ABG

WHAT INFORMATION IS OBTAINED FROM AN ABG:

• Acid base status

• Oxygenation• Dissolved O2 (pO2)

• Saturation of hemoglobin

• CO2 elimination

• Electrolytes – Na, K, Cl, Ca

WHICH ARTERY IS COMMONLY CHOSEN?

• The radial artery is superficial, has collaterals and is easily compressed. It is almost always the first choice.

• Other arteries (femoral, dorsalis pedis, brachial) can be used in emergencies.

Diagram showing the Anatomical position of the radial artery

and its relations

The above photograph shows an ABG analyzer and the technique

of needle placement in this machine

CAUTION

• Colour of the blood cannot be used to identify arterial blood rather it’s the flow

• Sluggish filling indicates the needle is in the vein

• If the patient experiences pain shooting distally, a nerve may have been hit

• Pain should always be avoided. Causes hyperventilation and change in parameters

• Ventilatory parameters are changed – wait for 15 minutes for equilibrium to be reached

CONDITIONS INVALIDATING OR MODIFYING ABG RESULTS

1. DELAYED ANALYSIS

• As the Consumption of O2 & Production of CO2 continues after blood drawn into syringe, reading may change with TIME.

• PaCO2 ∆ 3-10 mmHg/hour

• PaO2 ∆ at a rate related to initial value & dependant on Hb Sat

• When transportion time is > 5 mins, ice packs should be used

• Maximum time the changes can be prevented is 20 mins.

Parameter 37 C (Change every 10

min)

4 C (Change every 10

min)

pH 0.01 0.001

PCO2 1 mm Hg 0.1 mm Hg

PO2 0.1 vol % 0.01 vol %

2. Effect of Room Temperature on ABG values – increase in

temperature increases the rate of these time bound changes

3. EXCESSIVE HEPARIN

•Dilutional effect

•Heparin is acidic

Only 0.05 ml is required to anticoagulate 1 ml ofblood.

Dead space volume of a standard 5 ml syringe with 1

inch 22 guage needle is 0.2 ml, filling the syringe dead

space with heparin provides sufficient volume to

anticoagulate a 4 ml blood sample.

4. AIR BUBBLES

Mixing with sample lead to PaO2 & PaCO2i. Mixing/Agitation diffusion more erroneous resultsii. Avoid excess suctioniii. So, discard sample if excessive air bubbles presentiv. Seal with cork/cap immediately after taking sample

5. FEVER OR HYPOTHERMIA i. Most ABG analyzers report data at N body tempii. If severe hyper/hypothermia, values of pH & PCO2 at

37 C can be significantly diff from pt’s actual valuesiii. When blood is cooled, CO2 becomes more soluble

reducing its PCO2 by about 4.5% per ˚C fall in temperature and the pH rises by about 0.015 per ˚C fall in temperature.

iv. Pt’s temp should be mentioned while sending sample & lab should mention whether values being given in report at 37 C/pts actual temp

6. WBC COUNT

0.1 ml of O2 consumed/dL of blood in 10 min in ptswith Normal TLC

Marked increase in consumption may be seen in pts withvery high TLC/platelet counts – hence immediatechilling/analysis is essential

7. TYPE OF SYRINGEi. pH & PCO2 values unaffected

ii. PO2 values drop more rapidly in plastic syringes

Glass syringes may be preferred because

i. No diffusion of gases

ii. Less resistance of the plunger

INFORMATION CONTAINED IN AN ABG• Measured values

• pO2,pCo2,pH,Hct,Na,K,Ca,Cl

• Derived values

• HCo3(std & Actual), Buffer base, Base excess, standard base excess, A-a Do2, p50

• Feeded values

• Hb%, RespQ, Atm pressure, temperature

• Patients particulars• Name, age,

In case a machine does not measure a particular value it needs for

calculation, it uses a prefixed value for calculation of derived values.

Ex - if a machine does not measure Hematocrit, it uses a standard

value in its measurements

BICARBONATE (HCO3-)

Std HCO3-: HCO3

- levels MEASURED in lab considering that PCO2 was 40 mm Hg and Hb was 100% saturated with oxygen.

Normal: 22-27mmol/L Mean: 24mmol/L

Actual HCO3-: HCO3

- levels CALCULATED from pH & PCO2directly.

In standard normal subject: AB=SB

• Reflection of non respiratory (metabolic) acid-basestatus.

BASE EXCESS/BASE DEFICIT

• Definition: The amount of a acid or base in meq needed to titrate 1 litre of blood to a pH of 7.40.

• [std. BE - Assumes 100% oxygenation, 37oC, and pCO2 of 40]

Normal = 0 (-2 to +2)

Used to calculate the metabolic component of an acid-base disturbance.

Calculated from pH, PaCO2 and HCT

STANDARD BASE EXCESS

Also known as “Base Excess of ECF.” ECF includes plasma, red cells, and the surrounding interstitial fluid. It’s where the action takes place in the body regarding acid-base movement.

Blood-gas machines calculate SBE as:

SBE = 0.9287 * (HCO3- - 24.4 + (14.83 * (pH – 7.4)))

Arterial blood

Venous blood

pH 7.38 – 7.42 7.36 – 7.39

PaO2 80 - 100 38 - 42

PaCO2 36 – 44 (40) 44 – 48 (46)

HCO3- 22 – 26 (24) 20 – 24 (22)

SaO2 95 – 100% 75%

ASSESSMENT OF ACID-BASE STATUS

LOOK FOR ERROR

• Compare directly measured HCO3- in laboratory to that calculated in ABG.

• Insert H+, PaCO2, and HCO3- into Henderson equation …. If error >10%, its substantial

• Calculate anion gap – very low or negative suggests some error

THE SIX STEP APPROACH TO SOLVING ACID-BASE DISORDERS

ACID – BASE BALANCESTEP 1. ASSESSING THE PH

pH 1 7 14

Acidic Neutral Alkaline

pH = 7.35 – 7.45

Respiratory Metabolic

CO2=Acidosis HCO3=Acidosis

CO2=Alkalosis HCO3=Alkalosis

STEP 2: IS THE PRIMARY DISTURBANCE RESPIRATORY OR METABOLIC?

• Assess the paCO2 level.

• If pH and PaCO2 move in opposite direction – primaryproblem is respiratory.

• If pH and PaCO2 move in same direction - primary problem is metabolic

• Assess HCO3 value

• If pH increases the HCO3 should also increase

• If pH decreases HCO3 should also decrease

For primary – uncompensated response

IMPORTANT

• No compensatory phenomena is strong enough to fully compensate and make the pH normal

• If either pH or PaCO2 is normal - mixed metabolic and respiratory abnormality is present

• If pH is normal, PaCO2 identifies the respiratory abnormality

• If PaCO2 is normal, pH identifies the metabolic abnormality

RULE 1 : THE 1 FOR 10 RULE FOR ACUTE RESPIRATORY

ACIDOSIS

• The [HCO3] will increase by 1 mmol/l for every 10 mmHg elevation in pCO2 above 40 mmHg.

• Expected [HCO3] = 24 + { (Actual pCO2 - 40) / 10 }

• Example:

• A patient with an acute respiratory acidosis (pCO2 60mmHg) has an actual [HCO3] of 31mmol/l.

• The expected [HCO3] for this acute elevation of pCO2 is 24 + 2 = 26mmol/l.

• The actual measured value is higher than this indicating that a metabolic alkalosis must also be present. – (COMBINED DISORDER)

RULE 2 :THE 4 FOR 10 RULE FOR CHRONIC RESPIRATORY

ACIDOSIS• The [HCO3] will increase by 4 mmol/l for every 10 mmHg

elevation in pCO2 above 40mmHg.

• Expected [HCO3] = 24 + 4 { (Actual pCO2 - 40) /

• Example: • A patient with a chronic respiratory acidosis (pCO2 60mmHg) has an

actual [HCO3] of 31mmol/l.

• The expected [HCO3] for this chronic elevation of pCO2 is 24 + 8 = 32mmol/l. T

• There is no evidence indicating a second acid-base disorder.

• A level above 32 would indicate metabolic alkalosis as a second/combined disorder

RULE 3 :THE 2 FOR 10 RULE FOR ACUTE RESPIRATORY

ALKALOSIS

• The [HCO3] will decrease by 2 mmol/l for every 10 mmHg decrease in pCO2 below 40 mmHg.

• Expected [HCO3] = 24 - 2 { ( 40 - Actual pCO2) / 10 }

• Comment:

• In practice, this acute physicochemical change rarely results in a [HCO3] of less than about 18 mmol/s.

• (After all there is a limit to how low pCO2 can fall as negative values are not possible!)

• So a [HCO3] of less than 18 mmol/l indicates a coexisting metabolic acidosis. (COMBINED DISORDER)

RULE 4 :THE 5 FOR 10 RULE FOR A CHRONIC RESPIRATORY

ALKALOSIS• The [HCO3] will decrease by 5 mmol/l for every 10

mmHg decrease in pCO2 below 40 mmHg.

• Expected [HCO3] = 24 - 5 { ( 40 - Actual pCO2 ) / 10 } ( range: +/- 2)

• Comments:• It takes 2 to 3 days to reach maximal renal compensation

• The limit of compensation is a [HCO3] of about 12 to 15 mmol/l

• Values below 10 usually indicate associated metabolic acidosis

RULE 5 : THE ONE & A HALF PLUS 8 RULE - FOR A METABOLIC

ACIDOSIS(WINTER’S FORMULA)

• Expected pCO2 = 1.5 x [HCO3] + 8 (range: +/- 2)

• Comments:

• Maximal compensation may take 12-24 hours to reach

• The limit of compensation is a pCO2 of about 10 mmHg

• Example:

• A patient with a metabolic acidosis ([HCO3] 14mmol/l) has an actual pCO2 of 30mmHg.

• The expected pCO2 is (1.5 x 14 + 8) which is 29mmHg. This basically matches the actual value of 30 so compensation is maximal.

• If the actual pCO2 was 45mmHg and the expected was 29mmHg, then this difference (45-29) would indicate the presence of a respiratory acidosis and indicate its magnitude.

RULE 6 : THE POINT SEVEN PLUS TWENTY ONE RULE FOR A

METABOLIC ALKALOSIS

• Expected pCO2 = 0.7 [HCO3] + 21

(range: +/- 2)

STEP 4

RULE OUT COMBINED DISORDERS

pHN

or

N

pCO2 , HCO3 Comp Met Alkalosis

pCO2 N, HCO3 N N Acid Base Homeostasis

pCO2 , HCO3 Met acidosis

+

Resp alkalosis

Comp Met Acidosis

Comp Resp Alkalosis

Comp Resp Acidosis

Resp Acidosis

+

Met Alkalosis

STEP 5.IN METABOLIC ACIDOSIS CHECK ANION GAP

ANION GAP

• AG traditionally used to assess acid-base status esp in D/D of met acidosis

• AG & HCO3- used to assess mixed acid-base

disorders

AG based on principle of electroneutrality:

• Total Serum Cations = Total Serum Anions

• Na + (K + Ca + Mg) = HCO3 + Cl + (PO4 + SO4 + Protein + Organic

Acids)

• Na + UC = HCO3 + Cl + UA

• Na – (HCO3 + Cl) = UA – UC

• Na – (HCO3 + Cl) = AG Normal value of AG = 12 +/- 4 meq/L

STEP 6.CALCULATE DELTA ANION GAP (GAP-GAP)

• AG - HCO3- RELATIONSHIP - used to assess mixed acid-

base disorders in setting of high AG Met Acidosis:

AG/ HCO3- = 1 Pure High AG Met Acidosis

AG/ HCO3- > 1 Assoc Metabolic Alkalosis

∆AG/ HCO3- < 1 Assoc N AG Met Acidosis

Based on assumption that for each 1 meq/L increase in AG, HCO3

will fall by 1 meq/L

ILLUSTRATIVE CASE

• Paramedic brought a 20-yr old man to the Casualty.

• He was found lying in an alley with an empty liquor bottle nearby.

• BP 120/80, pulse rate 110/min, RR 28/min, Temp 98.6 F.

• Unresponsive, pupils minimally reactive to light. DTR brisk, symmetrical, Plantars normal

• Bilateral basal crepitations.

• History suggestive of ingestion of toxin- may be associated with acid base disorder.

STEP 2• Obtain ABG

• pH 7.1

• PaCO2 34

• PaO2 90

• HCO3- 12

• Cl- 97

• Na 145

• K 5

• BUN 30

• Cr 1.5

• Glucose 110

STEP 3: IDENTIFY THE PRIMARY DISTURBANCE

• pH <7.35 Acidemia

• pH > 7.45 Alkalemia

• Primary process is metabolic: Changes in HCO3- or

respiratory (changes in PaCO2)

• Looks like metabolic acidosis!

EVALUATION OF ABG

Both HCO3- & PaCO2 are low

Primary disorder- metabolic.

Compensation = Change in PaCO2 = 1.5 HCO3

- +8

= 1.5 12+8 = 26 mm Hg

PaCO2 should have been = 26.

But it is 34

Concomitant Respiratory acidosis is also present

pH 7.1

PaCO2 34

PaO2 90

HCO3- 12

Cl- 97

Na 145

K 5

BUN 30

Cr 1.5

Glucose 110

PaCO2= 1.5 x (HCO3-) + 8

EVALUATION OF ABG

AG = 145-(97+12)

AG = 36

High anion gap MA

pH 7.1

PaCO2 34

PaO2 90

HCO3- 12

Na 145

K 5

Cl- 97

BUN 30

Cr 1.5

Glucose 110

For D/d of metabolic acidosis: Anion Gap.

CONCEPT OF DELTA ANION GAPAG EXCESS/HCO3 DEFICIT

Delta ratio

= (Observed AG – normal AG) / (24 - HCO3

-)

(36-12) / (24 -12)

24/12 = 2.0

AG has out of proportion to decrease in HCO3

- (Suggest presence of concomitant metabolic alkalosis)

Additional metabolic alkalosis is also present

pH 7.1

PaCO2 34

PaO2 90

HCO3- 12

Na 145

K 5

Cl- 97

BUN 30

Cr 1.5

Glucose 110

IT’S NOT MAGIC UNDERSTANDING ABG’S, IT

JUST TAKES A LITTLE PRACTICE!