(V)ABG interpretation

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(V)ABG interpretation

• Kristian Hecht PGY-3 EM
• With thanks to Marc, Mark and Dr. Rigby

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Outline

• Why is everyone using VBGs anyway?
• Basic Review
• Lists that you have to remember (Doh!)
• Calculations you can do at the bedside
• A BS-free approach to the ABG
• Cases
• Special circumstances

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Basic Review
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Why do we care about ABGs?

• Aids in diagnosis
• Provides clues about clinically unrecognized
  disorders
• May indicate what treatments are needed
• Helps assess progress of illness or therapy

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Whats bad about ABGs

• ABGs are invasive
• Painful, even with lido!
• Have potential complications
• Local hematoma
• Arterial dissection
• thrombosis (rarely)
• Technically difficult, esp. in kids and elderly,
  thus, several attempts may be required.

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ABG Vs. VBG

• Can you use a venous gas to replace an ABG in the
  ED?
• What are the mean differences between arterial
  and venous samples?
• Are they clinically significant?

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• Canadian prospective observational study in the
  ED (CJEM January 2002 Vol 4, No 1)
• N218 pts
• Pts requiring ABG simultaneous venous sampling
• Correlation coefficients and mean differences
  were calculated
• Also 45 academic ED physicians were surveyed to
  determine the minimal clinically important
  difference in each variable

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• The mean differences (95 CI) in arterial and
  venous samples were
• pH 0.036 (0.030 - 0.042)
• pCO2 6.0 (5.0 - 7.0) mmHg
• HCO3 1.5 (1.3 - 1.7) mEq/L

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• These differences were considered greater than
  the minimum clinically significant differences
  identified in the survey
• Concluded that although highly correlated, the
  differences between them preclude using them
  interchangeably
• Can be used to follow trends

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Since 2002

• Arterial Blood Gas Analysis Are Its Values
  Needed for the Management of Diabetic
  Ketoacidosis?
• Ann Emerg Med. 200545550-551
• Good correlation between arterial and venous pH
  and HCO3
• The case for venous rather than arterial blood
  gases in diabetic ketoacidosis
• Emerg Med Australas. 2006 Feb18(1)64-7
• Review article analyzing the validity of venous
  BG sampling in DKA
• In patients with DKA the weighted average
  difference between arterial and venous pH was
  0.02 pH units
• Venous HCO3- was 1.88 higher than arterial

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Principles of Acid-Base

• Normal serum pH is maintained within a very
  narrow range of 7.36-7.44
• Equal to H 447 - 355µM
• pHgt7.8 or lt6.8 is incompatible w/life

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Principles of Acid-Base

• pH is maintained by 3 systems
• Physiologic buffers
• Lungs
• Kidneys
• Disorders in any of these systems leads to
  alterations in blood pH

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Physiologic Buffers

• 1) Bicarbonate-carbonic acid buffer system
• H HCO3- ? H2CO3 ? H2O CO2
• 2) Intracellular blood protein buffers
• Hemoglobin
• w/o this venous blood would be pH 4.5
• 3) Bone
• Reservoir of bicarb and phosphate

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Lungs

• Changes in pH sensed by chemoreceptors
• Peripherally (carotid bodies)
• Centrally (medulla oblongata)
• Drop in pH
• Increased minute ventilation
• Lowers PaCO2
• Increase in pH
• Decreased ventilatory effort
• Increases PaCO2

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Kidneys

• Play no role in acute compensation
• 6-12hrs Acidosis
• Active excretion of H
• Retention of HCO3-
• gt6hrs of Alkalemia
• Active excretion of HCO3-
• Retention of H

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Normal ABG parameters

• pH 7.40
• PCO2 40 mmHg
• HCO3 24 mM
• Anion Gap 12 - 15

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Terminology

• Acidemia blood pH lt 7.35
• Acidosis a physiologic process that, occurring
  alone, tends to cause acidemia
• e.g. metabolic acidosis from increased ketoacid
  production in DKA
• If the patient also has an alkalosis at the same
  time, the resulting blood pH may be low, normal
  or high

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Terminology

• Alkalemia blood pH gt 7.45
• Alkalosis a primary physiologic process that,
  occurring alone, tends to cause alkalemia
• i.e. respiratory alkalosis from
  hyperventilation
• If the patient also has an acidosis at the same
  time, the resulting blood pH may be high, normal
  or low.

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Terminology

• Primary acid-base disorder One of the four
  acid-base disturbances that is manifested by an
  initial change in HCO3- or PaCO2.
• Compensation The change in HCO3- or PaCO2 that
  results from the primary event. Compensatory
  changes are not classified by the terms used for
  the four primary acid-base disturbances.
• You cannot overcompensate for an Acid-Base
  disturbance

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Acid-Base Disorders

• Respiratory disorders
• Alter the serum PaCO2
• Metabolic disorders
• Alter the serum HCO3-

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Lists you have to remember.

• Doh

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Respiratory Disorders

• ACIDOSIS
• Hypoventilation
• Pulmonary pathology
• Airway obstruction
• Decreased respiratory drive

• ALKALOSIS
• ? minute ventilation
• CNS disease
• Hypoxemia
• Anxiety
• Toxic states
• Hepatic insufficiency
• Assisted ventilation

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Metabolic Disorders

• ACIDOSIS
• Anion gap metabolic acidosis
• Non-AG metabolic acidosis

• ALKALOSIS
• Saline responsive
• Saline resistant

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Anion Gap Metabolic Acidosis

• Addition of exogenous acids
• or
• Creation of endogenous acids

• Cat Mudpiles
• Carbon monoxide/cyanide
• Alcohol/AKA
• Toluene
• Methanol
• Uremia
• DKA
• Paraldehyde
• INH/Iron
• Lactic Acidosis
• Ethylene glycol
• Salicylates

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Normal AG Metabolic Acidosis

• Excessive loss of HCO3-
• or
• Inability to excrete H

• Hard ups
• Hyperalimentation/ Hyperventilation
• Acids/Addisons/ Acetazolamide
• RTA
• Diarrhea/Dehydration/ Diuretics
• Uterosigmoidostomy
• Pancreatic fistula or drainage
• Saline (large amounts)

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Saline-responsive metabolic alkalosis

• Volume contracted
• Contraction of the ECF around the constant plasma
  HCO3-
• Relative Excess
• Urinary chloride level lt10 mEq/L

1. Vomiting/Gastric Suction
2. Diuretics
3. Ion-deficient baby formula
4. Colonic adenomas

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Saline-resistant metabolic alkalosis

• Associated with mineralcorticoid excess
• Leads to ? Na reabsorption
• Secretion of K and H to maintain neutrality
• Urinary chloride gt10mEq/L

1. Primary aldosteronism
2. Exogenous steroids
3. Adenocarcinoma
4. Bartters Syndrome
5. Cushings disease
6. Ectopic adrenocorticotropic hormone

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Calculations that can help you
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Henderson-Hasselbalch equation

• Check validity of laboratory measurements
  obtained
• H 24 x PaCO2 HCO3 40 nEq/L
• HCO3 calculated on ABG with HH eqn
• HCO3 measued on Chem 6

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Respiratory Compensation

• Compensation PaCO2 HCO3-
• Acute Resp Acidosis 10 1
• Acute Resp Alkalosis 10 2
• Chronic Resp Acidosis 10 3
• Chronic Resp Alkalosis 10 4

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Metabolic Compensation

• Compensation PaCO2 HCO3-
• Metabolic Acidosis 10 10
• Metabolic Alkalosis 10 7.5

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The Corey Slovis approach to acid-base
abnormalities

• A no bull_at_ approach
• for non-nephrologist

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Slovis 6-step approach to ABG

1. Check the numbers
2. Apply the ABG rules
3. Calculate the AG
4. If Acidosis apply the rule of 15 (/- 2)
5. If Acidosis apply the delta gap (/- 4)
6. Check the osmolar gap

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Check the numbers

• Need both Chem 6 and blood gas
• Know your normal values
• Does the blood gas make sense?
• Are there any immediate hints to the diagnosis

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The ABG rules

• 1) Is it an Acidosis or Alkalosis
• Look at the pH (gt7.45, lt7.35)
• 2) Is it Respiratory or Metabolic
• Metabolic pCO2 pH ? in same direction
• Resp pCO2 pH ? in opposite direction
• 3) Is it a pure respiratory acidosis?
• ?pCO2 ?pH 11

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Calculate the AG

• Na HCO3- Cl
• Normal 5-12
• Upper limit of normal is 15

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Unmeasured ions
Mg2 Ca2 K
Albumin PO43- Acetate
HCO3-
Na
Cl-
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Anion Gap
HCO3-
Na
Cl-
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Anion Gap

• For example give me an M
• Methanol intoxication
• Methanol oxidized to formic acid
• Formate- H HCO3- ? Formate- ? CO2 H2O

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Add Formic acid
Anion Gap
HCO3-
Na
Cl-
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Formate-
Anion Gap
HCO3-
Na
Cl-
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Narrow AG?

• Sure, add more unmeasured cations, as carbonate
  or chloride
• e.g. FeCl2 MgCl2

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Add MgCl2
Anion Gap
HCO3-
Na
Cl-
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Mg2
Anion Gap
HCO3-
Na
Cl-
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Rule of 15
HCO3- 15 pCO2 pH (last 2 digits)
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Rule of 15

• Used in acidosis
• Derived from the Henderson Hasselbalch equation
• It predicts what resp compensation will do to the
  pCO2 and the pH
• If the Rule is broken then another process other
  than just resp compensation exists

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Rule of 15

• Creates a new set point for the pCO2
• pCO2 appropriate normal compensation
• pCO2 too low superimposed primary resp
  alkalosis
• pCO2 too high superimposed primary resp
  acidosis
• Note as HCO3 falls below 10 you need to use the
  formula
• HCO3 x 1.5 8 expected pCO2

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Examples of rule of 15

• 1) HCO320, pCO235 pH 7.35
• Pure wide gap metabolic acidosis with an
  appropriate 2ndary resp alkalosis
• 2) HCO310, pCO220 pH 7.32
• pCO2 is too low. Superimposed primary resp
  alkalosis
• 3) HCO310, pCO232 pH 7.14
• pCO2 is too high. Superimposed primary resp
  acidosis

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Delta Gap

• Checks for hidden metabolic process
• Based on the 11 concept that
  ?AG ?HCO3
• Upper limit of AG 15
• Normal HCO3 24
• Bicarb too high metabolic alkalosis
• Bicarb too low Non-gap metabolic acidosis

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Examples of delta gap

• AG20 HCO319
• ?AG 5 and ?HCO3 5
• No hidden process
• AG22 HCO38
• ?AG 7 and ?HCO3 16
• Bicarb too low additional normal AG metabolic
  acidosis
• AG26 HCO320
• ?AG 11 and ?HCO3 4
• Bicarb too high superimposed metabolic
  alkalosis

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Osmolar Gap

• Use if an unexplained anion gap acidosis
• 2Na BUN Glucose calculated gap
• OG Measured calculated
• Upper limit of normal is 10
• If higher consider toxic alcohols

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Intermission

• http//www.youtube.com/watch?vRcL6DwSufMI

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Confused???? Lets hit the cases
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Case 1

• 19yo male presents with 2 week hx of abdominal
  pains and blurred vision
• Na 135 BUN 30 pH 7.30
• Cl 100 Glucose 38 pCO2 30
• K 6.0 pO2 100
• HCO3 15

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Approach

• Check the numbers
• Apply the ABG rules
• Calculate the AG
• If Acidosis apply the rule of 15 (/- 2)
• If Acidosis apply the delta gap (/- 4)
• Check the osmolar gap

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Case 1

• Anion Gap Metabolic acidosis with appropriate
  resp compensation
• DDx MUDPILES
• Diagnosis DKA

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Case 2

• 36yo M presents with altered LOC. He is markedly
  agitated, febrile and hyperventilating
• Na 140 pH 7.32
• Cl 100 pCO2 20
• K 3.8 pO2 80
• HCO3 10

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Approach

• Check the numbers
• Apply the ABG rules
• Calculate the AG
• If Acidosis apply the rule of 15 (/- 2)
• If Acidosis apply the delta gap (/- 4)
• Check the osmolar gap

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Case 2 cont

• Anion gap metabolic acidosis
• And Resp alkalosis
• Two immediate things you have to think about?
• ASA overdose
• Sepsis

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Case 3

• 84yo F found down in her apartment with altered
  mental status
• Na 140 pH 7.16
• Cl 104 pCO2 64
• K 3.2 pO2 80
• HCO3 28

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Approach

• Check the numbers
• Apply the ABG rules
• Calculate the AG
• If Acidosis apply the rule of 15 (/- 2)
• If Acidosis apply the delta gap (/- 4)
• Check the osmolar gap

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Case 3

• Pure respiratory acidosis
• DDx
• Pulmonary pathology
• Airway obstruction
• Decreased respiratory drive

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Case 4

• 48yo known diabetic presents with 4d hx of
  abdominal pains, vomiting and severe diarrhea
• Not eating so stopped insulin
• Na 130 BUN 40 pH 7.30
• Cl 105 Glucose 29 pCO2 30
• K 4.8 pO2 100
• HCO3 15

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Approach

• Check the numbers
• Apply the ABG rules
• Calculate the AG
• If Acidosis apply the rule of 15 (/- 2)
• If Acidosis apply the delta gap (/- 4)
• Check the osmolar gap

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Case 4 cont

• Is this DKA?
• No!
• Non-AG Metabolic Acidosis
• DDx HARDUPS
• Most likely secondary to severe diarrhea

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Case 5

• 22yo F presents with retrosternal chest pain and
  describes SOB during her biology exam
• Na 135 BUN 9 pH 7.46
• Cl 101 Glucose 7.8 pCO2 35
• K 4.0 pO2 100
• HCO3 23

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Approach

• Check the numbers
• Apply the ABG rules
• Calculate the AG
• If Acidosis apply the rule of 15 (/- 2)
• If Acidosis apply the delta gap (/- 4)
• Check the osmolar gap

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Case 5 cont

• Respiratory alkalosis
• DDx
• CNS disease
• Hypoxemia
• Anxiety
• Toxic states
• Hepatic insufficiency
• Assisted ventilation

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Case 5 cont

• Respiratory alkalosis
• DDx
• CNS disease
• Hypoxemia
• Anxiety
• Toxic states
• Hepatic insufficiency
• Assisted ventilation

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Case 6

• You are about to place the ETT in a crashing
  patient when the RT shoves the following ABG into
  your face with no patient history at all
• Na 138 pH 7.25
• Cl 108 pCO2 25
• K 5.0 pO2 100
• HCO3 10

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Approach

• Check the numbers
• Apply the ABG rules
• Calculate the AG
• If Acidosis apply the rule of 15 (/- 2)
• If Acidosis apply the delta gap (/- 4)
• Check the osmolar gap

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Case 6 cont

• You explain to her that this is
• obviously.
• Wide gap metabolic acidosis with appropriate
  respiratory compensation
• DDx MUDPILES
• Delta gap indicating an additional non-AG
  metabolic acidosis
• DDx HARDUPS

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Case 7

• 35-year-old man with renal insufficiency admitted
  to hospital with pneumonia and the following lab
  values
• Na 145 pH 7.52
• Cl 98 pCO2 30
• K 2.9 pO2 62
• HCO3 21

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Approach

• Check the numbers
• Apply the ABG rules
• Calculate the AG
• If Acidosis apply the rule of 15 (/- 2)
• If Acidosis apply the delta gap (/- 4)
• Check the osmolar gap

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Case 7

• Three separate acid-base disorders !!!
• 1) Acute respiratory alkalosis
• Acute hyperventilation due to pneumonia
• 2) Concomitant metabolic acidosis
• From renal disease
• 3) Hypokalemic metabolic alkalosis
• From excessive diuretic therapy
• The result of all this acid-base abnormality?
  Blood gas values that are indistinguishable from
  those of simple acute respiratory alkalosis.

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Case 8

• Elderly man from nursing home with hx of RA
• Profound weakness and areflexia poor oral
  intake for days
• Current meds
• Sleeping pills PRN
• Prednisone 45mg daily
• Na 145 pH 7.58 Urine Cl 74 mmol/L
• Cl 86 pCO2 49
• K 1.9 pO2 84
• HCO3 45

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Approach

• Check the numbers
• Apply the ABG rules
• Calculate the AG
• If Acidosis apply the rule of 15 (/- 2)
• If Acidosis apply the delta gap (/- 4)
• Check the osmolar gap

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Case 8 Cont

• Metabolic Alkalosis w/ Resp alkalosis (107.5)
• ? Saline responsive or resistant
• Resistant
• DDx?
• Primary aldosteronism
• Exogenous steroids
• Adenocarcinoma
• Bartters Syndrome
• Cushings disease
• Ectopic adrenocorticotropic hormone
• Why is the K so low?

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Case 9

• EMS called for 38yo male increasingly agitated
  and incoherent
• paramedics noted he appeared "drunk" but normal
  vital signs and 02 Sats
• BP 110/70, HR 72, T 36C, RR 24, Sat 97 RA
• Thirty minutes later
• GCS fell to 9 (E2/M4/V3)
• RR ? 30 breaths/min
• No focal neurologic signs
• Physical examination was otherwise unremarkable
• PEA arrest requiring resuscitation with Epi

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Case 9

• Labs
• Na 153 BUN 5.9 pH 6.49
• Cl 108 Glucose 6.0 pCO2 62
• K 5.4 Cr 174 pO2 100
• HCO3 5

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Approach

• Check the numbers
• Apply the ABG rules
• Calculate the AG
• If Acidosis apply the rule of 15 (/- 2)
• If Acidosis apply the delta gap (/- 4)
• Check the osmolar gap

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Case 9 Cont

• What would be appropriate resp compensation for
  this metabolic acidosis?
• HCO3 x 1.5 8 expected pCO2
• pCO2 should 16
• Acid-Base abnormality?
• Severe AG metabolic acidosis
• Secondary severe Resp Acidosis

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Case 9 Cont

• Anything else you would like?
• Serum Osmolarity 487 mOsm
• Serum EtOH lt 2.2mmol/L
• What is the Osmolar Gap?
• 169 mEq/L
• Diagnosis?
• Severe methanol intoxication
• Serum methanol 37mmol/L
• Patient died

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Case 10

• 60yo male seriously ill on arrival to ED
• Vomiting dark brown fluid every hour or two for
  about a day plus several episodes of melena
• Past history of alcoholism, cirrhosis, portal
  hypertension
• Examination
• Jaundiced, sweaty, clammy and tachypnoeic
• BP 98/50, pulse 120/min
• Peripheries were cool
• Abdomen soft and nontender
• Signs of chronic liver disease present

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Case 10 Cont

• Labs
• Na 131
• Cl 85
• K 4.2
• Glucose 2.88 mmol/L
• BUN 8 mmol/L
• Creatinine 78 umol/L
• Lactate 20.3 mmol/l
• Hgb 62 g/L
• Albumin 20g/L

• ABG
• pH 7.10
• pCO2 14 mmHg
• pO2 103 mmHg
• HCO3 4 mmol/l

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Approach

• Check the numbers
• Apply the ABG rules
• Calculate the AG
• If Acidosis apply the rule of 15 (/- 2)
• If Acidosis apply the delta gap (/- 4)
• Check the osmolar gap

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Case 10 Cont

• WG metabolic acidosis with appropriate
  respiratory compensation
• Likely lactic acidosis
• Is there a secondary metabolic process?
• ?AG 27 and ?HCO3 20
• But

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Case 10 Cont

• Does a low serum albumin affect the measurement
  of the anion gap?
• Yes!
• If albumin lt40 g/l for every decline of 10 g/l
  subtract 4 from the normal value of the AG
• Therefore the ?AG ?HCO3 and it is a pure WG
  metabolic acidosis

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Case 11

• 28yo F known asthmatic and 8 months pregnant
  presents with increasing SOB over 24hrs
• She has been taking her inhalers with no effect
• Exam
• In resp distress, diaphoretic, and looking very
  tired
• Auscultation reveals no wheezing

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Case 11 Cont

• ABG
• pH 7.36
• PO2 90
• PCO2 45
• HCO3 22
• Are you concerned about her?

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Case 11

• Physiologic changes of pregnancy
• Physiological hyperventilation results in
  respiratory alkalosis with compensatory renal
  excretion of bicarbonate
• These changes alter normal ABG values
• pH 7.4-7.45
• PO2 95-105 mm Hg
• PCO2 28-32 mm Hg,
• HCO3 18-21 mEq/L.

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Case 11 Cont

• Even though the ABG does not at first glance
  appear worrisome
• A pCO2 of 45 at this stage in pregnancy likely
  represents a significant degree of CO2 retention
• Potentially impending resp failure!!!

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Name that Acidosis

• Distinctive Breath
• DKA
• Renal Failure
• Uremia
• Refractory Seizures
• INH
• Xray diagnosis
• Iron ingestion

• Blindness
• Methanol
• 1 Resp Alkalosis
• ASA
• GI Bleed
• Lactic acidosis
• U/A diagnosis
• Ethylene glycol

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Thanks!