K. Marcinkowski University of Medical Sciences, Department of Nephrology, Al. Przybyszewskiego 49, Pozna, Poland
Sir,
I have read with great interest the article by Movilli et al. [1] and a comment of Iorio et al. [2], concerning the relationships between PCR, DPI and metabolic acidosis. With the aim of adding further information on this issue, I would like to present the results of PCR, DPI and blood gas parameters obtained in continuous ambulatory peritoneal dialysis (CAPD) patients.
In our uraemic patients (n=55; Female, 16 and Male, 39; age 45±12 years) treated with CAPD through 127 (19±11) months, values of PCR and DPI, daily energy intake (DEI), blood acidbase parameters and nitrogen balance were evaluated every 3 months. PCR was calculated according to Randerson et al. [3] and normalized to total body mass (TBM), ideal body mass (IBM), TBW/0.58 (TBW, total body water) and lean body mass (LBM) estimated according to three methods: (i) from creatinine kinetics using the method of Keshaviah et al. [4]LBMcr; (ii) from anthropometric measurementsLBManthr; and (iii) from the relationship LBM=TBW/0.73. Values of DPI and DEI were obtained on the basis of diet histories taken by an experienced dietetician and checked against coloured pictures of individual foods and meals for accuracy. DPI and DEI were calculated using the computer program FOOD version 1.0 (The Institute of Nutrition and Food, Poland, licence 017). DPI was normalized as aforementioned for PCR. TBW was estimated according to the method of Watson et al. [5]. Parameters of acidbase balance were obtained from arterialized capillary blood using Blood Gas Analyzer Compact 1 (Austria). Daily nitrogen balance was calculated as the difference between nitrogen intake and nitrogen loss. Nitrogen intake was calculated as DPI/6.25 (g/day). Nitrogen loss was calculated as a sum of total nitrogen removal in dialysate and urine, basing on the nitrogen determinations performed by the modified Kjeldahl method [6]. A fixed amount of 1.824 g was taken into consideration accounting for nitrogen from protein (1.504 g) and amino acids (0.32 g) lost through the gastrointestinal tract and skin daily [7,8]. This fixed amount of nitrogen was also subtracted from values of nitrogen intake. Positive values of nitrogen balance indicate uptake of nitrogen (anabolic processes), negative values of nitrogen balance mean a loss of nitrogen (catabolic processes). Total DEI was calculated as oral DEI taken from diet record analysis plus DEI from glucose absorbed from the peritoneal cavity minus energy from glucose lost with daily urine. Total DEI was normalized to TBM.
Our earlier analyses revealed no significant differences in respective values of PCR, DPI and concentrations of H+ and HCO3- in the course of CAPD [9,10]. In the current study, mean result of each parameter was obtained in every patient as representative for her/his entire CAPD course, then mean±SD for all respective parameters were calculated for the entire group. The correlation analysis was performed using Spearman method. The P value <0.05 was defined as significant.
Results (mean±SD) of PCR and DPI are presented in Table 1. Mean blood gas parameters in the course of CAPD were as follows: H+ concentration 42.7±3.5 µmol/l; partial pressure of CO2 (pCO2) 38.6±2.8 mmHg; HCO3- concentration 21.9±2.2 mmol/l. HCO3- concentration <22 mmol/l was shown in 28 patients, including 32% uraemics with HCO3-<19.9 mmol/l. Metabolic alkalosis (HCO3- concentration >26 mmol/l) occurred in 7% of patients. Significant correlations between examined parameters are expressed in Table 2
. PCR (g/day) negatively correlated with DPI normalized to LBM. When both PCR and DPI were normalized to LBMcr, a positive relationship was shown, however, a mathematical coupling cannot be excluded. PCR negatively correlated with HCO3- concentration. There were no significant correlations between DPI and blood gas parameters. Nitrogen balance in the course of CAPD was 5.14±3.48 g/day. Total DEI was 35.8±11.5 kcal/kg TBM/day.
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In conclusion, our studies support the idea that PCR is influenced by acidbase status. PCR reflects DPI poorly under conditions of unbalanced nitrogen turnover.
References
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