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Measuring Intramucosal pH in Very Low Birth Weight Infants

Maintenance of adequate perfusion is essential for health of the intestinal mucosa. Methods available to assess intestinal perfusion provide information on mesenteric blood flow, which may differ from mucosal flow. Intramucosal pH (pHi) is influenced by tissue oxygenation and perfusion. Gastric pHi can be measured using the technique of tonometry. A prospective observational clinical study was performed to examine relationships between measured gastric pHi and mucosal CO2 (mCO2), and acid-base balance, gastrointestinal complications (necrotizing enterocolitis and perforation), and death in infants <1500 g birth weight. A nasogastric tonometry catheter (size 5F) was inserted into the stomach of infants, and pHi was calculated from mCO2 levels measured using saline tonometry. Measurements were performed at 3, 12, 24, and 48 h, then daily until arterial access was unavailable. Two hundred eleven sets of measurements were performed on 38 infants [birth weight (mean ±SD), 863 ± 241 g; gestation, 26.5 ± 1.8 wk; and median Clinical Risk Index for Babies score, 8.0 (interquartile range, 5.0–10.75)]. Mean pHi was 7.27 (95% confidence interval, 7.26–7.28) and mean mCO2 was 47.0 mm Hg (95% confidence interval, 45.7–48.3 mm Hg). pHi and mCO2 correlated significantly with arterial pH (pHa), arterial Pco2 (Paco2), and arterial base excess. There were no significant relationships between pHa and pH gap (pHa−pHi) or CO2 gap (mCO2−Paco2). Recurrent low pHi (<7.2 on more than one occasion) and an mCO2/Paco2 ratio of ≥1.29 were significantly associated with an increase in gastrointestinal complications. There were no statistically significant associations with death. In conclusion, changes in pH gap and CO2 gap can occur without alteration in pHa. Abnormalities in pHi might predict gastrointestinal complications in infants <1500 g.