Effects of hypoxia on the complexity of respiratory patterns during maturation

Intermittent hypercapnic hypoxia effects on the nicotinic acetylcholine receptors in the developing piglet hippocampus and brainstem

This study investigated the effects of acute (1 day) vs repeated (4 days) exposure to intermittent hypercapnic hypoxia (IHH) on the immunohistochemical expression of α2, α3, α5, α7, α9 and β2 nicotinic acetylcholine receptor (nAChR) subunits in the developing piglet hippocampus and brainstem medulla, and how prior nicotine exposure alters the response to acute IHH. Five piglet groups included: 1day IHH (1D IHH, n=9), 4days IHH (4D IHH, n=8), controls exposed only to air cycles for 1day (1D Air, n=6) or 4days (4D Air, n=5), and pre-exposed to nicotine for 13days prior to 1day IHH (Nic+1D IHH, n=7). The exposure period alternated 6min of HH (8%O₂, 7%CO₂, balance N₂) and 6min of air over 48min, while controls were switched from air-to-air. Results showed that: 1. repeated IHH induces more changes in nAChR subunit expression than acute IHH in both the hippocampus and brainstem medulla, 2. In the hippocampus, α2 and β2 changed the most (increased) following IHH and the CA3, CA2 and DG were mostly affected. In the brainstem medulla, α2, α5, α9 and β2 were changed (decreased) in most nuclei with the hypoglossal and nucleus of the solitary tract being mostly affected. 3. Pre-exposure to nicotine enhanced the changes in the hippocampus but dampened those in the brainstem medulla. These findings indicate that the nAChRs (predominantly with the α2/β2 complex) are affected by IHH in critical hippocampal and brainstem nuclei during early brain development, and that pre-exposure to nicotine alters the pattern of susceptibility to IHH.

Predicting response to leuprolide of women with premenstrual dysphoric disorder by daily mood rating dynamics

Approximately 60-70 percent of women with premenstrual dysphoric disorder (PMDD) show symptomatic improvement in response to the GnRH agonist leuprolide acetate, which suppresses ovarian function. However, it has been very difficult to either predict or understand why some women respond, while others do not. We applied several complementary statistical methods to the dynamics of pre-treatment mood rating data to determine possible predictors of response for women with PMDD. We compared responders (n = 33) to nonresponders (n = 12) in clinical trials of leuprolide (three months in duration) as a treatment for PMDD, on the basis of pre-trial daily self-ratings of sadness, anxiety, and irritability. We analyzed both sequential irregularity (approximate entropy, ApEn) and a quantification of spikiness of these series, as well as a composite measure that equally weighted these two statistics. Both ApEn and Spikiness were significantly smaller for responders than nonresponders (P ≤ 0.005); the composite measure was smaller for responders compared with nonresponders (P ≤ 0.002) and discriminated between the subgroups with high sensitivity and specificity. In contrast, mean symptom levels were indistinct between the subgroups. Relatively regular and non-spiky pre-trial dynamics of mood ratings predict a positive response to leuprolide by women with PMDD with high probability, moreover based on typically less than 3 months of daily records. The statistical measures may have broad and direct applicability to behavioral studies for many psychiatric disorders, facilitating both accurate diagnosis and the prediction of response to treatment.

Information flow to assess cardiorespiratory interactions in patients on weaning trials

Nonlinear processes of the autonomic nervous system (ANS) can produce breath-to-breath variability in the pattern of breathing. In order to provide assess to these nonlinear processes, nonlinear statistical dependencies between heart rate variability and respiratory pattern variability are analyzed. In this way, auto-mutual information and cross-mutual information concepts are applied. This information flow analysis is presented as a short-term non linear analysis method to investigate the information flow interactions in patients on weaning trials. 78 patients from mechanical ventilation were studied: Group A of 28 patients that failed to maintain spontaneous breathing and were reconnected; Group B of 50 patients with successful trials. The results show lower complexity with an increase of information flow in group A than in group B. Furthermore, a more (weakly) coupled nonlinear oscillator behavior is observed in the series of group A than in B.

Brainstem amino acid neurotransmitters and ventilatory response to hypoxia in piglets

The ventilatory response to hypoxia is influenced by the balance between inhibitory (GABA, glycine, and taurine) and excitatory (glutamate and aspartate) brainstem amino acid (AA) neurotransmitters. To assess the effects of AA in the nucleus tractus solitarius (NTS) on the ventilatory response to hypoxia at 1 and 2 wk of age, inhibitory and excitatory AA were sampled by microdialysis in unanesthetized and chronically instrumented piglets. Microdialysis samples from the NTS area were collected at 5-min intervals and minute ventilation (VE), arterial blood pressure (ABP), and arterial blood gases (ABG) were measured while the animals were in quiet sleep. A biphasic ventilatory response to hypoxia was observed in wk 1 and 2, but the decrease in VE at 10 and 15 min was more marked in wk 1. This was associated with an increase in inhibitory AA during hypoxia in wk 1. Excitatory AA levels were elevated during hypoxia in wk 1 and 2. Changes in ABP, pH, and ABG during hypoxia were not different between weeks. These data suggest that the larger depression in the ventilatory response to hypoxia observed in younger piglets is mediated by predominance of the inhibitory AA neurotransmitters, GABA, glycine, and taurine, in the NTS.

Sensitively recorded breathing signals of rats and their nonlinear dynamics

Nonlinear dynamical properties of sensitively recorded breathing signals (SRBS), which include cardiac induced air flow pulsations so-called pneumocardiogram (PNCG) signals, are investigated, in this methodological study. For this purpose, we assessed the SRBS of laboratory rat. The nonlinear behaviors of SRBS were investigated by the reconstructing phase space, using the autocorrelation function and the false nearest neighbor method. The chaotic SRBS attractors were discussed from the point of view of the cardiopulmonary system. This method can be used to assess the heart performance and respiratory mechanics, and might be useful to design for the physiological studies of cardiorespiratory system in small laboratory animals.

Biocomplexity of respiratory neural network during early maturation

Previous studies in various animal models have shown that respiratory premotor and motor neurons undergo rapid changes in biochemical and bioelectric properties during the first month of postnatal life. Early in postnatal life, there is an increase in the complexity of the morphology of the dendritic tree of respiratory neurons as it changes from a bipolar to a multipolar morphology. During normal breathing (eupnea), the phrenic nerve has a slow, ramping output which reflects the orderly recruitment of phrenic motoneurons throughout inspiration when viewed in the time domain. Hypercapnia stimulates the respiratory system increasing both the respiratory frequency and amplitude. During severe hypoxia, the output of the phrenic nerve initially falls to zero and then returns with a completely different firing pattern, gasping. During gasping the phrenic nerve fires with an abrupt onset and rises rapidly to maximal neural activity with a decrementing decline. During recovery from hypoxia a variety of respiratory patterns between eupnea and gasping are seen in the time domain. Both the time and frequency analysis methods, however, give little information about the system generating the output. In contrast, the nonlinear dynamic neural analysis method we propose has been found to be a useful method for quantifying the complexity (irregularity) of both physical systems and physiological signals. The respiratory motor output depends on the integrated properties of the neural network.

Analysis of the nonlinear autodependencies of respiratory pattern in patients on weaning trials

Traditional time domain techniques of data analysis are often not sufficient to characterize the nonlinear dynamics of respiration. In this study, the respiratory pattern variability was analyzed using auto mutual information measures. These provide access to nonlinear statistical autodependencies of respiratory pattern variability. A group of 20 patients on weaning trials from mechanical ventilation were studied at two different pressure support ventilation levels, in order to obtain respiratory volume signals with different variability. Time series of breathing duration, inspiratory time, fractional inspiratory time, tidal volume and mean inspiratory flow were analyzed. Different measures based on auto-mutual information were studied to characterize the respiratory pattern variability with regard to its complex organization.

Influence of peripheral chemodenervation on the complexity of respiratory patterns during early maturation

Previous studies in humans have revealed that, during development, the fetus/neonate may be susceptible to environmental perturbations such as overheating, smoking, hypercapnia and hypoxia (LEWIS and BOSQUE, 1995; MASKERY, 1995). In particular, alterations in behavioural states during early development can result in permanent alterations in their organisational states and subsequent abnormalities in the regulation of the cardiovascular and respiratory systems. The influence of the peripheral chemoreceptor afferent input on the approximate entropy (complexity) of the phrenic neurogram in the piglet was investigated in three different age groups: 3-7 days (n = 7), 10-16 days (n = 6) and 25-31 days (n = 4). The phrenic neurogram was recorded from piglets during control (40% O2) and severe hypoxia (gasping) (5-10% O2), before and after peripheral chemodenervation, and was analysed using the approximate entropy (ApEn) method. The results show that the complexity values of the phrenic neurogram during eupnea and gasping did not change significantly before and after chemodenervation, regardless of postnatal age. The complexity values during gasping were not significantly influenced by the carotid chemodenervation for the 3-7 day-old group, but they were significantly decreased by the carotid chemodenervation for the 10-16 day-old age group (p < 0.01) and the 25-31 day-old age group (p < 0.05). However, the complexity values significantly decreased when the O2 concentration was shifted from eupnea to gasping (p < 0.001), both before and after the chemodenervation (p < 0.001), regardless of the postnatal age. These results suggest that the peripheral chemodenervation reduces the complexity of the phrenic neurograms during gasping only for the 10-16 day-old and 25-31 day-old age groups, and it has no significant influence on the 3-7 day-old age group. Therefore it is speculated that the peripheral chemoreceptors may be inactive for the first seven days of postnatal life and become more active after seven days.

The Effects of Hypercapnia on Early and Later Phases of Phrenic Neurogram During Early Maturation

In this paper, we investigate the influence of hypercapnia on the early and late phases of the phrenic neurogram using the matching pursuit (MP) method in the decebrated piglets. The phrenic neurogram was recorded from 8 piglets (4-7 days old) during control (40% O2 with 5% end-tidal CO2), the mild hypercapnia (40% O2 with 7% end-tidal CO2), and the severe hypercapnia (40% O2 with 15% end-tidal CO2). The time-frequency representations, atoms, of the phrenic neurogram are calculated from the 5 consecutive phrenic neurogram burst for each piglet for each condition using the MP method after vagotomy and chemodenervation. Our results show that the energy percentage of atoms representing the nonperiodic neural activities (NPNAs) significantly increased when the CO2 concentration was shifted from 7% to 15% in the early phase (the first half) of the phrenic neurogram. In addition, the energy percentage of atoms representing the periodic neural activities (PNAs) decreased in the late phase (the second half) when the CO2 concentration was shifted from 7% to 15% (p < 0.01). As a summary, our result suggest that hypercapnia results in significant changes in the phrenic neurogram, an output of the respiratory neural networks in the medulla, both in time and frequency domians during early maturation.

Influence of the vagus nerve on respiratory patterns during early maturation

Nonlinear dynamical analysis was performed on the phrenic neurogram before and after vagotomy in order to study the influence of the vagus nerve on the complexity of the phrenic neurogram in piglets in three age groups: 3-7 days (n = 7); 11-19 days (n = 6); and 29-34 days (n = 8). The phrenic neurogram, generated by the respiratory neural networks in the medulla, projects on the diaphragm muscles and initiate the respiratory movement. On the other hand, the vagus nerves carry the information from mechanoreceptors, located at the lower airway and lungs, to the medulla. The data was recorded during normal breathing (eupnea) before and after vagotomy while piglets were ventilated with 40% O2 in N2 and analyzed using the approximate entropy (ApEn) method. The mean values of the approximate entropy before and after vagotomy during the first 7 days of the postnatal age were 1.32 +/- 0.1 (standard deviation) and 1.34 +/- 0.07, respectively. These values before and after vagotomy during the 11-19 days age group were 1.15 +/- 0.09 and 1.12 +/- 0.05, respectively. For the 29-34 days age group, they were 1.14 +/- 0.05 before vagotomy and 1.19 +/- 0.08 after vagotomy. These differences in the ApEn (complexity) values of the phrenic neurogram before and after vagotomy are not statistically different at each age group. However, the mean mean approximate entropy (complexity) values between the 3-7 days age group and the other two groups were significantly different both before and after vagotomy (p < 0.05) using an analysis off variance test. These results suggest that the vagus nerve may not be mature during early maturation in piglets.

Optimized Symbolic Dynamics Approach for the Analysis of the Respiratory Pattern

Traditional time domain techniques of data analysis are often not sufficient to characterize the complex dynamics of respiration. In this paper, the respiratory pattern variability is analyzed using symbolic dynamics. A group of 20 patients on weaning trials from mechanical ventilation are studied at two different pressure support ventilation levels, in order to obtain respiratory volume signals with different variability. Time series of inspiratory time, expiratory time, breathing duration, fractional inspiratory time, tidal volume and mean inspiratory flow are analyzed. Two different symbol alphabets, with three and four symbols, are considered to characterize the respiratory pattern variability. Assessment of the method is made using the 40 respiratory volume signals classified using clinical criteria into two classes: low variability (LV) or high variability (HV). A discriminant analysis using single indexes from symbolic dynamics has been able to classify the respiratory volume signals with an out-of-sample accuracy of 100%.

Chemical activation of pre-Bötzinger complex in vivo reduces respiratory network complexity

In the in vivo anesthetized adult cat model, multiple patterns of inspiratory motor discharge have been recorded in response to chemical stimulation and focal hypoxia of the pre-Bötzinger complex (pre-BötC), suggesting that this region may participate in the generation of complex respiratory dynamics. The complexity of a signal can be quantified using approximate entropy (ApEn) and multiscale entropy (MSEn) methods, both of which measure the regularity (orderliness) in a time series, with the latter method taking into consideration temporal fluctuations in the underlying dynamics. The current investigation was undertaken to examine the effects of pre-BötC-induced excitation of phasic phrenic nerve discharge, which is characterized by high-amplitude, rapid-rate-of-rise, short-duration bursts, on the complexity of the central inspiratory neural controller in the vagotomized, chloralose-anesthetized adult cat model. To assess inspiratory neural network complexity, we calculated the ApEn and MSEn of phrenic nerve bursts during eupneic (basal) discharge and during pre-BötC-induced excitation of phasic inspiratory bursts. Chemical stimulation of the pre-BötC using DL-homocysteic acid (DLH; 10 mM; 10–20 nl; n = 10) significantly reduced the ApEn from 0.982 ± 0.066 (mean ± SE) to 0.664 ± 0.067 ( P < 0.001) followed by recovery (∼1–2 min after DLH) of the ApEn to 1.014 ± 0.067; a slightly enhanced magnitude reduction in MSEn was observed. Focal pre-BötC hypoxia (induced by sodium cyanide; NaCN; 1 mM; 20 nl; n = 2) also elicited a reduction in both ApEn and MSEn, similar to those observed for the DLH-induced response. These observations demonstrate that activation of the pre-BötC reduces inspiratory network complexity, suggesting a role for the pre-BötC in regulation of complex respiratory dynamics.

CO2 sensitivity of the complexity of phrenic neurograms in the piglet during early maturation

In this paper, we investigate the influence of hypercapnia on the dynamics of the phrenic neurogram in the piglet in two different age groups: 3-7 days (n = 11) and 10-16 days (n = 9). The phrenic neurogram was recorded from 17 piglets (3-16 days old) during control (40% O(2) with 3-5% end-tidal CO(2)), mild hypercapnia (40% O(2) with 7% CO(2)) and severe hypercapnia (40% O(2) with 15% CO(2)) and analyzed using the approximate entropy (ApEn) method. The mean values of the approximate entropy (complexity) of phrenic neurograms during the first 7 days of the postnatal age were 1.56 +/- 0.1 (standard deviation) during normal breathing, 1.51 +/- 0.1 during mild hypercapnia and 1.37 +/- 0.08 during severe hypercapnia. These values for the 10-16 days age group were 1.51 +/- 0.1 during control, 1.49 +/- 0.11 during mild hypercapnia and 1.38 +/- 0.05 during severe hypercapnia. The mean values of phrenic neurogram durations during the first 7 days of the postnatal age were 0.82 +/- 0.03 (standard deviation) s during normal breathing, 0.85 +/- 0.007 s during mild hypercapnia and 0.65 +/- 0.05 s during severe hypercapnia. These values for the 10-16 days age group were 0.97 +/- 0.09 s during control, 1.10 +/- 0.05 during mild hypercapnia and 0.78 +/- 0.05 s during severe hypercapnia. Our results show that the complexity values of the phrenic neurogram were significantly decreased when the CO(2) concentration was shifted from control or mild to severe hypercapnia (p < 0.05) for both the 3-7 days old and the 10-16 days old groups. In addition, the duration of the phrenic neurogram decreased when the concentration was shifted from control or mild to severe hypercapnia (p < 0.05). But no significant changes in the duration of the phrenic neurogram were observed between control and mild hypercapnia concentration. These results suggest that severe hypercapnia can be characterized with a significant decrease of the complexity values and durations of the phrenic neurogram during inspiration during early maturation.

Brain-derived neurotrophic factor mRNA and protein in the piglet brainstem and effects of Intermittent Hypercapnic Hypoxia

Brain-derived neurotrophic factor (BDNF) is a neurotrophin essential for the development of normal respiratory rhythm and ventilatory control. Chronic exposure to Intermittent Hypercapnic Hypoxia (IHH) has been shown to alter ventilatory responses of piglets. This study investigated changes in BDNF distribution and expression in seven nuclei of the caudal medulla, from piglets exposed to IHH for 1, 2, 3, or 4 days before death, using non-radioactive in situ hybridisation (for mRNA) and immunohistochemistry (for protein). Compared to controls, BDNF mRNA was markedly increased across the entire medulla of the brainstem, after all durations of IHH (1-4 days). In contrast, BDNF protein expression increased after 1 day of exposure to IHH (p=0.003), but, thereafter, was not different to controls. Amongst individual nuclei, neurons of the dorsal motor nucleus of the vagus (DMNV) showed increased BDNF mRNA (p<0.01), but decreased protein expression (p=0.05) after all durations of IHH. In the ION, both mRNA and protein for BDNF were significantly increased after 1 day IHH (p<0.01 and p=0.001, respectively), but these increases were not sustained. This study is the first to investigate changes in BDNF expression in response to environmental challenges during postnatal development in the brainstem. Implications of the wide distribution of BDNF in the piglet caudal medulla and increased expression after IHH exposure are discussed, with particular reference to roles for BDNF-dependent neurons at this stage of development.

The Effects of Maturation on Early and Late Phases of Phrenic Neurogram During Eupnea

In this paper, we investigate the effects of maturation on the early and late phases of the phrenic neurogram. We have used the matching pursuit (MP) method to examine the effects of maturation on breathing patterns in both time and frequency domains. The MP was chosen since the wavelet transform method may not represent signals whose Fourier transforms have a narrow high-frequency support. The phrenic neurogram was recorded from 25 piglets (3-35 days) during eupnea (normal breathing) at three postnatal age groups: young (3-7 days (n = 9)), (middle) 10-21 days (n = 6), and old (29-35 days (n = 10)). The energy percentage of atoms representing the nonperiodic neural activities (NPNAs) significantly decreased from young age to middle age groups (p<0.01) and from young age to old age groups (p<0.01), and from middle age to old age groups (p<0.055) in the early phase (the first half) of the phrenic neurogram, but these changes were not statistically significant in the late phase (the second half) of the phrenic neurogram as maturation proceeded. However, the energy percentage of atoms representing the periodic neural activities (PNAs) decreased with maturation, but these changes were not statistically significant in the early phase of the phrenic neurogram. The energy percentage of (PNAs) increased in the late phase of the phrenic neurogram as maturation proceeded although these changes were only significant between young and old age groups (p<0.01). These results suggest that the significant decrease of the NPNAs in the early phase and the increase in the late phase of the phrenic neurogram could be a sign of maturation in piglets.

Investigating the complexity of respiratory patterns during recovery from severe hypoxia

Progressive hypoxemia in anesthetized, peripherally chemodenervated piglets results in initial depression of the phrenic neurogram (PN) culminating in phrenic silence and, eventually, gasping. These changes reverse after the 30 min reoxygenation (recovery) period. To determine if changes in the PN patterns correspond to changes in temporal patterning, we have used the approximate entropy (ApEn) method to examine the effects of maturation on the complexity of breathing patterns in chemodenervated, vagotomized and decerebrated piglets during severe hypoxia and reoxygenation. The phrenic neurogram in piglets was recorded during eupnea (normal breathing), severe hypoxia (gasping) and recovery from severe hypoxia in 31 piglets (2-35 days). Nonlinear dynamical analysis of the phrenic neurogram was performed using the ApEn method. The mean ApEn values for a recording of five consecutive breaths during eupnea, a few phrenic neurogram signals during gasping, the beginning of the recovery period, and five consecutive breaths at every 5 min interval for the 30 min recovery period were calculated. Our data suggest that gasping resulted in reduced duration of the phrenic neurogram, and the gasp-like patterns exist at the beginning of the recovery. But, the durations of phrenic neurograms during recovery were increased after 10 min postreoxygenation, but were restored 30 min post recovery. The ApEn (complexity) values of the phrenic neurogram during eupnea were higher than those of gasping and the early (the onset of) recovery from severe hypoxia (p < 0.01), but were not statistically different than 5 min post recovery regardless of the maturation stages. These results suggest that hypoxia results in a reversible reconfiguration of the central respiratory pattern generator.

Variability analysis of the respiratory volume based on non-linear prediction methods

This work proposed and studied a method of automatically classifying respiratory volume signals as high or low variability by means of non-linear analysis of the respiratory volume. The analysis used volume signals generated by the respiratory system to construct a model of its dynamics and to estimate the quality of the predictions made with the model. Different methods of prediction evaluation, prediction horizons and embedding dimensions were also analysed. Assessment of the method was made using a database that contained 40 respiratory volume signals classified using clinical criteria into two classes: low or high variability. The results obtained using the method of surrogate data provided evidence of non-linear determinism in the respiratory volume signals. A discriminant analysis carried out using non-linear prediction variables classified the respiratory volume signals with an accuracy of 95%.