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Effect of carbon dioxide on background cerebral electrical activity and fractional oxygen extraction in very low birth weight infants

Presented at the Neonatal Society 2003 Summer Meeting (programme).

Victor S1, Appleton RE2, Beirne M2, Weindling AM1

1 Department of Child Health, University of Liverpool, Liverpool, UK
2 Department of Neurology, Alder Hey Children's Hospital, Liverpool, UK

Introduction: The effect of arterial carbon dioxide tension (pCO2) on cerebral blood flow is well established (1). Changes in cerebral blood flow have also been positively related to changes in amplitude of electroencephalography (EEG) (2). However the relationship between cerebral electrical activity and pCO2 has not been established in very low birth weight (VLBW) infants. The purpose of this study was to test the hypothesis that changes in blood gases in VLBW infants was associated with changes in background cerebral electrical activity and cerebral fractional oxygen extraction.

Methods: This was a prospective, observational study on infants of VLBW and gestation less than 30 weeks. Four channel digital EEG recordings were performed for one hour each day for the first three days after birth. Cerebral fractional oxygen extraction (CFOE) using near infrared spectroscopy and arterial blood gases were measured midway through each recording. Spectral analysis of the EEG signal between 0 and 30 Hz was performed using Fast Fourier Transformation. Absolute and relative power of each frequency band [delta (0-3.5 Hz), theta (4-7.5 Hz), alpha (8-12.5 Hz) and beta (13-30 Hz)] and interburst intervals (IBI) were calculated. Stepwise regression was used for analysis.

Results: 26 infants without serious intraventricular haemorrhage were studied. Median (range) pCO2 was 40 (24 -68) mmHg on day 1. pCO2 was negatively correlated to relative power of delta (r =-0.6, p=0.001), and positively correlated to maximum IBI (r =0.77, p<0.001) and percentage IBI (r =0.57, p=0.002) on day 1. Hypocarbia was associated with increased CFOE (r =-0.43, p=0.008) on day 1. No significant associations were noted on day 2 and 3 of recording. pH, pO2 and mean blood pressure did not have any effect on EEG for the ranges studied.

Conclusion: EEG becomes more discontinuous with hypercarbia (pCO2 >55 mmHg) and less discontinuous with hypocarbia (pCO2 <35mmHg) on day 1. The increased continuity of EEG at low levels of pCO2 may be evidence of cerebral hypoxia; evidence supported by increased CFOE. Adaptation to pCO2 occurs after 24 hours.

1. Pryds O, Greisen G, Skov LL, Friis-Hansen B Pediatr Res 1990; 27(5): 445-449
2. Greisen G, Pryds O Brain Dev 1989; 11(3): 164-168.

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