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ACID-BASE SITUATIONS

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Objectives After today’s presentation you will: List the primary causes of respiratory acidosis List the primary causes of respiratory alkalosis Given a set of electrolytes, determine the anion gap List the primary causes of a metabolic acidosis with an increased anion gap List the primary causes of a metabolic acidosis with a normal anion gap List the primary causes of a metabolic alkalosis

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ACID-BASE DISTURBANCES

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Respiratory Disturbances

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Metabolic Disturbances

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Metabolic Acidosis

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General Causes of Metabolic Acidosis Failure of kidneys to excrete metabolic acids normally found in the body. Formation of excess quantities of metabolic acids in the body. Addition of metabolic acids to the body by ingestion or infusion of acids. Loss of base from the body fluids.

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Anion Gap Two ways to evaluate Na + - Cl - - HCO 3 - Normal is 6 to 12 mEq/L Na + + K + - Cl - - HCO 3 - Normal is 10 to 16

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Anion Gap mlhttp://www.thedrugmonitor.com/acidbase.ht ml

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Increased Anion Gap Fixed AcidsGreater than 20 = Accumulation of Fixed Acids NORMAL CHLORIDE LEVELNORMAL CHLORIDE LEVEL MUD PILERS Methanol Uremia (Azotemic Renal Failure) Elevated BUN/Creatinine Diabetic Ketoacidosis Paraldehyde (Formaldehyde and Toluene) Isopropyl alcohol Lactic (and Formic) Acidosis Ethylene Glycol Rhabdomyolysis Salicylates

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Methanol Paraldehyde Isopropyl Alcohol Ethylene Glycol Salicylates ARF Lactic Acidosis DKA Rhabdomyolysis

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Normal Anion Gap INCREASED CHLORIDE LEVEL – LOSS OF BASEINCREASED CHLORIDE LEVEL – LOSS OF BASE Renal Tubular Acidosis No reabsorption of HCO 3 - Enteric Drainage Tubes Small intestine drainage – Large amounts of base in stool Diarrhea Urinary Diversion Surgical alteration of ureters Carbonic Anhydrase Inhibitors Poor reabsorption of bicarbonate

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Low Anion Gap Look at albumin Hypoalbuminemia causes a low anion gap. Normal Albumin is 4.4 g/dL For every 0.4 g/dL decrease in albumin, the anion gap will decrease by 1 mEq/L Albumin (g/dL)Maximum Anion Gap (mEq/L)

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Example of Extreme Compensation On 2 L / min NC pH: 6.96 P a CO 2 : 6.8 mm Hg P a O 2 : 158 mm Hg HCO 3 - : 1.5 mEq/L BE: mEq/L

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Metabolic Alkalosis Most common acid-base abnormality? Aggressive treatment of partially compensated respiratory acidosis? Causes Loss of Acid Vomiting NG Drainage Gain of Alkali Increased ingestion of alkaline substances Excessive licorice ingestion Hypokalemia

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Advance Acid Base Interpretation

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Classification vs. Interpretation Classification: Identification of acid-base disturbance and the causative element along with any compensation that may be present. Interpretation: Use of calculations to determine if compensation is appropriate or if multiple disorders are present.

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Compensation There is no such thing as overcompensation. This usually means there is a second primary disorder at work in the opposite direction. If there is no compensation OR the compensation is less than expected: Compensation is not possible because the compensatory organ is not functioning appropriately. There has not been sufficient time for compensation (renal). Another primary disorder is present and is working in the same direction.

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PaCO 2 pHHCO 3 - RESPIRATORY ACIDOSIS Acute ↑10↓ 0.08↑ 1 mEq/L Chronic ↑10↓ 0.03↑ 4 mEq/L RESPIRATORY ALKALOSIS Acute ↓10↑ 0.08↓ 2 mEq/L Chronic ↓10↑ 0.03↓ 5 mEq/L HCO 3 - pHP a CO 2 METABOLIC ACIDOSIS ↓ 1↓ 0.015↓ 1.2 METABOLIC ALKALOSIS ↑1↑0.015↑ 0.7

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Degree of Variation Allow for some degree of variation as follows: pH: P a CO 2 : + 5 mm Hg

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Oakes’ Approach Primary Problem (Acidemia or Alkalemia) Primary Cause CO 2 HCO 3 - Compensation Initial Classification (Technical and Functional) Determine extent of compensation and the presence of other abnormalities Determine Anion Gap for metabolic acidemia Determine Oxygenation Assess Patient Check for Accuracy Final Interpretation

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Factors That May Complicate Acid-Base Determination Chronic Lung Disease Chronic Renal Disease Therapeutic interventions Mixed Acid-Base Problems

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COPD Typical picture is fully compensated, respiratory acidosis. pH: 7.38 P a CO 2 : 55 mm Hg HCO 3 - : 31 mEq/L BE: 5 mEq/L P a O 2 : 55 mm Hg Let’s go through the process….

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7.38, 55, 31 Acidosis CO 2 is elevated, so there is a respiratory cause. HCO 3 - is out of normal range, but is not the cause. The body is compensating. Technical Classification: Fully compensated respiratory acidosis. Functional Classification: Chronic respiratory acidosis.

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7.38, 55, 31 Change in CO 2 : +15 torr Change in pH: Change in HCO 3 - : +7 mEq/L BASELINEDisorder #1Disorder #2 pH: 7.40 P a CO 2 : 40 torr 55 HCO 3 -: 24 mEq/L

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7.38, 55, 31 Change in CO 2 : +15 torr Change in pH: Change in HCO 3 - : +7 mEq/L BASELINEDisorder #1Disorder #2 pH: 7.40 P a CO 2 : 40 torr 55 HCO 3 -: 24 mEq/L For every 10 torr change in P a CO 2 there is a 0.03 change in pH. pH = (1.5 x.03)= =7.36

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7.38, 55, 31 Change in CO 2 : +15 torr Change in pH: Change in HCO 3 - : +7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 55 HCO 3 -: 24 mEq/L For every 10 torr change in P a CO 2 there is a 0.03 change in pH. pH = (1.5 x.03)= =7.36

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7.38, 55, 31 Change in CO 2 : +15 torr Change in pH: Change in HCO 3 - : +7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 55 HCO 3 -: 24 mEq/L For every 10 torr change in P a CO 2 there is a 4 mEq/L change in HCO 3 -. HCO 3 - = (1.5 x 4)=6 24+6=30 mEq/L

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7.38, 55, 31 Change in CO 2 : +15 torr Change in pH: Change in HCO 3 - : +7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 55 HCO 3 -: 24 mEq/L 30 For every 10 torr change in P a CO 2 there is a 4 mEq/L change in HCO 3 -. HCO 3 - = (1.5 x 4)=6 24+6=30 mEq/L

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7.38, 55, 31 Change in CO 2 : +15 torr Change in pH: Change in HCO 3 - : +7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 55 HCO 3 -: 24 mEq/L 3031 Actual HCO 3 - (31) doesn’t match the predicted change in HCO 3 -, so there must be an underlying metabolic alkalosis superimposed on the compensation.

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7.38, 55, 31 Change in CO 2 : +15 torr Change in pH: Change in HCO 3 - : +7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 55 HCO 3 -: 24 mEq/L 3031 For every 1 mEq/L change in HCO 3 - there is a change in pH. pH = (1 x.015) = 0.15~ = 7.38

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7.38, 55, 31 Change in CO 2 : +15 torr Change in pH: Change in HCO 3 - : +7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 55 HCO 3 -: 24 mEq/L 3031 For every 1 mEq/L change in HCO 3 - there is a change in pH. pH = (1 x.015) = 0.15~ = 7.38

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7.38, 55, 31 Change in CO 2 : +15 torr Change in pH: Change in HCO 3 - : +7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 55 HCO 3 -: 24 mEq/L 3031

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COPD So our final interpretation is a fully compensated, respiratory acidosis with a secondary metabolic alkalemia. pH: 7.38 P a CO 2 : 55 mm Hg HCO 3 - : 31 mEq/L BE: 5 mEq/L P a O 2 : 55 mm Hg This may be due to hypochloremia and hypokalemia associated with steroids and diuretics.

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COPD – Relative Hyperventilation If the patient below develops a hypoxemic episode (e.g. pneumonia), the resulting hypoxemia may cause the patient to hyperventilate, the P a CO 2 to return to “normal”, and a metabolic alkalosis to be diagnosed. This is often seen when first starting the patient on BiPAP or mechanical ventilation pH: 7.38 P a CO 2 : 55 mm Hg HCO 3 - : 31 mEq/L BE: 5 mEq/L P aO 2 : 55 mm Hg pH: 7.52 P a CO 2 : 40 mm Hg HCO 3 - : 31 mEq/L BE: 5 mEq/L P a O 2 : 50 mm Hg Solution is to only correct the P a CO 2 to 52 mm Hg

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COPD with Lactic Acidosis COPD + Reduced Cardiac Output = Cellular hypoxia and Lactic Acidosis Don’t be fooled by “normal” ABGs pH: 7.38 P a CO 2 : 40 mm Hg HCO 3 - : 24 mEq/L BE: 1 mEq/L P a O 2 : 44 mm Hg pH: 7.38 P a CO 2 : 55 mm Hg HCO 3 - : 31 mEq/L BE: 5 mEq/L P aO 2 : 55 mm Hg Loss of HCO 3 - due to lactic acidosis.

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Chronic Renal Failure Chronic renal failure can distort ABG results. Renal ability to manipulate HCO 3 -, electrolyte and fluid levels is impaired. Always evaluate with a metabolic acidosis as a possible cause.

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Therapeutic Interventions Multiple therapeutic interventions can affect acid-base balance: Diuretics Steroids Electrolytes Oxygen Sodium Bicarbonate Mechanical Ventilation

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Let’s try another… Interpret this ABG: pH: 7.44 P a CO 2 : 18 mm Hg [BE]: -12 mEq/L [HCO 3 - ]: 12 mEq/L P a O 2 : 64 mm Hg

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7.44, 18, 12 Normal, but on alkalotic side. CO 2 is reduced, so there is a respiratory cause. HCO 3 - is out of normal range, but is not the cause as a lowered HCO 3 - would not cause an alkalosis. The body is compensating. Technical Classification: Fully compensated respiratory alkalosis. Functional Classification: Chronic respiratory alkalosis.

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7.44, 18, 12 Change in CO 2 : -22 torr Change in pH: Change in HCO 3 - : -12 mEq/L BASELINEDisorder #1Disorder #2 pH: 7.40 P a CO 2 : 40 torr 18 HCO 3 -: 24 mEq/L

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7.44, 18, 12 Change in CO 2 : -22 torr Change in pH: Change in HCO 3 - : -12 mEq/L BASELINEDisorder #1Disorder #2 pH: 7.40 P a CO 2 : 40 torr 18 HCO 3 -: 24 mEq/L For every 10 torr change in P a CO 2 there is a 0.03 change in pH. pH = (2.2 x.03)= =7.47

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7.44, 18, 12 Change in CO 2 : -22 torr Change in pH: Change in HCO 3 - : -12 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 18 HCO 3 -: 24 mEq/L For every 10 torr change in P a CO 2 there is a 0.03 change in pH. pH = (2.2 x.03)= =7.47

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7.44, 18, 12 Change in CO 2 : -22 torr Change in pH: Change in HCO 3 - : -12 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 18 HCO 3 -: 24 mEq/L For every 10 torr change in P a CO 2 there is a 5 mEq/L change in HCO 3 -. HCO 3 - = (2.2 x 5)= =13 mEq/L

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7.44, 18, 12 Change in CO 2 : -22 torr Change in pH:+0.04 Change in HCO 3 - : -12 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 18 HCO 3 -: 24 mEq/L 13 For every 10 torr change in P a CO 2 there is a 5 mEq/L change in HCO 3 -. HCO 3 - = (2.2 x 5)= =13 mEq/L

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7.44, 18, 12 Change in CO 2 : -22 torr Change in pH: Change in HCO 3 - : -12 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 18 HCO 3 -: 24 mEq/L 1312 Actual HCO 3 - (12) doesn’t match the predicted change in HCO 3 -, so there must be an underlying metabolic acidosis superimposed on the compensation.

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7.44, 18, 12 Change in CO 2 : -22 torr Change in pH: Change in HCO 3 - : -12 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 18 HCO 3 -: 24 mEq/L 1312 For every 1 mEq/L change in HCO 3 - there is a change in pH. pH = (1 x.015) = 0.15~ = 7.45

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7.44, 18, 12 Change in CO 2 : -22 torr Change in pH: Change in HCO 3 - : -12 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 18 HCO 3 -: 24 mEq/L 1312 For every 1 mEq/L change in HCO 3 - there is a change in pH. pH = (1 x.015) = 0.15~ = 7.45

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7.44, 18, 12 Change in CO 2 : -22 torr Change in pH: Change in HCO 3 - : -12 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 18 HCO 3 -: 24 mEq/L 1312 The predicted pH of 7.45 is within the degree of variation of for pH.

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Interpretation Fully compensated respiratory alkalosis with simultaneous metabolic acidosis.

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Mixed Acid-Base Disturbances Sometimes a simple compensatory mechanism isn’t the reason for the normalized acid-base status. If two opposing problems co-exist (ARF causing metabolic acidosis and pain causing respiratory alkalosis), you may have what looks like one compensating for the other.

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Interpret this ABG: pH: 7.38 P a CO 2 : 20 mm Hg [HCO 3 - ]: 17 mEq/L P a O 2 : 89 mm Hg

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7.38, 20, 17 Normal, but on acidotic side. CO 2 is reduced, which would not cause an alkalosis. HCO 3 - is reduced and would cause an alkalosis. The body is compensating. Technical Classification: Fully compensated metabolic acidosis. Functional Classification: Metabolic acidosis.

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7.38, 20, 17 Change in CO 2 : -20 torr Change in pH: Change in HCO 3 - : -7 mEq/L BASELINEDisorder #1Disorder #2 pH: 7.40 P a CO 2 : 40 torr HCO 3 -: 24 mEq/L 17 For every 1 mEq/L change in HCO 3 - there is a change in pH. pH = (7 x.015)= =7.30

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7.38, 20, 17 Change in CO 2 : -20 torr Change in pH: Change in HCO 3 - : -7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr HCO 3 -: 24 mEq/L 17 For every 1 mEq/L change in HCO 3 - there is a change in pH. pH = (7 x.015)= =7.30

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7.38, 20, 17 Change in CO 2 : -20 torr Change in pH: Change in HCO 3 - : -7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr HCO 3 -: 24 mEq/L 17 The calculated change in pH (7.30) differs from the actual measured pH, so there must be some additional acid- base disturbance.

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7.38, 20, 17 Change in CO 2 : -20 torr Change in pH: Change in HCO 3 - : -7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr HCO 3 -: 24 mEq/L 17 For every 1 mEq/L change in HCO 3 - there is a 1.2 change in P a CO 2. P a CO 2 = (7 x 1.2)= =31.6~32

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7.38, 20, 17 Change in CO 2 : -20 torr Change in pH: Change in HCO 3 - : -7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 32 HCO 3 -: 24 mEq/L 17 The calculated change in P a CO 2 (32) differs from the actual measured P a CO 2, so there must be a respiratory disturbance present.

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7.38, 20, 17 Change in CO 2 : -20 torr Change in pH: Change in HCO 3 - : -7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr HCO 3 -: 24 mEq/L 17 The difference between the calculated change in P a CO 2 (32) and the actual P a CO 2 (20), indicates that there must be a respiratory disturbance present.

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7.38, 20, 17 Change in CO 2 : -20 torr Change in pH: Change in HCO 3 - : -7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr HCO 3 -: 24 mEq/L 17 For every 10 torr change in P a CO 2 there is a 0.08 change in pH. pH = (1.2 x.08)= =7.396

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7.38, 20, 17 Change in CO 2 : -20 torr Change in pH: Change in HCO 3 - : -7 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr HCO 3 -: 24 mEq/L 17 The predicted pH is within the level of variability of 0.03 units.

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Interpretation Mixed metabolic acidosis and simultaneous respiratory alkalosis.

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Can you have a triple disorder?

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Anion & Bicarbonate Gaps Anion Gap: Na + - Cl - - HCO 3 - Normal value is 6 to 12 Above 20 considered High Bicarbonate Gap: HCO (AG-12) Normal is 20 to 28 – AG Metabolic Acidosis Less than 20 – AG Metabolic Acidosis + Non- AG Metabolic Acidosis Greater than 28 – AG Metabolic Acidosis + Metabolic Alkalosis

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7.52, 30, 21 Alkalemia. CO 2 is reduced, which would cause an alkalosis. Primary respiratory alkalosis. HCO 3 - is reduced and would not cause an alkalosis. The body is compensating (HCO 3 - decreasing) Technical Classification: Partially compensated respiratory alkalosis. Functional Classification: Chronic respiratory alkalosis.

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7.52, 30, 21 Change in CO 2 : -10 torr Change in pH: Change in HCO 3 - : -3 mEq/L BASELINEDisorder #1Disorder #2 pH: 7.40 P a CO 2 : 40 torr 30 HCO 3 -: 24 mEq/L For every 10 torr change in P a CO 2 there is a 0.03 change in pH. pH = (1 x.03)= =7.43

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7.52, 30, 21 Change in CO 2 : -10 torr Change in pH: Change in HCO 3 - : -3 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 30 HCO 3 -: 24 mEq/L For every 10 torr change in P a CO 2 there is a 0.03 change in pH. pH = (1 x.03)= =7.43

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7.52, 30, 21 Change in CO 2 : -10 torr Change in pH: Change in HCO 3 - : -3 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 30 HCO 3 -: 24 mEq/L The calculated change in pH (7.43) differs from the actual measured pH, so there must be some additional acid- base disturbance.

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7.52, 30, 21 Change in CO 2 : -10 torr Change in pH: Change in HCO 3 - : -3 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 30 HCO 3 -: 24 mEq/L For every 10 torr change in P a CO 2 there is a 5 mEq/L change in HCO 3 -. HCO 3 - = (1 x 5)=5 24-5=19 mEq/L

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7.52, 30, 21 Change in CO 2 : -10 torr Change in pH: Change in HCO 3 - : -3 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 30 HCO 3 -: 24 mEq/L 19 For every 10 torr change in P a CO 2 there is a 5 mEq/L change in HCO 3 -. HCO 3 - = (1 x 5)=5 24-5=19 mEq/L

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7.52, 30, 21 Change in CO 2 : -10 torr Change in pH: Change in HCO 3 - : -3 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 30 HCO 3 -: 24 mEq/L 1921 The calculated change in HCO 3 - differs from the actual measured HCO 3 -, so there must be some additional acid-base disturbance.

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7.52, 30, 21 Change in CO 2 : -10 torr Change in pH: Change in HCO 3 - : -3 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 30 HCO 3 -: 24 mEq/L 1921 For every 1 mEq/L change in HCO 3 - there is a change in pH. pH = (2 x.015)= =7.46

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7.52, 30, 21 Change in CO 2 : -10 torr Change in pH: Change in HCO 3 - : -3 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 30 HCO 3 -: 24 mEq/L 1921 For every 1 mEq/L change in HCO 3 - there is a change in pH. pH = (2 x.015)= =7.46

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7.52, 30, 21 Change in CO 2 : -10 torr Change in pH: Change in HCO 3 - : -3 mEq/L BASELINEDisorder #1Disorder #2 pH: P a CO 2 : 40 torr 30 HCO 3 -: 24 mEq/L 1921 Note that the pH still is not fully explained by the change in bicarbonate due to compensation. Some additional metabolic alkalosis must be present.

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Look to Anion & Bicarbonate Gaps Na + : 142 mEq/L Cl - : 98 mEq/L AG: Na + - Cl - - HCO 3 - = = 23 This means we have an Anion Gap Metabolic Acidosis BG: HCO (AG-12) = 21 + (23-12) = = 32 AG Metabolic Acidosis + Metabolic Alkalosis

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Interpretation Primary respiratory alkalosis and metabolic acidosis and metabolic alkalosis. Huh? An example would be a patient with pneumonia who is hyperventilating secondary to hypoxemia, who also has azotemic renal failure and has hypokalemia secondary to aggressive diuretic therapy.

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7.52, 30, 21 Change in CO 2 : -10 torr Change in pH: Change in HCO 3 - : -3 mEq/L BASELINEDisorder #1Disorder #2 pH: 7.40 P a CO 2 : 40 torr 30 HCO 3 -: 24 mEq/L

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Can you have a quadruple disorder?

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Acid-Base Map Fig 14-1

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pH: 7.35 P a CO 2 : 60 mm Hg

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pH: 7.60 P a CO 2 : 30 mm Hg

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pH: 7.38 P a CO 2 : 70 mm Hg

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pH: 7.35 P a CO 2 : 30 mm Hg

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Fun reading… Last part of Chapter 14 in Malley.

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7.43, 34, 22 Change in CO 2 : +15 torr Change in pH: Change in HCO 3 - : +7 mEq/L BASELINEDisorder #1Disorder #2 pH: 7.40 P a CO 2 : 40 torr 55 HCO 3 -: 24 mEq/L

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