Reference models for clinical studies by isotope dilution

Total water intake guidelines are sufficient for optimal hydration in United States adults

PURPOSE

Recent studies suggest that 24-h urine osmolality (UOsm) for optimal water intake should be maintained < 500 mmol·kg^-1. The purpose of this study was to determine the total water intake (TWI) requirement for healthy adults to maintain optimal hydration as indicated by 24-h urine osmolality < 500 mmol·kg^-1.

METHODS

Twenty-four-hour UOsm was assessed in 49 men and 50 women residing in the United States (age: 41 ± 14 y, body mass index: 26.3 ± 5.2 kg·m^-2). TWI was assessed from 7-day water turnover, using a dilution of deuterium oxide, corrected for metabolic water production. The diagnostic accuracy of TWI to identify UOsm < 500 mmol·kg^-1 was evaluated using receiver operating characteristic (ROC) analysis in men and women separately.

RESULTS

Twenty-four-hour UOsm was 482 ± 229 and 346 ± 182 mmol·kg^-1 and TWI was 3.57 ± 1.10 L·d^-1 and 3.20 ± 1.27 L·d^-1 in men and women, respectively. ROC analysis for TWI detecting 24-h UOsm < 500 mmol·kg^-1 in men yielded an area under the curve (AUC) of 77.4% with sensitivity, specificity, and threshold values of 83.3%, 64.5%, and 3.39 L·d^-1, respectively. The AUC was 82.4% in women with sensitivity, specificity, and threshold values of 85.7%, 72.1%, and 2.61 L·d^-1.

CONCLUSION

Considering threshold values in men and women of 3.4 L·d^-1 and 2.6 L·d^-1, respectively, maintaining TWI in line with National Academy of Medicine guidelines of 3.7 L·d^-1 in men and 2.7 L·d^-1 in women should be sufficient for most individuals in the United States to maintain 24-h UOsm < 500 mmol·kg^-1.

Validity and Reliability of a Water Frequency Questionnaire to Estimate Daily Total Water Intake in Adults

The purpose of this investigation was to assess the validity and reliability of a seven-day water frequency questionnaire (TWI-FQ) to estimate daily total water intake (TWI) in comparison to a water turnover objective reference value via deuterium oxide (D₂O). Data collection occurred over 3 weeks, with a wash-out period during week two. Healthy adults (n = 98; 52% female; 41 ± 14 y; BMI, 26.4 ± 5.5 kg·m^-2) retrospectively self-reported consumption frequencies of 17 liquids and 35 foods with specified volumes/amounts for weeks one and three via TWI-FQ. Standard water content values were utilized to determine the volume of water consumed from each liquid and food for calculation of mean daily TWI for each week. Diet records were completed daily during week two to estimate metabolic water production. To assess validity of the TWI-FQ, participants consumed D₂O at the start of each week and provided urine samples immediately before ingestion, the following day, and at the end of the week to calculate water turnover. Metabolic water was subtracted from water turnover to estimate TWI. TWI-FQ validity was assessed via Bland-Altman plot for multiple observations. Reliability was assessed via intraclass correlation and Pearson's correlation between weeks. TWI-FQ significantly underestimated D₂O TWI by -350 ± 1,431 mL·d^-1 (95% confidence interval (CI): -551, -149 mL·d^-1). TWI-FQ TWI was significantly correlated (r = 0.707, P <0.01) and not different (198 ± 1,180 mL·d^-1, 95% CI: -38, 435 mL·d^-1) between weeks. TWI-FQ intraclass correlation = 0.706 was significant [95% CI: 0.591, 0.793; F _{(97, 98)} = 5.799], indicating moderate test-retest reliability. While this tool would not be suitable for individual TWI assessment, the magnitude of bias may be acceptable for assessment at the sample-level.

Validation Testing Demonstrates Efficacy of a 7-Day Fluid Record to Estimate Daily Water Intake in Adult Men and Women When Compared with Total Body Water Turnover Measurement

Background: Mean daily water intake from fluids (WATER-FL) has proven to be difficult to measure because of a range of nonvalidated data collection techniques. Few questionnaires have been validated to estimate WATER-FL against self-reported diaries or urinary hydration markers, which may limit their objectivity.Objectives: The goals of this investigation were 1) to assess the validity of a 7-d fluid record (7dFLR) to measure WATER-FL (WATER-FL-7dFLR) through comparison with WATER-FL as calculated by measuring deuterium oxide (D₂O) disappearance (WATER-FL-D₂O), and 2) to evaluate the reliability of the 7dFLR in measuring WATER-FL.Methods: Participants [n = 96; 51% female; mean ± SD age: 41 ± 14 y; mean ± SD body mass index (in kg/m²): 26.2 ± 5.1] completed body water turnover analysis over 3 consecutive weeks. They completed the 7dFLR and food diaries during weeks 2 and 4 of the observation. The records were entered into nutritional software to determine the water content of all foods and fluids consumed. WATER-FL-D₂O was calculated from water turnover (via the D₂O dilution method), minus water from food and metabolic water. The agreement between the 2 methods of determining WATER-FL were compared according to a Bland-Altman plot at week 2. The test-retest reliability of 7dFLR between weeks 2 and 4 was assessed via intraclass correlation (ICC).Results: The mean ± SD difference between WATER-FL-7dFLR and WATER-FL-D₂O was -131 ± 845 mL/d. In addition, no bias was observed (F_[1,94] = 0.484; R² = 0.006; P = 0.488). When comparing WATER-FL-7dFLR from weeks 2 and 4, no significant difference (mean ± SD difference: 71 ± 75 mL/d; t_[79] = 0.954; P = 0.343) and an ICC of 0.85 (95% CI: 0.77, 0.90) was observed.Conclusions: The main findings of this study were that the use of the 7dFLR is an effective and reliable method to estimate WATER-FL in adults. This style of questionnaire may be extremely helpful for collecting water intake data for large-scale epidemiologic studies.

Hemorrhage and operation cause a contraction of the extracellular space needing replacement—evidence and implications? A systematic review

BACKGROUND

Hemorrhagic hypotension or operative trauma is believed to cause a contraction of the extracellular fluid volume (ECV) beyond the measured fluid losses. The aim of this review was to explore the evidence and implications of ECV loss.

METHODS

We performed a systematic review of original trials measuring ECV changes during hemorrhage or operation. PubMed, relevant periodicals, and reference lists were searched until no further original articles appeared. The quality of both the scientific and the technical methods of the trials were evaluated.

RESULTS

A total of 61 original articles were found. The pattern appeared that all investigators reporting shock or operation to cause a disparate reduction of the ECV had measured the ECV with the same method. The ECV was calculated from very few blood samples that were withdrawn after 20 to 30 minutes of equilibration of a tracer (the (35)SO(4)-tracer). Trials calculating ECV from multiple blood samples, after longer equilibration times, or using other tracers did not find a contraction of the ECV. On the contrary, trials using a bromide tracer found the ECV to be expanded after operation.

CONCLUSIONS

The evidence supporting the idea that hemorrhage or operation cause a contraction of the ECV is weak, and probably a result of flawed methodology.

Bedside red cell volumetry by low-dose carboxyhaemoglobin dilution using expiratory gas analysis

BACKGROUND

We developed a non-invasive, continuous, high-resolution method of measuring carboxyhaemoglobin fraction (COHb%) using expiratory gas analysis (EGA). We assessed whether application of EGA to carboxyhaemoglobin dilution provides red cell volume (RCV) measurement with accuracy equivalent to that of CO-haemoximetry, with a smaller infusion volume of carbon-monoxide-saturated autologous blood (COB). Method. We assessed the agreement between repeated COHb% measurements by EGA and simultaneous measurement by CO-haemoximetry, using Bland and Altman plot, in healthy subjects and patients with artificially controlled ventilation and no radiological evidence of pulmonary oedema or atelectasis. We assessed the agreement between RCV measurements by EGA with infusion of 20 ml of COB (RCVEGA) and RCV measurements by CO-haemoximetry with infusion of 100 ml of COB (RCVHEM), in healthy subjects.

RESULTS

The 'limits of agreement' between COHb% measurement by EGA (1 min average) and CO-haemoximetry were -0.09 and 0.08% in healthy subjects, and -0.11 and 0.09% in patients. Given the resolution of CO-haemoximetry (0.1%), the accuracy of EGA was equivalent to or greater than that of CO-haemoximetry. The 'limits of agreement' between RCVEGA and RCVHEM were -0.14 and 0.15 litre. Given the average resolution of RCVHEM (0.14 litre), the accuracy of RCVEGA was equivalent to that of RCVHEM.

CONCLUSION

EGA provided non-invasive, accurate, continuous, high-resolution COHb% measurements. Applying EGA to carboxyhaemoglobin dilution, we achieved RCV measurements with accuracy equivalent to that of CO-haemoximetry, with one-fifth of the COB infusion volume. However, clinical application of the method is limited to patients with no radiological evidence of pulmonary oedema or atelectasis.

Accuracy of carboxyhemoglobin dilution method for the measurement of circulating blood volume

PURPOSE

The management of circulating blood volume (BVc) is crucial in intensive care unit (ICU) patients. The purpose of this study was to verify the accuracy and precision of the carbon monoxide-labeled hemoglobin (CO-Hb) dilution method (CO method) by comparing it with the 51Cr-labeled erythrocyte dilution method (51Cr method) for the measurement of BVc.

METHODS

A prospective study was performed in 18 patients who underwent coronary artery bypass grafting (CABG) under mild hypothermic cardiopulmonary bypass (CPB). The BVc was measured by both the CO method and the 51Cr method at 24 hr after ICU admission in order to verify the accuracy and precision of the CO method. Paired data were assessed in absolute terms, and percentage errors were calculated by the degree of agreement.

RESULTS

Small mean differences and standard deviations between the CO method and the 51Cr method (-70.2 +/-184.8 mL) and small percentage errors (-0.49+/-1.29%) indicated the accuracy and precision of the CO method, and a close correlation was observed (r = 0.97).

CONCLUSION

The CO method can measure BVc with a similar degree of accuracy as the 51Cr method. It is simple, repeatable and safe without the risk of exposure to radioactivity in the ICU.

Blood volume determination by the carbon monoxide method using a new delivery system: accuracy in critically ill humans and precision in an animal model

OBJECTIVE

To evaluate accuracy and repeatability of blood volume determinations made by the carbon monoxide method, using a ventilator-driven administration system.

DESIGN

Prospective within-patient comparison, using simultaneous measurements by two methods to determine accuracy. Prospective laboratory investigation in animals to estimate repeatability.

SUBJECTS

For accuracy: Nineteen ventilated critically ill patients in a university hospital intensive care unit. For repeatability: Six anesthetized, mechanically ventilated normovolemic pigs because this is impossible to perform in humans.

INTERVENTIONS

In the accuracy study, a small mass of carbon monoxide was administered via a closed breathing system and arterial blood samples were taken from existing cannulas. In the repeatability study, an intramuscular sedative was given, followed by an inhalational anesthetic induction and mechanical ventilation via a tracheal tube. Left axillary artery and external jugular vein cannulas were sited. Anesthesia was maintained using an intravenous infusion. Five sequential circulating hemoglobin and blood volume estimations were made using the carbon monoxide method.

MEASUREMENTS AND MAIN RESULTS

The small carboxyhemoglobin increase produced by uptake of a small, known mass of carbon monoxide was used to estimate the circulating blood volume. Simultaneous measurement, using 51Cr-labeled red blood cells, was performed. Twenty measurements were made in 19 patients. The bias (mean difference between blood volume measurements by the two methods) was 397 mL (5.53 mL x kg(-1)) +/-415 mL (+/-5.95 mL x kg(-1)); the limits of agreement (mean difference +/-2 SD) were -433 mL and 1227 mL (-6.36 mL x kg(-1) and 17.42 mL x kg(-1)). Therefore, 95% of expected differences will lie between these limits. The mean blood volume was 75.8 mL x kg(-1) in the animals. The coefficient of variation of repeated estimates was 9.49%. Mean circulating hemoglobin mass was 7.31 mmol with a coefficient of variation of 10.18%. The mean hemoglobin concentration, by co-oximetry, was 5.014 mmol x L(-1), coefficient of variation, 2.99%.

CONCLUSION

This arrangement is a potential bedside method of estimating blood volume and circulating hemoglobin mass. We have rendered the technique more acceptable clinically by creating a ventilator-driven administration system.

Measurement of total body water in intensive care patients with fluid overload

The measurement of total body water (TBW) in critically ill intensive care patients with greatly expanded TBW allows body composition studies to be undertaken in such patients with potentially important clinical consequences. Previous workers in this field have stressed the importance of the distortion of compartmental specific activity resulting from continued intravenous (IV) fluid administration during the period of equilibration and have made attempts to predict the equilibrium value of specific activity from the early arterial kinetics. In this paper a method for the measurement of TBW in critically ill intensive care patients is presented together with results of 16 studies on 11 such patients (mean TBW 54.61). It is shown that the effect of continued IV fluid administration in association with prolonged equilibration is small and that the prediction of TBW from analysis of the early (first hour) arterial kinetics is inappropriate. It is concluded that in such patients the volume of distribution of the isotope is constant after four hours from IV injection and that TBW can be measured with a mean precision of 0.7% (SD) from the fourth, fifth, and sixth hour measurements.

Plasma water and 51Cr EDTA equilibration volumes of different tissues in the rat

The EDTA space and the plasma water volume of the whole animal and selected tissues were investigated with constant EDTA infusions of different duration in the rat. The calculated EDTA space increased with the duration of the infusion both in the whole animal and the tissues sampled. The plasma water volume remained constant, and therefore the calculated increase of the EDTA space suggests a slow intracellular accumulation of the tracer. As intracellular accumulation of EDTA increases with experimental time, it is necessary to use as short an experimental interval as possible to reduce this error when determining extracellular fluid volumes.

Age-related changes in ouabain pharmacology. Ouabain exhibits a different volume of distribution in adult and young dogs

To better understand why one must administer much higher doeses of digitalis glycosides to immature than to mature humans and animals, we studied ouabain pharmacokinetics in adult and young dogs. Consistent with reported observations that ouabain-binding, metabolism, and excretion do not change with age, we found no significant differences in the transfer coefficients in a linear two-compartment open model for ouabain pharmacokinetics following a bolus of 0.05 mg/kg. We did find, however, that young dogs had nearly twice the ouabain volumes of distribution per kilogram of body weight as adults (155.4 +/- 1.2 (SE) ml/kg vs. 80 +/- 0.6, P less than 0.0005) and that one could account for this difference with the fact that young dogs had nearly twice the plasma volume 108 +/- 9.8, vs. 68 +/- 7.3, P = 0.001) and interstitial fluid space (318 +/- 35 vs. 190 +/- 6.5, P = 0.006) as the adults. For the same dose per kilogram, left and right ventricular ouabain concentrations were inversely related to the volume of distribution, with the adults having significantly higher tissue levels and incidence of arrhythmias. One must give more ouabain to a young dog to get the same plasma concentration as in an adult because the mass of ouabain in rapid equilibrium with the plasma is diluted in a larger volume of distribution.

Regression of body compositional abnormalities of heart failure after intracardiac operations

Alterations in body composition are present in patients with heart disease and congestive heart failure, and additional changes occur acutely after corrective surgery. The reversibility and time course of reversion to normal body composition have not been demonstrated. Nine patients were studied preoperatively and 2 to 4 weeks and 6 to 12 mo postoperatively. Measurements were made, using the isotope-dilution technic, of the volumes of total body water, extracellular water, plasma, and red cells, and of the amounts of total exchangeable sodium and potassium. Values were derived for body fat, indices of body cell mass, and the intracellular and extracellular distribution of water and potassium. Significant changes ( P < 0.05) occurring 2 to 4 weeks after operation were increased in total body water (8.2%), intracellular water (15.0%), exchangeable sodium (7.1%), and intracellular potassium content (7.7%), and decrease in body weight (4.6%) and body fat (21.6%); 6 to 12 mo postoperatively there was increase in exchangeable potassium (6.8%), body weight (7.8%), and decrease in exchangeable sodium (16.1%), extracellular fluid (7.8%), plasma (8.0%), and the ratio of exchangeable sodium to exchangeable potassium (23.6%). Late postoperative values were similar to predicted normal values and indicate that postoperative recovery from valvular replacement in these patients is accompanied by resolution of most of the body compositional abnormalities of heart failure.

Acute alterations of body composition after open intracardiac operations

The body composition of patients with heart disease is acutely altered by surgical intervention. Interrelations and causes of the alterations have not been clear. Twenty patients were studied preoperatively and again 2 to 4 days after open intracardiac operations. In nine, measurements were made of the volumes of total body water, extracellular water, plasma, and red cells, and of the amounts of total exchangeable sodium and potassium. In an additional four patients total exchangeable sodium and potassium were measured, and in another seven only exchangeable sodium was measured. Calculations were made of the intracellular and extracellular distribution of water and potassium. The significant changes ( P < 0.05) occurring during the interval between studies were increase in extracellular water (7%), interstitial water (12.5%), and exchangeable sodium (9.4%), decrease in total exchangeable potassium (8.5%) and amount (8.7%), and concentration (6.9%) of calculated intracellular potassium. Uptake of sodium and water by the patient during cardiopulmonary bypass and postoperative loss of intracellular potassium with its secondary renal excretion are postulated as being etiologic. These findings imply that sodium intake in the early postoperative period should be low, water intake no more than urinary and insensible losses, and that potassium should be administered.