Occult risk factor for the development of cerebral edema in children with diabetic ketoacidosis: possible role for stomach emptying

The incidence of cerebral edema during therapy of diabetic ketoacidosis (DKA) in children remains unacceptably high-this suggests that current treatment may not be ideal and that important risk factors for the development of cerebral edema have not been recognized. We suggest that there are two major sources for an occult generation of osmole-free water in these patients: first, fluid with a low concentration of electrolytes that was retained in the lumen of the stomach when the patient arrived in hospital; second, infusion of glucose in water at a time when this solution can be converted into water with little glucose. In a retrospective chart review of 30 patients who were admitted with a diagnosis of DKA and a blood sugar > 900 mg/dL (50 mmol/L), there were clues to suggest that some of the retained fluid in the stomach was absorbed. To minimize the likelihood of creating a dangerous degree of cerebral edema in patients with DKA, it is important to define the likely composition of fluid retained in the stomach on admission, to look for signs of absorption of some of this fluid during therapy, and to be especially vigilant once fat-derived brain fuels have disappeared, because this is the time when glucose oxidation in the brain should increase markedly, generating osmole-free water.

Antenatal Bartter's syndrome: why is this not a lethal condition?

There are four themes in this teaching exercise for Professor McCance. The first challenge was to explain how a premature infant with Bartter's syndrome could survive despite having such a severe degree of renal salt wasting. Second, the medical team wanted to know why there was such a dramatic decrease in the natriuresis in response to therapy, despite the presence of a permanent molecular defect that affected the loop of Henle. Third, Professor McCance was asked why this patient seemed to have a second rare disease, AQP2 deficiency type of nephrogenic diabetes insipidus. The fourth challenge was to develop a diagnostic test to help the parents of this baby titrate the dose of indomethacin to ensure an effective dose while minimizing the likelihood of developing nephrotoxicity. The missing links in this interesting story emerge during a discussion between the medical team and its mentor.

Uncovering the basis of a severe degree of acidemia in a patient with diabetic ketoacidosis

In this teaching exercise, the goal is to demonstrate how an application of principles of physiology can reveal the basis for a severe degree of acidaemia (pH 6.81, bicarbonate <3 mmol/l (P(HCO(3))), PCO(2) 8 mmHg), why it was tolerated for a long period of time, and the issues for its therapy in an 8-year-old female with diabetic ketoacidosis. The relatively low value for the anion gap in plasma (19 mEq/l) suggested that its cause was both a direct and an indirect loss of NaHCO(3). Professor McCance suggested that ileus due to hypokalaemia might cause this direct loss of NaHCO(3), and that an excessive excretion of ketoacid anions without NH(4)(+) in the urine accounted for the indirect loss of NaHCO(3). In addition, he suspected that another factor also contributing to the severity of the acidaemia was a low input of alkali. He was also able to explain why there was a 16-h delay before there was a rise in the P(HCO(3)) once therapy began. The missing links in this interesting story, including a possible basis for the hypokalaemia, emerge during the discussion between the medical team and Professor McCance.

A hyperglycaemic hyperosmolar state in a young child: diagnostic insights from a quantitative analysis

This teaching exercise demonstrates how the application of principles of physiology can identify the cause of a severe degree of hyperglycaemia (plasma glucose concentration 80 mmol/l) in a very young patient with newly diagnosed diabetes mellitus, determine whether the patient has diabetic ketoacidosis, and highlight the potential risks for this patient on admission and during initial therapy. A consultation with Professor McCance was sought to determine whether this patient had an unusual degree of 'insulin resistance'. There were also uncertainties regarding the acid-base diagnosis. The patient did not appear to have an important degree of metabolic acidosis as judged from his pH of 7.39 and plasma bicarbonate concentration of 20 mmol/l in arterial blood; hence the diagnostic impression was that he had a hyperglycaemic hyperosmolar state. However, his plasma anion gap was significantly elevated, and remained so for 60 h, despite the administration of insulin. Issues in management concerning the basis for this severe degree of hyperglycaemia and how to minimize the risk of developing cerebral oedema are addressed. The missing links in this interesting story emerge during a discussion between the medical team and their mentor, Professor McCance.

The patient with a severe degree of metabolic acidosis: a deductive analysis

This teaching exercise demonstrates how principles of physiology might help in identifying the cause of a particularly severe case of metabolic acidosis and making appropriate decisions about therapy. The patient's plasma pH was 7.00 and their plasma bicarbonate concentration was 2 mmol/l. Because the time course of the patient's illness was believed to be <24 h, this suggested that a large quantity of acid had been added to the body in this short time period, but the medical team managing the case could not identify any acid that could have been produced rapidly by endogenous processes, or was ingested by the patient. Moreover, there was a question about how such a very low arterial PCO(2) (8 mmHg) could be sustained. Even once the diagnosis was made, there were issues to resolve concerning therapy. These included questions about how much sodium bicarbonate to administer, and what dangers might arise during this therapy. The missing links in this interesting story emerge during a discussion between the medical team and their imaginary mentor, Professor McCance.

An improved approach to the patient with metabolic acidosis: a need for four amendments

Clinicians should identify life-threatening issues in patients with metabolic acidosis. These threats may be present before therapy begins and/or anticipated after therapy commences. By adding four amendments, short-comings in the commonly used clinical approaches for the diagnosis of metabolic acidosis can be overcome. First, a definition of metabolic acidosis should consider not only the concentration of bicarbonate but also the content of bicarbonate in the extra cellular fluid compartment. The latter requires a quantitative estimate of the ECF volume, which can be obtained using the hematocrit and/or the total protein concentration in plasma. Second, to determine if the basis for metabolic acidosis was the addition of acids or the loss of NaHCO 3 , one must hunt for new anions, not only in plasma, but also in the urine. Third, it is important to measure the venous as well as the arterial PCO2 to assess the capacity to buffer H+ while minimizing H + binding to intracellular proteins. Fourth, to assess the role of the kidney in a patient with metabolic acidosis, the urine osmolal gap and the concentration of creatinine in the urine should be measured to provide an estimate of the rate of excretion of ammonium.

Recurrent uric acid stones

A 46-year-old female had a history of recurrent uric acid stone formation, but the reason why uric acid precipitated in her urine was not obvious, because the rate of urate excretion was not high, urine volume was not low, and the pH in her 24-h urine was not low enough. In his discussion of the case, Professor McCance provided new insights into the pathophysiology of uric acid stone formation. He illustrated that measuring the pH in a 24-h urine might obscure the fact that the urine pH was low enough to cause uric acid to precipitate during most of the day. Because he found a low rate of excretion of NH(4)(+) relative to that of sulphate anions, as well as a high rate of citrate excretion, he speculated that the low urine pH would be due to a more alkaline pH in proximal convoluted tubule cells. He went on to suspect that there was a problem in our understanding of the function of renal medullary NH(3) shunt pathway, and he suggested that its major function might be to ensure a urine pH close to 6.0 throughout the day, to minimize the likelihood of forming uric acid kidney stones.

A conceptual approach to the patient with metabolic acidosis. Application to a patient with diabetic ketoacidosis

We shall illustrate that management of patients with an acid-base disorder could be improved if the acid-base analysis was based on a better understanding of basic concepts of physiology. Three concepts of acid-base physiology and their clinical implications are emphasized in a patient with diabetic ketoacidosis. First, when an acid is produced from neutral precursors in the body, there is a net increase in the number of hydrogen ions (H(+)) and new anions. The corollary is that H(+) will be removed when the accompanying anion is metabolized to a neutral end-product or is excreted in the urine with H(+) or ammonium (NH(4)(+)). Second, buffering of H(+) is beneficial if H(+) are removed by bicarbonate rather than being able to bind to proteins. This latter function depends on having a low tissue PCO(2), due to a combination of hyperventilation plus an adequate blood flow rate to vital organs. Third, the kidneys add new bicarbonate to the body when NH(4)(+) is excreted with chloride ions.

Effect of a FastSkin suit on submaximal freestyle swimming

PURPOSE

Nine male collegiate swimmers swam three 183-m freestyle trials at "moderate, moderately hard, and hard" paces while wearing a traditional brief-style suit and on another occasion while wearing a newly designed suit covering the torso and legs with a material designed to reduce drag (FS).

METHODS

Postswim blood lactate concentration, V0(2), and rating of perceived exertion were measured. Average stroke length and rate, and breakout distance were determined for each swimming trial. Passive drag and buoyant force were also determined on swimmers while wearing both suits.

RESULTS

Swimmers swam at a higher mean velocity while wearing the FS (pooled mean % difference = 2%), but this was accompanied by a significant increase in V0(2) (4% difference, P< 0.05) and blood lactate concentration (10% difference, P< 0.05). Comparison of physiological responses at standardized freestyle swimming speeds of 1.4 and 1.6 m.s revealed no significant difference between the two suit conditions. Passive drag of the swimmers while being towed was not significantly different between the suits. Swimmers were significantly more buoyant while wearing the brief-style suit than the FS suit (P< 0.05).

CONCLUSION

These findings provide no evidence of either physical or physiological benefits of wearing these suits during submaximal freestyle swimming.

A patient with partial central diabetes insipidus: clarifying pathophysiology and designing treatment

Studies were undertaken in a 32-year-old man who developed polyuria (4 L/d) a few days after a basal skull fracture; the condition persisted 1 year after the accident. The other major features were thirst, a plasma sodium of 143 mmol/L, 24-hour urine osmolality of 221 mOsm/kg H(2)O, and levels of vasopressin in plasma that were less than 0.5 pg/mL on 20 separate occasions. The 24-hour urine volume implied that the diagnosis was partial rather than complete central diabetes insipidus; however, several random urine samples had a much higher osmolality. An infusion of hypertonic saline led to the release of vasopressin and the excretion of concentrated urine. We propose that the basis for the lesion may be the transection of some, but not all, of the fibers connecting the osmostat and vasopressin release center. This partial transection could permit vasopressin to be secreted in response to a larger rise in plasma sodium concentration. This pathophysiologic analysis provided the basis for therapy to minimize the degree of polyuria.

Effect of NaHCO3 on cardiac energy metabolism and contractile function during hypoxemia

OBJECTIVE

To examine the impact of administration of NaHCO3 on contractility and energy metabolism of the myocardium during hypoxemia.

METHODS

Regional myocardial hypoxia was induced in the left anterior descending (LAD) artery myocardium in anesthetized, open-chest dogs, using a perfusion circuit between the right atrium and the LAD artery, and a membrane oxygenator. The rate of flow in LAD artery was maintained constant with the use of a roller pump. During hypoxia, eight dogs were administered isotonic NaHCO3 in the circuit and six other dogs received equimolar NaCl. Myocardial contractile function was assessed using sonomicrometry for measurement of percentage of systolic shortening and preload recruitable stroke work. Oxygen consumption and the rate of appearance of lactate were measured. Clamp-frozen tissue samples were obtained at the end of the experiment from the hypoxic LAD myocardium and the nonhypoxic circumflex myocardium for measurement of tissue lactate level.

RESULTS

During hypoxia, there was a significant decrease in oxygen consumption by the LAD myocardium (35 +/- 7 micromol/min in the NaCl group and 40 +/- 7 micromol/min in the NaHCO3 group during hypoxia vs. 131 +/- 11 micromol/min during aerobic perfusion). There was also a significant decrease in myocardial contractility as measured by percentage of systolic shortening (14 +/- 3% to -8 +/- 3%); NaHCO3 infusion during hypoxia did not improve myocardial contractility (-7 +/- 2%). Similar results were obtained with measurements of preload recruitable stroke work. The rate of production of lactate during hypoxia was substantially lower than expected, based on the calculated oxygen deficit, and was not significantly increased by the administration of NaHCO3 (33 +/- 9 micromol/min in the NaCl group and 51 +/- 5 micromol/min in the NaHCO3 group). Tissue lactate was not statistically different in the hypoxic myocardium supplied by the LAD artery and the nonhypoxic myocardium supplied by the circumflex artery in either group.

CONCLUSION

The response of the myocardium to hypoxia is to decrease its mechanical work and metabolic demand. The infusion of NaHCO3 did not enhance myocardial contractile function or flux in glycolysis during hypoxia. We speculate that this diminished mechanical work and metabolic demand may represent an adaptive response to preserve cellular integrity until oxygen delivery is restored.

Studies on the pathogenesis of hypokalemia in Gitelman's syndrome: role of bicarbonaturia and hypomagnesemia

OBJECTIVE

Hypokalemia and renal potassium (K) wasting are hallmarks of the group of disorders called Bartter's syndrome. The presence of hypomagnesemia and a low rate of excretion of calcium are currently used to characterize a subgroup of these patients as having Gitelman's syndrome (GS) in which the molecular lesion is a defect in the thiazide-sensitive NaCl cotransporter in the distal convoluted tubule. This study was undertaken to examine whether bicarbonaturia or hypomagnesemia exacerbates the kaliuresis in patients with GS.

METHODS

Six patients with most of the diagnostic features of GS were examined. To examine the role of bicarbonaturia, the transtubular K concentration gradient (TTKG) was assessed before and after an oral load of NH4Cl which caused the urine pH to be < 6. To evaluate the role of hypomagnesemia, the TTKG was examined after an infusion of enough magnesium (Mg) to achieve normal levels of Mg in plasma for close to 24 h.

RESULTS

The TTKG remained very high even when the pH of the urine was < 6.0. An infusion of Mg caused the TTKG to approach expected values for hypokalemia in 4 of 6 patients. The infusion of Mg was extended in 1 patient who had a sustained high TTKG for 24 h; the TTKG remained elevated for 96 h despite normal plasma Mg levels.

CONCLUSIONS

Bicarbonaturia does not play a critical role in maintaining the very high TTKG in these patients. The K wasting in 4 of 6 of these patients could largely be attributed to hypomagnesemia and/or Mg depletion. The plasma aldosterone level tended to be higher in patients who did not respond to the infusion of Mg. Therefore, these patients may not represent a homogeneous group with regard to the pathophysiology of their renal K wasting.

Dynamic interactions between integrative physiology and molecular medicine: the key to understand the mechanism of action of aldosterone in the kidney

Our objective is to illustrate how an approach that integrates new insights from molecular biology and traditional physiology can lead to the development of new concepts. This dynamic interaction is illustrated by examining the steps taken to improve our understanding of the renal actions of aldosterone. We began by defining the big picture of what aldosterone does in the kidney. This led to the conclusion that aldosterone must at times become a sodium chloride-retaining hormone, while at other times it must function primarily or exclusively as a kaliuretic hormone. The second step was to define the major molecular actions of this hormone. Acting on the principal cells in the cortical collecting duct (CCD), aldosterone leads to the insertion of active epithelial sodium ion channels (ENaC) in their luminal membranes. This active ENaC, however, does not distinguish between the two major renal actions of aldosterone. Accordingly, we returned to integrative physiology and examined a possible role of renal and non-renal events. We implicated the potential importance of the delivery of bicarbonate ions to the CCD to determine which effect of aldosterone will become manifest. This, however, required that we reconsider some of the traditional views in interpretation of acid-base balance. At the clinical level, this global view can help us understand why, for example, a low dietary intake of potassium salts might predispose a person to an elevated blood pressure. Using a similar approach, it is possible to understand how the risk of the formation of kidney stones can be minimized.

Potassium

In a logical, stepwise approach to patients presenting with hypokalaemia or hyperkalaemia the clinician must first recognise circumstances in which the dyskalaemia represents a clinical emergency because therapy then takes precedence over diagnosis. If a dyskalaemia has been present for a long time, there is an abnormal renal handling of K+. The next step to analyse is the rate of excretion of K+ and, if necessary, its two components (urine flow rate and K+ concentration in the cortical collecting duct [CCD]) analysed independently. If the K+ concentration in the CCD is not in the expected range, its basis should be defined at the ion-channel level in the CCD from clinical information that can be used to deduce the relative rates of reabsorption of Na+ and Cl- in the CCD. This analysis provides the basis for diagnosis and may indicate where non-emergency therapy should then be directed.

Control of excretion of potassium: lessons from studies during prolonged total fasting in human subjects

A deficit of K+ of close to 300 mmol develops in the first 2 wk of fasting, but little further excretion of K+ occurs, despite high levels of aldosterone and the delivery of ketoacid anions that are not reabsorbed in the distal nephron. Our purpose was to evaluate how aldosterone could have primarily NaCl-retaining, rather than kaliuretic, properties in this setting. To evaluate the role of distal delivery of Na+, four fasted subjects received an acute infusion of NaCl to induce a natriuresis. To assess the role of distal delivery of HCO3-, five fasted subjects were given an infusion containing NaHCO3. The natriuresis induced by an infusion of NaCl caused only a small rise in the rate of excretion of K+ (0.8 +/- 0.1 to 1.9 +/- 0.3 mmol/h); in contrast, when HCO3- replaced Cl- in the infusate, K+ excretion rose to 8.3 +/- 2.2 mmol/h, despite little excretion of HCO3- (urine, pH 5.8) and similar rates of excretion of Na+. The transtubular K+ concentration gradient was 19 +/- 3 with HCO3- and 6 +/- 2 with NaCl. We conclude that the infusion of NaHCO3 led to an increase in K+ excretion, likely reflecting an increased rate of distal K+ secretion. With a low distal delivery of HCO3-, aldosterone acts as a NaCl-retaining, rather than a kaliuretic, hormone.

A new classification for renal defects in net acid excretion

The traditional classification of the group of disorders called renal tubular acidosis (RTA) into proximal and distal subclasses is based on which nephron segment is thought to have an abnormal function. Nevertheless, such a distinction may not be correct and also does not characterize the pathophysiology of the renal acidosis in each patient. In this article, we propose an alternative classification, one that is based on the component of net acid excretion that is abnormal. We also suggest expanding the definition of net acid excretion to include a term that describes the renal handling of metabolizable organic anions because their loss in the urine represents the loss of "potential bicarbonate." Because a low rate of excretion of ammonium (NH4+) is present in patients with both distal and isolated proximal RTA, our initial clinical step in patients with hyperchloremic metabolic acidosis (HCMA) is to evaluate the rate of excretion of NH4+. The basis for a low rate of excretion of NH4+ is shown by examining the urine pH. If the urine pH is low, further studies are performed to determine why the availability of NH3 is low; if the urine pH is high, further investigations are initiated to examine if the defect in H+ secretion involves the proximal or the distal nephron. Conversely, if the rate of excretion of NH4+ is high in a patient with HCMA, a component of the degree of acidosis could be attributable to a high rate of excretion of metabolizable organic anions. Case examples are provided to illustrate the approach and its implications for future molecular studies.

Does the administration of carbicarb lead acutely to back-titration of non-bicarbonate buffers?

OBJECTIVE

We have recently demonstrated in a model of acute metabolic acidosis in rats that the administration of NaHCO3 does not lead acutely to back-titration of non-bicarbonate buffers. Carbicarb is a new alkalinizing agent that has been proposed as a replacement for NaHCO3 in the treatment of metabolic acidosis. Hence, the purpose of this study was to examine the impact of carbicarb on back-titration of non-bicarbonate buffers.

METHODS

Rats were anaesthetized, intubated and ventilated to a PCO2 of approximately 30 mmHg. Acute metabolic acidosis was induced by the infusion over 1 hour of 3.5 mmol of hydrochloric acid. After a 20 min equilibration period, 3 groups of rats (n = 7 in each group) were examined. Rats in groups I received 0.75 meq of Na from 1 M carbicarb solution as an intravenous bolus, rats in group II received equimolar NaCl, while rats in group III were used as time controls. Calculations were performed to quantitate the amount of HCO3- that was retained in the ECF volume and the amount that was titrated with H+ and excreted as "acid-base" CO2. "Acid-base" CO2 was considered as the amount of CO2 that was excreted in excess of what would be produced during metabolism.

RESULTS

As compared to the NaCl and the time control groups, the administration of carbicarb led to significant alkalinization of the ECF, pH rose from 7.23 +/- 0.02 to 7.34 +/- 0.03. Of the 0.75 mmol of carbicarb that was administered, 0.61 +/- 0.05 mmol (70%) was retained in ECF. There was virtually no "acid-base" CO2 produced.

CONCLUSIONS

The administration of carbicarb does not lead acutely to back-titration of non-bicarbonate buffers especially under conditions of fixed ventilation.

Effect of alkali therapy with NaHCO3 or THAM on cardiac contractility

We examined the impact of alkali therapy on myocardial contractility in a model of myocardial ischemia in dogs using direct measurements of myocardial contractile function. Myocardial ischemia in the left anterior descending (LAD) artery territory was induced using a perfusion circuit from the internal carotid artery to the LAD artery. Myocardial contractile function was assessed using sonomicrometry for measurement of percent systolic shortening (%SS), preload recruitable stroke work (PRSW) slope, and end-systolic pressure-length relationship (ESPLR) area. Because the blood flow in LAD artery was diminished by approximately 70%, there was a significant decrease in O2 delivery and uptake by the ischemic myocardium. Ischemia led to a significant fall in LAD regional contractile function with %SS decreasing from 15 +/- 2 to 7 +/- 2%, PRSW slope from 82 +/- 10 to 37 +/- 5 mmHg, and ESPLR area from 121 +/- 2 to 48 +/- 14 mmHg.mm (P < 0.05). In six dogs, the intracoronary administration of NaHCO(3) resulted in a significant increase in pH in LAD arterial and venous blood. There was, however, no significant increase in %SS (6 +/- 2), PRSW slope (43 +/- 10 mmHg), or ESPLR area (60 +/- 13 mmHg.mm). Since administration of NaHCO(3) resulted in a significant increase in PCO2 in LAD arterial and venous blood, similar experiments were carried out in five dogs, but with the intracoronary infusion of the amine buffer THAM [tris(hydroxymethyl)aminomethane (Tris) buffer; 2-amino-2-hydroxyl-1,3-propandiol] instead of NaHCO3. Although administration of THAM resulted in a significant increase in pH and a significant decrease in PCO2, in both LAD arterial and venous blood, there was no significant improvement in any of the parameters used to assess myocardial contractile function. In conclusion, administration of alkali (NaHCO3 or THAM) does not enhance the contractile function of the ischemic myocardium.

The acute impact of NaHCO3 in treatment of metabolic acidosis on back-titration of non-bicarbonate buffers: a quantitative analysis

OBJECTIVE

The major non-bicarbonate buffers are intracellular proteins, a detrimental effect of severe acidosis could be their titration with H+. This in turn would lead to their net charge becoming more positive, and possibly, to changes in their shape and function. Since NaHCO3 is a treatment option in patients with severe metabolic acidosis, the purpose of this study was to examine the acute effect of the administration of NaHCO3 on back-titration of non-bicarbonate buffers in metabolic acidosis.

DESIGN

Prospective, controlled, non-randomized laboratory study.

SETTING

Research laboratory.

SUBJECTS

21 male Wistar rats.

INTERVENTION

Rats were anesthetized, intubated and ventilated. Ventilation was adjusted at the beginning of the experiment to a PCO2 of approximately 30 mmHg, no further adjustments were made thereafter. Acute metabolic acidosis was induced by the infusion of 3.5 mmol of hydrochloric acid over 1 hour. After an equilibration period, 3 groups of seven rats were studied; group I received 0.75 mmol NaHCO3, group II received equimolar NaCl, and group III served as time control.

MEASUREMENTS AND MAIN RESULTS

Measurements were made to enable quantitation of how much HCO3 was retained in the ECF and how much was titrated with H+ and was excreted as "acid-base" CO2. Since there are so few H+ present in the ECF in a free or a bound form, and in the absence of an increase in endogenous acid-production, the source of this H+ is from proteins in the ICF. As compared to the NaCl and the time control groups, the administration of NaHCO3 led to significant alkalinization of the ECF, pH rose from 7.22 +/- 0.03 to 7.34 +/- 0.02. Of the 0.75 mmol of NaHCO3 that was administered, 67% or 0.52 +/- 0.08 mmol was retained in ECF. Only a small amount (0.07 +/- 0.09 mmol) of acid-base CO2 was excreted.

CONCLUSIONS

The administration of NaHCO3 does not acutely lead to a significant back-titration of non-bicarbonate buffers, especially under conditions of fixed ventilation.

Disorders of potassium homeostasis: an approach based on pathophysiology

Disorders of potassium (K+) homeostasis are frequently encountered in clinical medicine and may have serious sequelae, particularly cardiac arrhythmias. Since long-term K+ balance depends on regulation of renal excretion of K+, the focus of this paper is to provide a novel way to analyze the K+ excretory process at the bedside in a noninvasive fashion. A fundamental aim was to incorporate recent new advances in K+ physiology to the clinical analysis of K+ disorders. In so doing, we have tried to replace eponyms and largely descriptive terms with more specific, but hypothetical pathophysiologic diagnoses. The approach we used focuses on an assessment of the components of K+ excretion in vivo. If the rate of excretion of K+ differs from the "expected" value for the stimulus of hypokalemia or hyperkalemia, one should determine whether the fault is with the flow rate and/or the [K+] in the terminal cortical collecting duct. The former is influenced primarily by the rate of excretion of osmoles when antidiuretic hormone acts, whereas the [K+] in the cortical collecting duct is determined by factors that modulate rate of electrogenic reabsorption of Na+ in that segment and its conductance for K+. By examining the extracellular fluid (ECF) volume status, the plasma renin activity, and the renal response to the induction of ECF volume contraction, we attempted to deduce whether the change in electrogenic reabsorption of Na+ was due to an altered Na+ transport or apparent permeability to chloride in the cortical collecting duct. We believe that an approach which draws heavily on pathophysiology can be of practical use at the bedside and, in addition, indicate areas in which more research could be fruitful. To illustrate these points, two clinical cases with hypokalemia and two with hyperkalemia were analyzed. Nevertheless, it is important to emphasize that the approach provided is speculative.

Hyperkalemia with mild ECF volume contraction: studies to provide a possible physiologic interpretation

A 30-y-old female presented with a history of hypertension and a modest degree of hyperkalemia. There was a mild degree of contraction of her ECF volume on clinical examination, with elevated levels of renin and aldosterone in plasma. No causes for secondary hypertension were found. Laboratory investigations revealed a slightly reduced glomerular filtration rate (GFR) and a subnormal kaliuretic response to exogenous mineralocorticoids. When a further degree of ECF volume contraction was induced, she was unable to conserve Na+ and Cl- appropriately. Moreover, expansion of the ECF volume led to a significant suppression of the levels of both renin and aldosterone in plasma. We speculate that these findings could be explained by a diminished net rate of reabsorption of Na+ in the cortical collecting duct. Such a reduction could lead to a diminished generation of an electrical gradient to favour the net secretion of K+ and lead to hyperkalemia with renal salt wasting. The resultant contraction of the extracellular fluid volume with the release of renin and aldosterone (and probably other vasoactive hormones) might have predisposed her to hypertension. This hypothesis was supported by the finding that NaCl supplements led to a significant drop in her blood pressure. This case could represent a new syndrome of hyperkalemia and "salt sensitive" hypertension.

Treatment of hyponatremia: a quantitative analysis

The bulk of clinical and experimental evidence seems to support the view that a catastrophic outcome can accompany rapid correction of chronic hyponatremia. However, the dilemma that faces the clinicians is controlling the rate at which the serum sodium concentration is increased during the treatment of hyponatremia. We present a case of severe hyponatremia and hypokalemia to illustrate, in a quantitative fashion, that the increase in serum sodium concentration will be determined not only by the tonicity of infused fluids, but also, and to a major degree, by the rate of excretion of free water by the kidney. A rapid increase in serum sodium concentration would occur if antidiuretic hormone (ADH) release is inhibited and water diuresis ensues. We suggest that the administration of potassium chloride (orally or via a central line) would have been the appropriate therapy for this patient.

Is accelerated oxidation of lactate required for dichloroacetate to lower the level of lactate in blood?

We examined mechanisms by which dichloroacetate (DCA), an activator of pyruvate dehydrogenase (PDH), led to a decrease in the concentration of lactate in blood in a unique "metabolic setting," where the concentration of lactate in blood was 5.4 +/- 0.5 mmol/L. Elevated levels of lactate were induced in anaesthetized rabbits by the administration of a large dose of insulin. The rate of consumption of oxygen was 1.2 +/- 0.1 mmol/min, the respiratory quotient was close to unity, and close to half of the PDH was in its active form; therefore, virtually all ATP synthesis should require flux through PDH. Hence, we predicted that DCA should not cause a significant decrease in the concentration of lactate in blood in this model. In contrast, if DCA was effective, new insights could be obtained into its mechanisms of action, at least in this setting. During steady-state hyperlactatemia, DCA was given as its sodium salt, 2 mmol/kg (n = 10); a control group (n = 5) received equimolar NaCl. Forty minutes later, the level of lactate in blood in the DCA group was 1.3 +/- 0.2 mmol/L, significantly lower than in the NaCl group (4.2 +/- 0.6 mmol/L). To determine the organ(s) responsible for removing lactate, arteriovenous differences were measured in organs drained by the jugular, femoral, and hepatic veins. There was no net uptake of lactate in these drainage beds after DCA was administered. From a quantitative analysis of the rate of removal of lactate and the rate of consumption of oxygen, it seems unlikely that the majority of the decrease in lactate could be directly attributed to an increase in its oxidation.

Persistent hyperkalemia in a patient with diabetes mellitus: a reversible defect in kaliuresis during bicarbonaturia

The purpose of this report is to apply recent advances in the understanding of the physiology of the excretion of potassium to a patient who had hyperkalemia due to a low rate of excretion of potassium. The defect was first suspected during therapy for diabetic ketoacidosis, when the concentration of potassium in plasma was unusually high (7.3 mmol/l) on admission and the deficit of potassium, as judged from the quantity of potassium infused to maintain normokalemia (40 mmol/24 h), was much less than expected. After recovery from diabetic ketoacidosis, hyperkalemia persisted despite near-normal values for creatinine and glucose in plasma. Excretion of potassium was low, considering the stimulus of hyperkalemia, and did not rise appreciably after the acute or chronic administration of a mineralocorticoid. The transtubular potassium concentration gradient (TTKG) did not exceed 6 after a large dose of fludrocortisone (200 micrograms) was administered. Notwithstanding, the TTKG rose to 14.4 following the intake of acetazolamide. We speculate that the basis for the hyperkalemia was type II hypoaldosteronism.

Might distal renal tubular acidosis be a proximal tubular cell disorder?

Incomplete renal tubular acidosis (RTA) and overt distal RTA may be different stages of the same underlying pathophysiology in certain individuals. The rationale that draws these conditions together is the relatively alkaline pH of the urine, hypocitraturia, and a possible familial association. The rate of excretion of ammonium (NH4+), on the other hand, suggests that these conditions stem from fundamentally different lesions. To explain this difference, we suggest that two possible disorders may result in the evolution from incomplete RTA to overt distal RTA. One subgroup could have gradient-limited distal RTA, while the other subgroup may have a lower pH of the intracellular fluid of the proximal convoluted tubular epithelium. Indices of proximal intracellular pH (rates of excretion of NH4+, NH3, and citrate) were culled from the literature spanning the years 1959 to 1991 on patients with incomplete RTA and overt distal RTA. Three points emerge: (1) the rate of excretion of NH4+ was lower in patients with overt distal RTA than in normals following an acute acid load (23 +/- 1 v 49 +/- 3 mumol/min); (2) the concentration of NH3 in the urine was almost 25-fold higher in incomplete RTA than in normals (69 +/- 14 v 3 +/- 0.4 nmol/min); and (3) in incomplete RTA, the pH of the urine fell to very low values (4.9 +/- 0.1) when high urine flows were induced with furosemide. The low pH of the urine would therefore suggest that many of these patients do not gradient-limited distal RTA, but more likely have proximal renal epithelial cell acidosis. We hypothesize that this high rate of excretion of NH4+ and low rate of excretion of citrate in the absence of acidosis or hypokalemia is consistent with proximal cell acidosis. To explain a transition from incomplete RTA to overt distal RTA, we speculate that toxicity of high concentrations of NH3 in the medullary interstitium as well as nephrolithiasis and nephrocalcinosis due to low urinary citrate and possibly an alkaline medullary interstitium may lead to damage of structures in this region.

Studies to determine the basis for hyperkalemia in recipients of a renal transplant who are treated with cyclosporine

Hyperkalemia is commonly encountered in patients who receive a renal transplant and the immunosuppressive drug, cyclosporine. There is also a high incidence of hypertension (which is thought to be due to expansion of the extracellular fluid volume) and hyperchloremic metabolic acidosis in this group of patients. This constellation of findings led to the suspicion of the possibility that their basis might be type II hypoaldosteronism. To test this hypothesis, 12 patients with hyperkalemia (plasma K+, 5.1 +/- 0.2 mmol/L at the time of study) while receiving cyclosporine were studied. Patients who had diabetes mellitus, those receiving drugs known to cause hyperkalemia (e.g., beta blockers, angiotensin-converting enzyme inhibitors, K(+)-sparing diuretics), or those with a serum creatinine greater than 200 mumol/L were excluded. The renal response to hyperkalemia was inappropriate because the transtubular K+ concentration gradient (TTKG) was only 4.3 +/- 0.4 compared with a TTKG of 13 +/- 1, 2 h after 50 mmol of KCl was given to normal subjects. The TTKG, after administration of 200 micrograms of fludrocortisone, was still very low (5.6 +/- 0.6) in the patients compared with that of controls (12 +/- 1). After administration of 250 to 500 mg of acetazolamide to increase the delivery of bicarbonate to the distal nephron, the TTKG rose significantly to 11 +/- 1 in patients on cyclosporine, compared with 17 +/- 1 in the controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Lactic acidosis, ketoacidosis, and energy turnover: "figure" you made the correct diagnosis only when you have "counted" on it--quantitative analysis based on principles of metabolism

Three cases are presented to illustrate that quantitative analysis based on physiologic principles can help resolve certain controversies in clinical medicine. For example, in case 1, a patient with severe hypoxia, the rate of production of lactic acid is so high that only restoration of delivery of oxygen is rational therapy. If the degree of hypoxia exceeds 5.6% of demand, dichloroacetate will not lessen the degree of acidosis. Further, even when delivery of oxygen is returned to normal, the rate of fall in lactate and rise in bicarbonate in plasma will be relatively slow. In case 2, a patient with diabetic ketoacidosis, our discussion stresses that the rate of production of ketoacids is not that rapid and that the degree of ketoacidosis is influenced to a major degree by decreasing the rate of oxidation of ketoacids in brain and kidneys. Case 3, a patient with severe hyperglycemia, illustrates that insulin will only promote the oxidation of glucose at a rapid rate once the levels of fatty acids and ketoacids decline to low levels. Accelerated transport of glucose by insulin is only a permissive action for the oxidation of glucose.

Interpretation of the urine osmolality: the role of ethanol and the rate of excretion of osmoles

One purpose of this report is to illustrate that calculating the rate of excretion of osmoles in the urine can be of value in the differential diagnosis of hypernatremia and polyuria. A second purpose is to illustrate a clinical example where the osmolality of the urine did not reflect the lack of action of ADH. A patient with ethanol intoxication seemed to have central diabetes insipidus on clinical grounds. However, the osmolality of the urine was 287 mosm/kg H2O, a value which made this diagnosis unlikely. Since the concentration of ethanol in plasma was 119 mmol/L, we suspected that the urine contained an appreciable quantity of alcohol; this might obscure the lack of action of ADH. A study was performed to document the quantitative relationship between the concentrations of ethanol in plasma and urine. The concentration of ethanol in the urine was approximately 1.4-fold greater than in plasma. Using this correction factor, the osmolality of the urine adjusted for ethanol in the patient was only 120 mosm/kg H2O, a value more consistent with the diagnosis of central diabetes insipidus.

Modulation of the secretion of potassium by accompanying anions in humans

In animals, secretion of potassium (K) in the cortical collecting duct (CCD) is modulated by the properties of the accompanying anion. In humans, results are inconclusive as previous studies have not differentiated between a kaliuresis due to a rise in the concentration of K from one due to an increase in the volume of urine. Our purpose was to study the effects of chloride (Cl) and bicarbonate on the secretion of K in the CCD in humans using the transtubular K concentration gradient (TTKG), a semi-quantitative index of secretion of K in the terminal CCD. After control blood and urine samples were obtained, all subjects ingested 0.2 mg fludrocortisone to ensure that mineralocorticoids were not limiting the secretion of K. The anionic composition of the urine was varied using three protocols: Normal subjects (N = 11) ingested cystine and methionine to induce sulfaturia; nine subjects with a contracted ECF volume (to lower the concentration of Cl in the urine) were also studied during sulfaturia following the ingestion of cystine and methionine; 13 normovolemic subjects were studied during bicarbonaturia following the ingestion of acetazolamide. When the concentration of Cl in the urine was greater than 15 mmol/liter, sulfate had no effect on the TTKG. With lower concentrations of Cl in the urine, the TTKG rose 1.5-fold. The TTKG rose 1.8-fold in the presence of bicarbonaturia despite concentrations of Cl in the urine that were greater than 15 mmol/liter, suggesting that bicarbonate has additional effects on this K secretory process. At comparable concentrations of sulfate and bicarbonate in the urine, the TTKG was increased only with bicarbonaturia.(ABSTRACT TRUNCATED AT 250 WORDS)

Glue-sniffing and distal renal tubular acidosis: sticking to the facts

An index case is presented to introduce the subject of the acid-base and electrolyte abnormalities resulting from toluene abuse. These include metabolic acidosis associated with a normal anion gap and excessive loss of sodium and potassium in the urine. The major question addressed is, what is the basis for the metabolic acidosis? Overproduction of hippuric acid resulting from the metabolism of toluene plays a more important role in the genesis of the metabolic acidosis than was previously believed. This conclusion is supported by the observation that the rate of excretion of ammonium was not low during metabolic acidosis in six of eight patients, suggesting that distal renal tubular acidosis was not an important acid-base abnormality in most cases where ammonium was measured. The excretion of hippurate in the urine unmatched by ammonium also mandates an enhanced rate of excretion of the cations, sodium and potassium. The loss of sodium causes extracellular fluid volume contraction and a fall in the glomerular filtration rate, which may transform the normal anion gap type of metabolic acidosis into one with a high anion gap (accumulation of hippurate and other anions). Continuing loss of potassium in the urine leads to hypokalemia. An understanding of the metabolism of toluene provides the basis for the unusual biochemical abnormalities seen with abuse of this solvent.

Removal of an inorganic acid load in subjects with ketoacidosis of chronic fasting

When a large inorganic acid load is ingested by normals, the proton load is eliminated because the rate of excretion of ammonium can rise to 200 to 300 mmol/day. In subjects with ketoacidosis of chronic fasting, such a large increase in the rate of excretion of ammonium might not be possible because of ATP balance considerations in proximal cells. Subjects with ketoacidosis of chronic fasting excreted less net acid as defined in the conventional way when they consumed a large inorganic acid load (136 +/- 6 vs. 176 +/- 26 mmol/day in control fasted subjects). Nevertheless, the vast majority of this inorganic acid load was eliminated because they were in steady state and had only a slightly lower concentration of bicarbonate (13 +/- 0.6 vs. 15 +/- 0.5 mmol/liter) and ketoacid anions (3.3 +/- 0.2 vs. 5.5 +/- 0.2 mmol/liter) in their blood. Using a definition of net acid excretion where the component of bicarbonate loss was expanded to include "potential bicarbonate" (ketoacid anions) in the urine, the rate of excretion of net acid was higher in subjects who ingested the inorganic acid load, owing to a much lower rate of excretion of ketoacid anions (9 +/- 2 vs. 120 +/- 7 mmol/day). This lower rate of excretion was not only due to a lower filtered load, but also to a higher fractional reabsorption of ketoacid anions during acidosis (97 +/- 0.1 vs. 77 +/- 0.2%). This higher fractional reabsorption could not be explained by a lower filtered load of ketoacid anions or to a restricted intake of sodium.(ABSTRACT TRUNCATED AT 250 WORDS)

The excretion of ammonium ions and acid base balance

The role of the kidney in acid base balance is to generate "new" bicarbonate ions, largely as a result of the excretion of ammonium ions. Three points will be covered in this review. First, we challenge the traditional view that the proximal nephron reclaims filtered bicarbonate ions, whereas, the distal nephron generates "new" bicarbonate ions. Virtually all "new" bicarbonate ions are generated in the proximal convoluted tubule during glutamine metabolism; very little is formed at distal sites. Second, the excretion of ammonium ions plays an important role in acid base balance only during chronic ketoacidosis, in response to diarrhea, in chronic renal insufficiency, and in distal renal tubular acidosis. Third, although the excretion of ammonium ions is said to signal the addition of bicarbonate ions to the extracellular fluid, the anion excreted with the ammonium cation is also important for acid base balance.

The transtubular potassium concentration in patients with hypokalemia and hyperkalemia

It is advantageous to make an independent assessment of the potassium (K) secretory process and the luminal flow rate in the renal cortex to evaluate K handling by the kidney during hypokalemia or hyperkalemia. The transtubular potassium concentration gradient (TTKG) is a semiquantitative index of the activity of the K secretory process. The purpose of this study was to define expected values for the TTKG in normal subjects with hypokalemia or following an acute K load. During hypokalemia of non-renal origin, the TTKG was 0.9 +/- 0.2; in contrast, the TTKG was significantly higher during the hypokalemia of hyperaldosteronism, 6.7 +/- 1.3. The TTKG was 11.8 +/- 3.6, 2 hours after normokalemic subjects received 0.2 mg 9 alpha-fludrocortisone (9 alpha-F). To obtain expected values during hyperkalemia, normal subjects ingested 50 mmol potassium chloride; 2 hours later, the TTKG was 13.1 +/- 3.8. Therefore, the expected value for the TTKG must be interpreted relative to the concentration of K in the plasma. Circumstances were also defined where the TTKG is low despite hyperaldosteronism, namely, during a water diuresis and pre-existing hypokalemia.

Urine electrolytes and osmolality: when and how to use them

The purpose of this review is to provide an update on the use of the urine electrolyte and osmolality measurements in patients with disorders of fluid, electrolytes, and/or acid-base metabolism. It is critical to appreciate that there are no 'normal values' for these parameters, only 'expected values' relative to clinical situations. Pitfalls in the interpretation of each electrolyte in the urine are also provided. To detect a mild to moderate degree of reduction of the 'effective' intravascular volume, both urine sodium (Na) and chloride (Cl) concentrations should be measured. Pitfalls in this assessment are abnormal renal and adrenal function and the use of diuretics. Insights into the etiology of the low 'effective' intravascular volume can be deduced by comparing the urine Na, potassium (K), and Cl concentrations. The urine net charge (Cl vs. Na + K) is the most reliable way to estimate the urine ammonium concentration short of its direct measurement, an assay that is not provided by most laboratories. This measurement is important in the differential diagnosis of hyperchloremic metabolic acidosis. To examine the renal response to hypokalemia or hyperkalemia, the two components of K excretion (K secretion and urine flow rate) should be examined separately. The former is evaluated using the transtubular K, concentration gradient. The urine osmolality is used to assess antidiuretic hormone action and the osmolality of the renal medulla and to determine the etiology of polyuria and/or hypernatremia. The urine osmolality can also be used to assess the ammonium concentration, using the urine osmolal gap, and to detect unusual urine osmoles.

Low dose ciclosporin from the early postoperative period yields potent immunosuppression after renal transplantation

This study sought to determine if low doses of ciclosporin (CS) designed to give fasting serum levels of 50-100 ng/ml achieve effective immunosuppression when used from the early postoperative period after renal transplantation. Ninety-four primary renal transplant recipients were studied. Group 1 patients were treated with CS 100 ng/ml and prednisone (0.15 mg/kg/day). Group 2 patients received CS 50 ng/ml, prednisone (0.15 mg/kg/day) and azathioprine (1 mg/kg/day). These patients were compared to a control group of 26 patients (group 3) maintained on only prednisone and azathioprine. CS-treated patients suffered significantly fewer rejection episodes than control subjects (rejection episodes per patient in first year: group 1: 0.3 +/- SD 0.6; group 2: 0.7 +/- SD 0.7; group 3: 1.3 +/- SD 1.1, p less than 0.005). In addition, a greater number of CS-treated patients were completely free of rejection episodes during the first year posttransplant (group 1: 63%; group 2: 64%; group 3: 19%, p less than 0.005). Patient and graft survival were similar in all groups after 1 year (group 1: 92 and 92% respectively; group 2: 95 and 87% respectively; group 3: 96 and 85% respectively). These data suggest that the dose of CS required for effective immunosuppression in vivo is lower than has been previously thought.

Ammonium excretion in chronic metabolic acidosis: benefits and risks

The expected renal response to a chronic acid load is an enhanced rate of ammonium production and excretion. Notwithstanding, high rates of ammonium production and/or excretion on a chronic basis may have detrimental consequences to patients. Examples discussed include the loss of extra lean body mass during chronic fasting, an accelerated rate of progression of renal insufficiency and possibly destruction of the medullary area of the kidney owing to local alkalinization.

Urine electrolytes in the assessment of extracellular fluid volume contraction

The purpose of this study was to determine which urine electrolytes should be measured to confirm that the extracellular fluid (ECF) volume is depleted. ECF volume contraction was induced by furosemide administration to rats consuming an electrolyte-free diet. An external potassium balance was achieved by replacing potassium losses with KHCO3 and KCl so that the sodium and chloride deficits were comparable (equivalent to a 30% reduction in ECF volume). As expected, the urine sodium and chloride concentrations fell to 2 +/- 0.3 mmol/l and 3 +/- 0.3 mmol/l, respectively. Rats were then randomized to receive 50-75% of their sodium or chloride deficit as either: NaCl (control group), NH4Cl or NaHCO3 to mimic clinical situations associated with ECF volume contraction. In the NaCl group, the urine sodium and chloride concentrations remained low (6 +/- 2 mmol/l and 7 +/- 2 mmol/l), consistent with persistent ECF volume contraction. Although the NH4Cl group continued to have a low urine sodium concentration (2 +/- 0.2 mmol/l), there was now a marked increase in the urine chloride concentration (51 +/- 7 mmol/l; p less than 0.01 vs. NaCl group). In contrast, although the NaHCO3 group continued to have a low urine chloride concentration (2 +/- 1 mmol/l), there was a significant increase in the urine sodium concentration (19 +/- 3 mmol/l; p less than 0.01 vs. NaCl group). We conclude that the clinical assessment of ECF volume by urine electrolytes requires an evaluation of both the urine sodium and chloride concentrations.

What is the underlying defect in patients with isolated, proximal renal tubular acidosis?

Our aim in this article is to propose a new hypothesis concerning the etiology of renal tubular acidosis (RTA) in that subgroup of patients who have the isolated, primary type of proximal RTA. We suggest that their underlying disorder is a more alkaline intracellular pH of the proximal convoluted tubule. Increased alkalinity of proximal tubular cells would explain the low rate of bicarbonate reabsorption per liter glomerular filtration and the decreased rate of ammonium excretion despite a low urine pH and the presence of chronic metabolic acidosis. Additional diagnostic tests to evaluate this hypothesis in this specific subgroup of patients with proximal RTA are also outlined.