Cerebral autoregulation of preterm neonates--a non-linear control system?

Low frequency cerebral arterial and venous flow oscillations in healthy neonates measured by NeoDoppler

BACKGROUND

A cerebroprotective effect of low frequency oscillations (LFO) in cerebral blood flow (CBF) has been suggested in adults, but its significance in neonates is not known. This observational study evaluates normal arterial and venous cerebral blood flow in healthy neonates using NeoDoppler, a novel Doppler ultrasound system which can measure cerebral hemodynamics continuously.

METHOD

Ultrasound Doppler data was collected for 2 h on the first and second day of life in 36 healthy term born neonates. LFO (0.04-0.15 Hz) were extracted from the velocity curve by a bandpass filter. An angle independent LFO index was calculated as the coefficient of variation of the filtered curve. Separate analyses were done for arterial and venous signals, and results were related to postnatal age and behavioral state (asleep or awake).

RESULTS

The paper describes normal physiologic variations of arterial and venous cerebral hemodynamics. Mean (SD) arterial and venous LFO indices (%) were 6.52 (2.55) and 3.91 (2.54) on day one, and 5.60 (1.86) and 3.32 (2.03) on day two. After adjusting for possible confounding factors, the arterial LFO index was estimated to decrease by 0.92 percent points per postnatal day (p < 0.001). The venous LFO index did not change significantly with postnatal age (p = 0.539). Arterial and venous LFO were not notably influenced by behavioral state.

CONCLUSION

The results indicate that arterial LFO decrease during the first 2 days of life in healthy neonates. This decrease most likely represents normal physiological changes related to the transitional period. A similar decrease for venous LFO was not found.

Influence of Induced Blood Pressure Variability on the Assessment of Cerebral Autoregulation in Patients after Cardiac Arrest

Objective

To determine if increasing variability of blood pressure influences determination of cerebral autoregulation.

Methods

A prospective observational study was performed at the ICU of a university hospital in the Netherlands. 13 comatose patients after cardiac arrest underwent baseline and intervention (tilting of bed) measurements. Mean flow velocity (MFV) in the middle cerebral artery and mean arterial pressure (MAP) were measured. Coefficient of variation (CV) was used as a standardized measure of dispersion in the time domain. In the frequency domain, coherence, gain, and phase were calculated in the very low and low frequency bands.

Results

The CV of MAP was significantly higher during intervention compared to baseline. On individual level, coherence in the VLF band changed in 5 of 21 measurements from unreliable to reliable and in 6 of 21 measurements from reliable to unreliable. In the LF band 1 of 21 measurements changed from unreliable to reliable and 3 of 21 measurements from reliable to unreliable. Gain in the VLF and LF band was lower during intervention compared to baseline.

Conclusions

For the ICU setting, more attention should be paid to the exact experimental protocol, since changes in experimental settings strongly influence results of estimation of cerebral autoregulation.

Repeatability of Bolus Kinetics Ultrasound Perfusion Imaging for the Quantification of Cerebral Blood Flow

Ultrasound perfusion imaging (UPI) can be used for the quantification of cerebral perfusion. In a neuro-intensive care setting, repeated measurements are required to evaluate changes in cerebral perfusion and monitor therapy. The aim of this study was to determine the repeatability of UPI in quantification of cerebral perfusion. UPI measurement of cerebral perfusion was performed three times in healthy patients. The coefficients of variation of the three bolus injections were calculated for both time- and volume-derived perfusion parameters in the macro- and microcirculation. The UPI time-dependent parameters had overall the lowest CVs in both the macro- and microcirculation. The volume-related parameters had poorer repeatability, especially in the microcirculation. Both intra-observer variability and inter-observer variability were low. Although UPI is a promising tool for the bedside measurement of cerebral perfusion, improvement of the technique is required before implementation in routine clinical practice.

Low spontaneous variability in cerebral blood flow velocity in non-survivors after cardiac arrest

OBJECTIVE

To investigate spontaneous variability in the time and frequency domain in mean flow velocity (MFV) and mean arterial pressure (MAP) in comatose patients after cardiac arrest, and determine possible differences between survivors and non-survivors.

METHODS

A prospective observational study was performed at the ICU of a tertiary care university hospital in the Netherlands. We studied 11 comatose patients and 10 controls. MFV in the middle cerebral artery was measured with simultaneously recording of MAP. Coefficient of variation (CV) was used as a standardized measure of dispersion in the time domain. In the frequency domain, the average spectral power of MAP and MFV were calculated in the very low, low and high frequency bands.

RESULTS

In survivors CV of MFV increased from 4.66 [3.92-6.28] to 7.52 [5.52-15.23] % at T=72h. In non-survivors CV of MFV decreased from 9.02 [1.70-9.36] to 1.97 [1.97-1.97] %. CV of MAP was low immediately after admission (1.46 [1.09-2.25] %) and remained low at 72h (3.05 [1.87-3.63] %) (p=0.13). There were no differences in CV of MAP between survivors and non-survivors (p=0.30). We noticed significant differences between survivors and non-survivors in the VLF band for average spectral power of MAP (p=0.03) and MFV (p=0.003), whereby the power of both MAP and MFV increased in survivors during admission, while remaining low in non-survivors.

CONCLUSIONS

Cerebral blood flow is altered after cardiac arrest, with decreased spontaneous fluctuations in non-survivors. Most likely, these changes are the consequence of impaired intrinsic myogenic vascular function and autonomic dysregulation.

Intraaortic Balloon Pump Counterpulsation and Cerebral Autoregulation: an observational study

Background

The use of Intra-aortic counterpulsation is a well established supportive therapy for patients in cardiac failure or after cardiac surgery. Blood pressure variations induced by counterpulsation are transmitted to the cerebral arteries, challenging cerebral autoregulatory mechanisms in order to maintain a stable cerebral blood flow. This study aims to assess the effects on cerebral autoregulation and variability of cerebral blood flow due to intra-aortic balloon pump and inflation ratio weaning.

Methods

Cerebral blood flow was measured using transcranial Doppler, in a convenience sample of twenty patients requiring balloon counterpulsation for refractory cardiogenic shock (N = 7) or a single inotrope to maintain mean arterial pressure following an elective placement of an intra-aortic balloon pump for cardiac surgery (N = 13). Simultaneous blood pressure at the aortic root was recorded via the intra-aortic balloon pump. Cerebral blood flow velocities were recorded for six minute intervals at a 1:1 balloon inflation-ratio (augmentation of all cardiac beats) and during progressive reductions of the inflation-ratio to 1:3 (augmentation of one every third cardiac beat). Real time comparisons of peak cerebral blood flow velocities with systolic blood pressure were performed using cross-correlation analysis. The primary endpoint was assessment of cerebral autoregulation using the time delay between the peak signals for cerebral blood flow velocity and systolic blood pressure, according to established criteria. The variability of cerebral blood flow was also assessed using non-linear statistics.

Results

During the 1:1 inflation-ratio, the mean time delay between aortic blood pressure and cerebral blood flow was -0.016 seconds (95% CI: -0.023,-0.011); during 1:3 inflation-ratio mean time delay was significantly longer at -0.010 seconds (95% CI: -0.016, -0.004, P < 0.0001). Finally, upon return to a 1:1 inflation-ratio, time delays recovered to those measured at baseline. During inflation-ratio reduction, cerebral blood flow irregularities reduced over time, whilst cerebral blood flow variability at end-diastole decreased in patients with cardiogenic shock.

Conclusions

Weaning counterpulsation from 1:1 to 1:3 inflation ratio leads to a progressive reduction in time delays between systolic blood pressure and peak cerebral blood flow velocities suggesting that although preserved, there is a significant delay in the establishment of cerebral autoregulatory mechanisms. In addition, cerebral blood flow irregularities (i.e. surrogate of flow adaptability) decrease and a loss of cerebral blood flow chaotic pattern occurs during the end-diastolic phase of each beat in patients with cardiogenic shock.

Elevated cerebral pressure passivity is associated with prematurity-related intracranial hemorrhage

OBJECTIVES

Cerebral pressure passivity is common in sick premature infants and may predispose to germinal matrix/intraventricular hemorrhage (GM/IVH), a lesion with potentially serious consequences. We studied the association between the magnitude of cerebral pressure passivity and GM/IVH.

PATIENTS AND METHODS

We enrolled infants <32 weeks' gestational age with indwelling mean arterial pressure (MAP) monitoring and excluded infants with known congenital syndromes or antenatal brain injury. We recorded continuous MAP and cerebral near-infrared spectroscopy hemoglobin difference (HbD) signals at 2 Hz for up to 12 hours/day and up to 5 days. Coherence and transfer function analysis between MAP and HbD signals was performed in 3 frequency bands (0.05-0.25, 0.25-0.5, and 0.5-1.0 Hz). Using MAP-HbD gain and clinical variables (including chorioamnionitis, Apgar scores, gestational age, birth weight, neonatal sepsis, and Score for Neonatal Acute Physiology II), we built a logistic regression model that best predicts cranial ultrasound abnormalities.

RESULTS

In 88 infants (median gestational age: 26 weeks [range 23-30 weeks]), early cranial ultrasound showed GM/IVH in 31 (37%) and parenchymal echodensities in 10 (12%) infants; late cranial ultrasound showed parenchymal abnormalities in 19 (30%) infants. Low-frequency MAP-HbD gain (highest quartile mean) was significantly associated with early GM/IVH but not other ultrasound findings. The most parsimonious model associated with early GM/IVH included only gestational age and MAP-HbD gain.

CONCLUSIONS

This novel cerebrovascular monitoring technique allows quantification of cerebral pressure passivity as MAP-HbD gain in premature infants. High MAP-HbD gain is significantly associated with GM/IVH. Precise temporal and causal relationship between MAP-HbD gain and GM/IVH awaits further study.

Fluctuating pressure-passivity is common in the cerebral circulation of sick premature infants

Cerebral blood flow pressure-passivity results when pressure autoregulation is impaired, or overwhelmed, and is thought to underlie cerebrovascular injury in the premature infant. Earlier bedside observations suggested that transient periods of cerebral pressure-passivity occurred in premature infants. However, these transient events cannot be detected reliably by intermittent static measurements of pressure autoregulation. We therefore used continuous bedside recordings of mean arterial pressure (MAP; from an indwelling arterial catheter) and cerebral perfusion [using the near-infrared spectroscopy (NIRS) Hb difference (HbD) signal) to detect cerebral pressure-passivity in the first 5 d after birth in infants with birth weight <1500 g. Because the Hb difference (HbD) signal [HbD = oxyhemoglobin (HbO2) - Hb] correlates with cerebral blood flow (CBF), we used coherence between MAP and HbD to define pressure-passivity. We measured the prevalence of pressure-passivity using a pressure-passive index (PPI), defined as the percentage of 10-min epochs with significant low-frequency coherence between the MAP and HbD signals. Pressure-passivity occurred in 87 of 90 premature infants, with a mean PPI of 20.3%. Cerebral pressure-passivity was significantly associated with low gestational age and birth weight, systemic hypotension, and maternal hemodynamic factors, but not with markers of maternal infection. Future studies using consistent serial brain imaging are needed to define the relationship between PPI and cerebrovascular injury in the sick premature infant.

Objective selection of signals for assessment of cerebral blood flow autoregulation in neonates

A number of different system identification techniques have been proposed to assess dynamic cerebral autoregulation in critically ill patients. From these methods, the response to a standard stepwise change in blood pressure can be estimated. Responses lacking physiological consistency are a common occurrence and could be the consequence of particular system identification procedures or, alternatively, caused by measurements with a poor signal-to-noise ratio. A multi-observer approach was adopted in this paper to classify cerebral blood flow velocity (CBFV) step responses to spontaneous changes in arterial blood pressure in a group of 43 neonates with a mean gestational age of 33.7 weeks (range 24-42 weeks) and a mean birthweight of 1,980 g (range 570-3,910 g). Three experienced observers independently analysed the estimated step responses in 191 recordings each lasting 100 s; for an autoregressive (ARX) model, 124 (65%) of the step responses were accepted by at least two of the three observers. Two other system identification methods, transfer function analysis and the moving average Wiener-Laguerre model, gave 90 (45%) and 98 (51%) acceptable responses, respectively. Only 54 epochs (28%) were accepted with all three methods. With 88 (46%) responses rejected by at least two methods, it can be concluded that signal quality was the main reason for nonphysiological step responses. To avoid the need for subjective visual selection, an automatic procedure for classifying step responses was implemented leading to sensitivities and specificities in the range 85-90%, with respect to the agreement with subjective evaluations. Objective selection of CBFV step responses is thus feasible and could also be adapted for other physiological measurement techniques relying on system identification methods.

Language lateralization in young children assessed by functional transcranial Doppler sonography

Compared to adults, children show superior recovery of language function after damage to the dominant brain hemisphere. Possible explanations are that children have different patterns of language representation or display different patterns of reorganization. Information about language lateralization in children could provide insights into the repair mechanisms of the young brain. While functional magnetic resonance imaging (fMRI) is usually difficult to perform in children younger than 5 years, functional transcranial Doppler sonography (fTCD) is nonfrightening and readily applicable in young and very young children. However, for serial examinations, sufficient validity and reliability are required. To this end, we designed a picture-description language task (PDLT) for fTCD examinations in children, compared the outcome to established protocols and determined the 1 month retest-reliability of the measurement in 16 children aged 2-9 years. The dependent variable was the task-related hemispheric perfusion difference based on averaged relative cerebral blood flow velocity (CBFV) increases in the middle cerebral arteries. This picture-description language lateralization index was compared to language lateralization by a phonetic word generation task (PWGT) in adults revealing good intermethod validity (r=0.70; P <or= 0.05). The 1 month retest-reliability of the PDLT in the children was r=0.87 (P <or= 0.05). With this degree of reliability, fTCD seems a promising tool for the assessment of changes in hemispheric involvement in language in young and very young children.

Psychosocial effects of intensive care on infants and families after discharge

The neonatal intensive care environment exposes the developing immature newborn to many sources of stress and pain at a time when the infant is developmentally least able to cope with it. Animal and human evidence suggest that effects of stress, mediated through permanent changes in the brain and neuroendocrine responses, may result in changes in behaviour and information processing, which persist throughout childhood. These changes impact on the dynamics of the mother infant dyad and infant learning. Interactional styles arising in the newborn period tend to persist throughout childhood but may be amenable to intervention focusing on maternal recognition of infant cues, social stimulation of the infant, and family integration. Developmental care may promote better family, infant and child outcomes by both reducing neonatal stress and its neurobiological sequelae, and fostering an appropriate interactional relationship between mother and infant.

The repeatability of cerebral autoregulation assessment using sinusoidal lower body negative pressure

A forced periodic variation in blood pressure produces a similar variation in cerebral blood velocity. The amplitudes and phases of the pressure and velocity waveforms are indicative of the dynamic response of the cerebral autoregulation. The phase of the velocity leads the pressure; the greater the phase difference the faster the autoregulation response. Various techniques have been employed to oscillate arterial blood pressure but measurement reproducibility has been poor. The purpose of this study was to assess the reproducibility of phase measurements when sinusoidal lower body negative pressure is used to vary blood pressure. Five healthy volunteers were assessed at two vacuum levels on each of eight visits. For each measurement a 12 s sinusoidal cycle was maintained for 5 min. The Fourier components of blood pressure and the middle cerebral artery velocity were determined at the oscillation frequency. The phase of velocity consistently led the pressure. The mean phase difference was 42+/-13 degrees for the stronger vacuum and 36+/-42 degrees for the weaker vacuum. The variation given is the within-subjects standard deviation estimated from a one-way analysis of variance. Sinusoidal lower body negative pressure is a useful stimulus for investigating autoregulation; it has advantages over other methods. High vacuums show good reproducibility but are too uncomfortable for patient use.

Behavior, pain perception, and the extremely low-birth weight survivor

This article explores the literature concerning responses to pain of both premature and term-born newborn infants, the evidence for short-term and long-term effects of pain, and behavioral sequelae in individuals who have experienced repeated early pain in neonatal life as they mature. There is no doubt that pain causes stress in babies and this in turn may adversely affect long-term neurodevelopmental outcome. Although there are methods for assessing dimensions of acute reactivity to pain in an experimental setting, there are no very good measures available at the present time that can be used clinically. In the clinical setting repeated or chronic pain is more likely the norm rather than infrequent discrete noxious stimuli of the sort that can be readily studied. The wind-up phenomenon suggests that, exposed to a cascade of procedures as happens with clustering of care in the clinical setting in an attempt to provide periods of rest for stressed babies, an infant may in fact perceive procedures that are not normally viewed as noxious, as pain. Pain exposure during lifesaving intensive medical care of ELBW neonates may also affect subsequent reactivity to pain in the neonatal period, but behavioral differences are probably not likely to be clinically significant in the long term. Prolonged and repeated untreated pain in the newborn period, however, may produce a relatively permanent shift in basal autonomic arousal related to prior NICU pain experience, which may have long-term sequelae. In the long run, the most significant clinical effects of early pain exposure may be on neurodevelopment, contributing to later attention, learning, and behavior problems in these vulnerable children. Although there is considerable evidence to support a variety of adverse effects of early pain, there is less information about the long-term effects of opiates and benzodiazepines on the developing central nervous system. Current evidence reviewed suggests that judicious use of morphine for adjustment to mechanical ventilation may ameliorate the altered autonomic response. It may be very important, however, to distinguish stress from pain. Animal evidence suggests that the neonatal brain is affected differently when exposed to morphine administered in the absence of pain than in the presence of pain. Pain control may be important for many reasons but overuse of morphine or benzodiazepines may have undesirable long-term effects. This is a rapidly evolving area of knowledge of clear relevance to clinical management likely to affect long-term outcomes of high-risk children.

Effects of perinatal pain and stress

Neonatal intensive care exposes preterm neonates to a series of repeated, randomly occurring invasive procedures and handling, resulting in acute pain, chronic pain, and prolonged stress during a critical window associated with epochal brain development. Characteristics of the immature pain system in preterm neonates (such as a low pain threshold, prolonged periods of windup, overlapping receptive fields, immature descending inhibition) predisposes them to greater clinical and behavioral sequelae from inadequately treated pain than older age groups. Evidence for developmental plasticity in the neonatal brain suggests that repetitive painful experiences during this period or prolonged exposure to analgesic drugs may alter neuronal and synaptic organization permanently. Traditionally, clinicians have chosen the perspective that routine use of analgesic or sedative drugs in preterm neonates may create more problems than minimal therapy. However, the immediate and long-term consequences of inadequately treated pain have forced them to reconsider the risk-benefit ratios for such therapy. Whereas the short-term consequences of prolonged analgesic therapy in human neonates are well-known (tolerance, withdrawal, ventilator dependency), long-term consequences are relatively unknown. Advances in the study of repetitive pain associated with routine NICU care have challenged the perspective that prolonged pain and stress were inevitable consequences of premature birth.

Cyclical fluctuations in blood pressure, heart rate and cerebral blood volume in preterm infants

Many recently published papers describe cyclical changes of cerebral circulatory variables, mainly in cerebral blood flow velocity (CBFV) performed with Doppler sonography. In this paper we focus on another important variable of cerebral circulation: on cerebral blood volume (CBV) measured by near infrared spectrophotometry (NIRS). In a retrospective analysis of NIRS measurements in 20 preterm infants (median 27 3/7 weeks of gestation), the dominating frequencies and prevalence of cyclical changes of CBV and its possible correlation with peripheral circulatory variables (mean arterial pressure and heart rate) was examined. In 19 out of the 20 infants cyclical changes of CBV were found within a frequency range of 2-4.7 cycles/min which is comparable to the results of the Doppler studies describing fluctuations in CBFV. A dominating frequency of heart rate (HR), was found only in 12 out of 20 infants, and it was with 2.1-3.8 cycles/min in a similar range compared to CBV. In mean arterial blood pressure (MABP), however we detected cycles with longer periods every 1-2.5 min in 14 out of 20 infants. There was a significant coherence between MABP/CBV and HR/CBV. The area under the coherence curve, however, was significantly larger between MABP and CBV as compared to HR and CBV (P = 0.0007, Wilcoxon signed-rank test).

Sleep state changes associated with cerebral blood volume changes in healthy term newborn infants

In order to assess the possible effects of sleep states on cerebral haemodynamics in healthy term infants, we measured cerebral oxyhaemoglobin, deoxyhaemoglobin and total haemoglobin concentration using near infrared spectroscopy. Thirty-seven sleep state changes in seventeen infants (gestational age: 37 to 41 4/7 weeks), aged between two and eight days were continuously registrated during 1-3 h. Transcutaneous PaO2, PaCO2, arterial O2 saturation and heart rate were simultaneously recorded and sleep states were clinically defined. There was a close relationship between sleep state changes and changes in total cerebral haemoglobin concentration, which increased from active to quiet sleep and decreased from quiet to active sleep. Changes in total cerebral haemoglobin were due, in the most part, to changes in the cerebral oxyhaemoglobin concentration. In conclusion, sleep states influence the cerebral haemoglobin concentration. Studies on cerebral haemodynamics should take sleep state into account in term newborn infants.

Assessment of cerebral pressure autoregulation in humans--a review of measurement methods

Assessment of cerebral autoregulation is an important adjunct to measurement of cerebral blood flow for diagnosis, monitoring or prognosis of cerebrovascular disease. The most common approach tests the effects of changes in mean arterial blood pressure on cerebral blood flow, known as pressure autoregulation. A 'gold standard' for this purpose is not available and the literature shows considerable disparity of methods and criteria. This is understandable because cerebral autoregulation is more a concept rather than a physically measurable entity. Static methods utilize steady-state values to test for changes in cerebral blood flow (or velocity) when mean arterial pressure is changed significantly. This is usually achieved with the use of drugs, shifts in blood volume or by observing spontaneous changes. The long time interval between measurements is a particular concern in many of the studies reviewed. Parallel changes in other critical variables, such as pCO2, haematocrit, brain activation and sympathetic tone, are rarely controlled for. Proposed indices of static autoregulation are based on changes in cerebrovascular resistance, on parameters of the linear regression of flow/velocity versus pressure changes, or only on the absolute changes in flow. The limitations of studies which assess patient groups rather than individual cases are highlighted. Newer methods of dynamic assessment are based on transient changes in cerebral blood flow (or velocity) induced by the deflation of thigh cuffs, Valsalva manoeuvres, tilting and induced or spontaneous oscillations in mean arterial blood pressure. Dynamic testing overcomes several limitations of static methods but it is not clear whether the two approaches are interchangeable. Classification of autoregulation performance using dynamic methods has been based on mathematical modelling, coherent averaging, transfer function analysis, crosscorrelation function or impulse response analysis. More research on reproducibility and inter-method comparisons is urgently needed, particularly involving the assessment of pressure autoregulation in individuals rather than patient groups.

Frequency-domain analysis of cerebral autoregulation from spontaneous fluctuations in arterial blood pressure

The dynamic relationship between spontaneous fluctuations of arterial blood pressure (ABP) and corresponding changes in cerebral blood flow velocity (CBFV) is studied in a population of 83 neonates. Static and dynamic methods are used to identify two subgroups showing either normal (group A, n = 23) or impaired (group B, n = 21) cerebral autoregulation. An FFT algorithm is used to estimate the coherence and transfer function between CBFV and ABP. The significance of the linear dependence between these two variables is demonstrated by mean values of squared coherence > 0.50 for both groups in the frequency range 0.02-0.50 Hz. However, group A has significantly smaller coherences than group B in the frequency ranges 0.02-0.10 Hz and 0.33-0.49 Hz. The phase response of group A is also significantly more positive than that of group B, with slopes of 9.3 +/- 1.05, and 1.80 +/- 1.2 rad Hz-1, respectively. The amplitude frequency response is also significantly smaller for group A in relation to group B for the frequency range 0.25-0.43 Hz. These results suggest that transfer function analysis may be able to identify different components of cerebral autoregulation and also provide a deeper understanding of recent findings by other investigators.

Transfer function analysis of dynamic cerebral autoregulation in humans

To test the hypothesis that spontaneous changes in cerebral blood flow are primarily induced by changes in arterial pressure and that cerebral autoregulation is a frequency-dependent phenomenon, we measured mean arterial pressure in the finger and mean blood flow velocity in the middle cerebral artery (VMCA) during supine rest and acute hypotension induced by thigh cuff deflation in 10 healthy subjects. Transfer function gain, phase, and coherence function between changes in arterial pressure and VMCA were estimated using the Welch method. The impulse response function, calculated as the inverse Fourier transform of this transfer function, enabled the calculation of transient changes in VMCA during acute hypotension, which was compared with the directly measured change in VMCA during thigh cuff deflation. Beat-to-beat changes in VMCA occurred simultaneously with changes in arterial pressure, and the autospectrum of VMCA showed characteristics similar to arterial pressure. Transfer gain increased substantially with increasing frequency from 0.07 to 0.20 Hz in association with a gradual decrease in phase. The coherence function was > 0.5 in the frequency range of 0.07-0.30 Hz and < 0.5 at < 0.07 Hz. Furthermore, the predicted change in VMCA was similar to the measured VMCA during thigh cuff deflation. These data suggest that spontaneous changes in VMCA that occur at the frequency range of 0.07-0.30 Hz are related strongly to changes in arterial pressure and, furthermore, that short-term regulation of cerebral blood flow in response to changes in arterial pressure can be modeled by a transfer function with the quality of a high-pass filter in the frequency range of 0.07-0.30 Hz.

Cross-spectral analysis of cerebral autoregulation dynamics in high risk preterm infants during the perinatal period

In preterm infants intraventricular hemorrhage occurs predominantly within the perinatal period, which may be due to a "lost autoregulation" of cerebral blood flow (CBF). In this study, perinatal autoregulation dynamics were investigated in high risk preterm infants by cross-spectral analysis (CSA), which is a statistical tool in the analysis of time series. In 15 ventilated preterm infants of 25-32 gestational weeks, a total number of 30 records were made between 24 and 96 h of life. Doppler-derived CBF velocity (CBFv), used as a quantitative measure for CBF, and direct mean arterial blood pressure (MABP) were measured continuously for 10 min. The spectral power of low frequency (LF, 0.02-0.2 Hz) oscillations in CBFv and MABP was quantified by spectral analysis. From the results of CSA, a LF phase-shift between the CBFv and MABP LF oscillations was calculated in each record. Within the study group, the LF spectral power of CBFv and MABP was initially low and increased significantly until 96 h of life. The LF phase-shift was about 0 degrees at 24 h and increased significantly to 55 degrees at 96 h of life. The initially low LF spectral power of CBFv and MABP may indicate a perinatal depression of autonomic nervous centers, which are thought to control LF oscillations of vital parameters. In the light of a high pass filter model for autoregulation, the initially low LF phase-shift may indicate an initially impaired autoregulation, which supports the "lost autoregulation" hypothesis.

Frequency dependence of cerebrovascular impedance in preterm neonates: a different view on critical closing pressure

The nonproportional relationship between instantaneous arterial blood pressure (BP) and cerebral blood flow velocity (CBFv) is well explained by the concept of critical closing pressure (CCP). We aimed to determine the frequency response of the neonatal cerebrovascular system, and to establish the exact mathematical relationship between cerebrovascular impedance and CCP under physiologic conditions. In 10 preterm neonates (gestational age, 25-32 weeks; birth weight, 685-1,730 g; age 1-7 days) we Doppler-traced CBFv of the internal carotid artery. Blood pressure was traced simultaneously. Critical closing pressure was graphically determined. Cerebrovascular impedance was calculated as the square root of the ratio of the corresponding peaks in the power spectra of BP and CBFv at zero frequency, and at heart rate (H) and harmonics (xH). Uniformly, the impedance between H and 3H (2 to 6 Hz) was reduced about fivefold, compared with the impedance at zero frequency. The cerebrovascular system behaves like a high-pass filter, leading to a reduction of the DC (direct current) component of CBFv (analogous to current) relative to that of the driving force BP (analogous to voltage). The frequency response of cerebrovascular impedance reflects the ratio of CCP and DC BP. A mathematical derivation of this relationship is given matching the observed results. Thus, both the CCP and the impedance approach are valid.

Spectral pattern of neonatal cerebral blood flow velocity: comparison with spectra from blood pressure and heart rate

The cerebral blood flow velocity (CBFV) frequency spectra were studied in 106 premature and term newborns (gestational age range. 24-42 wk) and compared with the heart rate (HR) and mean arterial blood pressure (BP) spectra over the 0.005-0.5 Hz range. CBFV, BP, and HR were shown to have similar but not identical spectral patterns. Adjustment of a l/f model to these spectra produced highly significant fittings, but the residuals were not independent. This condition was met only by the CBFV and BP spectra over a limited frequency range (0.005-0.06 Hz). These results provide a characterization of the CBFV spectra for a much larger population of newborns than hitherto available, indicating that under certain conditions CBFV and BP might show the properties of chaotic systems. In infants without major complications, gestational age (GA) did not have a significant influence on the CBFV spectrum, whereas the spectral power to 0.5 Hz of both BP and HR was found to increase with GA. The spectral power increased over the first 24 h of postnatal life for all three variables: only CBFV showed a significant spectral change in the low frequency (LF, 0.02-0.08 Hz) range. A matched group comparison, adjusted for GA and postnatal age, indicated a reduction in CBFV LF power for term infants with birth asphyxia when compared with normal infants, which was not reproduced in the HR spectra.

Are adult transcranial Doppler systems suitable for application in neonates?

Transcranial Doppler systems have not been available for monitoring of cerebral blood flow velocities in neonates because of potential hazardous effects of energy output from standard instruments developed for adult application. Aim of the study was to test commercially available transcranial Doppler instruments for their applicability in neonates and to develop guidelines for adaptation for safe neonatal use. Energy output of five commercially available transcranial Doppler instruments was measured with a hydrophone system and a radiation force balance. At the highest setting and at the nominal 10% attenuation level, five out of five and two out of five instruments, respectively, had an energy output above the recommended limits. Power reduction was not linear in one instrument. Evaluation of safety devices (alarm, freeze mode, energy reduction facilities, display of energy values) showed that none of the tested instruments had an optimal setting for safe neonatal application.

CONCLUSION

Commercially available transcranial Doppler instruments should be evaluated critically for their energy output prior to their application in neonates. Special software for neonatal application of transcranial Doppler systems should be developed in order to provide extremely low energy output levels and devices for indication of duration of Doppler insonation and energy output.

Ventilator-associated sinus arrhythmia in a preterm neonate--an indicator for a mature autonomic nervous system?

According to control theory, the interactions between respiration and heart rate (i.e. respiratory sinus arrhythmia, RSA, and breath amplitude sinus arrhythmia, BASA) reflect the inner workings of the physiological control systems of respiration and circulation. This paper reports on a preterm neonate (28.5 weeks old, 940 g) who showed the presence of ventilation-associated sinus arrhythmia (VASA) under moderate artificial ventilation. His heart rate variability was entrained to the ventilatory stimulus. VASA and entrainment suggest that the parasympathetic part of the autonomic nervous system might be more mature than expected in some preterm neonates.

Cyclic variation pattern of cerebral blood flow velocity and postconceptional age

In preterm neonates, the risk for intracerebral haemorrhage is linked to immaturity of cerebral autoregulation. The preterm's 2-5/min cyclic variation pattern of cerebral blood flow velocity is thought to reflect the degree of immaturity of autoregulation--a speculation to be tested. In a cross-sectional study 15 infants (gestational age 26-40 weeks, postconceptional age (PCA) 26-42 weeks, age 1-99 days were investigated. We performed a 10 min pulsed Doppler tracing on an internal carotid artery by means of a computer controlled 5 MHz Duplex device. Systolic velocity (Vs) was recorded pulse by pulse. After appropriate data transformation, in all infants the Fast Fourier Transform of the time course of Vs revealed the presence of a 2-5/min cyclic variation pattern (one sample z-test, P < 0.0001). There was no significant correlation between proportionate spectral power of the 2-5/min frequency band and either PCA (r = 0.23, P = 0.42) or age (r = 0.41, P = 0.13). Between 26 and 42 weeks PCA, the cycling phenomenon is constant thus not reflecting cerebral maturation, and its presence does not mean immaturity of cerebral autoregulation.