Clinical utility of serum biochemical variables for predicting acid-base balance in critically ill horses

Acid-base variables in acute and chronic form of nontuberculous mycobacterial infection in growing goats experimentally inoculated with Mycobacterium avium subsp. hominissuis or Mycobacterium avium subsp. paratuberculosis

In current literature, data assessing the acid-base equilibrium in animals and humans during bacterial infection are rare. This study aimed to evaluate acid-base deteriorations in growing goats with experimentally induced NTM (nontuberculous mycobacteria) infections by application of the traditional Henderson-Hasselbalch approach and the strong ion model. NTM-challenged animals were orally inoculated with either Mycobacterium avium subsp. hominissuis (MAH; n = 18) or Mycobacterium avium subsp. paratuberculosis (MAP; n = 48). Twenty-five goats served as non-infected controls. Until 51^st week post-inoculation (wpi), blood gas analysis, serum biochemical analysis, and serum electrophoresis were performed on venous blood. Fifty percent (9/18) of goats inoculated with MAH developed acute clinical signs like apathy, fever, and diarrhea. Those animals died or had to be euthanized within 11 weeks post-inoculation. This acute form of NTM-infection was characterized by significantly lower concentrations of sodium, calcium, albumin, and total protein, as well as significantly higher concentrations of gamma globulin, associated with reduced albumin/globulin ratio. Acid-base status indicated alkalosis, but normal base excess and HCO₃^- concentrations, besides significantly reduced levels of SID (strong ion difference), A_{tot Alb} (total plasma concentration of weak non-volatile acids, based on albumin), A_{tot TP} (A_tot based on total protein) and markedly lower SIG (strong ion gap). The remaining fifty percent (9/18) of MAH-infected goats and all goats challenged with MAP survived and presented a more sub-clinical, chronic form of infection mainly characterized by changes in serum protein profiles. With the progression of the disease, concentrations of gamma globulin, and total protein increased while albumin remained lower compared to controls. Consequently, significantly reduced albumin/globulin ratio and lower A_{tot Alb} as well as higher A_{tot TP} were observed. Changes were fully compensated with no effect on blood pH. Only the strong ion variables differentiated alterations in acid-base equilibrium during acute and chronic NTM-infection.

Agreement of 2 electrolyte analyzers for identifying electrolyte and acid-base disorders in sick horses

BACKGROUND

Use of different analyzers to measure electrolytes in the same horse can lead to different interpretation of acid-base balance when using the simplified strong ion difference (sSID) approach.

OBJECTIVE

Investigate the level of agreement between 2 analyzers in determining electrolytes concentrations, sSID variables, and acid-base disorders in sick horses.

ANIMALS

One hundred twenty-four hospitalized horses.

METHODS

Retrospective study using paired samples. Electrolytes were measured using a Beckman Coulter AU480 Chemistry analyzer (PBMA) and a Nova Biomedical Stat Profile (WBGA), respectively. Calculated sSID variables included strong ion difference, SID₄ ; unmeasured strong ions, USI; and total nonvolatile buffer ion concentration in plasma (A_tot ). Agreement between analyzers was explored using Passing-Bablok regression and Bland-Altman analysis. Kappa (κ) test evaluated the level of agreement between analyzers in detecting acid-base disorders.

RESULTS

Methodologic differences were identified in measured Na⁺ and Cl^- and calculated values of SID₄ and USI. Mean bias (95% limits of agreement) for Na⁺ , Cl^- , SID₄ , and USI were: -1.2 mmol/L (-9.2 to 6.8), 4.4 mmol/L (-4.4 to 13), -5.4 mmol/L (-13 to 2), and -6.2 mmol/L (-14 to 1.7), respectively. The intraclass correlation coefficient for SID₄ and USI was .55 (95%CI: -0.2 to 0.8) and .2 (95%CI: -0.15 to 0.48), respectively. There was a poor agreement between analyzers for detection of SID₄ (κ = 0.20, 95%CI, 0.1 to 0.31) or USI abnormalities (κ = -0.04, 95%CI, -0.11 to 0.02).

CONCLUSIONS AND CLINICAL IMPORTANCE

Differences between analyzer methodology in measuring electrolytes led to a poor agreement between the diagnosis of acid-base disorders in sick horses when using the sSID approach.

Age-dependent physiological dynamics in acid-base balance, electrolytes, and blood metabolites in growing goats

There is a paucity of published data reporting acid-base equilibrium in goats, and no information is available on how the acid-base complexity changes when suckling goat kids become ruminants. The aims of this study were to evaluate young healthy goats for age-related changes in serum proteins, metabolites, and electrolytes; differences in results when the Henderson-Hasselbalch equation or strong ion approaches were used were also assessed. To assess biological variability and reproducibility, two consecutive long-term studies, each lasting from the 6th to 56th week of life (wl), were performed in 15 (Study 1) and 10 (Study 2) animals. Blood gas analysis, serum biochemical analysis, and electrophoresis were performed on venous blood, and acid-base information was obtained using the traditional Henderson-Hasselbalch approach, Stewart's strong ion model, and Constable's simplified strong ion model. In all goats within the first 4-5 months, serum concentrations of glucose, l-lactate, and inorganic phosphate decreased significantly, while serum concentrations of total protein, albumin, and gamma globulin increased. Consequently, nonvolatile weak acids (A_{tot Alb} and A_{tot TP}) increased. At the end of this 'adaptation period', i.e. when milk was replaced by purely plant-based food, significantly lower bicarbonate and base excess values were accompanied by blood pH that shifted towards acidosis. Electrolytes (Na⁺, K⁺, Ca²⁺, and Cl^-), anion gap, strong ion difference, and strong ion gap did not show age-dependent trends. In conclusion, somatic growth and development of gastro-intestinal fermentation in growing goats act as complex sources of physiological variability on acid-base equilibrium that was not reflected by the Henderson-Hasselbalch equation only.

Effect of Intravenously Administered Crystalloid Solutions on Acid-Base Balance in Domestic Animals

Intravenous fluid therapy can alter plasma acid-base balance. The Stewart approach to acid-base balance is uniquely suited to identify and quantify the effects of the cationic and anionic constituents of crystalloid solutions on plasma pH. The plasma strong ion difference (SID) and weak acid concentrations are similar to those of the administered fluid, more so at higher administration rates and with larger volumes. A crystalloid's in vivo effects on plasma pH are described by 3 general rules: SID > [HCO3-] increases plasma pH (alkalosis); SID < [HCO3-] decreases plasma pH (alkalosis); and SID = [HCO3-] yields no change in plasma pH. The in vitro pH of commercially prepared crystalloid solutions has little to no effect on plasma pH because of their low titratable acidity. Appreciation of IV fluid composition and an understanding of basic physicochemical principles provide therapeutically valuable insights about how and why fluid therapy can produce and correct alterations of plasma acid-base equilibrium. The ideal balanced crystalloid should (1) contain species-specific concentrations of key electrolytes (Na+ , Cl- , K+ , Ca++ , Mg++ ), particularly Na+ and Cl- ; (2) maintain or normalize acid-base balance (provide an appropriate SID); and (3) be isosmotic and isotonic (not induce inappropriate fluid shifts) with normal plasma.

Physicochemical Approach to Determine the Mechanism for Acid-Base Disorders in 793 Hospitalized Foals

BACKGROUND

The quantitative effect of strong electrolytes, unmeasured strong anions (UAs), pCO2, and plasma protein concentrations in determining plasma pH can be demonstrated using the physicochemical approach. Plasma anion gap (AG) and strong ion gap (SIG) are used to assess UAs in different species.

HYPOTHESES

Strong ions are a major factor influencing changes in plasma pH of hospitalized foals. AG and SIG accurately predict severe hyper-L-lactatemia ([L-lac(-)] > 7 mmol/L).

ANIMALS

Seven hundred and ninety three hospitalized foals < 7 days old.

METHODS

Retrospective study. The relationship between measured pH and physicochemical variables, and the relationship between plasma [L-lac(-)] and AG and SIG, were determined using regression analyses. Optimal AG and SIG cut points to predict hyper-L-lactatemia were identified using an ROC curve analysis.

RESULTS

Combined, the measured strong ion difference and SIG accounted for 54-69% of the changes in the measured arterial pH of hospitalized foals. AG and SIG were significantly associated with plasma [L-lac(-)] (P < .0001). The receiver operator characteristics (ROC) AUC of AG and SIG for prediction of severe hyper-L-lactatemia were 0.89 (95%CI, 0.8-0.95; P < .0001) and 0.90 (95%CI, 0.81-0.96; P < .0001), respectively. Severe hyper-L-lactatemia was best predicted by AG > 27 mmol/L (sensitivity 80%, 95%CI, 56-94, specificity 85%, 95%CI, 73-93; P < .0001) and SIG <-15 mmol/L (sensitivity 90%, 95%CI, 68-98; specificity 80%; 95%CI, 68-90; P < .0001).

CONCLUSION AND CLINICAL RELEVANCE

Altered concentrations of strong ions (Na(+), K(+), Cl(-)) and UAs were the primary cause of acidemia of hospitalized foals. AG and SIG were good predictors of hyper-L-lactatemia and could be used as surrogate tests.

Quantitative accuracy of the simplified strong ion equation to predict serum pH in dogs

Background

Electrochemical approach to the assessment of acid‐base states should provide a better mechanistic explanation of the metabolic component than methods that consider only pH and carbon dioxide.

Hypothesis/Objectives

Simplified strong ion equation (SSIE), using published dog‐specific values, would predict the measured serum pH of diseased dogs.

Animals

Ten dogs, hospitalized for various reasons.

Methods

Prospective study of a convenience sample of a consecutive series of dogs admitted to the Massey University Veterinary Teaching Hospital (MUVTH), from which serum biochemistry and blood gas analyses were performed at the same time. Serum pH was calculated (Hcal+) using the SSIE, and published values for the concentration and dissociation constant for the nonvolatile weak acids (A_tot and K_a), and subsequently Hcal+ was compared with the dog's actual pH(Hmeasured+). To determine the source of discordance between Hcal+ and Hmeasured+, the calculations were repeated using a series of substituted values for A_tot and K_a.

Results

The Hcal+ did not approximate the Hmeasured+ for any dog (P = 0.499, r² = 0.068), and was consistently more basic. Substituted values A_tot and K_a did not significantly improve the accuracy (r² = 0.169 to <0.001). Substituting the effective SID(Atot−[HCO3−]) produced a strong association between Hcal+ and Hmeasured+ (r² = 0.977).

Conclusions and clinical importance

Using the simplified strong ion equation and the published values for A_tot and K_a does not appear to provide a quantitative explanation for the acid‐base status of dogs. Efficacy of substituting the effective SID in the simplified strong ion equation suggests the error lies in calculating the SID.