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Restoration of fluid balance after exercise-induced hypohydration avoids the detrimental effects of a body water deficit on subsequent exercise performance and physiological function. Key issues in restoring fluid balance are consumption of a volume of fluid greater than that lost in sweat and replacement of electrolyte losses, particularly sodium.
Acid-base balance is a general term for the way in which the body maintains a relatively constant pH despite continuous production of metabolic end products and is fundamental to physiologic homeostasis. Disease states of animals lead to irregularities of body fluid, electrolyte, and acid-base balance. With the advent of less expensive, accurate, as well as portable blood gas and electrolyte analyzers, acid-base assessment has become a standard of care in veterinary medicine. It is important that the veterinary practitioner understand acid-base pathophysiology to assure successful treatment of these disease states. The purpose of this article is to provide a simple and concise description of traditional and modern acid-base data interpretation and assist the practitioner with application of these methods.
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Body position exerts considerable influence on transcapillary balance of protein-poor fluid, i.e., on the whole-body Starling equilibrium. Movement into upright position causes hemoconcentration whereas supine position makes hemodilution. This means that the alteration of blood and plasma volume influences, e.g., plasma protein concentration, hematocrit, and blood hemoglobin content. The application of the mechanical oscillator technique for high-precision density measurements on capillary blood and plasma, especially to quantify the time-course of alterations caused by posture, is discussed in this paper. Tilting into upright position (70 degrees) 45 min after recumbency led to an average increase of 6.6% after 10, and of 11.1% of plasma volume after 30 min in 12 test persons. In 13 test persons, tilting back into supine position (0 degrees) after 30-60 min of standing (70 degrees) caused a mean increase of plasma volume amounting 6.5% after 10, and of 10.5% after 30 min. Postural blood density variations showed shapes similar to that of plasma density, indicating blood volume alterations ranging between 5 and 10%. It is emphasized that, as a consequence for clinical practice, the body positioning before and during blood sampling must be taken into consideration especially in case of precise controls of the course of hematological variables, and in case of statistical comparisons among several test groups.
An MR method is described for estimating overhydration in patients suffering from renal failure. In 29 healthy volunteers, the fat content of a selected slice of the lower limb was correlated to the overall T1-relaxation time (r = -0.97, p < 0.000001). When compared with the normal T1 of the voluntary group, delta T1 was calculated having regard to the individual fat content in 74 MR studies of 22 overhydrated patients. MR results were correlated with the difference between clinically estimated dry and the actual body weight (r = 0.6, p < 0.000001). In an intraindividual follow-up (11 studies) MRI was shown to provide good correlation to the actual body weight (r = 0.9, p < 0.0005) and fluid balance (r = 0.87, p < 0.001).
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Segmental bioelectrical impedance analysis (BIA) was conducted in five patients who underwent esophageal resections. Resistance values fitted at zero frequency (R0) in each body segment (arm, trunk and leg) were determined before the induction of anesthesia, at the end of surgery and on the second or third postoperative day. Extracellular water volume (ECW) in each body segment was estimated using the equation derived from the cell suspension theory. ECW in whole body was obtained from the sum of each body segment. R0 in trunk and leg significantly decreased at the end of surgery compared to the values before the induction of anesthesia (P < 0.05). The change ratio of R0 in trunk before the induction of anesthesia was significantly lower at the end of surgery than that in arm (P < 0.05), resulting from the most striking fluid accumulation in the trunk. Postoperatively, R0 in all body segments, however, appeared to decrease similarly compared to the values before the induction of anesthesia, suggesting the redistribution phenomena of extracellular water among body segments. The correlation (r = 0.90, P < 0.001) and good agreement [bias = 0.01 (L)] between net fluid balances and estimates of ECW changes in whole body suggest that BIA allows close monitoring of tissue hydration during perioperative period by providing estimates of ECW in body segments.
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OBJECTIVES: To determine whether routine nutrition assessment parameters and body composition change after nutritional support in intensive care unit (ICU) patients and whether the changes, if any, are related to cumulative energy and fluid balances. DESIGN: A prospective study. SETTING: A university teaching hospital. PATIENTS: Forty-five mechanically ventilated medical and surgical patients admitted to the ICU who received nutritional support for 7 days (group 1) and 9 patients of this group who received nutritional support for 3 weeks or longer (group 2). INTERVENTIONS: Enteral and parenteral nutritional support prescribed on the basis of metabolic cart measurements of energy expenditure. OUTCOME MEASURES: Routine nutrition assessment, including determinations of weight, serum albumin and prealbumin, and lymphocyte count and body composition, including measurements of body cell mass, extracellular fluid and body fat, determined from bioelectric impedance analysis. RESULTS: In group 1 patients, weight, albumin and prealbumin levels, and extracellular mass changed, but there was no change in lymphocyte count, body cell mass or body fat. Changes in weight and extracellular mass were slightly related to cumulative fluid balance; changes in albumin and prealbumin levels were not related to cumulative energy or fluid balance. The findings were similar for group 2 patients. CONCLUSIONS: Changes in routine nutrition assessment parameters and body composition are slightly affected by fluid balance but not by energy balance; thus, they are not specific indicators of the adequacy of nutritional support in ICU patients. Improved nutrition assessment parameters are required to better monitor the response to nutritional support in critically ill patients.
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In addition to regulating developmental changes in body fluid content, the newborn kidney must maintain a positive sodium balance to ensure adequate body growth. Mechanisms by which the developing organism perceives changes in volume and the manner in which the kidney responds to these changes have been reviewed. Perception of changes in ECF volume is sensed by volume receptors that involve central nervous system signal processing (e.g., low- and high-pressure volume receptors) before influencing the kidney by way of the renal nerves or that are directly coupled with the kidney and do not involve central nervous system processing (e.g., juxtaglomerular apparatus, ANF, hepatic factors). Results presently available demonstrate that these mechanisms are functional early in life and that their sensitivity changes during development in accordance with the needs of the organism. In addition, the developing kidney has unique characteristics that allow it to maintain a positive sodium balance necessary for growth.
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