Osmotic and Nonosmotic Sodium Storage during Acute Hypertonic Sodium Loading

Acute sodium bicarbonate administration improves ventilatory efficiency in experimental respiratory acidosis: clinical implications

Administering sodium bicarbonate (NaHCO3) to patients with respiratory acidosis breathing spontaneously is contraindicated because it increases carbon dioxide load and depresses pulmonary ventilation. Nonetheless, several studies have reported salutary effects of NaHCO3 in patients with respiratory acidosis but the underlying mechanism remains uncertain. Considering that such reports have been ignored, we examined the ventilatory response of unanesthetized dogs with respiratory acidosis to hypertonic NaHCO3 infusion (1 N, 5 mmol/kg) and compared it with that of animals with normal acid-base status or one of the remaining acid-base disorders. Ventilatory response to NaHCO3 infusion was evaluated by examining the ensuing change in PaCO2 and the linear regression of the PaCO2 vs. pH relationship. Strikingly, PaCO2 failed to increase and the ΔPaCO2 vs. ΔpH slope was negative in respiratory acidosis, whereas PaCO2 increased consistently and the ΔPaCO2 vs. ΔpH slope was positive in the remaining study groups. These results cannot be explained by differences in buffering-induced decomposition of infused bicarbonate or baseline levels of blood pH, PaCO2, and pulmonary ventilation. We propose that NaHCO3 infusion improved the ventilatory efficiency of animals with respiratory acidosis, i.e., it decreased their ratio of total pulmonary ventilation to carbon dioxide excretion (VE/VCO2). Such exclusive effect of NaHCO3 infusion in animals with respiratory acidosis might emanate from baseline increased VD/VT (dead space/tidal volume) caused by bronchoconstriction and likely reduced pulmonary blood flow, defects that are reversed by alkali infusion. Our observations might explain the beneficial effects of NaHCO3 reported in patients with acute respiratory acidosis.

Hypertonic Saline Infusion for Hyponatremia: Limitations of the Adrogué-Madias and Other Formulas

Hypertonic saline infusion is used to correct hyponatremia with severe symptoms. The selection of the volume of infused hypertonic saline ( VInf ) should address prevention of overcorrection or undercorrection. Several formulas computing this VInf have been proposed. The limitations common to these formulas consist of (1) failure to include potential determinants of change in serum sodium concentration ([ Na ]) including exchanges between osmotically active and inactive sodium compartments, changes in hydrogen binding of body water to hydrophilic compounds, and genetic influences and (2) inaccurate estimates of baseline body water entered in any formula and of gains or losses of water, sodium, and potassium during treatment entered in formulas that account for such gains or losses. In addition, computing VInf from the Adrogué-Madias formula by a calculation assuming a linear relation between VInf and increase in [ Na ] is a source of errors because the relation between these two variables was proven to be curvilinear. However, these errors were shown to be negligible by a comparison of estimates of VInf by the Adrogué-Madias formula and by a formula using the same determinants of the change in [ Na ] and the curvilinear relation between this change and VInf . Regardless of the method used to correct hyponatremia, monitoring [ Na ] and changes in external balances of water, sodium, and potassium during treatment remain imperative.

Determinants of hypokalemia following hypertonic sodium bicarbonate infusion

The hypokalemic response to alkali infusion has been attributed to the resulting extracellular fluid (ECF) expansion, urinary potassium excretion, and internal potassium shifts, but the dominant mechanism remains uncertain. Hypertonic NaHCO₃ infusion (1 N, 5 mmol/kg) to unanesthetized dogs with normal acid-base status or one of the four chronic acid-base disorders decreased plasma potassium concentration ([K⁺]_p) at 30 min in all study groups (Δ[K⁺]_p, - 0.16 to - 0.73 mmol/L), which remained essentially unaltered up to 90-min postinfusion. ECF expansion accounted for only a small fraction of the decrease in ECF potassium content, (K⁺)_e. Urinary potassium losses were large in normals and chronic respiratory acid-base disorders, limited in chronic metabolic alkalosis, and minimal in chronic metabolic acidosis, yet, ongoing kaliuresis did not impact the stability of [K⁺]_p. All five groups experienced a reduction in (K⁺)_e at 30-min postinfusion, Δ(K⁺)_e remaining unchanged thereafter. Intracellular fluid (ICF) potassium content, (K⁺)_i, decreased progressively postinfusion in all groups excluding chronic metabolic acidosis, in which a reduction in (K⁺)_e was accompanied by an increase in (K⁺)_i. We demonstrate that hypokalemia following hypertonic NaHCO₃ infusion in intact animals with acidemia, alkalemia, or normal acid-base status and intact or depleted potassium stores is critically dependent on mechanisms of internal potassium balance and not ECF volume expansion or kaliuresis. We envision that the acute NaHCO₃ infusion elicits immediate ionic shifts between ECF and ICF leading to hypokalemia. Thereafter, maintenance of a relatively stable, although depressed, [K⁺]_e requires that cells release potassium to counterbalance ongoing urinary potassium losses.

Edelman Revisited: Concepts, Achievements, and Challenges

The key message from the 1958 Edelman study states that combinations of external gains or losses of sodium, potassium and water leading to an increase of the fraction (total body sodium plus total body potassium) over total body water will raise the serum sodium concentration ([Na]_S), while external gains or losses leading to a decrease in this fraction will lower [Na]_S. A variety of studies have supported this concept and current quantitative methods for correcting dysnatremias, including formulas calculating the volume of saline needed for a change in [Na]_S are based on it. Not accounting for external losses of sodium, potassium and water during treatment and faulty values for body water inserted in the formulas predicting the change in [Na]_S affect the accuracy of these formulas. Newly described factors potentially affecting the change in [Na]_S during treatment of dysnatremias include the following: (a) exchanges during development or correction of dysnatremias between osmotically inactive sodium stored in tissues and osmotically active sodium in solution in body fluids; (b) chemical binding of part of body water to macromolecules which would decrease the amount of body water available for osmotic exchanges; and (c) genetic influences on the determination of sodium concentration in body fluids. The effects of these newer developments on the methods of treatment of dysnatremias are not well-established and will need extensive studying. Currently, monitoring of serum sodium concentration remains a critical step during treatment of dysnatremias.