(v)abg interpretation kristian hecht pgy-3 em with thanks to marc, mark and dr. rigby

(V)ABG (V)ABG interpretationinterpretationKristian Hecht PGY-3 EMKristian Hecht PGY-3 EM

With thanks to Marc, Mark With thanks to Marc, Mark and Dr. Rigbyand Dr. Rigby

OutlineOutline

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

Basic ReviewBasic Review

Why do we care about ABGs?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

What’s bad about ABG’sWhat’s bad about ABG’s

• ABG’s 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. VBGABG 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)

– N=218 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:

1) pH = 0.036 (0.030 - 0.042)

2) pCO2 = 6.0 (5.0 - 7.0) mmHg

3) 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 2002Since 2002

• Arterial Blood Gas Analysis: Are Its Values Needed for the Management of Diabetic Ketoacidosis?– Ann Emerg Med. 2005;45:550-551– 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 Feb;18(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-BasePrinciples of Acid-Base

• Normal serum pH is maintained within a very narrow range of 7.36-7.44

• Equal to [H+] 447 - 355μM

• pH>7.8 or <6.8 is incompatible w/life

Principles of Acid-BasePrinciples of Acid-Base

• pH is maintained by 3 systems1)Physiologic buffers

2)Lungs

3)Kidneys

• Disorders in any of these systems leads to alterations in blood pH

Physiologic BuffersPhysiologic 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

LungsLungs

• 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

KidneysKidneys

• Play no role in acute compensation

• 6-12hrs Acidosis– Active excretion of H+

– Retention of HCO3-

• >6hrs of Alkalemia– Active excretion of HCO3

-

– Retention of H+

Normal ABG parametersNormal ABG parameters

• pH 7.40

• PCO2 40 mmHg

• [HCO3] 24 mM

• Anion Gap = 12 - 15

Terminology Terminology

• Acidemia: blood pH < 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 Terminology

• Alkalemia: blood pH > 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.

TerminologyTerminology• 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 overcompensateovercompensate for an Acid-Base disturbance

Acid-Base DisordersAcid-Base Disorders

• Respiratory disorders– Alter the serum PaCO2

• Metabolic disorders– Alter the serum HCO3

-

Lists you have to Lists you have to remember….remember….

D’oh…D’oh…

Respiratory DisordersRespiratory Disorders

ACIDOSIS• Hypoventilation

– Pulmonary pathology– Airway obstruction– Decreased respiratory

drive

ALKALOSIS• ↑ minute ventilation

– CNS disease– Hypoxemia– Anxiety– Toxic states– Hepatic insufficiency– Assisted ventilation

Metabolic DisordersMetabolic Disorders

ACIDOSIS

1) Anion gap metabolic acidosis

2) Non-AG metabolic acidosis

ALKALOSIS

1) Saline responsive

2) Saline resistant

Anion Gap Metabolic AcidosisAnion 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 AcidosisNormal AG Metabolic Acidosis

• Excessive loss of HCO3

-

or

• Inability to excrete H+

“Hard ups”• Hyperalimentation/

Hyperventilation• Acids/Addison’s/

Acetazolamide• RTA• Diarrhea/Dehydration/

Diuretics• Uterosigmoidostomy• Pancreatic fistula or drainage• Saline (large amounts)

Saline-responsive metabolic Saline-responsive metabolic alkalosisalkalosis

• Volume contracted• Contraction of the

ECF around the constant plasma HCO3

-

• Relative Excess• Urinary chloride level

<10 mEq/L

1) Vomiting/Gastric Suction

2) Diuretics

3) Ion-deficient baby formula

4) Colonic adenomas

Saline-resistant metabolic alkalosisSaline-resistant metabolic alkalosis

• Associated with mineralcorticoid excess

• Leads to ↑ Na+

reabsorption• Secretion of K+ and

H+ to maintain neutrality

• Urinary chloride >10mEq/L

1) Primary aldosteronism2) Exogenous steroids3) Adenocarcinoma4) Bartter’s Syndrome5) Cushing’s disease6) Ectopic

adrenocorticotropic hormone

Calculations that can Calculations that can help youhelp you

Henderson-Hasselbalch Henderson-Hasselbalch equationequation

• 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 CompensationRespiratory 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 CompensationMetabolic Compensation

Compensation PaCO2 : HCO3-

Metabolic Acidosis 10 : 10

Metabolic Alkalosis 10 : 7.5

““The Corey Slovis approach The Corey Slovis approach to to

acid-base abnormalities”acid-base abnormalities”A no bull@#$% approach A no bull@#$% approach

for non-nephrologistfor non-nephrologist

Slovis 6-step approach to ABG 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

Check the numbersCheck 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 rulesThe ABG rules1) Is it an Acidosis or Alkalosis

• Look at the pH (>7.45, <7.35)

2) Is it Respiratory or Metabolic– Metabolic = pCO2 + pH ∆ in samesame direction

– Resp = pCO2 + pH ∆ in oppositeopposite direction

3) Is it a pure respiratory acidosis?

↑pCO2 : ↓pH = 1:1

Calculate the AGCalculate the AG

• Na+ – [HCO3- + Cl]

• Normal = 5-12

• Upper limit of normal is 15

Na+

Cl-

HCO3-

Albumin PO4

3- Acetate

Mg2+ Ca2+ K+

Unmeasured ions

Na+

Cl-

HCO3-

Anion Gap

Anion GapAnion Gap

• For example: give me an ‘M’

• Methanol intoxication

• Methanol oxidized to formic acid

• Formate- + H+ + HCO3- Formate- + CO2 + H2O

Na+

Cl-

HCO3-

Anion Gap

Add Formic acid

Na+

Cl-

Anion Gap

Formate-

HCO3-

Narrow AG?Narrow AG?

• Sure, add more unmeasured cations, as carbonate or chloride

• e.g. FeCl2 MgCl2

Na+

Cl-

HCO3-

Anion Gap

Add MgCl2

Na+

Cl-

HCO3-

Anion Gap

Mg2+

Rule of 15Rule of 15

HCO3- + 15 = pCO2 = pH (last 2 digits)


• 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


• 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 15Examples of rule of 15

1) HCO3=20, pCO2=35 pH= 7.35

• Pure wide gap metabolic acidosis with an appropriate 2ndary resp alkalosis

2) HCO3=10, pCO2=20 pH= 7.32

• pCO2 is too low. Superimposed primary resp alkalosis

3) HCO3=10, pCO2=32 pH= 7.14

• pCO2 is too high. Superimposed primary resp acidosis

Delta GapDelta Gap

• Checks for “hidden” metabolic process• Based on the 1:1 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 gapExamples of delta gap

• AG=20 HCO3=19– ∆AG = 5 and ∆HCO3 = 5 – No hidden process

• AG=22 HCO3=8– ∆AG = 7 and ∆HCO3 = 16– Bicarb too low = additional normal AG metabolic

acidosis• AG=26 HCO3=20

– ∆AG = 11 and ∆HCO3 = 4– Bicarb too high = superimposed metabolic alkalosis

Osmolar GapOsmolar Gap

• Use if an unexplained unexplained anion gap acidosis

• 2Na + BUN + Glucose = calculated gap

• OG = Measured – calculated

• Upper limit of normal is ~10

• If higher consider toxic alcohols

IntermissionIntermission

http://http://www.youtube.com/watch?vwww.youtube.com/watch?v=RcL6DwSufMI=RcL6DwSufMI

Confused????Confused????

Lets hit the casesLets hit the cases

Case #1Case #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