(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

Basic Review

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

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.

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?

- 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

- 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

- 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

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

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

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

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

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

Normal ABG parameters

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

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

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.

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

Acid-Base Disorders

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

Lists you have to remember.

- Doh

Respiratory Disorders

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

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

Metabolic Disorders

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

- ALKALOSIS
- Saline responsive
- Saline resistant

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

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)

Saline-responsive metabolic alkalosis

- Volume contracted
- Contraction of the ECF around the constant plasma

HCO3- - Relative Excess
- Urinary chloride level lt10 mEq/L

- Vomiting/Gastric Suction
- Diuretics
- Ion-deficient baby formula
- Colonic adenomas

Saline-resistant metabolic alkalosis

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

- Primary aldosteronism
- Exogenous steroids
- Adenocarcinoma
- Bartters Syndrome
- Cushings disease
- Ectopic adrenocorticotropic hormone

Calculations that can help you

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

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

Metabolic Compensation

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

The Corey Slovis approach to acid-base

abnormalities

- A no bull_at_ approach
- for non-nephrologist

Slovis 6-step approach to ABG

- 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

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

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

Calculate the AG

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

Unmeasured ions

Mg2 Ca2 K

Albumin PO43- Acetate

HCO3-

Na

Cl-

Anion Gap

HCO3-

Na

Cl-

Anion Gap

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

Add Formic acid

Anion Gap

HCO3-

Na

Cl-

Formate-

Anion Gap

HCO3-

Na

Cl-

Narrow AG?

- Sure, add more unmeasured cations, as carbonate

or chloride - e.g. FeCl2 MgCl2

Add MgCl2

Anion Gap

HCO3-

Na

Cl-

Mg2

Anion Gap

HCO3-

Na

Cl-

Rule of 15

HCO3- 15 pCO2 pH (last 2 digits)

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

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

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

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

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

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

Intermission

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

Confused???? Lets hit the cases

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

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

Case 1

- Anion Gap Metabolic acidosis with appropriate

resp compensation - DDx MUDPILES
- Diagnosis DKA

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

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

Case 2 cont

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

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

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

Case 3

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

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

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

Case 4 cont

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

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

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

Case 5 cont

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

Case 5 cont

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

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

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

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

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

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

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.

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

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

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?

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

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

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

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

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

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

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

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

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

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

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

Case 11 Cont

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

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.

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!!!

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

Thanks!