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Acid Base Electrolytes KISS Method

Acid Base Electrolytes KISS Method. Beth Adams MS PAS Wayne State University. Introduction. Acid-base is a subject that is somewhat intimidating because:

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Acid Base Electrolytes KISS Method

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  1. Acid Base Electrolytes KISS Method Beth Adams MS PAS Wayne State University

  2. Introduction • Acid-base is a subject that is somewhat intimidating because: • chemists screwed it up: While every other medically important ion is measured in simple, easy to understand units, hydrogen ions are measured on a non-linear negative log rhythmic scale i.e. pH • When we are taught about acid base it is usually without benefit of knowing the patient and clinical situation and we work backwards • When we are actually working with patients with these disturbances they need quick action and interpretation

  3. Objectives • to provide a simple, systematic approach to interpreting acid base electrolyte disturbances. • Understand there are variations and that you must develop an approach that works for you and then practice, practice and more practice makes perfect

  4. A few not so simple rules: Simplified • Henderson-Hasselbalch not just a good idea. • It’s the law: Simplified: where 6.1 is the pKa (negative logarithm of the acid dissociation constant) for carbonic acid (H2CO3) and 0.03, the factor which relates PCO2 to the amount of CO2 dissolved in plasma.

  5. Sesame Street Simple • Acidity=Bicarbonate/Carbon Dioxide • Or A = B/CD • Four disturbances can occur and the disturbance is categorized by the primary defect (what happened first): • Decrease in bicarbonate=metabolic acidosis • Increase in bicarbonate=metabolic alkalosis • Decrease in carbon dioxide=respiratory alkalosis • Increase in carbon dioxide=respiratory acidosis

  6. Things to remember • the body attempts to minimize changes in pH by keeping our cations equal to our anions. • Therefore electrolytes are a component of acid base disturbances • The organs systems involved in homeostasis (respiratory, GI and renal have 2 options- hold on or let go of ions. • Disorders generally occur when these systems either get rid of too much or hold on to too much.

  7. Compensation • In order to remain in health, the body attempts to minimize changes in pH. • Faced with a change in one component the other factor changes in the same direction so that the fraction remains nearly constant. • For example the body responds to a fall in bicarbonate by decreasing carbon dioxide. This minimizes changes in the ratio that determines the pH.

  8. Compensation Occurs as the body begins to correct the acid base imbalance pH will be normal or near normal if total compensation pH will be abnormal if partial compensation Both the PaCo2 & HCO3 will be abnormal (when compensation occurs)

  9. Compensation Respiratory imbalances are compensated for by the renal system Metabolic imbalances are compensated for by the respiratory system

  10. As simple as possible

  11. Know what’s normal • pH: Normal generally considered as 7.35-7.45. Use 7.40 as number to remember • pCO2: Normal 35-45mmHg. Use 40 as number to remember • Bicarb(HCO3): Normal 22-26 mmol/L. Use 24 as number to remember • Anion Gap: Normal 12+/- 2 mmol/L. Use 12 as number to remember

  12. ABC’s of ABG’s • Golden Rule: • Review history • Physical exam findings if present • Then assess the following: • Acidosis or Alkalosis • Initial or Primary problem: Metabolic or Respiratory • Is compensation occurring • Prediction formula’s as indicated

  13. Step 1: Assess pH • < 7.40 is acidemic = primary disturbance is acidosis • > 7.40 is alkalotic = primary disturbance is alkalosis • Compensation is not perfect and overcompensation does not occur • The pH will approach 7.40 but never reach it in the setting of acid base disturbances • When you assess pH and its normal-your not off the hook. You need to look at all the numbers

  14. Step 2: Assess Primary Disorder • Assess HCO3 (normal = 24) and pCO2 (normal = 40) • low bicarbonate (< 24) and low pCO2 (< 40) suggests a metabolic acidosis. • a high bicarbonate (> 24) and high pCO2 (> 40) suggests that the primary disorder is a respiratory one. • bicarbonate (> 24) and pCO2 (> 40) supports a metabolic alkalosis • while low bicarbonate (< 24) and low pCO2 (<40) supports a respiratory alkalosis.

  15. A little practice makes perfect • Determine the primary acid-base disturbance: • 1. pH = 7.27 / pCO2 = 34 / HCO3 = 12 • Answer: Metabolic Acidosis • 2. pH = 7.34 / pCO2 = 50 / HCO3 = 26 • Answer: Respiratory Acidosis • 3. pH = 7.45 / pCO2 = 48 / HCO3 = 32 • Answer: Metabolic Alkalosis • 4. pH = 7.45 / pCO2 = 18 / HCO3 = 12 • Answer: Respiratory Alkalosis

  16. Metabolic Acidosis

  17. Respiratory Acidosis

  18. Metabolic Alkalosis

  19. Respiratory Alkalosis

  20. Two Additional Rules • In determining primary problem • In metabolic disorders: pH, HCO3 and pCO2 all move in the same direction • In respiratory disorders: pH, HCO3 and pCO2 move in discordant directions

  21. Problem pH of 7.27 pCO2 of 27 mm Hg HCO3 of 10 mEq/l Note that both the PCO2 and HCO3 are low

  22. Breaking it down Low pH = acidosis Low pCO2 = alkalosis Low HCO3 = acidosis HCO3 corresponds with the pH This is a metabolic problem Metabolic acidosis with partial compensation

  23. Step 3: Additional Abnormalities • Calculate anion gap it may be your first indicator of acid base disturbance • In Respiratory Acidosis the acid that is accumulating is known = CO2 • In Metabolic Acidosis the acid can be any anion • Metabolic Acidosis is characterized by the type of acid that is consuming the bicarbonate • In Medicine there are really only 2 types of anions: • Chloride • Not Chloride • Anion Gap allows us to determine if the excess anion is chloride or not • Anion Gap = Na – (Cl + HCO3) and normal is 6 +/- 3 • As positive must equal negative the anion gap indicates the presence of unmeasured anions (normally albumin, phosphate and others. • When AG is elevated, there has been an increase in unmeasured anions to maintain homeostasis

  24. Step 3: Additional Abnormalities • Remember to correct for each 1 mg/dl decrease in albumin there will be a 2.4 mEq/L decrease in AG (normal AG is lower) • If AG > 20 rest assured there is a primary metabolic acidosis regardless of pH or HCO3 levels • Calculate the delta gap: • Patient’s gap – normal gap (12) + patient’s HCO3 • If > 30 there is an underlying metabolic alkalosis • If < 23 there is an underlying non-gap metabolic acidosis

  25. Practice: Example 1 • Your called to assess a G3P2 woman at 35 weeks with left leg swelling. Your resident was concerned about DVT/PE and performed an ABG • pH = 7.50 (? high or low) = high = alkalemia • pCO2 = 29 (? high or low) = low =alkalemia • HCO3 = 22 ( ? High or low) = near normal • Primary disturbance is likely Acute Respiratory Alkalosis

  26. Acute Respiratory Alkalosis Pregnancy Lung disease Drugs (salicylate, catecholamines) Sepsis Anxiety Any cause of hyperventilation Hypoxia Hepatic encephalopathy Mechanical ventilation

  27. Example 2 • Your in the ER when the EMS unit brings in a young man with suspected drug overdose. An ABG is taken and then Narcan is administered • pH is 7.25 = low =acidemia • pCO2 is 60 = high = acidosis • Bicarb is 26 = nearly normal • Likely acute respiratory acidosis (normal bicarb indicates no compensation yet

  28. Acute Respiratory Acidosis • CNS depression: opiods, other drugs, CNS events, CO2 retention in severe COPD • Neuromuscular disorders (Guillian Barre, other myopathies and neuropathies • Acute airway obstruction • Laryngospasm or bronchospasm • PE • Impaired lung motion • Homo 1 or pneumothorax 2

  29. Example 3 • Your called to evaluate a patient with known history of poorly controlled COPD. She ran out of her meds and can’t afford them. She also refuses to use a prescribed BIPAP machine. She looks very uncomfortable • pH 7.34 = low = acidemia • pCO2 = 60 = high = respiratory acidosis • HCO3 = 31 = high = metabolic alkalosis • High HCO3 indicates compensation taking place therefore problem is chronic - e.g. lung disease, muscular disorder, respiratory center depression

  30. Step 4: Assess Compensation • Predictability of compensation used to determine if additional primary disorder present • If pH approaching 7.40 with significant disturbance in primary respiratory or metabolic category then you know there has been some compensation • If the degree of compensation falls in the predicted range then there is no additional acid-base disturbance.

  31. Step 4: Assess Compensation • Each primary acid-base disturbance has its own equation to calculate the predicted degree of compensation • If the prediction equation explains the compensation then you have a simple acid base disorder. • If the prediction equation does not explain the compensation then a second primary disorder exists. • Respiratory compensation for metabolic disorders is essentially instant while metabolic (renal) compensation for respiratory disorders follows a slowed process. • In metabolic disorders, if the actual pCO2 is less than the predicted pCO2 there is an additional respiratory alkalosis. If the actual pCO2 is greater than the predicted pCO2 there is an additional respiratory acidosis • In respiratory disorders, if the actual HCO3 is greater than the predicted HCO3 there is an additional metabolic alkalosis. If the actual HCO3 is less than the predicted HCO3 there is an additional metabolic acidosis.

  32. Predictability Table

  33. Acute vs Chronic

  34. Using Prediction -Examples • Metabolic acidosis • Suppose a patient has a pH of 7.37, HCO3 of 10 and a pCO2 of 18. • All three variables are lower than normal so the patient has a metabolic disturbance. • The pH is decreased so this is metabolic acidosis. • To look for a second primary condition the first step is to use Winter’s formula to see if the compensation is appropriate. • With a bicarbonate of 10, Winter’s formula predicts a pCO2 of 23±2. • The actual pCO2 is 18, below the predicted pCO2 so this patient has an additional primary respiratory alkalosis. • If the actual pCO2 was 24, then the patient would have physiologically compensated metabolic acidosis without a second primary respiratory disorder. • If the actual pCO2 was 28 then the patient would have a pCO2 that was higher than predicted or an additional primary respiratory acidosis.

  35. Using Prediction -Examples • Metabolic alkalosis • Suppose a patient has a pH of 7.50, HCO3 of 36 and pCO2 of 48. • All three variables are higher than normal so the patient has a metabolic disturbance. • The pH is increased so this is metabolic alkalosis • To look for a second primary condition first determine what the expected compensation should be. • In metabolic alkalosis the pCO2 rises 0.7 for every 1 mmol/L increase in HCO3. • An HCO3 of 36 is an increase of 12 from normal. This should be compensated by an increase in pCO2 of 8 • The actual pCO2 is 48, so this patient has an isolated metabolic alkalosis with appropriate respiratory compensation. • If the pCO2 was 58, the patient would have an additional primary respiratory acidosis • If the pCO2 was 28, the patient would have an additional primary respiratory alkalosis

  36. Using Prediction -Examples • Respiratory acidosis • a patient has a pH of 7.35, HCO3 of 30 and a pCO2 of 56. • The pH is decreased and both the HCO3 and pCO2 are elevated. Since the variables move in discordant direction it is a respiratory disturbance. • The pH is decreased so this is respiratory acidosis. • To look for a second primary condition the first step is to determine the expected bicarbonate. • The pCO2 is 16 above normal which corresponds to an expected increase in HCO3 of 2 in acute respiratory acidosis and 5 in chronic respiratory acidosis • So the expected bicarbonate is 26 if the respiratory acidosis is acute and 29 if it is chronic. • The actual HCO3 is 30 so there is an additional metabolic alkalosis if the patient has acute disease and a pure respiratory acidosis if the condition is chronic • Note the equation can not tell you if the disorder is acute or chronic this is your decision based on history

  37. Using Prediction -Examples • Respiratory alkalosis • Suppose a patient has a pH of 7.56, HCO3 of 23 and a pCO2 of 22. • The pH is increased and the HCO3 and pCO2 are both decreased. Since the variables move in discordant direction it is a respiratory disturbance. • The pH is increased so this is respiratory alkalosis. • To look for a second primary condition the first step is to determine the expected bicarbonate. • The pCO2 is 18 below normal which corresponds to an expected decrease in HCO3 of 4 in acute respiratory alkalosis and 8 in chronic respiratory alkalosis. • So the expected bicarbonate is 20 if the respiratory acidosis is acute and 16 if it is chronic. • The actual HCO3 is 23 so this is a respiratory alkalosis with metabolic alkalosis regardless if it is acute or chronic.

  38. Example 4 • Your assigned to surgery service and the patient assigned to you has a small bowel obstruction. She has been vomiting for 2 days and just had an NG tube placed to suction. You notice her bicarb was 36 that am so you decide to check an ABG • pH 7.50 = high =alkalemia • pCO2 = 48 = high favoring resp acidosis • HCO3 = 36 = high favoring metabolic alkalosis • Likely a case of primary metabolic alkalosis with a mildly elevated pCO2 due to respiratory compensation • Good reminder to look at your am HCO3 and monitor trends

  39. Example 4 continued • Metabolic Alkalosis has 2 flavors • Low urine chloride < 10: • vomiting, NG tube suction, past diuretic use, post hypercapnia • High urine chloride >20: • current or recent diuretic use, refeeding alkalosis, Cushing’s syndrome, exogenous steroids, primary hyperaldosteronism. CHF, ascites, hypokalemia, Conn’s syndrome, licorice ingestion, excess alkali administration • Note remember this you lose acid from the gut and base from the butt

  40. Example 5 • A 61 yo diabetic man has been admitted to your internal med service. His last HA1C was 7.2. His blood sugar is now 457 and he has a productive cough and temp of 39. His O2 sat is 93% • pH 7.20 = low = acidemia • pCO2 = 21= low = resp alkalosis • HCO3 = 8 = metabolic acidosis • You correctly conclude this is a primary metabolic acidosis with a compensatory resp. alkalosis • The electrolyte panel is pending so you do not have an AG yet

  41. Elevated AG Acidosis M ethanol U remia D iabetic or alcoholic ketoacidosis P araldehyde I NH, iron toxicity L actic acid E thylene glycol R habdomyolosis S alicylates

  42. Osmolar Gap • If AG elevated you need to calculate an osmolar gap to r/o methanol, ethylene glycol or ethanol as a source • Calculated serum osm = 2(Na + K) + glucose/18 + BUN/2.8 • Osmolar Gap = measured Osm- calculated • If >10 then think methanol, ethylene glycol, sorbitol, mannitol, renal failure, or radiocontrast dye

  43. Non Gap Acidosis GI bicarbonate losses: diarrhea (base from butt), ureteral diversion Renal bicarb losses: RTA, early renal failure, aldosterone and carbonic anhydrase inhibitors Post hypocapnia Hyperchloremic Metabolic Acidosis after large volume NS resuscitation

  44. Non Gap Acidosis continued • To determine the cause you must distinguish between renal and non renal (usually GI) sources • Urine Anion Gap = Una + Uk – Ucl • Negative UAG implies kidney compensating and therefore non-renal cause • Highly positive UAG implies renal impairment

  45. Delta Gap • Calculate the delta gap: • patient’s Gap- normal gap(12) + patient’s bicarb • IF >30, there is an underlying metabolic alkalosis • If <23, there is an underlying non-gap metabolic acidosis

  46. Example 6 • You are in Cat 1 when a patient is transferred. She is breathing fast and is lethargic • pH 7.50 = high = alkalemia • pCO2= 20 = low = resp alkalosis • HCO3 = 16 = low = metabolic acidosis • Na=140 • Cl = 103 • AG= 140 – (103 + 15) = 22 = AG Metabolic Acidosis with a resp alkalosis • Delta Gap = 22-12+15 =25 likely no other problem • ASA is markedly elevated and this is a classic picture of salicylate overdose: AG metabolic acidosis with a resp alkalosis

  47. Example 7 • You are called to write admission orders for a woman for a woman in CKD clinic presenting with 3 days hx of vomiting • pH 7.40 = normal but your not done yet • pCO2 = 40 = normal • Bicarb = 24 = normal (Check the anion gap) • AG= 21 therefore a primary AG metabolic acidosis (uremia) is present despite normal parameters • Delta gap = 21-12+24=33, therefore and underlying metabolic alkalosis (vomiting) is also present

  48. déjà vu back to Example 5 The electrolyte profile you were waiting for was lost and now your waiting for the repeat, your DM patient received insulin and IVF but became minimally responsive and his RR decreased to 12 so a repeat ABG was obtained

  49. déjà vu back to Example 5 continued pH 7.10 = low = acidemia pCO2 = 50 = high = resp acidosis HCO3 = 15 = low = metabolic acidosis AG = 30, primary AG metabolic acidosis Delta Gap = 33 therefore metabolic alkalosis also present Primary AG acidosis due to DKA with underlying pneumonia, resp acidosis secondary to obtunded state and hypoventilation and metabolic alkalosis secondary to vomiting

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