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Gastro-oesophageal reflux (GOR) is the regurgitation of gastric contents into the oesophagus. It is alleged to be the cause of many clinical problems in premature infants, such as failure to thrive, apnoea, desaturation, bradycardia, and stridor. (1,2)
Twenty four hour oesophageal pH monitoring is currently regarded as the optimal method of diagnosing occult GOR in infants with respiratory events. European and North American working parties have produced guidelines for the methodology and interpretation of oesophageal pH studies in infants and children. (3,4) However, pH monitoring in preterm infants has shortcomings, which makes the diagnosis of GOR in this group more difficult and its clinical significance more uncertain than in older children. This review seeks to highlight these shortcomings and to discuss the future of pH monitoring in preterm infants.
The lack of published normal values for reflux variables in preterm infants makes interpretation of pH monitoring difficult. A reflux index (% of time pH [less than] 4) of [greater than] 10% is widely accepted to be indicative of pathological GOR in infants, but this value is based on a study of term infants. (5) There are only a few small studies involving preterm infants, with variable results (6-10); they are summarised in table 1. Inconsistencies in feeding methods, ventilation, and positioning are reflected in the varying reflux indices.
Defining upper limits of normal is further hampered by the reporting of mean values for reflux indices rather than ranges. (6,7,9,10) Vandenplas et al, (5) who studied the largest series of mature infants, showed that the reflux index is not normally distributed in this population. (5) Attempting to define the upper limit of normal in preterm infants using mean (SD) is unlikely to be meaningful for this reason. It may be more appropriate to divide the population into centiles to define the upper limit of normal.
The detection of GOR with a pH probe in the lower oesophagus depends on the acidity of the refluxate. The probe is passed through the nares or mouth to measure oesophageal pH continuously for 24 hours. It depends on the principle that refluxed gastric contents will cause a dip in oesophageal pH to[less than] 4. The percentage of time that the oesophageal pH is[less than] 4 (reflux index) is the principal parameter used when defining pathological GOR.
Preterm infants are capable of hydrogen ion secretion, (11) but frequent milk feeds can buffer gastric acid, making the gastric pH high. (12-15) If the gastric pH is [greater than]4, episodes of GOR will not be picked up by conventional oesophageal pH monitoring.
Washington et al (13) looked at mature infants (mean age 4 months) who were on standard feeds for their age, and found that the gastric pH was[less than] 4 for a mean of 42% of the time (range 1.7-98.8%). Mitchell et al (14) studied a mixed group of term and preterm infants who were milk fed, and found that the gastric pH was[less than] 4 for a mean of only 24.5% of the time (range 0.6-69.1%). Our own work studying only preterm infants who were exclusively milk fed showed even greater buffering, with a median gastric pH of[less than] 4 for 8.2% of the time (range 2.0-41.2%). (15) This may be because preterm infants are fed more frequently.
It is possible to measure gastric and oesophageal pH simultaneously with a dual channel pH probe. The analysis of the oesophageal pH recording is then restricted …