9. Approach to Acid-Base Disorders (Chaffer) PDF

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W. Jacob Cobb TAMHSC COM – Class of 2017 Dr. Chaffer Approach to Acid-Base Disorders   Note: Dr. Chaffer states that this lecture is NOT necessarily testable material. However, Dr. Wesson will be teaching us the same material next week which WILL be testable, so this is a great preview of the mate...

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W. Jacob Cobb TAMHSC COM – Class of 2017 Dr. Chaffer

Approach to Acid-Base Disorders 

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Note: Dr. Chaffer states that this lecture is NOT necessarily testable material. However, Dr. Wesson will be teaching us the same material next week which WILL be testable, so this is a great preview of the material. Dr. Wesson’s approach is different to Dr. Chaffer’s approach. BOTH ARE CORRECT. I like this approach. General Discussion of Acid-Base Disorders: o We know that there are 3 responses to disturbances in acid-base balance:  Initial response (occurs in a few seconds): Buffering w/ existing HCO3 Secondary response (occurs w/i a few minutes): Respiratory compensation  Tertiary response (occurs w/i hours to a few days): Renal compensation via increased uptake/synthesis of HCO3- and excretion of H+ (depending on the situation) o Further, we know that there are 4 BASIC Acid-Base Disorders, summarized in the table below: Disorder

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Bicarb Level (compared to 24 = normal serum bicarb)

Metabolic Acidosis ↓ Metabolic Alkalosis ↑ Respiratory Acidosis ↑ Respiratory Alkalosis ↓ Explanation (Disclaimer  my explanations may or may not be FACTUALLY correct, but they get you the right answer either way.):  Long version: o In a respiratory disorder, as the name suggests, the primary point of dysfunction is the respiratory system. Examine the equation we all should know by now: CO2 + H2O  H2CO3  H+ + HCO3 In a RESPIRATORY ALKALOSIS, we have a HIGH pH. How can this occur?  Breathing rapidly  Blowing off CO2 “pulls” this equation to the LEFT which uses UP the bicarb making it LOWER.  In Respiratory ACIDOSIS, the opposite happens: NOT BREATHING ENOUGH:  Build up of CO2 “pushes” the equation to the right thereby making H2CO3 which dissociates into H+ and HCO3-, so we have a LOW pH and a HIGH bicarb. o In a metabolic disorder, the problem is NOT within the lungs, so the ABOVE EQUATION DOES NOT APPLY ANYMORE (at least for the way I think about these problems).  This is taking place in the blood and SERUM. So, if we have a METABOLIC ACIDOSIS = Low pH. We have bicarb floating around

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in our serum to buffer. In the presence of Acid, it WILL BE USED UP giving us a LOW bicarb In a metabolic ALKALOSIS, we have too much BASE. It will combine with the free H+ in the serum and leave the bicarb alone, giving us a high pH and a high bicarb.

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Short version: If you are simply comfortable with memorizing a general rule,  SAME = Metabolic (remember this because the word SAME has Me for metabolic)  This means the ARROWS go in the same direction. I.E. metabolic acidosis = LOW pH and LOW bicarb  REVERSE = Respiratory (remember this because Reverse starts w/ R and so does respiratory)  This means the arrows go in the OPPOSITE direction. I.E. Respiratory alkalosis = HIGH pH and Low BICARB o However, truth be told, the respiratory disorders can be broken down into 2 categories for each primary disorder:  Acute and Chronic o Physiologic responses:  Below, I have listed how we know when the response is appropriate. This will be important shortly when we consider application of these principles.  If we have a RESPIRATORY DISORDER, the physiologic response will be RENAL  RESPIRATORY ACIDOSIS o Increase the bicarb production in the kidney to buffer this increase in H+ o Acute Response:  Appropriateness: For each 10 mmHg rise in PaCO2 (baseline = 40), we expect a 1 mEq/L rise in tCO2 from baseline (24 mEq/L) o Chronic Response:  Appropriateness: For each 10 mmHg rise in PaCO2, we expect a 3 mEq/L rise in tCO2 from baseline (24)  RESPIRATORY ALKALOSIS o Begin to excrete Bicarb in the kidney in order to get rid of the extra base in the blood. o Higher numbers o Acute Response:  Appropriateness: For each 10 mmHg drop in PaCO2 (baseline = 40), we expect a 2 mEq/L DROP in tCO2 (baseline = 24). o Chronic Response:  Appropriateness: For each 10 mmHg drop in PaCO2 from baseline (40), we expect a 4 mEq/L DROP in baseline tCO2 from 24

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If we have a METABOLIC DISORDER, the physiologic response will be RESPIRATORY  METABOLIC ACIDOSIS: o ***MOST COMMON – KNOW THE FORMULA*** o Increase our BREATHING in order to blow off CO2 and decrease H+ - respiratory compensation. o How to calculate the appropriateness of the physiologic response:  Albert-Dell-Winter’s Formula:  Expected PaCO2 = 1.5 (tCO2) + 8 (± 2)  if actual < expected  respiratory alkalosis (less acid than expected)  If actual > expected respiratory acidosis (more acid than expected)  METABOLIC ALKALOSIS o Cause: ingestion of lots of alkali

o DECREASE our breathing in order to RAISE our PaCO2 to increase the H+ and decrease the pH – respiratory compensation o How to tell if the response is appropriate:  For EACH 10 mEq/L rise in tCO2 (above 24), expect a 7 mmHg rise in PaCO2 from baseline (40 mmHg under normal conditions)  This translates to a 0.7 mmHg rise in PaCO2 for each 1 mEq/L rise in bicarb. Dr. Chaffer’s 8 Step Approach to Diagnosing Acid-Base Disorders: o 1: Get a complete history and physical examination of the patient  Important b/c it can tell you important clues that will help you down the line  Ask about NV/D, vitamins, supplements, intake and output to make sure it fits with the MAc, RAc, MAlk, RAlk o 2: Are the Arterial Blood Gas (ABG) and Serum Bicarb (total serum CO2) INTERNALLY CONSISTENT?  Minimal laboratory data you need to make a diagnosis =  ABG – CALCULATED number  Serum Bicarb (measured as tCO2 or Total Serum CO2 NOT PaCO2) – MEASURED number  Samples must be obtained at the same time bc data can change rapidly  Internally consistent means w/i 10% of each other (bc the ABG is calculated)  If they are NOT in agreement, you have to repeat the studies (both at same time)  Ex: ABG Bicarb = 19; tCO2 = 12; NOT GOOD, repeat studies  Assume they are consistent on exams o 3: Is it Acidemia or Alkalemia?  pH of 7.4 = alkalemia

o 4: What is the anion gap (AG)?  Difference between primary measured cations (Na+/K+) and primary measured anions (Cl-/HCO3-)  AG = Na+ - (Cl- + tCO2)  Expected value = 12 ± 2  tCO2 = HCO3 Notice that we DO NOT include things such as K+ or proteins in this calculation.  Adjustments that must be made:  Albumin (protein anion)– this is what makes up most of the anion gap, so it must be considered 

If Albumin is >4.5, we must add 2 to the expected AG for each g/dL rise in albumin o Ex: Expected AG = 12; Albumin = 6.7g/dL; Corrected AG = 16  If Albumin is 2, there is LESS of a change in tCO2 than you expect for that change in AG. This means there is excess bicarb around = ADDITIONAL METABOLIC ALKALOSIS  If < 1, there is MORE of a change in tCO2 than you expect (bicarb is being used up more than expected) = ADDITIONAL NON-ANION GAP METABOLIC ACIDOSIS.  Often thought of as a hyperchloremic metabolic acidosis (high Cl-) – bc the anion accounted for in anion gap equation = Cl- - if pathology related to consumption of excessive HCO3-/losses of HCO3-  high Cl-, but there is no elevated AG bc proton is being buffered by HCO3 Patients who receive a lot of IV fluids, they may become hyperchloremic bc there is high osmolarity of Cl- than physiologic levels o 8: Is there an Osmolality gap?  Only used in AGMA to help narrow differential diagnosis (we are not responsible for this yet). Common causes of Elevated Anion Gap Acidosis: o MUDPILES  Methanol, Uremia, Diabetic Ketoacidosis, Paraldehyde (anti-convulsant rarely used in infa nts), isoniazid/iron, Lactate, Ethylene Glycol, Salicylates...