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Editor—We read with interest the paper by Nelle at al.1 The authors report that nasogastric bolus feeding in preterm infants provokes a considerable decrease in cerebral blood flow velocity (CBFV) while blood pressure and heart rate remain unchanged. In this study 20 to 40 ml of milk were delivered over 5 minutes to infants with a mean postmenstrual age of 35 weeks.
Was the feeding tube inserted before tube feeding or was it indwelling? Insertion itself could have significantly affected CBFV, heart rate, and oxygen saturation during the feeding period.2-4
What position were the infants in during the study? There are clear differences in oxygen saturation and heart rate between the prone and supine positions.5
Apart from the periods when the infants woke up because of the blood pressure measurements, it is difficult to see how the infants remained in a quiet sleep state throughout the investigation. Behavioural state significantly affects CBFV.6 We recently studied the effects of orogastric feeding with infants in the prone position.7 After feeding (recorded until 20 minutes postprandially), most of the infants were in quiet sleep state, but before or during feeding the quiet sleep state was recorded in only a few infants (fig 1). This increase in the incidence of the quiet sleep state after feeding had a beneficial effect on oxygenation.7
We agree that a 10% postprandial decrease in CBFV over 10 minutes is considerable when associated with oxygen desaturation. But it is debatable whether such a decrease is due to the insufficient increase in cardiac output. In our experience, the duration of milk infusion in this study was rather short for the amount of milk given. Fast gastric filling could have caused the greater stimulation of vagal afferents3 with an ensuing transient decrease of CBFV.4 The authors did not report any cardiorespiratory instability in association with feeding, as reported by others.2 3 We found no significant influence of intermittent orogastric tube feeding on CBFV in healthy preterm infants.7
Transcranial Doppler ultrasonography allowed Nelle et alto perform continuous CBFV measurements over the 1 hour period. However, the authors limited the information by analysing just the first five Doppler curves at the beginning of each recorded minute. In our opinion, this made the recordings similar to those assessed by conventional duplex Doppler ultrasound scanning.
Transcranial Doppler sonography has established the concept of slow variability of CBFV in neonates. During slow variability, physiological state dependent CBFV fluctuations of 25% per minute have been reported.6 Accordingly, if transcranial Doppler ultrasound measurements are analysed in the same way as duplex Doppler measurements, important information may be lost.
We suggest analysing the mean values of consecutive one minute recordings, if absolute values of CBFV are to be presented and if the expected change in CBFV takes place over a longer (10–60 minute) periods of time. For changes in CBFV that occur over a shorter (<10 minutes) or longer (>60 minutes) periods of time, mean values of other time units may be more appropriate (means of 5 second, or 10 second, or 5 minute periods, etc.).
Dr Nelle et al respond: An indwelling feeding tube was inserted at least 4 hours before taking the measurements in all infants. Thus a vagal response to tube insertion at the time of the measurements was unlikely.1-1 One minute after the start of tube feeding, CBFV fell to 34.8 (15.4) cm/s (p<0.05) and increased after 2 minutes to 37.7 (13.2) cm/s, and changed little during the following 3 minutes.
Measurements were carried out when the infants were in the supine position. The Doppler probe was fixed 30 minutes before starting the measurements, and care was taken to ensure that nursing or other routine care was not performed for at least an hour before investigations began. During measurements, infants were in the quiet sleep state, as reported.
We agree that behavioural state1-2 has a significant impact on CBFV. That is why we kept the environment as quiet and tranquil as possible, but we cannot rule out the possibility that some infants woke up during blood pressure measurements.
Haxhija and Rosegger question whether a decrease in CBFV is due to insufficient increase in cardiac output. Mean blood flow velocities in various intestinal arteries increase after tube feeding by about 100% in both full term and premature infants.1-3 To maintain cardiac output and blood pressure, the perfusion of other organs has to decrease in response to feeding.1-4 In preterm infants, peripheral (muscular and cutaneous) blood flow remains unchanged after feeding.1-5 Thus the increase in splanchnic blood flow in preterm infants after feeding requires either a substantial rise in cardiac output1-6 or redistribution of regional blood flow in favour of the splanchnic area at the expense of other organs.
We agree that gastric filling, as used in our study, could have stimulated vagal afferents and might have impaired CBFV.1-7
Lucas et al 1-8 found that plasma concentrations of gut hormones were significantly increased 30 to 60 minutes after starting feeding, rising well above plasma concentrations found in adults. Gastrointestinal hormones have an important role in local intestinal perfusion, thereby altering systemic and cerebral circulation.
We analysed only the first five beats at the beginning of each recorded minute, because we were not aiming to study CBFV variability within each minute. Rather, we wanted to study minute by minute changes in CBFV after tube feeding. This is not feasible, using Doppler devices. Nevertheless, our results suggest that under certain conditions—that is, infants at risk of cardiovascular or cerebrovascular disease, bronchopulmonary dysplasia, or apnoea–bradycardia syndrome—tube feeding may cause adverse outcomes, and this should be borne in mind when used in critically ill neonates.