Short-term rhythms of the cardiorespiratory system and their significance in neonatology

Relaxation Rate of the Respiratory Muscles and Prediction of Extubation Outcome in Prematurely Born Infants

BACKGROUND

Accurate prediction of extubation outcome could result in a significant reduction of respiratory morbidity in premature neonates.

OBJECTIVES

To assess whether the respiratory muscle time constant of relaxation (τ) predicted extubation outcome in mechanically ventilated, premature infants.

METHODS

Forty-six mechanically ventilated infants with a median gestational age of 26 (interquartile range [IQR] 25-29) weeks were prospectively studied. τ was calculated from the reciprocal of the slope of the decline in airway pressure as a function of time. Measurements of τ were done during 5-10 min of a spontaneous breathing test (SBT) prior to extubation. During the first and last minute of the SBT, τ1 and τ2, respectively, were assessed, and the difference between them was calculated (Δτ).

RESULTS

The median τ2 was significantly higher in infants whose extubation failed (20.7 [IQR 12.9-34.7] s/cm H2O) than in infants whose extubation succeeded (8.2 [IQR 6.2-17.8] s/cm H2O, p = 0.002). The median Δτ was significantly higher in infants whose extubation failed (10.3 [IQR 4.4-23.9] s/cm H2O) than in infants whose extubation succeeded (-1.63 [IQR -5.7 to 0.3] s/cm H2O, p = 0.001). Extubation failure was associated with τ2 (p = 0.011) and Δτ (p = 0.010) after correcting for postmenstrual age, patent ductus arteriosus, and intraventricular haemorrhage. Receiver operator characteristic curve analysis demonstrated that Δτ predicted extubation failure with an area under the curve of 0.937. A Δτ of +1.02 s/cm H2O predicted extubation failure with 94% sensitivity and 83% specificity.

CONCLUSIONS

The respiratory muscle time constant of relaxation during an SBT was significantly greater in infants whose extubation failed and could be used to predict extubation outcome in prematurely born infants.

Episodic hormone secretion: a comparison of the basis of pulsatile secretion of insulin and GnRH

Rhythms govern many endocrine functions. Examples of such rhythmic systems include the insulin-secreting pancreatic beta-cell, which regulates blood glucose, and the gonadotropin-releasing hormone (GnRH) neuron, which governs reproductive function. Although serving very different functions within the body, these cell types share many important features. Both GnRH neurons and beta-cells, for instance, are hypothesized to generate at least two rhythms endogenously: (1) a burst firing electrical rhythm and (2) a slower rhythm involving metabolic or other intracellular processes. This review discusses the importance of hormone rhythms to both physiology and disease and compares and contrasts the rhythms generated by each system.

Does air pollution trigger infant mortality in Western Europe? A case-crossover study

BACKGROUND

Numerous studies show associations between fine particulate air pollutants [particulate matter with an aerodynamic diameter ≤ 10 μm (PM₁₀)] and mortality in adults.

OBJECTIVES

We investigated short-term effects of elevated PM₁₀ levels on infant mortality in Flanders, Belgium, and studied whether the European Union (EU) limit value protects infants from the air pollution trigger.

METHODS

In a case-crossover analysis, we estimated the risk of dying from nontraumatic causes before 1 year of age in relation to outdoor PM₁₀ concentrations on the day of death. We matched control days on temperature to exclude confounding by variations in daily temperature.

RESULTS

During the study period (1998-2006), PM₁₀ concentration averaged 31.9 ± 13.8 μg/m³. In the entire study population (n = 2,382), the risk of death increased by 4% [95% confidence interval (CI), 0-8%; p = 0.045] for a 10-μg/m³ increase in daily mean PM₁₀. However, this association was significant only for late neonates (2-4 weeks of age; n = 372), in whom the risk of death increased by 11% (95% CI, 1-22%; p = 0.028) per 10-μg/m³ increase in PM₁₀. In this age class, infants were 1.74 (95% CI, 1.18-2.58; p = 0.006) times more likely to die on days with a mean PM₁₀ above the EU limit value of 50 μg/m3 than on days below this cutoff.

CONCLUSIONS

Even in an affluent region in Western Europe, where infant mortality is low, days with higher PM air pollution are associated with an increased risk of infant mortality. Assuming causality, the current EU limit value for PM₁₀, which may be exceeded on 35 days/year, does not prevent PM₁₀ from triggering mortality in late neonates.

Maturation of the autonomic nervous system: differences in heart rate variability in premature vs. term infants

AIMS

Heart rate variability (HRV) reveals information on the functional state of the autonomic nervous system (ANS). This study was initiated to assess the physiological- and maturational development of the ANS by comparing HRV data of healthy prematures with term infants.

METHODS

Short-term recordings of HRV in 39 premature healthy infants (29-35 weeks' gestation) were performed and compared with normative data of term infants. Frequency domain HRV parameters are computed in three frequency bands.

RESULTS

Gestational age of newborn infants is correlated with HRV. Prematures showed significant lower HRV parameters than term infants. The most significant differences were discerned for HRV parameters reflecting parasympathetic activity, whereas the pulse of the baro-receptor reflex was similar for prematures and term infants at 0.07 Hz (0.1 Hz adults). A respiratory peak in the HF-band according to respiratory sinus arrhythmia (RSA), as is common in adults, was not detected for prematures as it is not detected in term infants.

CONCLUSIONS

Maturation of the ANS is accompanied by increasing HRV with a pronounced increase of parasympathetic activity. These changes are measurable by short-term recordings. The physiological pulses concerning baroreceptor reflex activity and respiratory modulation were similar in prematures and term infants.

Gap analysis of pediatric reference intervals related to thyroid hormones and the growth hormone–insulin growth factor axis

Efficacy of laboratory medicine in assisting attending physicians in their diagnostic and follow-up endeavors is intimately linked to an access to meaningful and reliable reference values. Pediatrics is particularly sensitive to this problem as the processes, associated with growth and development, are imposing rapid discontinuous changes on the physiology of the individuals. Some developmental stages are more critical than others. The neonatal and the pubertal periods, for which we lack reference ranges, are two such examples. Beyond biological considerations, we realize that, over the last 2 decades, technology has evolved, both at the analytical and reagent levels. This technological evolution inexorably leads to the need in redefining reference values. It is for this reason that a group of clinical and medical biochemists have joined their efforts in creating the Canadian Laboratory Initiative in Paediatric Reference (CALIPER) which objective is to define a pan-Canadian set of reference values from birth to late adolescence. To illustrate the need of such a venture, a brief gap analysis for biochemical variables related to the thyroid function, and the growth hormone-insulin-like growth factors axis follows.

Heart rate variability during extracorporeal membrane oxygenation and recovery in severe neonatal disease

BACKGROUND

Heart rate variability (HRV) reveals information on the functional state of the autonomic nervous system (ANS) in neonates. During severe illness, heart rate variability is impaired.

AIM

This study was initiated to measure the changes in HRV in neonates during extracorporeal membrane oxygenation (ECMO) and recovery from severe respiratory and circulatory failure. Moreover, we compared our data with HRV data of healthy newborns and we investigated the differences in HRV parameters between ECMO-survivors and non-survivors.

STUDY DESIGN

This study is of an observational character. We performed short-term recordings of heart rate variability in 14 neonates during ECMO and recovery. We computed time- and frequency-domain HRV parameters.

RESULTS

ECMO significantly affects time-domain HRV parameters. Severe neonatal illness causes a significant reduction of all calculated HRV parameters; clinical recovery is accompanied by an increase of HRV. In comparison with normative data of healthy newborns, however, HRV remains impaired. The ECMO-development ratio separated the non-survivors from the survivors during ECMO therapy.

CONCLUSIONS

During severe neonatal illness, HRV is impaired. It remains to be clarified whether the impairment of HRV during severe illness can predict the neurological outcome. The ability of the E/D ratio as an HRV parameter to serve as a predictive tool has to be corroborated in larger group of patients.

Neonate capillary blood gas reference values

OBJECTIVES

Because biological data are instrument-dependent and because technology has evolved over the last two decades, the published capillary blood reference values for blood gases, lactate, ionized calcium (iCa) and glucose may not reflect the present day situation. Hence, we report such values for healthy term neonates at 48 +/- 12 h of life.

DESIGN AND METHODS

The Institution Ethics Review Board for Research on Human Subjects has accepted the protocol. Extra blood sample was obtained at the time heel-pricks were performed in the frame of the Quebec genetic screening program. One hundred twenty-six term neonates (39.6 +/- 1.2 weeks of gestation) were included in the study. pH, pO2, pCO2, lactate, ionized calcium and glucose were simultaneously measured with selective electrodes on the ABL 735 blood gas analyzer (Radiometer).

RESULTS

All variables exhibited a Gaussian distribution. Since there was no gender effect, all data were pooled.

Short term heart rate variability in healthy neonates: normative data and physiological observations

BACKGROUND

The measurement of heart rate variability (HRV) reveals information on the functional state of the autonomic nervous system (ANS). Moreover, several diseases are known to be accompanied by a reduction in HRV. Currently, there are no data on HRV within larger samples of healthy neonates.

AIM

This study was initiated to establish normative data of HRV parameters and to assess physiological observations in healthy newborn infants.

STUDY DESIGN

We performed 10 min recordings of HRV in 80 healthy neonates. We computed time domain parameters. Furthermore power spectrum analysis was calculated by Fourier Transformation in 3 frequency bands. The total power as well as the peak frequencies was estimated for each band.

RESULTS

The normative data of each HRV parameter for healthy newborns are presented as mean, standard deviation and the 5th and 95th percentiles. Increasing postnatal age is accompanied by a significant (p<0.05) elevation of HRV parameters. Baroreceptor loop fluctuations were delayed from 0.1 Hz (adults) to 0.07 Hz in healthy newborns. A respiratory peak according to respiratory sinus arrhythmia (RSA) as it is common in adults was not detectable.

CONCLUSIONS

Normative data of HRV allows the interpretation of clinical data in neonatal diseases affecting the ANS. Moreover, we observed physiological deviations in healthy newborns in comparison to adults.

Air pollution attributable postneonatal infant mortality in U.S. metropolitan areas: a risk assessment study

BACKGROUND

The impact of outdoor air pollution on infant mortality has not been quantified.

METHODS

Based on exposure-response functions from a U.S. cohort study, we assessed the attributable risk of postneonatal infant mortality in 23 U.S. metropolitan areas related to particulate matter <10 microm in diameter (PM10) as a surrogate of total air pollution.

RESULTS

The estimated proportion of all cause mortality, sudden infant death syndrome (normal birth weight infants only) and respiratory disease mortality (normal birth weight) attributable to PM10 above a chosen reference value of 12.0 microg/m3 PM10 was 6% (95% confidence interval 3-11%), 16% (95% confidence interval 9-23%) and 24% (95% confidence interval 7-44%), respectively. The expected number of infant deaths per year in the selected areas was 106 (95% confidence interval 53-185), 79 (95% confidence interval 46-111) and 15 (95% confidence interval 5-27), respectively. Approximately 75% of cases were from areas where the current levels are at or below the new U.S. PM2.5 standard of 15 microg/m3 (equivalent to 25 microg/m3 PM10). In a country where infant mortality rates and air pollution levels are relatively low, ambient air pollution as measured by particulate matter contributes to a substantial fraction of infant death, especially for those due to sudden infant death syndrome and respiratory disease. Even if all counties would comply to the new PM2.5 standard, the majority of the estimated burden would remain.

CONCLUSION

Given the inherent limitations of risk assessments, further studies are needed to support and quantify the relationship between infant mortality and air pollution.