with a median (range) gestational age (GA) 25.9 (23.9-29.4) weeks, bodyweight 0.95 (0.48-1.61) kg, and postnatal age (PNA) 17 (1-52) days at the start of continuous treatment. A population pharmacokinetic model was developed using non-linear mixed-effects modelling (NONMEM®).

RESULTS

A two-compartment model best described the pharmacokinetics of doxapram and keto-doxapram. PNA and GA affected the formation clearance of keto-doxapram (CL_{FORMATION KETO-DOXAPRAM}) and clearance of doxapram via other routes (CL_{DOXAPRAM OTHER ROUTES}). For a median individual of 0.95 kg, GA 25.6 weeks, and PNA 29 days, CL_{FORMATION KETO-DOXAPRAM} was 0.115 L/h (relative standard error (RSE) 12%) and CL_{DOXAPRAM OTHER ROUTES} was 0.645 L/h (RSE 9%). Oral bioavailability was estimated at 74% (RSE 10%).

CONCLUSIONS

Dosing of doxapram only based on bodyweight results in the highest exposure in preterm infants with the lowest PNA and GA. Therefore, dosing may need to be adjusted for GA and PNA to minimize the risk of accumulation and adverse events. For switching to oral therapy, a 33% dose increase is required to maintain exposure.

IMPACT

Current dosing regimens of doxapram in preterm infants only based on bodyweight result in the highest exposure in infants with the lowest PNA and GA. Dosing of doxapram may need to be adjusted for GA and PNA to minimize the risk of accumulation and adverse events. Describing the pharmacokinetics of doxapram and its active metabolite keto-doxapram following intravenous and gastroenteral administration enables to include drug exposure to the evaluation of treatment of AOP. The oral bioavailability of doxapram in preterm neonates is 74%, requiring a 33% higher dose via oral than intravenous administration to maintain exposure.

P2X3 receptor antagonism reduces the occurrence of apnoeas in newborn rats

Hyperreflexia of the peripheral chemoreceptors is a potential contributor of apnoeas of prematurity (AoP). Recently, it was shown that elevated P2X3 receptor expression was associated with elevated carotid body afferent sensitivity. Therefore, we tested whether P2X3 receptor antagonism would reduce AoP known to occur in newborn rats. Unrestrained whole-body plethysmography was used to record breathing and from this the frequency of apnoeas at baseline and following administration of either a P2X3 receptor antagonist - AF-454 (5 mg/kg or 10 mg/kg s.c.) or vehicle was derived. In a separate group, we tested the effects of AF-454 (10 mg/kg) on the hypoxic ventilatory response (10 % FiO₂). Ten but not 5 mg/kg AF-454 reduced the frequency of AoP and improved breathing regularity significantly compared to vehicle. Neither AF-454 (both 5 and 10 mg/kg) nor vehicle affected baseline respiration. However, P2X3 receptor antagonism (10 mg/kg) powerfully blunted hypoxic ventilatory response to 10 % FiO₂. These data suggest that P2X3 receptors contribute to AoP and the hypoxic ventilatory response in newborn rats but play no role in the drive to breathe at rest.

Doxapram Treatment and Diaphragmatic Activity in Preterm Infants

BACKGROUND

Doxapram is a treatment option for severe apnea of prematurity (AOP). However, the effect of doxapram on the diaphragm, the main respiratory muscle, is not known.

OBJECTIVES

To investigate the effect of doxapram on diaphragmatic activity measured with transcutaneous electromyography of the diaphragm (dEMG).

METHODS

A pilot study was conducted in a tertiary neonatal intensive care unit. Diaphragmatic activity was measured from 30 min before up to 3 h after the start of doxapram treatment. dEMG parameters were compared to baseline (5 min before doxapram treatment) and at 15, 60, 120 and 180 min after the start of doxapram infusion.

RESULTS

Eleven preterm infants were included with a mean gestational age of 25.5 ± 1.2 weeks and birth weight of 831 ± 129 g. The amplitudedEMG, peakdEMG and tonicdEMG values did not change in the 3 h after the start of doxapram infusion compared to baseline. Clinically, the number of apnea episodes in the 24 h after doxapram treatment decreased significantly.

CONCLUSION

Doxapram infusion does not alter diaphragmatic activity measured with transcutaneous dEMG in preterm infants with AOP, indicating that its working mechanism is primarily on respiratory drive and not on respiratory muscle activity.

Retrospective study shows that doxapram therapy avoided the need for endotracheal intubation in most premature neonates

AIM

Using doxapram to treat neonates with apnoea of prematurity might avoid the need for endotracheal intubation and invasive ventilation. We studied whether doxapram prevented the need for intubation and identified the predictors of the success.

METHODS

This was a retrospective study of preterm infants born from January 2006 to August 2014 who received oral or intravenous doxapram. Success was defined as no need for endotracheal intubation, due to apnoea, during doxapram therapy. Univariable and multivariable logistic regression analyses identified predictors of success during the first 48 hours of doxapram therapy.

RESULTS

Data on 203 patients with a median gestational age of 26.1 (interquartile range 25.1-27.4) weeks were analysed. During the first 48 hours of doxapram therapy, 157 (77%) patients did not need endotracheal intubation and 127 (63%) patients were successfully treated over the entire treatment course. The median postnatal age at the start of doxapram therapy was 20 days (interquartile range 12-30). Postnatal age and a lower fraction of inspired oxygen at the start of doxapram therapy were significant predictors of success (odds ratio 0.964, 95% confidence interval 0.938-0.991, p = 0.001).

CONCLUSION

Oral and intravenous doxapram effectively treated most cases of apnoea in preterm infants, avoiding the need for intubation.

Doxapram Treatment for Apnea of Prematurity: A Systematic Review

BACKGROUND

Apnea of prematurity (AOP) is a common complication of preterm birth, for which caffeine is the first treatment of choice. In case of persistent AOP, doxapram has been advocated as an additional therapy.

OBJECTIVE

To identify and appraise all existing evidence regarding efficacy and safety of doxapram use for AOP in infants born before 34 weeks of gestational age.

METHODS

All studies reporting on doxapram use for AOP were identified by searching electronic databases, references from relevant studies, and abstracts from the Societies for Pediatric Research. Two reviewers independently assessed study eligibility and quality, and extracted data on study design, patient characteristics, efficacy and safety outcomes.

RESULTS

The randomized controlled trials showed less apnea during doxapram treatment when compared to placebo, but no difference in treatment effect when compared to theophylline. No serious adverse effects were reported. We identified 28 observational studies consisting mainly of cohort studies and case series (n = 1,994). There was considerable heterogeneity in study design and quality. Most studies reported a positive effect of doxapram on apnea rate. A few studies reported on long-term outcomes with conflicting results. A range of possible doxapram-related short-term adverse effects were reported, sometimes associated with the use of higher doses.

CONCLUSION

Based on the limited number of studies and level of evidence, no firm conclusions on the efficacy and safety of doxapram in preterm infants can be drawn. For this reason, routine use cannot be recommended. A large multicenter randomized controlled trial is urgently needed to provide more conclusive evidence.

Long-Term Neurodevelopmental Outcome after Doxapram for Apnea of Prematurity

BACKGROUND

Doxapram has been advocated as a treatment for persistent apnea of prematurity (AOP).

OBJECTIVE

To evaluate the effect of doxapram on long-term neurodevelopmental outcome in preterm infants as its safety still needs to be established.

METHODS

From a retrospective cohort of preterm infants with a gestational age (GA) <30 weeks and/or a birth weight <1,250 g, born between 2000 and 2010, infants treated with doxapram (n = 142) and a nontreated control group were selected (n = 284). Patient characteristics and clinical and neurodevelopmental outcome data at 24 months' corrected age were collected. Neurodevelopmental delay (ND) was defined as having a Mental or Psychomotor Developmental Index (MDI/PDI) <-1 standard deviation (SD), cerebral palsy, or a hearing or visual impairment. Odds ratios (OR) were calculated using multiple logistic regression analyses adjusting for potential confounders.

RESULTS

Infants treated with doxapram had a lower GA compared to controls. The number of infants with a MDI or PDI <-1 SD was not different between the groups. The risk of the combined outcome death or ND was significantly lower in the doxapram group after adjusting for confounding factors (OR = 0.54, 95% CI: 0.37, 0.78). Doxapram-treated infants had a higher risk of bronchopulmonary dysplasia and patent ductus arteriosus, but a lower risk of spontaneous intestinal perforation. All other morbidities were not different between the groups.

CONCLUSIONS

This study suggests that doxapram is not associated with an increased risk of ND. These findings need to be confirmed or refuted by a large, well-designed, placebo-controlled randomized trial.

Amplitude-integrated electroencephalography shows that doxapram influences the brain activity of preterm infants

AIM

The aim of this study was to measure the brain activity of preterm infants treated with caffeine citrate and doxapram for preterm apnoea, using amplitude-integrated electroencephalography (aEEG), to identify any adverse effects on cerebral function.

METHODS

We analysed the aEEG tracings of 13 preterm infants <30 weeks of gestation before, during and after doxapram treatment, with regard to background activity (percentages of continuous and discontinuous patterns), occurrence of sleep-wake cycling and appearance of electrographic seizure activity. They were also compared with 61 controls without doxapram treatment.

RESULTS

During doxapram treatment, aEEG tracings showed an increase in continuous background activity (19 ± 30% before treatment, 38 ± 35% during treatment) and a decrease in discontinuous patterns. In addition, they showed more frequent electrographic seizure activity (0% before treatment, 15 ± 37% during treatment) and less frequent sleep-wake cycling (92 ± 27% before treatment, 85 ± 37% during treatment) could be observed. These results were confirmed when compared to the control group.

CONCLUSION

Doxapram treatment influences aEEG in preterm infants, showing higher percentages of continuous activity as well as more electrographic seizure activity and less sleep-wake cycling. It should, therefore, be used with caution in very preterm infants.

Doxapram and hypokalaemia in very preterm infants

BACKGROUND

We observed two preterm infants who developed severe hypokalaemia following doxapram. We therefore wished to review the possible association between doxapram and severe hypokalaemia.

STUDY DESIGN

A retrospective study of preterm infants born before 32 weeks of gestation and hospitalised in our intensive care unit in 2004. For each infant, treatment with doxapram or with any drug known to interfere with potassium metabolism, potassium intakes and episodes of hypokalaemia have been recorded.

RESULTS

Out of 105 infants, 54 received doxapram. Doxapram-treated infants were significantly younger and had a lower birth weight. Doxapram treated infants were more likely to receive caffeine, furosemide, insulin and mechanical ventilation. There was no difference between the two groups for the other parameters. Hypokalaemia was frequently encountered in our population since it occurred in 76% of enrolled patients and severe hypokalaemia (potassium plasma level below 3 mmol/l) was found in 41%. Bivariate analysis underlined several risk factors for severe hypokalaemia: use of doxapram, gestational age below 28 weeks, use of mechanical ventilation, furosemide, ibuprofen, insulin and postnatal corticosteroids. Cox model's multivariate analysis showed that administration of furosemide and doxapram significantly increased the occurrence of severe hypokalaemia with relative risks of 4.9 (95% CI 1.9 to 12.5) and 8.2 (95% CI 3.1 to 21.7), respectively.

CONCLUSIONS

This retrospective study underlines the high incidence of severe hypokalaemia in very preterm infants and an increased risk of severe hypokalaemia during doxapram treatment. We recommend potassium monitoring during any use of doxapram.

Apnea of prematurity: pathogenesis and management strategies

Apnea of prematurity (AOP) is a significant clinical problem manifested by an unstable respiratory rhythm reflecting the immaturity of respiratory control systems. This review will address the pathogenesis of and treatment strategies for AOP. Although the neuronal mechanisms leading to apnea are still not well understood, recent decades have provided better insight into the generation of the respiratory rhythm and its modulation in the neonate. Ventilatory responses to hypoxia and hypercarbia are impaired and inhibitory reflexes are exaggerated in the neonate. These unique vulnerabilities predispose the neonate to the development of apnea. Treatment strategies attempt to stabilize the respiratory rhythm. Caffeine remains the primary pharmacological treatment modality and is presumed to work through blockade of adenosine receptors A(1) and A(2). Recent evidences suggest that A(2A) receptors may have a greater role than previously thought. AOP typically resolves with maturation suggesting increased myelination of the brainstem.

Dynamic QT/RR relationship of cardiac conduction in premature infants treated with low-dose doxapram hydrochloride

Doxapram hydrochloride, a respiratory stimulant, has several undesirable side effects during high-dose administration, including second-degree atrioventricular (AV) block and QT prolongation. In Japan, this drug is contraindicated for newborn infants. Recent studies, however, have demonstrated the efficacy and safety of doxapram therapy for apnea of prematurity (AOP) using lower doses than those previously tested. As a result, approximately 60% of Japanese neonatologists continue to use this drug. This study used surface ECG recordings to assess the cardiac safety of low-dose doxapram hydrochloride (0.2 mg/kg/h) in fifteen premature very-low-birth-weight infants with idiopathic AOP. Cardiac intervals and number of apnea episodes were compared before and after drug administration. Low-dose doxapram hydrochloride resulted in approximately 90% reduction in the frequency of apnea without side effects. None of the infants developed QT or PR prolongation, arrhythmia, or other conduction disorders. In addition, there was no change in the slope of QT/RR before versus after administration of doxapram hydrochloride. We conclude that low-dose administration of doxapram hydrochloride did not have any undesirable effects on myocardial depolarization and repolarization.

[Sensory stimulations for the treatment of idiopathic apneas of prematurity]

Idiopathic apneas of prematurity remain an alarming problem in neonatology. Long-term effects on neurological development cannot be excluded. Two principal treatments are currently applied to prevent apneic attacks: analeptics (methylxanthines and doxapram) and continuous positive airway pressure. Several studies suggest the use of sensory stimulations for curative or preventive treatments. Protocols of tactile, kinesthetic and auditory stimulations are described and their respective efficacy is discussed. We also resume a recent preliminary study which suggested the use of an olfactory stimulation as preventive treatment. A beneficial effect, specifically on apnea associated with severe bradycardia, was observed. Complementary studies are necessary to confirm the benefit of such sensory stimulations for apneas of prematurity, and, if so, to allow clinical applications.

Brain Hemodynamic Effects of Doxapram in Preterm Infants

BACKGROUND

Doxapram is a respiratory stimulant widely used for the treatment of idiopathic apnea of prematurity, although it has been demonstrated that it can induce a transient decrease of cerebral blood flow and that isolated mental delay in infants weighing <1,250 g is associated with the total dosage and duration of doxapram therapy.

OBJECTIVES

To evaluate the effects of doxapram on cerebral hemodynamics in preterm infants using cerebral Doppler ultrasonography and near-infrared spectroscopy.

METHODS

Preterm infants who required treatment with doxapram for apnea of prematurity unresponsive to caffeine were treated with doxapram at an hourly dose of 0.5 mg x kg(-1).h(-1), followed by 1.5 and 2.5 mg x kg(-1).h(-1).

RESULTS

20 preterm infants were studied. Doxapram induced a significant decrease of oxygenated hemoglobin (O(2)Hb) and cerebral intravascular oxygenation (HbD = O(2)Hb - HHb) and an increase of HHb and CtOx concentrations, while cerebral blood volume and cerebral blood flow velocity did not change.

CONCLUSIONS

Doxapram infusion induces the increase of cerebral oxygen consumption and requirement and the contemporary decrease of oxygen delivery probably mediated by a decrease of cerebral blood flow. Caution must be recommended in prescribing this drug for apnea of prematurity.

Doxapram inhibits carotid sinus baroreceptors in rabbits

1 To clarify the effects of doxapram on the baroreflex, we recorded carotid sinus nerve (CSN) activity in isolated and perfused carotid artery bifurcations of rabbits. 2 The CSN activity due to chemoreceptor stimulation was blocked by resection of the nerve branches from the carotid body. After the resection, the CSN activity was correlated to increase of carotid sinus (CS) pressure. 3 Administration of doxapram reduced the CSN activity originating from baroreceptors. The effect of doxapram on baroreceptors was dose dependent and reversible. 4 It is unlikely that doxapram altered CS wall mechanics because CS pressure did not change in the presence of the drug. 5 We conclude that doxapram acts on the cardiovascular system in part by inhibiting the negative feedback loop that originates in CS baroreceptors.

Olfactory stimulation prevents apnea in premature newborns

OBJECTIVE

Methylxanthines and doxapram are currently used to treat apnea of prematurity but are not fully effective and often present undesirable side effects. The present study examines whether exposure to an odor known to modulate the infant's respiratory rate could reduce the frequency of apneic spells.

METHOD

Fourteen preterm newborns born at 24 to 28 gestational weeks presenting recurrent apnea despite caffeine and doxapram therapy were exposed to a pleasant odor diffused during 24 hours in the incubator. Efficiency of the olfactory treatment was judged by comparing frequency and severity of apneas occurring during the day of odorization with that observed the day before (baseline) and the day after (posttreatment control). Apnea was defined as any complete cessation of breathing movements for >20 seconds, or less if associated with hypoxia or bradycardia.

RESULTS

Concerning all types of apneas, a diminution of 36% was observed and seen in 12 of 14 infants. Apneas without bradycardia were reduced (44%) during the day with odorization, and this diminution affected all the infants. The frequency of apnea with moderate bradycardia (heart rate between 70 and 90 beats per minute) was maintained while the frequency of apnea associated with severe bradycardia (heart rate <70 beats per minute) decreased strongly (45%) and affected all the infants. No side effects were observed.

CONCLUSION

The introduction of a pleasant odor in the incubator is of therapeutic value in the treatment of apneas unresponsive to caffeine and doxapram.

Doxapram treatment for apnea in preterm infants

BACKGROUND

Recurrent apnea is common in preterm infants, particularly at very early gestational ages. These episodes of loss of effective breathing can lead to hypoxemia and bradycardia which may be severe enough to require resuscitation including use of positive pressure ventilation. Doxapram has been used to stimulate breathing and so prevent apnea and its consequences.

OBJECTIVES

In preterm infants with recurrent apnea, does treatment with Doxapram lead to a clinically important reduction in apnea and use of intermittent positive airways pressure (IPPV), without clinically important side effects?

SEARCH STRATEGY

Searches were made of the Oxford Database of Perinatal trials, the Cochrane Central Register of Controlled Trials (CENTRAL, The Cochrane Library, Issue 3, 2004), MEDLINE from 1966 - June 2004, EMBASE from 1980 - June 2001, CINAHL from 1982- June 2004. Text words 'doxapram', 'apnea or apnoea' and the MeSH term 'infant, premature' were used. Previous reviews including cross references, abstracts from conferences and symposia proceedings were also examined. Abstracts of the Society for Pediatric Research were searched from 1996 - 2004 inclusive.

SELECTION CRITERIA

All trials utilising random or quasi-random patient allocation, in which doxapram was used for the treatment of apnea in preterm infants were included.

DATA COLLECTION AND ANALYSIS

Each author evaluated the papers for quality and inclusion criteria. Independent data extraction was carried out.

MAIN RESULTS

Only one trial, which randomized 11 infants to intravenous doxapram and 10 infants to placebo, was found. There were fewer treatment failures after 48 hours in the group of preterm infants treated with doxapram (4/11) compared with the group treated with placebo (8/10). The wide confidence intervals made this result non-significant [RR 0.45 (0.20, 1.05)]. Only one infant, who was from the placebo group, was given IPPV. Of the seven responders by 48 hours in the group of 11 who received doxapram, five failed to respond between 48 hours and seven days after commencement of therapy. This gives a late failure rate of 9/11, similar to the short term failure rate in the placebo group of 8/10. It is not possible to evaluate the late responses of all those in the placebo group since they crossed over to a treatment arm.

REVIEWERS' CONCLUSIONS

Although intravenous Doxapram might reduce apnea within the first 48 hours of treatment, there are insufficient data to evaluate the precision of this result or to assess potential adverse effects. No long term outcomes have been measured. Further studies are needed to determine the role of this treatment in clinical practice.

Treatment of apnea of prematurity

In the last decade, knowledge regarding the neurodevelopment and functional aspects of the respiratory centers during postnatal maturation has increased substantially. However, an increase in such knowledge has not provided a basis for change in practice. The diagnosis of apnea of prematurity (AOP) is one of exclusion. All causes of secondary apnea must be ruled out before initiating treatment for AOP. Treatment will depend on the etiology as well as effectiveness and tolerability of the treatment by the patient. The primary goal of any treatment of AOP is to prevent the frequency of apnea lasting >20 seconds, and/or those that are shorter, but associated with cyanosis and bradycardia. The clinical management of AOP is not much different today than it was two decades ago, with pharmacologic and nonpharmacologic treatment options remaining the mainstay of therapy. Methylxanthines are still the most widely used pharmacologic agents. Due to the wider therapeutic index of caffeine and ease of once daily administration, it should be the preferred agent. Doxapram, or nonpharmacologic treatment measures such as nasal continuous positive airway pressure, may be considered in infants who are unresponsive to methylxanthine treatment alone. Treatment should be continued until there is complete resolution of apnea, and for some time thereafter. The choice of method for weaning treatment remains one of individual physician preference. Discharge from hospital after apnea requires close monitoring and some infants will require home apnea monitors. The decision to provide a home apnea monitor should be individualized for each patient, depending on the effectiveness of treatment and clinical response.

Low-dose doxapram therapy for idiopathic apnea of prematurity

BACKGROUND

Doxapram is contraindicated for newborn infants in Japan because of its serious side effects. However, because of encouraging results of recent studies regarding the efficacy and safety of therapy for apnea of prematurity (AOP) with lower doses of doxapram than those previously proposed, approximately 60% of Japanese neonatologists continue to use doxapram at small doses. Caution is warranted because the sample sizes of the former studies are inadequate to evaluate doxapram for both its beneficial and harmful effects. Therefore, we conducted the present study in order to investigate the efficacy and harmful events of low-dose doxapram therapy for idiopathic AOP in very low-birth weight (VLBW) infants in a larger population.

METHODS

One hundred and six VLBW infants with idiopathic AOP were treated with doxapram at a dose of 0.2-1.0 mg/kg per h in combination with methylxanthines and the frequency of apnea and secondary outcomes were compared with a group of control infants.

RESULTS

An approximate 80% reduction in the frequency of apnea was found with only minimal side effects following low-dose doxapram. Although there were no significant differences in secondary outcomes between the doxapram-treated and control groups, mortality in doxapram-treated infants was significantly lower than that in control infants.

CONCLUSIONS

Patients with AOP unresponsive to treatment with methylxanthines may benefit from the addition of low-dose doxapram.

Risks and benefits of therapies for apnoea in premature infants

Apnoea in infants can result from a wide range of causes, and requires thorough evaluation before deciding on appropriate treatment. Continuous monitoring of premature infants with apnoea is mandatory in order to define the pathophysiology and type of apnoea; selection of treatment involves careful assessment of aetiology, as well as efficacy and tolerability in each individual case. The objective of treatment is to prevent the deleterious consequences of apnoeas that last >20 seconds and/or are associated with bradycardia, cyanosis or pallor, and occur more often than once an hour over a 12-hour period. Apnoea management involves both pharmacological and nonpharmacological treatment. We suggest methylxanthines as first-line therapy for idiopathic apnoeas; evidence suggests that caffeine is better tolerated and as efficacious as theophylline (since it is particularly efficacious against the 'central' component of idiopathic apnoea of prematurity). If treatment fails, additional measures such as doxapram may be appropriate when hypoventilation is present, or nasal continuous positive airway pressure when upper airway instability or obstructive apnoeas are predominant. Apnoea prophylaxis is an additional reason to advocate prenatal maturation with betamethasone. Weaning from treatment is attempted 4 to 5 days after complete resolution of apnoea, beginning with the last treatment introduced. Monitoring should be maintained for 4 to 5 days to detect any relapse of recurrent and severe apnoeas, which would lead to the resumption of the most recently withdrawn treatment.

Prophylactic doxapram for the prevention of morbidity and mortality in preterm infants undergoing endotracheal extubation

BACKGROUND

When preterm infants have been given intermittent positive pressure ventilation (IPPV) for respiratory failure, weaning from support and tracheal extubation may be difficult. A significant contributing factor is thought to be the relatively poor respiratory effort and tendency to develop hypoventilation and apnea, particularly in very preterm infants. Doxapram stimulates breathing and appears to act via stimulation of both the peripheral chemoreceptors and the central nervous system. This effect might increase the chance of successful tracheal extubation.

OBJECTIVES

In preterm infants being weaned from IPPV and in whom endotracheal extubation is planned, does treatment with doxapram reduce the use of intubation and IPPV, or reduce other morbidity, without clinically important side effects? In this regard, how does doxapram compare with standard treatment or with an alternative treatment such as methylxanthine or CPAP? Subgroup analyses were prespecified according to birth weight and/or gestational age, use of co-interventions (methylxanthines or nasal CPAP), and route of administration (intravenous or oral).

SEARCH STRATEGY

The standard search strategy of the Neonatal Review Group as outlined in the Cochrane Library was used. This included searches of the Oxford Database of Perinatal Trials, Cochrane Controlled Trials Register, MEDLINE and EMBASE.

SELECTION CRITERIA

Eligible studies included published trials utilising random or quasi-random patient allocation in which preterm or low birth weight infants being weaned from IPPV were given doxapram compared with standard care or other treatments, to facilitate weaning from IPPV and endotracheal extubation. Trials were independently assessed by the authors before inclusion.

DATA COLLECTION AND ANALYSIS

The standard methods of the Cochrane Collaboration and its Neonatal Review Group were used. Each author extracted data separately; the results were compared and any differences resolved. The data were synthesized using the standard method of Neonatal Review Group with use of relative risk and risk difference.

MAIN RESULTS

Two trials involving a total of 85 infants compared doxapram and placebo. In both the individual trials and the meta-analyses there were no significant differences between the doxapram and placebo groups in any of the outcomes (failed extubation, death before discharge, respiratory failure, duration of IPPV, side effects, oxygen at 28 days or oxygen at discharge). There was a trend towards an increase in side effects (hypertension or irritability leading to cessation of treatment) in the doxapram group [summary RR 3.21 (0.53, 19.43). In one of these two trials (Huon 1998) an 'alarming rise in blood pressure' occurred in five infants in the doxapram group and none of the controls, although in only one was treatment withdrawn. One additional trial involving only eight infants compared doxapram with aminophylline, but there were insufficient data for meaningful analysis.

REVIEWER'S CONCLUSIONS

The evidence does not support the routine use of doxapram to assist endotracheal extubation in preterm infants who are eligible for methylxanthine and/or CPAP. The results should be interpreted with caution because the small number of infants studied does not allow reliable assessment of the benefits and harms of doxapram. Further trials are required to evaluate the benefits and harms of doxapram compared with no treatment or with other treatments, such as methylxanthines or CPAP, to evaluate whether it is more effective in infants not responding to these other treatments, and to assess whether the drug is effective when given orally.

Doxapram treatment for apnea in preterm infants

BACKGROUND

Recurrent apnea is common in preterm infants, particularly at very early gestational ages. These episodes of loss of effective breathing can lead to hypoxemia and bradycardia which may be severe enough to require resuscitation including use of positive pressure ventilation. Doxapram has been used to stimulate breathing and so prevent apnea and its consequences.

OBJECTIVES

In preterm infants with recurrent apnea, does treatment with doxapram lead to a clinically important reduction in apnea and use of Intermittent positive airways pressure (IPPV), without clinically important side effects?

SEARCH STRATEGY

Searches were made of the Oxford Database of Perinatal trials, the Cochrane Collaboration Clinical Trials Register, MEDLINE (using text words 'doxapram', 'apnea or apnoea' and Mesh term 'infant, premature') previous reviews including cross references, abstracts, conferences and symposia proceedings, expert informants, mainly in the English language. Also an expert informant's search in the Japanese language was made by Prof. Y. Ogawa in 1996.

SELECTION CRITERIA

All trials utilising random or quasi-random patient allocation, in which doxapram was used for the treatment of apnea in preterm infants were included.

DATA COLLECTION AND ANALYSIS

Each author evaluated the papers for quality and inclusion criteria. Independent data extraction was carried out.

MAIN RESULTS

Only one trial, which randomized 11 infants to intravenous doxapram and 10 infants to placebo, was found. There were fewer treatment failures after 48 hours in the group of preterm infants treated with doxapram (4/11) compared with the group treated with placebo (8/10). The wide confidence intervals made this result non-significant [RR 0.45 (0.20, 1.05)]. Only one infant, who was from the placebo group, was given IPPV. Of the seven responders by 48 hours in the group of 11 who received doxapram, five failed to respond between 48 hours and seven days after commencement of therapy. This gives a late failure rate of 9/11, similar to the short term failure rate in the placebo group of 8/10. It is not possible to evaluate the late responses of all those in the placebo group since they crossed over to a treatment arm.

REVIEWER'S CONCLUSIONS

Although intravenous doxapram might reduce apnea within the first 48 hours of treatment, there are insufficient data to evaluate the precision of this result or to assess potential adverse effects. No longterm outcomes have been measured. Further studies are needed to determine the role of this treatment in clinical practice.

Second-degree atrioventricular heart block after doxapram administration

Doxapram, a respiratory stimulant, is used to treat idiopathic apnea of prematurity. The side effects reported are minimal. We present three cases of second-degree atrioventricular block caused by QT interval prolongation associated with doxapram administration. All three infants returned to normal sinus rhythm after doxapram administration was stopped.

Drug therapy in the neonatal foal

The neonatal period in foals refers to the first 7 days of postnatal life. The effects of drugs (pharmacologic agents) may be different in neonatal foals, particularly during the first 3 days of postnatal life, from those in older foals and adult horses. The changed drug effects decrease as the physiologic processes that affect absorption, distribution, and elimination (metabolism and excretion) of drugs mature. Dosage regimens should take into account the altered pharmacokinetic profiles of drugs, and because of wide individual variation, the response to therapy should be closely monitored for signs of toxicity. In conjunction with the prudent use of drugs, good nursing care and the provision of supportive therapy are critical in the management of neonatal foal diseases. Over-crowding imposes stress upon young foals and predisposes them to an increased incidence of bacterial and parasitic infections. The collection of specimens for precise microbiologic diagnosis and correction of deficits in serum immunoglobulins should precede antimicrobial therapy. Although E. coli is by far the most common cause of bacterial infections in neonatal foals, other bacterial pathogens of unpredictable susceptibility often cause infection. The selection of an antimicrobial drug for specific therapy should be based on both the microbiologic (quantitative susceptibility) and pharmacologic (pharmacokinetic) properties of the drug. The use of an antimicrobial drug or combination of drugs that will produce a bactericidal effect is highly desirable. Whenever possible, a parenteral preparation that can be administered intravenously should be chosen. The bioavailability and selectivity of action of pharmacologic agents are influenced by the dosage form and route of administration. Diazepam is the sedative drug of choice for neonatal foals. Cimetidine, an H2-receptor antagonist, may be indicated in foals diagnosed to have gastric ulcers; hepatic microsomal oxidative metabolism of drugs administered concurrently with cimetidine is decreased. Nonsteroidal anti-inflammatory drugs (flunixin, phenylbutazone) have a higher incidence of toxicity in foals and, when indicated, should be used at lower dosage than in adult horses. Even though it is highly important to maintain hydration status and electrolyte balance, intravenous infusion should always be performed slowly. Immature renal function decreases the ability of the neonatal animal to excrete excess fluid. The use of drugs in neonatal foals requires greater precision in dosage, more attention to the route and rate of administration, and close monitoring of pharmacologic effects.

Obstructive, mixed, and central apnea in the neonate: physiologic correlates

In an attempt to determine physiologic responses to neonatal apnea, we evaluated changes in heart rate and oxygen saturation as measured by pulse oximetry during 2082 episodes of apnea lasting 15 seconds or more in 47 infants less than 34 weeks of gestational age with idiopathic apnea of prematurity. Of these episodes, 832 (39.9%) were central, 1032 (49.6%) were mixed, and 218 (10.5%) were obstructive. Oxygen saturation decreased with increasing duration of apnea regardless of type or treatment, and the decrease in saturation was correlated with preapnea saturation. The baseline heart rate was similar for all apnea types. Infants receiving doxapram had a lower baseline heart rate (137.8 +/- 10.5 beats/min) than did infants receiving no therapy (142.8 +/- 16.6 beats/min) and infants receiving theophylline (149.7 +/- 15.0 beats/min) (p = < 0.001). A heart rate fall to less than 100 beats/min was seen more frequently with central apnea than with mixed or obstructive events, and in infants who were not receiving therapy. Falls in heart rate were significantly less in infants receiving doxapram (27.8% +/- 18.0%) than in infants receiving theophylline (44.5% +/- 19.0%) or no therapy (48.4% +/- 18.3%) (p = < 0.001). The most common heart rate pattern overall was a gradual decrease interrupted by accelerations, whereas an initial heart rate acceleration was the most common pattern in obstructive apnea. We conclude that heart rate response to neonatal apnea is a complex and is dependent on therapy and on type and duration of apnea.

Ventilatory response to keto-doxapram in intact and carotid body denervated lambs

We aimed to investigate the role of the carotid bodies in the ventilatory response to keto-doxapram, and whether this response was dose-dependent; we studied two group (n = 5 per group) of awake, intact and carotid-body denervated (CBD) lambs, aged 10-15 days. At 20 min intervals, they received 0.5, 1.0, 2.5 and 5.0 m g/kg mg/kg of keto-doxapram as an intravenous bolus infusion. Plasma keto-doxapram was measured (HPLC). Ventilation was recorded via a face mask and a pneumotachograph. In intact lambs, an immediate dose-dependent increase in minute ventilation (V1) was observed. At 2 min, VI increased from baseline by 125 +/- 28, 212 +/- 49, 378 +/- 41 and 637 +/- 92 mol.kg 1.min 1 (mean +/- SE, P less than 0.01) corresponding to the foregoing incremental doses. A significant correlation was observed between the peak VI and the corresponding plasma keto-doxapram concentrations (r = 0.73, P less than 0.0003). In CBD lambs, VI increased significantly less than in intact animals. In conclusion, early ventilatory response to keto-doxapram depends mainly on intact carotid bodies, and the effect is dose-dependent.

Low-dose doxapram therapy in premature infants and its CSF and serum concentrations

The efficacy of low-dose doxapram therapy (0.2 mg/kg/h) in combination with methylxanthines was evaluated in 20 premature infants with idiopathic apnea unresponsive to methylxanthines alone, and in 13 premature infants with secondary apnea. The serum concentrations of doxapram and, in some infants, the simultaneous cerebrospinal fluid and serum concentrations were measured, and the correlation between cerebrospinal fluid and serum concentrations in the postnatal period was determined. The following results were obtained: 1) In idiopathic apnea of prematurity, low-dose doxapram therapy was as effective as a dose of 1.0-2.5 mg/kg/h and the side effects were few, mild, and reversible. 2) In premature infants over seven days of age, serum concentrations of doxapram were almost stable but were significantly lower than in infants within the first six days of life. 3) The ratio of the cerebrospinal fluid to serum doxapram concentration was 0.48 +/- 0.13 (mean +/- SD). There was a good correlation between cerebrospinal fluid and serum concentrations (r = 0.933, p less than 0.001). The initial doxapram dose can be set as low as 0.2 mg/kg/h in very young premature infants with idiopathic apnea of prematurity unresponsive to methylxanthines.

Respiratory control and stimulation of spontaneous breathing

Respiratory movements are controlled by two pairs of neuronal groups in the brain stem: nucl. tractus solitarius and nucl. paraambigualis. These neuronal pools receive drive inputs from the forebrain and hypothalamus, and from central and peripheral chemoreceptors (9). Whether there is endogenous spontaneous respiratory rhythmic activity has been a matter of controversy, but this has recently been discovered in brainstem preparations of the neonatal rat in studies by Onimaru and Homma (27).

A blinded, randomized, placebo-controlled trial to compare theophylline and doxapram for the treatment of apnea of prematurity

A blinded, randomized, placebo-controlled trial was conducted to evaluate the effectiveness of theophylline and doxapram therapy in 31 infants with significant apnea of prematurity. Of 10 infants, two had a short-term response to placebo, 8 of 10 infants to theophylline, and 7 of 11 infants to doxapram (placebo vs treatment with theophylline or doxapram: p = 0.01). The two infants who initially responded to placebo remained responsive for the duration of the study. Of the eight infants in whom treatment with placebo failed, five were randomly assigned to receive theophylline, for a total of 15 infants treated with theophylline, and two of the eight were randomly assigned to receive doxapram, for a total of 13 infants treated with doxapram; the remaining infant required tracheal intubation. Of the 15 infants randomly assigned to receive theophylline, seven responded for the duration of the study; of the eight infants who did not respond to treatment with theophylline, five responded to doxapram, one responded to a combination of theophylline and doxapram, and two remained resistant to treatment. Of the 13 infants randomly assigned to receive doxapram four responded for the duration of the study; of the nine who did not respond to doxapram, seven responded to theophylline, one responded to a combination of theophylline and doxapram, and one remained resistant to treatment. This study demonstrates that although therapy with theophylline or doxapram is associated with a significant short-term reduction in the incidence of apnea compared with that in placebo-treated infants, the long-term response to treatment is frequently incomplete and is not sustained more than 1 week.

Effects of caffeine and doxapram perfusion on local cerebral glucose utilization in conscious rats

The quantitative autoradiographic 2-[14C]deoxyglucose method was used to measure the effects of a continuous infusion of the respiratory stimulants, caffeine or doxapram, 18 mg/kg per h, on local cerebral glucose utilization in the adult male rat. Local cerebral glucose utilization was measured in 54 cerebral structures from different systems. Caffeine induced widespread increases in energy metabolism, resulting in a significant increase in glucose utilization in 25 structures out of the 54 studied. These increases were distributed within all systems studied, sensory, extrapyramidal motor, limbic and hypothalamic systems. In addition, caffeine induced a non-significant, 10-15%, increase in local cerebral glucose utilization in central respiratory areas. Doxapram infusion did not change the rates of glucose utilization in any of the structures. The rates of local cerebral glucose utilization were significantly lower after doxapram than after caffeine exposure in five cerebral areas, among which were three central respiratory areas. The results confirm the absence of side-effects of doxapram as compared to caffeine when used as respiratory stimulant, especially in neonates. These results also favor a preferentially central action of caffeine on respiratory areas and a more peripheral action of doxapram on chemoreceptors, at least at therapeutic levels.

Patient-controlled analgesia with a mixture of pethidine and doxapram hydrochloride. A comparison of the incidence of respiratory dysrhythmias with pethidine alone

Twenty-four patients who underwent elective cholecystectomy received double-blind increments of either pethidine 30 mg or a mixture of pethidine 30 mg and doxapram 45 mg delivered on a patient-controlled basis. A loading dose of doxapram 100 mg or saline was administered. There was no difference in respiratory depression as indicated by respiratory frequency and end tidal carbon dioxide concentration. The incidence of respiratory apnoea was similar in the two groups but apnoea was of shorter duration in patients who received doxapram, although the difference was not significant. Patients who received doxapram assessed overall pain as worse on a linear analogue scale (p less than 0.05) but demanded similar amounts of pethidine. There was no difference in pain on movement.

Serum doxapram and respiratory neuromuscular drive in normal man

To investigate the means by which doxapram affects the control of ventilation, ventilatory function and P0.1 have been related to serum doxapram concentration during a 45-min infusion of doxapram hydrochloride in 7 healthy, conscious subjects under normoxic conditions. Serum doxapram concentrations increased during the infusion: 1.88, 2.48, 3.42, and 3.97 micrograms/ml after 5, 10, 30 and 45 min, respectively. The majority of significant changes in the measurements from the baseline were observed at 30 and 45 min: VE, VT, P0.1, P0.1/end-tidal CO2 tension, VT/Ti and blood pressure were increased, and end-tidal CO2 tension was decreased. No significant changes in Pdimax, Ti/Ttot, VE/P0.1, and P0.1/(VT/Ti) were observed. A correlation was observed between the % increases in P0.1 and VE and doxapram concentration, and between VE and P0.1. The doxapram-induced increase in VE appears to be caused by increased neural drive. It is related to the serum drug concentration in the conscious subject.

The dose response of theophylline in the treatment of apnea of prematurity

In an effort to establish the minimum effective dose of theophylline in the treatment of idiopathic apnea of prematurity, a prospective trial of 22 infants with at least 0.33 episodes of apnea per hour were studied. Apnea was diagnosed exclusively by continuous recording of heart rate, respiratory impedance, end-tidal CO2, and either or both transcutaneous oxygen and pulse oximetry. Four discrete serum concentrations of theophylline (23 mumol/l or 4.2 mg/L, 47 mumol/L or 8.5 mg/L, 70 mumol/L or 12.7 mg/L, and 84 mumol/L or 15.3 mg/L) were attained by using repeated loading doses of 4 mg/kg and increasing the maintenance dose from 1 to 1.5 mg/kg to 2 to 2.5 mg/kg, given every 8 hours. Before treatment and 24 hours after each loading dose, airway occlusions and measures of tidal volume, minute ventilation, and respiratory timing were performed. The effectiveness of therapy was assessed by either a continuous computer data-acquisition system or paper recording for the duration of the study. Of the 22 infants, three responded at level 1, three at level 2, and 10 at level 3. One of the four infants loaded to the fourth level had a sustained response for a total cumulative response of 77%. The five remaining infants required additional treatment with doxapram or continuous positive airway pressure. There was a significant increase in inspiratory pressure 100 msec after airway occlusion, maximum inspiratory pressure during airway occlusion, tidal volume, ratio of tidal volume to inspiratory time (mean inspiratory flow), and minute ventilation from the pretreatment measurements to those at the maximum dose of theophylline. The apnea response did not correlate with these improvements in ventilation measures.