describe what happened to the concentration of ions in the urine

• Journal List
• Crit Care
• 5.10(two); 2006
• PMC1550906

Crit Care. 2006; ten(2): 137.

Strong ion difference in urine: new perspectives in acid-base assessment

Luciano Gattinoni

¹Dipartimento di Anestesia, Rianimazione, e Terapia del Dolore, Fondazione IRCCS – 'Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena' di Milano, Istituto di Anestesiologia eastward Rianimazione, Università degli Studi di Milano, Milano, Italy

Eleonora Carlesso

^twoIstituto di Anestesiologia due east Rianimazione, Università degli Studi di Milano, Milano, Italia

Paolo Cadringher

²Istituto di Anestesiologia east Rianimazione, Università degli Studi di Milano, Milano, Italy

Pietro Caironi

²Istituto di Anestesiologia e Rianimazione, Università degli Studi di Milano, Milano, Italian republic

Abstract

The plasmatic strong ion difference (SID) is the difference between positively and negatively charged strong ions. At pH 7.iv, temperature 37°C and fractional carbon dioxide tension 40 mmHg, the ideal value of SID is 42 mEq/l. The buffer base is the sum of negatively charged weak acids ([HCO_iii ^-], [A^-], [H₂PO_four ^-]) and its normal value is 42 mEq/50. Co-ordinate to the law of electroneutrality, the amount of positive and negative charges must exist equal, and therefore the SID value is equal to the buffer base value. The easiest assessment of metabolic acidosis/alkalosis relies on the base of operations backlog adding: buffer base of operations_actual - buffer base of operations_ideal = SID_actual - SID_platonic. The SID arroyo allows 1 to appreciate the human relationship between acid–base and electrolyte equilibrium from a unique perspective, and here nosotros describe a comprehensive model of this equilibrium. The extracellular volume is characterized past a given SID, which is a function of baseline conditions, endogenous and exogenous input (endogenous production and infusion), and urinary output. Of annotation, volume modifications vary the concentration of charges in the solution. An expansion of extracellular volume leads to acidosis (SID decreases), whereas a wrinkle of extracellular volume leads to alkalosis (SID increases). A thorough understanding of acid–base equilibrium mandates recognition of the importance of urinary SID.

Traditionally, the assessment of metabolic acidosis and alkalosis relies on measurement of the base backlog, which is the deviation between the 'ideal' buffer base [i] (i.east. the sum of the negatively charged forms of weak acids, [A^-] + [HCO₃ ^-] + [H₂PO₄ ^-], at standard weather condition (pH 7.4, temperature 37°C, fractional carbon dioxide tension forty mmHg) and the 'bodily' buffer base of operations [2]):

Base of operations excess = buffer base_bodily - buffer base of operations_ideal     (ane)

During the by few years a novel approach based on assessment of the strong ion difference (SID) has been introduced to evaluate metabolic acidosis and alkalosis. For simplicity, nosotros limit our discussion to these 2 disturbances.

Please notation that in the post-obit discussion we volition refer to the amount of potent ion difference as SID (mEq), while we will refer to the strong ion difference concentration as [SID] (mEq/l).

Past definition, strong ions are ever dissociated in a solution. In plasma, as well as in interstitial fluids, the sum of positively charged ions (primarily Na⁺, K⁺, Ca²⁺ and Mg²⁺) exceeds the sum of the negatively charged strong ions (primarily Cl^- and lactate^-) of about 42 mEq/50. This departure is called the SID, and according to the Stewart model [3] its variation is i of the determinants of acrid–base condition. Looking at Figure ​ 1, the connection between base excess and SID is credible. The buffer base and SID are equivalent. In fact, because the ideal SID is equal to 42 mEq/l (as is the normal buffer base of operations), it follows that

Gamblegram. The figure shows gamblegrams during ideal conditions and during acidosis. In ideal conditions the difference between positively and negatively charged strong ions is equal to 42 mEq/l (the stiff ion difference [SID]) and, according to the law of electro-neutrality, is equivalent to the buffer base of operations (BB; i.e. the sum of [HCO_three ^-], [A^-] and [H₂PO_four ^-], where A^- are the weak acids in dissociated course, mainly albumin). During acidosis, SID decreases merely the law of electro-neutrality is still satisfied. It follows that base excess = BB_actual - BB_ideal = SID_acidosis - SID_ideal.

Base of operations excess = SID_actual - SID_ideal = buffer base of operations_bodily - buffer base_ideal     (ii)

Because computation of the bodily SID is rather complicated, requiring the determination of all of the strong ion concentrations, we believe that the base excess arroyo may be easier, more than rapid and adequate for clinical purposes. Indeed, the frequent argue involving the comparing of the 'SID approach' with the 'base backlog arroyo' to assessment of metabolic acidosis [4,5] appears futile because their physiological meanings, as well equally their variations, are identical. In other words, the 2 approaches look at the same thing from different points of view.

The motion-picture show is different when ane considers the 'agreement' of acrid–base and electrolyte equilibria, which anybody has studied in separate capacity of the textbooks. The great merit of the Stewart arroyo is that it considers electrolytes and acid–base status in a common framework. Here, nosotros would like to propose a comprehensive model that may explicate, at least qualitatively, many of the findings observed in clinical practice and in the literature.

The SID reflects the difference in electrical charges of the potent ions in the volume of the extracellular compartment (V). At time 0, it will be equal to V(0) × [SID(0)]. For example, if at time 0 the SID is normal (i.e. 42 mEq/l) then the net amount of electrical charge in the extracellular fluid (fifteen fifty) will be 630 mEq. During a given flow of time there may be an improver of book to the system (eastward.grand. infusion of a solution) with its ain SID (SID_infusion). Consequently, a net amount of charge equal to 5_infusion × [SID_infusion] will be added to the system. Similarly, the urinary system will excrete a volume of urine (V_urine) with its own SID (SID_urine). The terminal variable that must be taken into account is endogenous production of SID (sulphates, phosphates, lactate and ketoacids, among other components). Information technology follows that the SID at a given time 't' may be derived from a serial of equations, which may announced to exist complicated in their expression merely unproblematic in their meaning. Eqn 3 (below) indicates that, in a arrangement, the cyberspace corporeality of electrical charges due to the strong ions is equal to the cyberspace electrical charge of the system at time zero plus the internet electrical charge added as a result of metabolism plus the net electric charge added with book infusion minus the net electrical charge extracted via urine.

where EPR(t) is the 'endogenous production rate' of SID (mEq/min), IR(t) is the volume infusion charge per unit and UPR(t) is the urine production rate. At a given fourth dimension 't', the net fluid book of the extracellular compartment is equal to the initial volume of the organization plus the volume added with infusion minus the book extracted in the form of urine.

Because what matters in terms of acrid–base status is the concentration, rather than the net amount of electrical accuse, the SID at a given time 't' may exist expressed from the above equations as shown in equation 5 at the foot of the page:

Information technology is important to call up that an increase in SID volition lead the organization to become more basic whereas a subtract in SID will pb the organization to get more than acidic. In full general, Eqn 5 indicates that metabolic acidosis or alkalosis may occur either by changing the net electrical charge at constant extracellular volume or by irresolute the extracellular volume at constant electric charge.

Looking at Eqn five, we may make several comments. To maintain the metabolic acid–base condition of a system (i.e. the baseline SID), ii weather condition must be satisfied: the input quantity of SID should equal the output quantity of SID; and the distribution volume of SID should remain constant. To the all-time of our cognition, the but studies in which the stiff ion remainder (input and output) was investigated were conducted in cows [half-dozen-viii]; different amounts of SID in the nutrition acquired corresponding changes in urinary SID. Unfortunately, no such investigation has been conducted in critically sick patients. Every bit discussed higher up, SID has been studied in comparison with base excess but without whatsoever physiological rationale [9]. The SID approach has been also proposed to explain metabolic acidosis during saline infusion (SID input) [10], simply but a few papers have tackled and discussed the problem of urinary SID (SID output) [11-13]. What we lack is the entire picture of the system; unfortunately, this requires frequent assessment of urine electrolytes.

Some clinical findings may exist viewed from the perspective of the general framework of Eqn 5. It is well known that rapid infusion of saline induces metabolic acidosis. This has been attributed to changes in SID due to hyperchloraemia [10]. By looking at Eqn 3 we derive a different point of view. Considering the SID of saline is equal to 0, information technology follows that, if the urinary output of electrical charge and metabolic production remain abiding, the cyberspace difference of electric charges in the organisation (i.eastward. the numerator in Eqn 5) does not modify. What causes the acidosis is the expansion of the extracellular volume (volume input greater than book output), which leads to decreased concentration of the net amount of electrical accuse (i.e. the SID).

Unfortunately, information technology is not like shooting fish in a barrel to consider the urinary SID. In fact, although 40–42 mEq/l of plasmatic negative charge may be derived from the dissociated weak acids ([A^-], [HCO₃ ^-] and [H_twoPO_iv ^-]), the amount of weak acids is far less in urine and, overall, the range of urinary pH is an order of magnitude greater than that in plasma. One time over again, the problem is simpler when 1 considers the entire picture. In fact, as far as the plasmatic acid–base equilibrium is concerned, nosotros must consider only the components of urinary [SID] that may affect the plasmatic [SID] (i.e. [K⁺], [Na⁺] and [Cl^-]). In fact, in urine

[Na⁺] + [Thou⁺] + [Un⁺] = [Cl^-] + [Un^-]     (6)

where Un⁺ and United nations^- are the positive and negative unmeasured ions. It follows that

[Na⁺] + [K⁺] - [Cl^-] = [Un^-] - [United nations⁺]     (seven)

Quantitatively, the most of import anion in urine is SO₄ ^two-, which is derived from the metabolism of sulphur amino acids, whereas the most important cation is NH₄ ⁺. In normal conditions, the sum of urinary [Na⁺] + [K⁺] - [Cl^-] amounts to 42 mEq/fifty [14]. It follows that the concentration of unmeasured anions exceeds the concentration of unmeasured cations of 42 mEq/l. When a strong ion such as lactate is added to the plasma, the plasmatic SID will subtract. Consequently, the urinary arrangement volition react by increasing its excretion of chloride, thereby decreasing the plasma chloride concentration (while [Na⁺] and [K⁺] must be maintained within normal ranges). The increased excretion of chloride will subtract the urinary SID. Therefore, the difference between [Un^-] and [United nations⁺] should subtract (Eqn seven). This is accomplished past increasing the excretion rate of NH₄ ⁺, which is a manner to augment elimination of Cl^- without Na⁺ [11,15].

Indeed, the furnishings of any volume infusion or other interventions cannot be understood if the urinary SID and volume are non taken into account. A merit of the study by Moviat and colleagues [xiii] is that, for the first time in critical care, attention is focused on the urinary system, which is the main regulator of SID. The authors plant that the increase in urinary SID (indirectly induced by blocking carbonic anhydrase) was the key commuter for correction of metabolic alkalosis. The message is important – urinary SID should exist a key component of global acrid–base of operations cess. Nosotros believe that urinary electrolyte monitoring may open a new perspective of inquiry in critical intendance. Acid–base of operations equilibrium, i of the oldest research areas in medicine, is still an open up field for new discoveries and approaches.

Abbreviations

SID = potent ion difference.

Competing interests

The authors declare that they have no competing interests.

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