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Eric Niederhoffer, Ph.D. SIU-SOM Biochemical basis of acidosis and alkalosis: evaluating acid base disorders.

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Presentation on theme: "Eric Niederhoffer, Ph.D. SIU-SOM Biochemical basis of acidosis and alkalosis: evaluating acid base disorders."— Presentation transcript:

1 Eric Niederhoffer, Ph.D. SIU-SOM Biochemical basis of acidosis and alkalosis: evaluating acid base disorders

2 Outline Approach history physical examination differentials clinical and laboratory studies compensation Respiratory acidosis alkalosis Metabolic acidosis alkalosis

3 Approach History - subjective information concerning events, environment, trauma, medications, poisons, toxins Physical examination - objective information assessing organ system status and function Differentials - potential reasons for presentation Clinical and laboratory studies - degree of changes from normal Compensation - assessment of response to initial problem

4 Evaluation of Acid-Base Conditions Examine serum electrolytes (increased or decreased total CO 2, increased AG, abnormal HCO 3 - gap) and ABGs (directional changes in pH, HCO 3 -, and P CO 2 ). Examine ABG data for mixed acid-base conditions. Complete clinical assessment of history, physical examination, previous ABGs and serum electrolytes, along with other laboratory data. Identify underlying clinical cause(s) for each acid-base disorder. Treat the clinical conditions.

5 Reference ranges and points ParameterReference rangeReference point Na mEq/L K mEq/L Cl mEq/L CO 2, total24-30 mEq/L pH P CO mm Hg40 mm Hg P O mm Hg HCO mEq/L24mEq/L Anion gap8-16 mEq/L12 mEq/L Bicarbonate gap-6-6 mEq/L Osmolar gap<10 mOsm/L

6 Evaluation of Serum Electrolytes Total CO 2 Increased, >30 mEq/L metabolic alkalosis HCO 3 - retention for respiratory acidosis Decreased, <24 mEq/L metabolic acidosis (AG or HCA) HCO 3 - excretion for respiratory alkalosis Normal Anion Gap NormalIncreased, >20 mEq/L consider potential cause Decreased, <8 mEq/L consider hypoproteinemia, abnormal proteins or cations Negative, < -6 mEq/L hyperchloremic acidosis (HCA) and/or HCO 3 - excretion for respiratory alkalosis Bicarbonate Gap Positive, > 6 mEq/L metabolic alkalosis and/or HCO 3 - retention for respiratory acidosis

7 Evaluation of Arterial Blood Gas Metabolic acidosis Metabolic alkalosis Respiratory acidosis Respiratory alkalosis Primary processCompensatory response

8 Delta ratio Assessment <0.4Hyperchloraemic normal anion gap acidosis 0.4 – 0.8 Combined high AG and normal AG acidosis Note that the ratio is often <1 in acidosis associated with renal failure Uncomplicated high-AG acidosis Lactic acidosis: average value 1.6 DKA more likely to have a ratio closer to 1 due to urine ketone loss (if patient not dehydrated) >2 Pre-existing increased [HCO 3 - ]: concurrent metabolic alkalosis pre-existing compensated respiratory acidosis ratio = Anion gap/[HCO 3 - ] = (AG – 12)/(24 - [HCO 3 - ])

9 Compensation Primary Disturbance pHHCO 3 - P CO 2 Compensation Respiratory acidosis<7.35Compensatory increase Primary increase Acute: 1-2 mEq/L increase in HCO 3 - for every 10 mm Hg increase in P CO 2 Chronic: 3-4 mEq/L increase in HCO 3 - for every 10 mm Hg increase in P CO 2 Respiratory alkalosis>7.45Compensatory decrease Primary decrease Acute: 1-2 mEq/L decrease in HCO 3 - for every 10 mm Hg decrease in P CO 2 Chronic: 4-5 mEq/L decrease in HCO 3 - for every 10 mm Hg decrease in P CO 2 Metabolic acidosis<7.35Primary decrease Compensatory decrease 1.2 mm Hg decrease in P CO 2 for every 1 mEq/L decrease in HCO 3 - Metabolic alkalosis>7.45Primary increase Compensatory increase mm Hg increase in P CO 2 for every 1 mEq/L increase in HCO 3 -, P CO 2 should not rise above 55 mm Hg in compensation

10 Respiratory acidosis P CO 2 greater than expected Acute or chronic Causes excess CO 2 in inspired air (rebreathing of CO 2 -containing expired air, addition of CO 2 to inspired air, insufflation of CO 2 into body cavity) decreased alveolar ventilation (central respiratory depression & other CNS problems, nerve or muscle disorders, lung or chest wall defects, airway disorders, external factors) increased production of CO 2 (hypercatabolic disorders)

11 R acid acute A 65-year-old man comes to the physician with a 3-hour history of shortness of breath after feeling ill for the past week. His BMI is 30 kg/m 2. His temperature is 38.3°C (101°F), pulse is 96/min, respirations are 20/min and shallow, and blood pressure is 145/90 mm Hg. Na mEq/LpH7.33 K mEq/LP O 2 61 mm Hg Cl mEq/LP CO 2 50 mm Hg CO 2, total28 mEq/LHCO mEq/L History suggests hypoventilation, supported by increased P CO 2 and lower than anticipated P O 2. Respiratory acidosis (acute) due to no renal compensation.

12 Description Na mEq/LpH7.33 K mEq/LP O 2 61 mm Hg Cl mEq/LP CO 2 50 mm Hg CO 2, total28 mEq/LHCO mEq/L AG = 11 mEq/LBG = 1 mEq/L 1-2 mEq/L increase in HCO 3 - for every 10 mm Hg increase in P CO 2. P CO 2 increase = = 10 mm Hg. HCO 3 - increase predicted = (1-2) x (10/10) = 1-2 mEq/L add to 24 mEq/L (reference point) = mEq/L

13 R acid chronic A 56-year-old woman with COPD is brought to the physician with a 3- hour history of shortness of breath. Her temperature is 37°C (98.6°F), pulse is 90/min, respirations are 22/min and shallow, and blood pressure is 135/80 mm Hg. Na mEq/LpH7.33 K mEq/LP O 2 52 mm Hg Cl - 99 mEq/LP CO 2 62 mm Hg CO 2, total34 mEq/LHCO mEq/L History suggests hypoventilation, supported by increased P CO 2. Respiratory acidosis (chronic) with renal compensation.

14 Description Na mEq/LpH7.33 K mEq/LP O 2 52 mm Hg Cl - 99 mEq/LP CO 2 62 mm Hg CO 2, total34 mEq/LHCO mEq/L AG = 14 mEq/LBG = 10 mEq/L 3-4 mEq/L increase in HCO 3 - for every 10 mm Hg increase in P CO 2. P CO 2 increase = = 22 mm Hg. HCO 3 - increase predicted = (3-4) x (22/10) = 7-9 mEq/L add to 24 mEq/L (reference point) = mEq/L

15 Respiratory alkalosis P CO 2 less than expected Acute or chronic Causes increased alveolar ventilation (central causes, direct action via respiratory center; hypoxaemia, act via peripheral chemoreceptors; pulmonary causes, act via intrapulmonary receptors; iatrogenic, act directly on ventilation)

16 R alk acute A 17-year-old woman is brought to the physician with a 3- hour history of epigastric pain and nausea. She admits taking a large dose of aspirin. Her respirations are 20/min and full. Na mEq/LpH7.55 K mEq/LP O mm Hg Cl mEq/LP CO 2 25 mm Hg CO 2, total23 mEq/LHCO mEq/L History suggests hyperventilation, supported by decreased P CO 2. Respiratory alkalosis (acute) due to no renal compensation.

17 Description Na mEq/LpH7.55 K mEq/LP O mm Hg Cl mEq/LP CO 2 25 mm Hg CO 2, total23 mEq/LHCO mEq/L AG = 12 mEq/LBG = -2 mEq/L 1-2 mEq/L decrease in HCO 3 - for every 10 mm Hg decrease in P CO 2. P CO 2 decrease = = 15 mm Hg. HCO 3 - decrease predicted = (1-2) x (15/10) = 2-3 mEq/L subtract from 24 mEq/L (reference point) = mEq/L

18 R alk chronic A 81-year-old woman with a history of anxiety is brought to the physician with a 2-day history of shortness of breath. She has been living at 9,000 ft elevation for the past 1 month. Her respirations are full at 20/min. Na mEq/LpH7.48 K mEq/LP O 2 69 mm Hg Cl mEq/LP CO 2 22 mm Hg CO 2, total17 mEq/LHCO mEq/L History suggests hyperventilation, supported by decreased P CO 2. Respiratory alkalosis (chronic) with renal compensation.

19 Description Na mEq/LpH7.48 K mEq/LP O 2 69 mm Hg Cl mEq/LP CO 2 22 mm Hg CO 2, total17 mEq/LHCO mEq/L AG = 12 mEq/LBG = -8 mEq/L 4-5 mEq/L decrease in HCO 3 - for every 10 mm Hg decrease in P CO 2. P CO 2 decrease = = 18 mm Hg. HCO 3 - decrease predicted = (4-5) x (18/10) = 7-9 mEq/L subtract from 24 mEq/L (reference point) = mEq/L

20 Metabolic acidosis Plasma HCO 3 - less than expected Gain of strong acid or loss of base Alternatively, high anion gap or normal anion gap metabolic acidosis Causes high anion-gap acidosis (normochloremic) (ketoacidosis, lactic acidosis, renal failure, toxins) normal anion-gap acidosis (hyperchloremic) (renal, gastrointestinal tract, other)

21 M acid increased AG A 75-year-old man with severe congestive heart failure is brought to the emergency department. He takes none of his prescribed medications. His respirations are 24/min and blood pressure is 80/50 mm Hg. He has decreased urine output; his baseline creatinine concentration has been 1.6 mg/dL. Na mEq/LpH7.19 K mEq/LP O 2 80 mm Hg Cl - 97 mEq/LP CO 2 21 mm Hg CO 2, total8 mEq/LHCO mEq/L Lactate20 mEq/L Urea54 mg/dL Creatinine2.5 mg/dL History suggests congestive heart failure (poor perfusion). Metabolic acidosis with appropriate respiratory compensation.

22 Description Na mEq/LpH7.19 K mEq/LP O 2 80 mm Hg Cl - 97 mEq/LP CO 2 21 mm Hg CO 2, total8 mEq/LHCO mEq/L Lactate20 mEq/L Urea54 mg/dLAG = 30 mEq/L Creatinine2.5 mg/dLBG = 2 mEq/L 1.2 mm Hg decrease in P CO 2 for every 1 mEq/L decrease in HCO 3 -. HCO 3 - decrease = 24-8 = 16 mEq/L P CO 2 decrease predicted = 1.2 x 16 = 19 mm Hg. subtract from 40 mm Hg (reference point) = 21 mm Hg

23 M acid normal AG A 2-year-old girl is brought to the physician because of a 1- week history of diarrhea. She is at the 30 th centile for height and weight. Physical examination shows no abnormalities. Laboratory studies show a fractional excretion of HCO 3 - of 2.5%. Na mEq/LpH7.34 K mEq/LP O 2 96 mm Hg Cl mEq/LP CO 2 29 mm Hg CO 2, total16 mEq/LHCO mEq/L Urine pH5.0 History suggests intestinal electrolyte loss. Metabolic acidosis with respiratory compensation.

24 Description Na mEq/LpH7.34 K mEq/LP O 2 96 mm Hg Cl mEq/LP CO 2 29 mm Hg CO 2, total16 mEq/LHCO mEq/L Urine pH5.0 FE HCO % AG = 12 mEq/LBG = -9 mEq/L 1.2 mm Hg decrease in P CO 2 for every 1 mEq/L decrease in HCO 3 -. HCO 3 - decrease = = 9 mEq/L P CO 2 decrease predicted = 1.2 x 9 = 11 mm Hg. subtract from 40 mm Hg (reference point) = 29 mm Hg

25 Metabolic alkalosis Plasma HCO 3 - greater than expected Loss of strong acid or gain of base Causes (2 ways to organize) loss of H + from ECF via kidneys (diuretics) or gut (vomiting) gain of alkali in ECF from exogenous source (IV NaHCO 3 infusion) or endogenous source (metabolism of ketoanions) or addition of base to ECF (milk-alkali syndrome) Cl - depletion (loss of acid gastric juice) K + depletion (primary/secondary hyperaldosteronism) Other disorders (laxative abuse, severe hypoalbuminaemia)

26 Urinary Chloride Spot urine Cl - less than 10 mEq/L often associated with volume depletion respond to saline infusion common causes - previous thiazide diuretic therapy, vomiting (90% of cases) Spot urine Cl - greater than 20 mEq/L often associated with volume expansion and hypokalemia resistant to therapy with saline infusion causes: excess aldosterone, severe K + deficiency, current diuretic therapy, Bartter syndrome

27 M alk low Urine Cl - An 24-year-old woman is brought to the physician with a 3- month history of weakness and fatigue. She has binges of eating followed by self-induced vomiting. Blood pressure is 90/60 mm Hg. Physical examination shows erosions of the lingual surface of the teeth. Na mEq/LpH7.52 K mEq/LP O 2 78 mm Hg Cl - 90 mEq/LP CO 2 49 mm Hg CO 2, total41 mEq/LHCO mEq/L Urine Cl - 5 mEq/L History and physical examination suggests bulimia nervosa. Metabolic alkalosis with respiratory compensation. The cause is most likely bulimia nervosa.

28 Description Na mEq/LpH7.52 K mEq/LP O 2 78 mm Hg Cl - 90 mEq/LP CO 2 49 mm Hg CO 2, total41 mEq/LHCO mEq/L Urine Cl - 5 mEq/L AG = 8 mEq/LBG = 11 mEq/L mm Hg increase in P CO 2 for every 1 mEq/L increase in HCO 3 -. HCO 3 - increase = = 15 mEq/L P CO 2 increase predicted = x 15 = 9-12 mm Hg. add to 40 mm Hg (reference point) = mm Hg

29 M alk high Urine Cl - An 83-year-old woman is brought to the physician with a 1- week history of weakness, nausea, and poor appetite. Her current medications are aspirin and hydrochlorothiazide. Her blood pressure is 110/70 mm Hg. Na mEq/LpH7.48 K mEq/LP O 2 66 mm Hg Cl - 77 mEq/LP CO 2 49 mm Hg CO 2, total38 mEq/LHCO mEq/L Urine Cl - 74 mEq/L History and physical examination suggest electrolyte imbalance. Metabolic alkalosis with respiratory compensation. The cause most likely is current diuretic therapy.

30 Description Na mEq/LpH7.48 K mEq/LP O 2 66 mm Hg Cl - 77 mEq/LP CO 2 49 mm Hg CO 2, total38 mEq/LHCO mEq/L Urine Cl - 74 mEq/L AG = 17 mEq/LBG = 17 mEq/L mm Hg increase in P CO 2 for every 1 mEq/L increase in HCO 3 -. HCO 3 - increase = = 12 mEq/L P CO 2 increase predicted = x 12 = 7-9 mm Hg. add to 40 mm Hg (reference point) = mm Hg

31 Review Questions What is an effective approach to acid base problems? What are the reference ranges and reference points? What are the anion, bicarbonate, and osmolar gap? What is the delta ratio? What is compensation? What are the characteristics of respiratory acidosis and alkalosis? What are the characteristics of metabolic acidosis and alkalosis? What is the utility of spot urine Cl - ?


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