AcidBase Disorders

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Acid-Base Disorders

• Katie Murphy MD
• August 2004

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Why Are Acid-Base Disorders So Frightening?

• When we learn about acid-base disorders, we are
  usually given the numbers out of context (without
  a patient and the clinical situation) and are
  expected to work backwards.
• When we are analyzing a blood gas of a patient,
  it is usually when she is very ill or in
  distress, exactly the time we have the most
  trouble thinking in a clear, methodical way.

3
How Can Acid-Base Be Less Frightening?

• Learn an approach that works for you.
• Practice
• Practice
• Practice
• Use that approach every time.

4
Things to Remember

• The body must remain electrostatically neutral.
  Our cations must equal our anions or very bad
  things happen.
• The organs systems involved in maintaining
  homeostasis (respiratory, GI and renal primarily)
  have just 2 options Hold on to ions or let go of
  ions.
• Disorders arise when the body has gotten rid of
  too much or held on to too much.

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Things to Remember Part 2

• The body is smart, but so are you. You do not
  have to memorize everything, but you should have
  an approach and know where to look the rest up,
  especially in the middle of the night.

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Know the Normal Values

• pH Normal 7.34 to 7.45. Use 7.40 as the number
  to remember.
• pCO2 Normal 35 to 45mmHg. Use 40 as the number
  to remember.
• Bicarb Normal 22-26mmol/L. Use 24 as number to
  remember
• Anion Gap Normal 12 /- 2mmol/L. Use 12 as
  number to remember

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Step 1 Look at the pH

• Less than 7.40 is acidemic
• Greater than 7.40 is alkalemic
• The body is good, but not perfect at
  compensation. It will also never overcompensate.
  So, the pH will approach 7.40 but never reach it
  in the setting of an acid base disturbance.
• Greater than 7.40---the primary disturbance is
  alkalosis.
• Less than 7.40--the primary process is an
  acidosis.

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Step 1 Continued

• When you look at the pH ask your self Is that
  high, low or normal?
• If normal, you are not done. You need to look at
  all of the numbers.

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Step 2 What is the primary process?

• Whichever abnormality caused the pH to shift in
  that direction is the primary process.
• pH lt7.40, primary disturbance is due to elevated
  pCO2 (respiratory acidosis) or to a lowered
  bicarb (metabolic acidosis).
• pHgt7.40, then the primary process is due to
  lowered pCO2 (respiratory alkalosis) or an
  elevated bicarb (metabolic alkalosis).

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Step 2 Continued

• You are looking at the pCO2 and the bicarb to
  determine if the value and its direction of
  change from normal can explain the change in the
  pH.
• Start with the pCO2 because it is listed first
  (and because it is less panic inducing- you can
  either breathe faster or slower than normal)

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Step 2 Continued

• Look at the pCO2 and ask yourself high, low or
  normal.
• Does the answer above correspond with the
  direction of the pH
• Example pH 7.34

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Step 2 Continued

• Look at the pCO2 and ask yourself high, low or
  normal.
• Does the answer above correspond with the
  direction of the pH
• Example pH 7.34 lowacidemia

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Step 2 Continued

• Look at the pCO2 and ask yourself high, low or
  normal.
• Does the answer above correspond with the
  direction of the pH
• Example pH 7.34 lowacidemia
• pCO2 60mmHg

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Step 2 Continued

• Look at the pCO2 and ask yourself high, low or
  normal.
• Does the answer above correspond with the
  direction of the pH
• Example pH 7.34 lowacidemia
• pCO2 60mmHg highacidosis
• Respiratory acidosis is your primary process

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Step 2 Continued

• Look at the pCO2 and ask yourself high, low or
  normal.
• Does the answer above correspond with the
  direction of the pH
• Example pH 7.34 lowacidemia
• pCO2 60mmHg highacidosis
• Respiratory acidosis is your primary process
• Example pH 7.20

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Step 2 Continued

• Look at the pCO2 and ask yourself high, low or
  normal.
• Does the answer above correspond with the
  direction of the pH
• Example pH 7.34 lowacidemia
• pCO2 60mmHg highacidosis
• Respiratory acidosis is your primary process
• Example pH 7.20 low acidemia

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Step 2 Continued

• Look at the pCO2 and ask yourself high, low or
  normal.
• Does the answer above correspond with the
  direction of the pH
• Example pH 7.34 lowacidemia
• pCO2 60mmHg highacidosis
• Respiratory acidosis is your primary process
• Example pH 7.20 low acidemia
• pCO2 21mmHg

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Step 2 Continued

• Look at the pCO2 and ask yourself high, low or
  normal.
• Does the answer above correspond with the
  direction of the pH
• Example pH 7.34 lowacidemia
• pCO2 60mmHg highacidosis
• Respiratory acidosis is your primary process
• Example pH 7.20 low acidemia
• pCO2 21mmHglowalkalosis
• Therefore you must have a primary metabolic
  process.

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Step 2 Continued

• Look at the bicarb and ask your self high, low or
  normal.
• Example pH 7.34

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Step 2 Continued

• Look at the bicarb and ask your self high, low or
  normal.
• Example pH 7.34 lowacidemia
• Bicarb 31mmol/L

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Step 2 Continued

• Look at the bicarb and ask your self high, low or
  normal.
• Example pH 7.34 lowacidemia
• Bicarb 31mmol/L highalkalosis
• Therefore, the primary process is respiratory.

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Step 3Additional Abnormalities

• Calculate the anion gap We should look at the
  anion gap anytime we order a BMP. It can be your
  first indicator of an acid base derangement.
• The anion gap is the difference between the
  concentration of cations (mostly Na) and anions
  (Cl Bicarb)
• AG Measured cations - measured anions
• OR
• AG Unmeasured anions - unmeasured cations
• As positive must equal negative, the anion gap
  indicates the presence of unmeasured anions
  (normally albumin, phosphate and others). When
  the AG is elevated, there has been an increase in
  unmeasured anions to maintain homeostasis.

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Step 3 Continued

• Remember to correct for hypoalbuminemia For each
  1mg/dl decrease in albumin, there will be a 2.4
  mEg/L decrease in anion gap. ( Your normal AG
  value is lower)
• If the anion gap is greater than 20, there is a
  primary metabolic acidosis present no matter what
  the pH or bicarb levels.
• The body does not generate an anion gap as
  compensation.
• Calculate the delta gap
• patients Gap- normal gap(12) patients bicarb
• IF gt30, there is an underlying metabolic
  alkalosis
• If lt23, there is an underlying non-gap metabolic
  acidosis

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Practice Example 1

• You are getting sign out on L and D call about a
  G3P2 _at_ 35 weeks with left leg swelling. The
  resident was worried about pulmonary embolism and
  performed an ABG.
• pH 7.50 (Is this high, low or normal)
• pCO2 29 (Is this high, low or normal)
• Bicarb22 ( IS this high, low or normal)

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Example 1 Continued

• pH 7.50

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Example 1 Continued

• pH 7.50 highalkalemia
• pCO2 29

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Example 1 Continued

• pH 7.50 highalkalemia
• pCO2 29 lowalkalemiaprimary process
• bicarb

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Example 1 Continued

• pH 7.50 highalkalemia
• pCO2 29 lowalkalemiaprimary process
• bicarb 22 near normal

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Example 1 Continued

• pH 7.50 highalkalemia
• pCO2 29 lowalkalemiaprimary process
• bicarb 22 near normal
• Acute Respiratory Alkalosis
• Pregnancy Lung disease
• Drug use ( salicylates, catecholamines)
• Sepsis Anxiety
• Any cause of hyperventilation
• hypoxia mechanical ventilation
• hepatic encephalopathy

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Example 2

• You are watching Trainspotting at home because
  Ewan Mc Gregor rocks. You imagine If I had done
  an ABG on Ewan right before the scene where he
  gets Narcan in the ED, what would his ABG have
  been?
• pH 7.25 (High, low or normal)
• pCO2 60 (High, low or normal)
• Bicarb 26 ( High, low or normal)

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Example 2 continued

• pH 7.25

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Example 2 continued

• pH 7.25 lowacidemia
• pCO2 60

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Example 2 continued

• pH 7.25 lowacidemia
• pCO2 60 high acidosis
• bicarb 26

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Example 2 continued

• pH 7.25 lowacidemia
• pCO2 60 high respiratory acidosis
• bicarb 26 nearly normal
• Acute Respiratory Acidosis, as nearly normal
  bicarb indicated no compensation yet.
• CNS depression ( opioids, other drugs, CNS
  events, CO2 retention in COPD)
• Neuromuscular disorders (Guillian Barre, other
  myopathies and neuropathies)
• Acute Airway Obstruction-laryngospasm,
  bronchospasm
• Severe PNA or pulmonary edema
• Impaired lung motion- hemo or pneumo thorax

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Example 3

• Sunday 3AM, you are called to the ED to evaluate
  S.A.,well known to our service for her poorly
  controlled COPD. She has run out of her meds and
  her BIPAP machine was stolen. She looks
  comfortable.
• pH 7.34(High, low, normal)
• pCO2 60 (High, low, normal)
• Bicarb 31 (High, low, normal)

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Example 3 Continued

• pH 7.34

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Example 3 Continued

• pH 7.34 low acidemia
• pCO2 60

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Example 3 Continued

• pH 7.34 low acidemia
• pCO2 60 high respiratory acidosis
• Bicarb31

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Example 3 Continued

• pH 7.34 low acidemia
• pCO2 60 high respiratory acidosis
• Bicarb31 high metabolic alkalosis
• High pCO2 consistent with pH, therefore primary
  respiratory acidosis.
• High bicarb indicates compensation is taking
  place, therefore we know we have a chronic
  problem
• Chronic lung disease Chronic NM
  disorder
• Chronic respiratory center depression- central
  hypoventilation

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What About those Compensation Formulas?

• The goal of compensation is to get as close to
  7.40 as possible, i.e., to maintain homeostasis.
• If the pH is approaching 7.40 in the face of a
  significant disturbance in the primary
  respiratory or metabolic category, you know there
  has been some degree of compensation

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Compensation Part 2

• That having been said, the compensation formulas
  are as follows
• Metabolic Acidosis
• PCO2 decreases 1.2 for each mmol/l change in
  HCO3 or pCO2 last 2 digits of pH
• Metabolic Alkalosis
• pCO2 increases 0.6 for each mmol/L change in HCO3
• Respiratory Acidosis
• acute HCO3 increases 0.1 for every mmHg change
  in pCO2
• chronic HCO3 increases 0.35 for every mmHg
  change in pCO2
• Respiratory alkalosis
• acute HCO3 decreases 0.22 for every mmHg change
  in pCO2
• chronic HCO3 decreases 0.5 for every mmHg change
  in pCO2

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Example 4

• A patient you are co-following on the surgery
  service has a small bowel obstruction. She was
  vomiting for 2 days and now has an NG tube to
  suction. You notice that her bicarb on the
  morning BMP was 36, so you recommend an ABG to
  the surgeons. They call you with the result.
• pH 7.50 (High, low, normal)
• pCO2 48 (high, low, normal)
• Bicarb 36 (High, low, normal)

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Example 4 Continued

• pH 7.50

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Example 4 Continued

• pH 7.50 high alkalemia
• pCO2 48

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Example 4 Continued

• pH 7.50 high alkalemia
• pCO2 48 high respiratory acidosis
• Bicarb 36

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Example 4 Continued

• pH 7.50 high alkalemia
• pCO2 48 high respiratory acidosis
• Bicarb 36 high metabolic alkalosis
• The bicarb is elevated, consistent with a primary
  metabolic alkalosis with a mildly elevated pCO2
  due to respiratory compensation.
• This is a good reminder to look at the bicarb on
  your morning BMP.

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Example 4 Continued

• Metabolic Alkalosis come in two flavors Low
  urinary chloride and normal or high urinary
  chloride. (this is when having a favorite spot to
  look things up comes in handy)
• Low Urinary Chloride ( lt10) Vomiting, NG tube
  suction, past diuretic use, post hypercapnia
• Normal or High Urinary Chloride(gt20)
  Current/recent diuretic use, refeeding alkalosis,
  Cushings syndrome, exogenous steroids, primary
  hyperaldo, CHF, ascites, hypokalemia, Conns
  syndrome, licorice ingestion, excess alkali
  administration

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Example 5

• You are admitting a 61 yo diabetic man with a Hgb
  A1c of 7.2. His blood sugar is 457, he is
  complaining of productive cough, his temperature
  is 39 and his O2 sat is 93
• pH 7.20 ( High, low, normal)
• pCO2 21 (High, low, normal)
• Bicarb 8 ( High, low, normal)

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Example 5 Continued

• pH 7.20

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Example 5 Continued

• pH 7.20 low acidemia
• pCO2 21

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Example 5 Continued

• pH 7.20 low acidemia
• pCO2 21 low respiratory alkalosis
• Bicarb 8

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Example 5 Continued

• pH 7.20 low acidemia
• pCO2 21 low respiratory alkalosis
• Bicarb 8 low metabolic acidosis
• You correctly conclude there is a primary
  metabolic acidosis with a compensatory
  respiratory alkalosis.
• The BMP is pending, so you can not calculate the
  anion gap yet.

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Elevated Anion Gap Acidosis

• M ethanol
• U remia
• D iabetic or alcoholic ketoacidosis
• P araldehyde
• I NH, iron toxicity
• L actic acid
• E thylene glycol
• R habdomyolisis
• S alicylates

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Osmolar Gap

• If an elevated anion gap is present, you need to
  calculate an osmolar gap to rule out methanol,
  ethylene glycol or ethanol as a source.
• Calculated serum osms 2(Na K) glucose/18
  BUN/2.8
• Osmolar Gap Measured Osm- calculated osm
• If gt10, think methanol, ethylene glycol,
  sorbitol, mannitol, renal failure or
  radiocontrast dye.

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Non-Gap Acidosis

• GI Bicarbonate Losses Diarrhea, ureteral
  diversion
• Renal Bicarb Losses RTA, Early renal failure,
  aldosterone and carbonic anhydrase inhibitors
• Posthypocapnia
• Hypercholremic Metabolic Acidosis after large
  volume NS resuscitation

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Non-Gap Acidosis Continued

• To determine the etiology of a non-gap
  acidosis,you must distinguish between renal and
  non-renal (usually GI) sources.
• Urine AG Una Uk-Ucl
• A negative UAG implies kidney is appropriately
  compensating for acidosis, and is therefore a
  non-renal cause.
• A highly positive UAG implies renal impairment.

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Example 6

• You are in Zone 3 on your last ED shift when a
  patient is transferred from PES, where she has
  been for several hours. She is now breathing fast
  and is lethargic.
• pH 7.50 (High, low, normal)
• pCO2 20 (high, low, normal)
• Bicarb 15 (High, low, normal)

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Example 6 Continued

• pH 7.50

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Example 6 Continued

• pH 7.50 high alkalemia
• pCO2 20

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Example 6 Continued

• pH 7.50 high alkalemia
• pCO2 20 low respiratory alkalosis
• Bicarb 15

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Example 6 Continued

• pH 7.50 high alkalemia
• pCO2 20 low respiratory alkalosis
• Bicarb 15 low metabolic acidosis
• Serum Na 140
• Serum Cl 103
• AG 140 -(10315) 22 Anion Gap Acidosis with
  a respiratory alkalosis

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Example 6 Continued

• pH 7.50 high alkalemia
• pCO2 20 low respiratory alkalosis
• Bicarb 15 low metabolic acidosis
• Serum Na 140
• Serum Cl 103
• AG 140 -(10315) 22 Anion Gap Acidosis with
  a respiratory alkalosis
• Delta Gap 22-121525, no other metabolic
  derangement
• Her ASA level comes back markedly elevated.
• This is the classic picture of salicylate
  overdose a respiratory alkalosis with a gap
  acidosis

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Example 7

• You are called to 5C to write admit orders for a
  direct admit from pre-dialysis clinic of a 54
  year old woman with 3 days of vomiting. An ABG
  was sent from clinic.
• pH 7.41 (High, low, normal)
• pCO2 40 (High, low, normal)
• Bicarb 24 (high, low, normal)
• Na 145
• Cl 100

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Example 7 Continued

• pH 7.40

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Example 7 Continued

• pH 7.41 normal. But you are not done yet.
• pCO2 40

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Example 7 Continued

• pH 7.40 normal. But you are not done yet.
• pCO2 40 normal
• Bicarb 24

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Example 7 Continued

• pH 7.40 normal. But you are not done yet.
• pCO2 40 normal
• Bicarb 24 normal
• Anion Gap 21, therefore a primary gap acidosis
  (uremia) is present despite the above values

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Example 7 Continued

• pH 7.40 normal. But you are not done yet.
• pCO2 40 normal
• Bicarb 24 normal
• Anion Gap 21, therefore a primary gap acidosis
  (uremia) is present despite the above values
• Delta Gap 21-12 24 33, therefore an
  underlying metabolic alkalosis (vomiting) is also
  present

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Example 5 Reprise

• That BMP you were waiting for was lost. While
  waiting for the second BMP, your diabetic patient
  received insulin and IVF. He became minimally
  responsive and his RR decreased to 12. A repeat
  ABG was sent
• pH 7.10 (High, low, normal)
• pCO2 50 (High, low, normal)
• Bicarb 15 ( High, low, normal)
• Na 145
• Cl 100

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Example 5 Reprise Continued

• pH 7.10

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Example 5 Reprise Continued

• pH 7.10 low acidemia
• pCO2 50

72
Example 5 Reprise Continued

• pH 7.10 low acidemia
• pCO2 50 high respiratory acidosis
• Bicarb 15

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Example 5 Reprise Continued

• pH 7.10 low acidemia
• pCO2 50 high respiratory acidosis
• Bicarb 15 low metabolic acidosis

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Example 5 Reprise Continued

• pH 7.10 low acidemia
• pCO2 50 high respiratory acidosis
• Bicarb 15 low metabolic acidosis
• Anion Gap 30, primary anion gap metabolic
  acidosis

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Example 5 Reprise Continued

• pH 7.10 low acidemia
• pCO2 50 high respiratory acidosis
• Bicarb 15 low metabolic acidosis
• Anion Gap 30, primary metabolic acidosis
• Delta Gap 33, therefore metabolic alkalosis is
  present.

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Example 5 Reprise Continued

• pH 7.10 low acidemia
• pCO2 50 high respiratory acidosis
• Bicarb 15 low metabolic acidosis
• Anion Gap 30, primary metabolic acidosis
• Delta Gap 33, therefore metabolic alkalosis is
  present.
• Primary AG acidosis due to DKA ( due to
  underlying pneumonia), respiratory acidosis
  secondary to obtunded state and hypoventilation
  and a metabolic alkalosis due to vomiting.

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Hey Katie, where can I go to get more information
on Acid Base Disorders ?

• Good question
• Have your FPIS Intern Teaching Handout on Acid
  Base Disorders with you.
• Habers A Practical Approach to Acid Base
  Disorders is an excellent discussion.
• Banker, et, al., Acid-Base Disturbances 5 Rules
  That Can Simplify Diagnosis, is another great
  resource.

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References

• Di Thomas, Michele, Acid-Base Disorders,
  Handout, August 2003.
• Haber, Richard, A Practical Approach to
  Acid-Base Disorders, Western Journal of
  Medicine, 1991 August 155 146-151.
• Banker, Dipesh, et al., Acid-Base Disturbances
  5 Rules That Can Simplify Diagnosis, Consultant,
  March 2004, 381-399.