Simultaneous Estimation of Neonatal Total Body Water by Antipyrine and H₂¹⁸O Dilution

Abstract

Total body water was estimated simultaneously by dilution of antipyrine (antipyrine space; APS) and H2¹⁸O (¹⁸O-space; ¹⁸OS) in 5 baboon neonates and 2 two-month-old lambs (total of 14 studies). Calculations of ¹⁸OS were made either from a single plasma δ¹⁸O measured 4 h after injection of the marker (¹⁸OS4) or by extrapolation to time zero of several plasma δ¹⁸O obtained at precisely timed interval (¹⁸OS₀). Mean ( ± SD) ¹⁸OS₄ was on the average 23% lower than mean ¹⁸OS₀ (796 ± 70 ml/kg vs. 650 ± 67 ml/kg, p < 0.001). Mean APS was not statistically different from mean ¹⁸OS₀ (664 ± 127 ml/kg vs. 658 ± 57 ml/kg), but APS ranged from 41 % lower to 30% higher than ¹⁸OS₀ These data and information from the literature suggest that antipyrine is of doubtful reliability and should be abandoned as a marker for total body water.

Pharmacokinetics

“…For different molecules, the apparent volume of distribution may range from about 0.04 l/kg to more than 20 l/kg. For this reason, antipyrine has been used as a marker of total body water volume and exhibits a volume of distribution of about 0.7 l/ kg [4]. For this reason, such compounds are used to estimate plasma volume [1] and hepatic blood flow [2].…”

Adme

“…Ions such as bromide do not readily pass through cell membranes but distribute rapidly throughout extracellular water volume, having a volume of distribution of about 0.4 L/kg [76]. On the other hand, neutral lipid-soluble drugs, such as antipyrine [77], readily diffuse through cell membranes and easily distribute throughout extra-and intracellular water volume and can have a volume of distribution of about 0.6 L/kg (i.e., total body water). Compounds such as basic drugs, like amlodipine, have a higher affinity for tissue than for plasma proteins, and can easily have volumes of distribution greater than 10 L/kg [78].…”